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ABSTRACT Seafood is an important resource for global nutrition and food security, with both land and marine aquaculture playing pivotal roles. High visual acuity is key for health and survival of farmed, cultured, and wild fish. Cleaner fish technology to control parasite infestation has become important in marine aquaculture and highlights the importance of visual acuity in the efficacy of cleaner fish species. New clinical diagnostic approaches towards understanding and optimising fish visual health could benefit both aquacultured and wild fish populations. Opportunities for developing and using advanced non‐invasive clinical assessment and diagnosis of ocular health in wild, cultured, and experimental fish are key to more rapidly realising how threats to eye health in these animals might be better understood and mitigated. Ophthalmoscopy can rapidly and non‐invasively image anatomical aspects of retinal and anterior ocular tissues and has been used in mammalian biomedicine since the turn of the 20th century. More now than ever, labour‐intensive post‐mortem approaches for ocular analysis such as histology are increasingly being replaced or supplemented by application of various forms of optical coherence tomography (OCT) imaging of ocular tissues in mammalian biomedicine. Advances and availability of other methodological approaches such as three‐dimensional printing and computer science make instrument customisation affordable and adaptable. This review article will outline how ophthalmoscopy, OCT, and other methodologies are being applied towards understanding ocular health in teleost fish species and will describe some of the future opportunities that technological advances might afford in advancing ocular imaging in fish health and disease in general. [ABSTRACT FROM AUTHOR]

The karyotype, which is the number and shape of chromosomes, is a fundamental characteristic of all eukaryotes. Karyotypic changes play an important role in many aspects of evolutionary processes, including speciation. In organisms with monocentric chromosomes, it was previously thought that chromosome number changes were mainly caused by centric fusions and fissions, whereas chromosome shape changes, that is, changes in arm numbers, were mainly due to pericentric inversions. However, recent genomic and cytogenetic studies have revealed examples of alternative cases, such as tandem fusions and centromere repositioning, found in the karyotypic changes within and between species. Here, we employed comparative genomic approaches to investigate whether centromere repositioning occurred during karyotype evolution in medaka fishes. In the medaka family (Adrianichthyidae), the three phylogenetic groups differed substantially in their karyotypes. The Oryzias latipes species group has larger numbers of chromosome arms than the other groups, with most chromosomes being metacentric. The O. javanicus species group has similar numbers of chromosomes to the O. latipes species group, but smaller arm numbers, with most chromosomes being acrocentric. The O. celebensis species group has fewer chromosomes than the other two groups and several large metacentric chromosomes that were likely formed by chromosomal fusions. By comparing the genome assemblies of O. latipes, O. javanicus, and O. celebensis, we found that repositioning of centromere‐associated repeats might be more common than simple pericentric inversion. Our results demonstrated that centromere repositioning may play a more important role in karyotype evolution than previously appreciated. [ABSTRACT FROM AUTHOR]

Teleost B cells producing neutralizing antibodies contribute to protection against salmonid alphavirus (SAV) infection, the etiological agent of pancreas disease, thereby reducing mortality and disease severity. Our previous studies show differences in B cell responses between the systemic immune tissues (head kidney (HK) and spleen) and the peritoneal cavity (PerC) after intraperitoneal SAV3 infection in Atlantic salmon (Salmo salar) where the response in PerC dominates at the late time points. By employing the same infection model, we aimed to further characterize these B cells. Immunophenotyping of teleost B cells is challenging due to limited availability of markers; however, RNA-seq opens an opportunity to explore differences in transcriptomic responses of these cells. Our analysis identified 334, 259 and 613 differentially expressed genes (DEGs) in Atlantic salmon IgM+IgD+ B cells from HK, spleen, and PerC, respectively, at 6 weeks post SAV3 infection. Of these, only 34 were common to all the three immune sites. Additionally, out of the top 100 genes with the highest fold change in expression, only four genes were common across B cells from the three sites. Functional enrichment analyses of DEGs using KEGG and GO databases demonstrated differences in enriched innate immune signaling and the cytokine-cytokine interaction pathways in B cells across the sites, with varying numbers of genes involved. Overall, these findings show the presence of transcriptionally distinct B cell subsets with innate immune functions in HK, spleen and PerC of SAV3-infected Atlantic salmon. Further, our data provide new insights into the immunoregulatory role of fish B cells through the differential expression of various cytokine ligands and receptors and will be a useful resource for further studies into B cell immune compartments. [ABSTRACT FROM AUTHOR]

Rapid global expansion of offshore wind farms, tidal, and wave technologies signifies a new era of renewable energy development. While a promising means to combat the impacts of climate change, such developments necessitate fine-scale monitoring of biological communities to determine impacts associated with construction, operation, and eventual decommission. Here, we evaluate the performance of a gridded, Innovasea Systems, Inc. fine-scale acoustic telemetry positioning system (FSPS, n = 20 acoustic receivers) for tracking behaviors of diverse, temperate fish assemblages in relation to a subsea cable route supporting the Ørsted offshore wind development in coastal New York. We examined array performance through positioning error derived from receiver reference transmitters and tracked animals (n = 260) comprising 17 species of teleost and elasmobranch. We evaluated the effects of environmental variables (temperature, tilt, noise, and depth), transmitter power, individual movement rates, and receiver loss on horizontal positioning error (HPE) and route mean squared error (RMSE). Across a 16-month deployment period, many positions were derived for Atlantic sturgeon (n = 2,612), black sea bass (n = 9,175), clearnose skate (n = 10,306), summer flounder (n = 13,304), and little skate (n = 15,186), suggesting that these species may serve as sentinel candidates for assessing behavioral changes following construction, operation, and decommission. We found that receivers placed at the boundary of the grid exhibited higher HPE and RMSE, however these errors did not significantly change despite large receiver losses (25%). Generalized Linear Models revealed that temperature, noise, tilt, and depth were often significant predictors of HPE and RMSE, however, a substantial amount of variance was not explained by the models (~ 70%). Average movement rates ranged from 1.1 m s−1 (common thresher shark) to 0.03 m s−1 (little skate and summer flounder) but had minimal effects on positioning error. Finally, we observed that higher transmitter powers (158 dB) may lead to higher and more variable HPE values. Overall, these findings provide new insight into the drivers of FSPS array performance and illustrate their broad utility for monitoring fish behavior associated with offshore marine developments. [ABSTRACT FROM AUTHOR]

Gonadotropins and progestins are the primary regulators of follicle maturation and ovulation in fish, and they require complex communication among the oocyte and somatic cells of the follicle. The major progestin and the maturation-inducing hormone in salmonids is 17α,20β-dihdroxy-4-pregnen-3-one (17,20βP), and traditional nuclear receptors and membrane steroid receptors for the progestin have been identified within the follicle. Herein, RNA-seq was used to conduct a comprehensive survey of changes in gene expression throughout the intact follicle in response to in vitro treatment with these hormones to provide a foundation for understanding the coordination of their actions in regulating follicle maturation and preparation for ovulation. A total of 5292 differentially expressed genes were identified from our transcriptome sequencing datasets comparing four treatments: fresh tissue; untreated control; 17,20βP-treated; and salmon pituitary homogenate-treated follicles. Extensive overlap in affected genes suggests many gonadotropin actions leading to the acquisition of maturational and ovulatory competence are mediated in part by gonadotropin induction of 17,20βP synthesis. KEGG analysis identified signaling pathways, including MAPK, TGFβ, FoxO, and Wnt signaling pathways, among the most significantly enriched pathways altered by 17,20βP treatment, suggesting pervasive influences of 17,20βP on actions of other endocrine and paracrine factors in the follicle complex. [ABSTRACT FROM AUTHOR]

The study describes the anatomy vertebrae of the Common Carp (Cyprinus carpio; Linnaeus, 1758) using 3D micro-CT and Semiquantitative Elemental Microanalysis (EDX), which helps to understand the ontogeny and factors that determine the biomechanics of bones in wild fish. The study of Common carp vertebrae revealed a complex and heterogeneous structure, with a variety of components contributing to their function and stability. The vertebral bodies were composed of lamellar trabeculae and internal hollow spaces, while the neural and hemal arches were composed of bony trabeculae and connected by a lace-like structure. The results of EDX analysis demonstrated the presence of oxygen, nitrogen, sodium, carbon, magnesium, and chlorine, among others. The calcium to phosphorus ratio was within the ranges described in teleost, suggesting good bone health. The presence of magnesium, sodium, potassium, chlorine, and nitrogen is due to their essential role in different biological processes. In addition, a difference was observed between the trabeculae present in the vertebrae of the abdominal and caudal segments, suggesting a relationship between the density of each bone and its function. These findings contribute to the understanding of the structure and function of vertebrae in wild Common carp and may have implications for understanding the evolution of the vertebral column in fish in general. [ABSTRACT FROM AUTHOR]

GnRH governs reproduction by regulating pituitary gonadotropins. Unlike most vertebrates, gnrh −/− zebrafish are fertile. To elucidate the role of the hypophysiotropic-Gnrh3 and other mechanisms regulating pituitary gonadotropes, we profiled the gene expression of all individual pituitary cells of wild-type and gnrh3 −/− adult female zebrafish. The single-cell RNA sequencing showed that LH and FSH gonadotropes express the 2 gonadotropin beta subunits with a ratio of 140:1 (lhb : fshb) and 4:1 (fshb : lhb), respectively. Lh gonadotropes predominantly express genes encoding receptors for GnRH (gnrhr2), thyroid hormone, estrogen, and steroidogenic factor 1. No GnRH receptor transcript was enriched in FSH gonadotropes. Instead, cholecystokinin receptor-b and galanin receptor-1b transcripts were enriched in these cells. The loss of the Gnrh3 gene in gnrh3−/− zebrafish resulted in downregulation of fshb in LH gonadotropes and upregulation of pituitary hormones like TSH, GH, prolactin, and proopiomelanocortin-a. Likewise, targeted chemogenetic ablation of Gnrh3 neurons led to a decrease in the number of fshb +, lhb + and fshb +/ lhb + cells. Our studies suggest that Gnrh3 directly acts on LH gonadotropes through Gnrhr2, but the outcome of this interaction is still unknown. Gnrh3 also regulates fshb expression in both gonadotropes, most likely via a non-GnRH receptor route. Altogether, while LH secretion and synthesis are likely regulated in a GnRH-independent manner, Gnrh3 seems to play a role in the cellular organization of the pituitary. Moreover, the coexpression of lhb and fshb in both gonadotropes provides a possible explanation as to why gnrh3−/− zebrafish are fertile. [ABSTRACT FROM AUTHOR]

Salinity can shape species' distributions via physiological costs. Understanding how and when these costs occur can help predict colonisation patterns, for example during species invasions. To understand the impacts of high salinity environments on the ongoing expansion of the invasive round goby (Neogobius melanostomus) towards the North Sea, fish from this invasion front were spawned in a common-garden setup at different salinities. Reproductive adults were acclimated to a control (20 PSU) salinity representing the current invasion front conditions, and a North Sea coastal salinity (30 PSU). In these salinities they were sorted into spawning groups, and spawning attempts and egg development was tracked during 1.5 months. The results showed a strong impact of the 30 PSU salinity treatment, with a complete developmental failure of all clutches spawned in this salinity. The failed clutches resulted in a significant increase of egg consumption by the nest-guarding males. In addition, sperm velocity showed a decrease with increasing salinity when tested in both spawning salinities and at an intermediate level. Few differences in adult physiology were found as a consequence of the salinity treatments. These results show that, despite adult acclimation to North Sea coastal conditions, the gametes of N. melanostomus cannot as of now cope with the salinities into which their spread is currently predicted. Though adaptation or transgenerational acclimation might allow for reproduction in salinities higher than the species currently occupy, we suggest current models to incorporate a reproductive limit for N. melanostomus at a salinity of 30 PSU. [ABSTRACT FROM AUTHOR]

The perception and discrimination of thousands of different odorants by the vertebrate olfactory system is unrivaled in terms of molecular complexity and involves activation of specific odorant receptors. A variety of olfactory receptor gene families are reported in teleost, all of which are the G-protein coupled receptors. The distribution of odorant receptors is uneven through the olfactory epithelium. Distinct odorant receptor neurons (ORNs) possess individual type of odorant receptor linked to specific G-proteins and activated in an odorant specific manner. Though teleost lack a vomeronasal organ yet the respective mammalian receptor homologs are expressed by one of the specialized types of ORN present in the olfactory epithelium. The gene repertoire sizes and their evolution vary greatly between the different odorant receptor gene families. This review foregathers the information on teleost odorant receptor repertoires, the spatial distribution of olfactory receptor neurons and their signal conveying mechanisms to the telencephalon. [ABSTRACT FROM AUTHOR]

Pectoral fins, the anterior paired fins in fish, have enhanced maneuvering abilities due to morphological changes. Teleosts have fewer radial bones in their pectoral fins than basal species, resulting in more-elaborate fins. The mechanism behind this radial constraint change in teleosts is unclear. Here, we found that mutations in hhip, which encodes an antagonist of Hedgehog signaling, led to an increase in radial bones in a localized region. Expression of the Shh genes, encoding ligands of Hedgehog signaling, coincided with notable hhip expression specifically during early development. We suggest that a negative feedback effect of Hedgehog signaling by hhip regulates the constraint of the pectoral fin in zebrafish. Additionally, re-analysis of hhip-related gene expression data in zebrafish and basal species revealed that the notable hhip expression during early development is a characteristic of zebrafish that is not observed in basal species. Region-specific expression of Hox13 genes in the zebrafish pectoral fin indicated that the median region, analogous to the region with abundant radials in basal species, is expanded in hhip-/-zebrafish. These data underscore potential morphological evolution through constrained diversity. [ABSTRACT FROM AUTHOR]

Simple Summary: Streptococcus agalactiae, a significant pathogen affecting humans and aquatic species, is causing high morbidity and mortality in fish, particularly Nile tilapia (Oreochromis niloticus). This study focuses on S. agalactiae infection in cultured O. niloticus, examining transmission, sources, risk factors, clinical signs, pathogenesis, virulence factors, and methods for diagnosis, treatment, control, and prevention. Streptococcus agalactiae (Group B Lancefield) has emerged as a significant pathogen affecting both humans and animals, including aquatic species. Infections caused by S. agalactiae are becoming a growing concern in aquaculture and have been reported globally in various freshwater and marine fish species, particularly those inhabiting warm water environments. This has led to numerous outbreaks with high morbidity and mortality in fish. Nile tilapia (Oreochromis niloticus), a member of the Cichlid family, is one of the severely affected fish species by S. agalactiae. The current study aims to focus on S. agalactiae infection in cultured O. niloticus with reference to its transmission and sources of infection; risk factors influencing GBS infection, disease clinical signs, lesions, and pathogenesis; S. agalactiae virulence factors; and how to diagnose, treat, control, and prevent infection including vaccination and herbal extract medication. [ABSTRACT FROM AUTHOR]

Chromatophoromas are neoplasms arising from pigment cells in vertebrates. Iridophoromas are a type of chromatophoroma that are reported in several teleost species. There are multiple case reports of this diagnosis in betta fish (Betta splendens), but all previously reported cases originated from the skin. This is the first report of an intra-ocular iridophoroma. An adult betta fish had buphthalmia of the right eye and enucleation was performed. The fish survived surgery but was found dead 2 days later. The eye and entire body were examined histologically, and the right eye was also examined ultrastructurally. Histologically, the uveal tract of the right eye was unilaterally and markedly expanded by a neoplasm that expanded and obliterated the iris and choroid and regionally invaded the cornea and sclera. The neoplasm was composed of spindle cells that contained pale green, birefringent, crystalline granules (iridophores). Transmission electron microscopy revealed that neoplastic cells contained a single elongated nucleus and one or more thin cytoplasmic bundles of reflecting plates that were oriented parallel to the basal lamina, further confirming the diagnosis of iridophoroma. This is the first reported case of an iridophoroma arising from the uveal tract. Most cases of iridophoromas and other chromatophoromas in fish are reported as benign. However, there are no established histological criteria of malignancy in these neoplasms. Despite the bland cellular morphology, most reported cases of iridophoromas and other chromatophoromas in bettas (including this case) had substantial tissue invasion and/or destruction. This suggests that iridophoromas and chromatophoromas in bettas may have local invasion, consistent with malignancy, despite bland cytological features. [ABSTRACT FROM AUTHOR]

Among vertebrates, ray-finned fishes (Actinopterygii) display the highest diversity in parental care, and their diversification has been hypothesized to be related to phylogenetic changes in fertilization modes. Using the most comprehensive, sex-specific data from 7,600 species of 62 extant orders of ray-finned fishes, we inferred ancestral states and transitions among care types and caring episodes (i.e. the stage of offspring development). Our work has uncovered 3 novel findings. First, transitions among different care types (i.e. male-only care, female-only care, biparental care, and no care) are common, and the frequencies of these transitions show unusually diverse patterns concerning fertilization modes (external, or internal via oviduct, mouth, or brood pouch). Second, both oviduct and mouth fertilization are selected for female-biased care, whereas fertilization in a brood pouch is selected for male-biased care. Importantly, internal fertilization without parental care is extremely unstable phylogenetically. Third, we show that egg care in both sexes is associated with nest building (which is male-biased) and fry care (which is female-biased). Taken together, the aquatic environment, which supports considerable flexibility in care, facilitated the diversification of parenting behavior, creating the evolutionary bases for more comprehensive parenting to protect offspring in semiterrestrial or terrestrial environments. [ABSTRACT FROM AUTHOR]

Carbonic anhydrase (CA) activity is ubiquitously found in all vertebrate species, tissues and cellular compartments. Most species have plasma-accessible CA (paCA) isoforms at the respiratory surfaces, where the enzyme catalyzes the conversion of plasma bicarbonate to carbon dioxide (CO2) that can be excreted by diffusion. A notable exception are the teleost fishes that appear to lack paCA at their gills. The present review: (i) recapitulates the significance of CA activity and distribution in vertebrates; (ii) summarizes the current evidence for the presence or absence of paCA at the gills of fishes, from the basal cyclostomes to the derived teleosts and extremophiles such as the Antarctic icefishes; (iii) explores the contribution of paCA to organismal CO2 excretion in fishes; and (iv) the functional significance of its absence at the gills, for the specialized system of O2 transport in most teleosts; (v) outlines the multiplicity and isoform distribution of membrane-associated CAs in fishes and methodologies to determine their plasma-accessible orientation; and (vi) sketches a tentative time line for the evolutionary dynamics of branchial paCA distribution in the major groups of fishes. Finally, this review highlights current gaps in the knowledge on branchial paCA function and provides recommendations for future work. [ABSTRACT FROM AUTHOR]

The fish gill is a multifunctional organ that is important in multiple physiological processes such as gas transfer, ionoregulation, and chemoreception. This characteristic organ of fishes has received much attention, yet an often-overlooked point is that larval fishes in most cases do not have a fully developed gill, and thus larval gills do not function identically as adult gills. In addition, large changes associated with gas exchange and ionoregulation happen in gills during the larval phase, leading to the oxygen and ionoregulatory hypotheses examining the environmental constraint that resulted in the evolution of gills. This review thus focuses exclusively on the larval fish gill of teleosts, summarizing the development of teleost larval fish gills and its function in gas transfer, ionoregulation, and chemoreception, and comparing and contrasting it to adult gills where applicable, while providing some insight into the oxygen vs ionoregulatory hypotheses debate. [ABSTRACT FROM AUTHOR]

In goldfish, spinal cord injury triggers the formation of a fibrous scar at the injury site. Regenerating axons are able to penetrate the scar tissue, resulting in the recovery of motor function. Previous findings suggested that regenerating axons enter the scar through tubular structures surrounded by glial elements with laminin-positive basement membranes and that glial processes expressing glial fibrillary acidic protein (GFAP) are associated with axonal regeneration. How glia contribute to promoting axonal regeneration, however, is unknown. Here, we revealed that glial processes expressing vimentin or brain lipid-binding protein (BLBP) also enter the fibrous scar after spinal cord injury in goldfish. Vimentin-positive glial processes were more numerous than GFAP- or BLBP-positive glial processes in the scar tissue. Regenerating axons in the scar tissue were more closely associated with vimentin-positive glial processes than GFAP-positive glial processes. Vimentin-positive glial processes co-expressed matrix metalloproteinase (MMP)-14. Our findings suggest that vimentin-positive glial processes closely associate with regenerating axons through tubular structures entering the scar after spinal cord injury in goldfish. In intact spinal cord, ependymo-radial glial cell bodies express BLBP and their radial processes express vimentin, suggesting that vimentin-positive glial processes derive from migrating ependymo-radial glial cells. MMP-14 expressed in vimentin-positive glial cells and their processes might provide a beneficial environment for axonal regeneration. [ABSTRACT FROM AUTHOR]

Aim: To identify the physiological role of the acid‐base sensing enzyme, soluble adenylyl cyclase (sAC), in red blood cells (RBC) of the model teleost fish, rainbow trout. Methods: We used: (i) super‐resolution microscopy to determine the subcellular location of sAC protein; (ii) live‐cell imaging of RBC intracellular pH (pHi) with specific sAC inhibition (KH7 or LRE1) to determine its role in cellular acid‐base regulation; (iii) spectrophotometric measurements of haemoglobin–oxygen (Hb‐O2) binding in steady‐state conditions; and (iv) during simulated arterial‐venous transit, to determine the role of sAC in systemic O2 transport. Results: Distinct pools of sAC protein were detected in the RBC cytoplasm, at the plasma membrane and within the nucleus. Inhibition of sAC decreased the setpoint for RBC pHi regulation by ~0.25 pH units compared to controls, and slowed the rates of RBC pHi recovery after an acid‐base disturbance. RBC pHi recovery was entirely through the anion exchanger (AE) that was in part regulated by HCO3−‐dependent sAC signaling. Inhibition of sAC decreased Hb‐O2 affinity during a respiratory acidosis compared to controls and reduced the cooperativity of O2 binding. During in vitro simulations of arterial‐venous transit, sAC inhibition decreased the amount of O2 that is unloaded by ~11%. Conclusion: sAC represents a novel acid‐base sensor in the RBCs of rainbow trout, where it participates in the modulation of RBC pHi and blood O2 transport though the regulation of AE activity. If substantiated in other species, these findings may have broad implications for our understanding of cardiovascular physiology in vertebrates. [ABSTRACT FROM AUTHOR]

Abstract Rapid global expansion of offshore wind farms, tidal, and wave technologies signifies a new era of renewable energy development. While a promising means to combat the impacts of climate change, such developments necessitate fine-scale monitoring of biological communities to determine impacts associated with construction, operation, and eventual decommission. Here, we evaluate the performance of a gridded, Innovasea Systems, Inc. fine-scale acoustic telemetry positioning system (FSPS, n = 20 acoustic receivers) for tracking behaviors of diverse, temperate fish assemblages in relation to a subsea cable route supporting the Ørsted offshore wind development in coastal New York. We examined array performance through positioning error derived from receiver reference transmitters and tracked animals (n = 260) comprising 17 species of teleost and elasmobranch. We evaluated the effects of environmental variables (temperature, tilt, noise, and depth), transmitter power, individual movement rates, and receiver loss on horizontal positioning error (HPE) and route mean squared error (RMSE). Across a 16-month deployment period, many positions were derived for Atlantic sturgeon (n = 2,612), black sea bass (n = 9,175), clearnose skate (n = 10,306), summer flounder (n = 13,304), and little skate (n = 15,186), suggesting that these species may serve as sentinel candidates for assessing behavioral changes following construction, operation, and decommission. We found that receivers placed at the boundary of the grid exhibited higher HPE and RMSE, however these errors did not significantly change despite large receiver losses (25%). Generalized Linear Models revealed that temperature, noise, tilt, and depth were often significant predictors of HPE and RMSE, however, a substantial amount of variance was not explained by the models (~ 70%). Average movement rates ranged from 1.1 m s−1 (common thresher shark) to 0.03 m s−1 (little skate and summer flounder) but had minimal effects on positioning error. Finally, we observed that higher transmitter powers (158 dB) may lead to higher and more variable HPE values. Overall, these findings provide new insight into the drivers of FSPS array performance and illustrate their broad utility for monitoring fish behavior associated with offshore marine developments.

Introduction Zebrafishes represent a proven model for human diseases and systems biology, exhibiting physiological and genetic similarities and having innate and adaptive immune systems. However, they are underexplored for human vaccinology, vaccine development, and testing. Here we summarize gaps and challenges.Areas covered Zebrafish models have four potential applications: 1) Vaccine safety: The past successes in using zebrafishes to test xenobiotics could extend to vaccine and adjuvant formulations for general safety or target organs due to the zebrafish embryos’ optical transparency. 2) Innate immunity: The zebrafish offers refined ways to examine vaccine effects through signaling via Toll-like or NOD-like receptors in zebrafish myeloid cells. 3) Adaptive immunity: Zebrafishes produce IgM, IgD,and two IgZ immunoglobulins, but these are understudied, due to a lack of immunological reagents for challenge studies. 4) Systems vaccinology: Due to the availability of a well-referenced zebrafish genome, transcriptome, proteome, and epigenome, this model offers potential here.Expert Opinion It remains unproven whether zebrafishes can be employed for testing and developing human vaccines. We are still at the hypothesis-generating stage, although it is possible to begin outlining experiments for this purpose. Through transgenic manipulation, zebrafish models could offer new paths for shaping animal models and systems vaccinology.

Histopathological studies of parasitic infections in fish from the natural environment of Brazilian Amazon, are quite scarce. In this study, we investigated the histopathological changes of the proximal intestine of specimens of the Amazonian fish Hoplias malabaricus infected by the hematophagous nematode Procamallanus (Spirocamallanus) paraensis. The prevalence of the infection was 60%, with an average abundance of 1.46 and an average intensity of 2.43 parasites/fish. No significant difference was observed in the prevalence of infection and fish size or sex, but larger fish showed greater infection intensity, which was also significantly higher in male hosts. Histological sections of the proximal intestine showed reduction and loss of the epithelial lining, exposure of the lamina propria where the nematode interacts with the intestine wall, through the insertion of the buccal capsule and fish cellular debris in the intestinal lumen. In addition, areas with bleeding and inflammatory infiltrate were observed, but no changes or presence of parasite structures were observed in the other tunics of the intestinal wall. [ABSTRACT FROM AUTHOR]

Abstract Background The family Batrachoididae are a group of ecologically important teleost fishes with unique life histories, behavior, and physiology that has made them popular model organisms. Batrachoididae remain understudied in the realm of genomics, with only four reference genome assemblies available for the family, with three being highly fragmented and not up to current assembly standards. Among these is the Gulf toadfish, Opsanus beta, a model organism for serotonin physiology which has recently been bred in captivity. Results Here we present a new, de novo genome and transcriptome assemblies for the Gulf toadfish using PacBio long read technology. The genome size of the final assembly is 2.1 gigabases, which is among the largest teleost genomes. This new assembly improves significantly upon the currently available reference for Opsanus beta with a final scaffold count of 62, of which 23 are chromosome scale, an N50 of 98,402,768, and a BUSCO completeness score of 97.3%. Annotation with ab initio and transcriptome-based methods generated 41,076 gene models. The genome is highly repetitive, with ~ 70% of the genome composed of simple repeats and transposable elements. Satellite DNA analysis identified potential telomeric and centromeric regions. Conclusions This improved assembly represents a valuable resource for future research using this important model organism and to teleost genomics more broadly.

Background: The family Batrachoididae are a group of ecologically important teleost fishes with unique life histories, behavior, and physiology that has made them popular model organisms. Batrachoididae remain understudied in the realm of genomics, with only four reference genome assemblies available for the family, with three being highly fragmented and not up to current assembly standards. Among these is the Gulf toadfish, Opsanus beta, a model organism for serotonin physiology which has recently been bred in captivity. Results: Here we present a new, de novo genome and transcriptome assemblies for the Gulf toadfish using PacBio long read technology. The genome size of the final assembly is 2.1 gigabases, which is among the largest teleost genomes. This new assembly improves significantly upon the currently available reference for Opsanus beta with a final scaffold count of 62, of which 23 are chromosome scale, an N50 of 98,402,768, and a BUSCO completeness score of 97.3%. Annotation with ab initio and transcriptome-based methods generated 41,076 gene models. The genome is highly repetitive, with ~ 70% of the genome composed of simple repeats and transposable elements. Satellite DNA analysis identified potential telomeric and centromeric regions. Conclusions: This improved assembly represents a valuable resource for future research using this important model organism and to teleost genomics more broadly. [ABSTRACT FROM AUTHOR]

Chemokines are cytokines that mediate leukocyte traffic between the lymphoid organs, the bloodstream, and the site of tissue damage, which is essential for an efficient immune response. In particular, the gamma interferon (IFN-g) inducible chemokines CXCL9, CXCL10, and CXCL11, and their receptor CXCR3, are involved in T cell and macrophage recruitment to the site of infection. The nature and function of these chemokines and their receptor are well-known in mammals, but further research is needed to achieve a similar level of understanding in fish immunity. Thus, in this study, we seek to identify the genes encoding the components of the Atlantic salmon (Salmo salar) CXCL9, CXCL10, CXCL11/CXCR3 axis (CXCL9-11/CXCR3), predict the protein structure from the amino acid sequence, and explore the regulation of gene expression as well as the response of these chemokines and their receptor to viral infections. The cxcl9, cxcl10, cxcl11, and cxcr3 gene sequences were retrieved from the databases, and the phylogenetic analysis was conducted to determine the evolutionary relationships. The study revealed an interesting pattern of clustering and conservation among fish and mammalian species. The salmon chemokine sequences clustered with orthologs from other fish species, while the mammalian sequences formed separate clades. This indicates a divergent evolution of chemokines between mammals and fish, possibly due to different evolutionary pressures. While the structural analysis of the chemokines and the CXCR3 receptor showed the conservation of critical motifs and domains, suggesting preserved functions and stability throughout evolution. Regarding the regulation of gene expression, some components of the CXCL9-11/CXCR3 axis are induced by recombinant gamma interferon (rIFN-γ) and by Infectious pancreatic necrosis virus (IPNV) infection in Atlantic salmon cells. Further studies are needed to explore the role of Atlantic salmon CXCL9-11 chemokines in regulating immune cell migration and endothelial activation, as seen in mammals. To the best of our knowledge, there have been no functional studies of chemokines to understand these effects in Atlantic salmon. [ABSTRACT FROM AUTHOR]

Skin microbiomes provide vital functions, yet knowledge about the drivers and processes structuring their species assemblages is limited—especially for non‐model organisms. In this study, fish skin microbiome was assessed by high throughput sequencing of amplicon sequence variants from metabarcoding of V3–V4 regions in the 16S rRNA gene on fish hosts subjected to the following experimental manipulations: (i) translocation between fresh and brackish water habitats to investigate the role of environment; (ii) treatment with an antibacterial disinfectant to reboot the microbiome and investigate community assembly and priority effects; and (iii) maintained alone or in pairs to study the role of social environment and inter‐host dispersal of microbes. The results revealed that fish skin microbiomes harbour a highly dynamic microbial composition that was distinct from bacterioplankton communities in the ambient water. Microbiome composition first diverged as an effect of translocation to either the brackish or freshwater habitat. When the freshwater individuals were translocated back to brackish water, their microbiome composition converged towards the fish microbiomes in the brackish habitat. In summary, external environmental conditions and individual‐specific factors jointly determined the community composition dynamics, whereas inter‐host dispersal had negligible effects. The dynamics of the microbiome composition was seemingly non‐affected by reboot treatment, pointing towards high resilience to disturbance. The results emphasised the role of inter‐individual variability for the unexplained variation found in many host‐microbiome systems, although the mechanistic underpinnings remain to be identified. [ABSTRACT FROM AUTHOR]

Fish, first jawed vertebrate group appearing in evolution, have immune system similar to those of mammals. These early animals' innate and adaptive immune systems are completely developed, having head kidney and thymus as primary, spleen and mucosa associated lymphoid tissue as secondary immune organs. The adaptive immunity performs key role in defence against recurring infections and elimination of fish pathogens with the help of its memory cells, T‐cell‐receptors and immunoglobulins. Epidermal secretions including antibacterial peptides, lysozymes, lectins, complement and C‐reactive proteins which function in disruption of antigens, phagocytosis, inflammatory response and mending of tissue injury play a significant role as elements of fish immune system. Fish, however, have greater innate immunity than humans do. Estrogens play crucial role in immunomodulation through nuclear oestrogen receptors, which are found in majority of immune cells and lymphoid organs in fish. There is mounting proof that xenoestrogens, that can attach as agonists to oestrogen receptors, pose significant ecotoxicological risk by disrupting the defence mechanism in fish. Though less potent than natural oestrogen, they can bioaccumulate to finally reach a substantial dose. This review's objective is to give an overview of the fish defence system, its estrogenic regulation and estrogenic‐endocrine disruption. [ABSTRACT FROM AUTHOR]

Simple Summary: Mercury represent a serious threat for marine ecosystems due to its persistence in the environment. Fishes are the most numerous and widely distributed group of vertebrates, living in the sea with different species often studied and used as bioindicators of the quality of aquatic systems being able to reflect even small changes in environmental parameters. Mitochondria are small cell organelles with their own DNA and the number of mitochondria within a cell is highly variable in different animal tissues, depending on metabolic requirements. Mitochondrial genome is vulnerable to reactive oxygen species (ROS) which in turn impair mitochondrial function. Therefore, the aim of the present study was the validation of the variation in the number of mitochondrial DNA copies (mtDNAcn) as biomarker of oxidative stress in aquatic environment. Three selected fish species were collected in Augusta Bay, a contaminated area remarkable by past Hg inputs, and in a control area (Marzamemi and Portopalo di Capo Passero), both in the South-East of Sicily. Based on the evidence found, the assessment of mtDNAcn variation emerges as a valid biomarker of oxidative stress deriving from contaminant exposure. Mercury (Hg) pro-oxidant role on biological systems and its biogeochemical cycle represent a serious threat due to its persistence in marine environment. As the mitochondrial genome is exposed to reactive oxygen species (ROS), the aim of the present study is the validation of the variation in the number of mitochondrial DNA copies (mtDNAcn) as biomarker of oxidative stress in aquatic environment. During summer 2021, three selected fish species (Mullus barbatus, Diplodus annularis and Pagellus erythrinus) were collected in Augusta Bay, one of the most Mediterranean contaminated areas remarkable by past Hg inputs, and in a control area, both in the south-east of Sicily. The relative mtDNAcn was evaluated by qPCR on specimens of each species from both sites, characterized respectively by higher and lower Hg bioaccumulation. M. barbatus and P. erythrinus collected in Augusta showed a dramatic mtDNAcn reduction compared to their control groups while D. annularis showed an incredible mtDNAcn rising suggesting a higher resilience of this species. These results align with the mitochondrial dynamics of fission and fusion triggered by environmental toxicants. In conclusion, we suggest the implementation of the mtDNAcn variation as a valid tool for the early warning stress-related impacts in aquatic system. [ABSTRACT FROM AUTHOR]

Simple Summary: The present study investigates the reproductive and immune defense systems in the gonads of black scorpionfish (Scorpaena porcus, Linnaeus 1758). This study concentrates on the initial phases of reproduction in both male and female fish and investigates the presence and function of an antimicrobial peptide, Piscidin-1. Histological analyses demonstrate morphological changes from the immature to developing stages in gonads. Immunohistochemical methods demonstrate strong reactivity to Piscidin-1 in germinal and somatic cells during early ontogeny, suggesting its crucial role in local defense mechanisms. This research contributes to the understanding of the interaction between reproductive biology and immune defense in S. porcus, offering insights that could benefit species conservation and management efforts. One of the most crucial biological indicators in tracking long-term variations in the reproductive cycle is sexual development. Scorpaena porcus (Linnaeus, 1758), commonly known as the black scorpionfish, is a small teleost from the family Scorpaenidae. Much is known about its ecology, but data on its reproductive and defense systems are still lacking. Antimicrobial peptides (AMPs), such as piscidins, are integral components of the innate immune system in fish. These peptides exhibit a wide range of activity against bacteria, fungi, viruses, and protozoa and act as the first line of host defense. This study aims to investigate the primary sexual development stages in male and female gonads of black scorpionfish, providing additional knowledge on the reproductive biology of this teleost while evaluating concomitant changes in the expression of a Piscidin-1 antimicrobial peptide. The results show a histological, morpho-structural change from the immature stage to the developing virgin stage. Immunohistochemical analyses show that germinal and somatic cells are strongly reactive to Piscidin-1 in both gonads at an early ontogeny stage. These data suggest that Piscidin-1 may play a key role in the local defense system of scorpionfish gonads at this delicate stage, which is critical for the continuation and maintenance of the species. The present findings are potentially useful for a better understanding of the reproductive cycle of this fish, improving our knowledge of the interaction between the immune system and reproduction. [ABSTRACT FROM AUTHOR]

Salmonids possess a unique respiratory system comprised of three major components: highly pH-sensitive hemoglobins, red blood cell (RBC) intracellular pH (pHi) protection, and a heterogeneous distribution of plasma accessible carbonic anhydrase (paCA), specifically with absence of paCA at the gills. These characteristics are thought to have evolved to enhance oxygen unloading to the tissues while protecting uptake at the gills. Our knowledge of this system is detailed in adults, but little is known about it through development. Developing rainbow trout (Oncorhynchus mykiss) express embryonic RBCs containing hemoglobins that are relatively insensitive to pH; however, availability of gill paCA and RBC pHi protection is unknown. We show that pre-hatch rainbow trout express gill paCA, which is lost in correlation with the emergence of highly pH-sensitive adult hemoglobins and RBC pHi protection. Rainbow trout therefore exhibit a switch in respiratory strategy with hatch. We conclude that gill paCA likely represents an embryonic trait in rainbow trout and is constrained in adults due to their highly pH-sensitive hemoglobins. [ABSTRACT FROM AUTHOR]

Vasoactive-intestinal peptide (Vip) is a pleiotropic peptide with a wide range of distribution and functions. Zebrafish possess 2 isoforms of Vip (a and b), in which Vipa is most homologous to the mammalian form. In female zebrafish, Vipa can stimulate LH secretion from the pituitary but is not essential for female reproduction, as vipa −/− females display normal reproduction. In contrast, we have found that vipa −/− males are severely subfertile and sex ratio of offspring is female-biased. By analyzing all aspects of male reproduction with wild-type (WT) males, we show that the testes of vipa −/− are underdeveloped and contain ∼70% less spermatids compared to WT counterparts. The sperm of vipa −/− males displayed reduced potency in terms of fertilization (by ∼80%) and motility span and duration (by ∼50%). In addition, vipa −/− male attraction to WT females was largely nonexistent, indicating decreased sexual motivation. We show that vipa mRNA and protein is present in Leydig cells and in developing germ cells in the testis of WT, raising the possibility that endogenous Vipa contributes to testicular function. Absence of Vipa in vipa −/− males resulted in downregulation of 3 key genes in the androgen synthesis chain in the testis, 3β-hsd, 17β-hsd1 , and cyp11c1 (11β-hydrogenase), associated with a pronounced decrease in 11-ketotestosterone production and, in turn, compromised reproductive fitness. Altogether, this study establishes a crucial role for Vipa in the regulation of male reproduction in zebrafish, like in mammals, with the exception that Vipa is also expressed in zebrafish testis. [ABSTRACT FROM AUTHOR]

The recent development of single cell sequencing technologies has revolutionized the state-of-art of cell biology, allowing the simultaneous measurement of thousands of genes in single cells. This technology has been applied to study the transcriptome of single cells in homeostasis and also in response to pathogenic exposure, greatly increasing our knowledge of the immune response to infectious agents. Yet the number of these studies performed in aquacultured fishspecies is still very limited. Thus, in the current study, we have used the 10x Genomics single cell RNA sequencing technology to study the response of rainbow trout (Oncorhynchus mykiss) peripheral blood leukocytes (PBLs) to infectious pancreatic necrosis virus (IPNV), an important trout pathogen. The study allowed us to obtain a transcriptomic profile of 12 transcriptionally distinct leukocyte cell subpopulations that included four different subsets of B cells, T cells, monocytes, two populations of dendritic-like cells (DCs), hematopoietic progenitor cells, non-specific cytotoxic cells (NCC), neutrophils and thrombocytes. The transcriptional pattern of these leukocyte subpopulations was compared in PBL cultures that had been exposed in vitro to IPNV for 24 h and mock-infected cultures. Our results revealed that monocytes and neutrophils showed the highest number of upregulated protein-coding genes in response to IPNV. Interestingly, IgM+IgD+ and IgT+ B cells also upregulated an important number of genes to the virus, but amuch fainter response was observed in ccl4+ or plasma-like cells (irf4+ cells). A substantial number of protein-coding genes and genes coding for ribosomal proteins were also transcriptionally upregulated in response to IPNV in T cells and thrombocytes. Interestingly, although genes coding for ribosomal proteins were regulated in all affected PBL subpopulations, the number of such genes transcriptionally regulated was higher in IgM+IgD+ and IgT+ B cells. A further analysis dissected which of the regulated genes were common and which were specific to the different cell clusters, identifying eight genes that were transcriptionally upregulated in all the affected groups. The data provided constitutes a comprehensive transcriptional perspective of how the different leukocyte populations present in blood respond to an early viral encounter in fish. [ABSTRACT FROM AUTHOR]

Simple Summary: As the most diverse vertebrate group, the unique adaptive expansion of euryhaline fishes is critical to understanding vertebrate evolution. In particular, the high degree of consistency of unique paired appendage structures across the extremely morphologically diverse group of teleost fishes has become a fascinating scientific question. Early teleost fishes provide a critical window into the study of this large taxon. Therefore, this study constructs high-quality chromosome-level genomes of Osteoglossiformes (Chitala ornata). It also explores the genomic features of early teleost fishes and traces the unique genetic basis of pectoral fin evolution in teleost fishes at the molecular level, which provides an important basis for understanding the evolution of the origin of early teleosts. Teleosts are the most prolific vertebrates, occupying the vast majority of aquatic environments, and their pectoral fins have undergone remarkable physiological transformations throughout their evolution. Studying early teleost fishes, such as those belonging to the Osteoglossiformes order, could offer crucial insights into the adaptive evolution of pectoral fins within this group. In this study, we have assembled a chromosomal-level genome for the Clown featherback (Chitala ornata), achieving the highest quality genome assembly for Osteoglossiformes to date, with a contig N50 of 32.78 Mb and a scaffold N50 of 40.73 Mb. By combining phylogenetic analysis, we determined that the Clown featherback diverged approximately 202 to 203 million years ago (Ma), aligning with continental separation events. Our analysis revealed the intriguing discovery that a unique deletion of regulatory elements is adjacent to the Gli3 gene, specifically in teleosts. This deletion might be tied to the specialized adaptation of their pectoral fins. Furthermore, our findings indicate that specific contractions and expansions of transposable elements (TEs) in teleosts, including the Clown featherback, could be connected to their adaptive evolution. In essence, this study not only provides a high-quality genomic resource for Osteoglossiformes but also sheds light on the evolutionary trajectory of early teleosts. [ABSTRACT FROM AUTHOR]

The most common method of estimating teleost ages is via sectioned otoliths. With diminishing funding and policies around cost-recovery from fishing industries, exploring cost-effective methods of estimating ages is warranted. The present study used 18 years of size-at-age data collected from monitoring of the commercial halfbeak (Hyporhamphus australis) fishery off New South Wales, Australia, to predict age classes from otolith weights, while considering other sources of variability such as sex, fish length, and year, month, and location of capture. We observed a significant linear relationship between age class and mean otolith weight. A generalized linear mixed model predicted 1-year olds with an 82% success rate; but was less successful for other ages. Year of sampling explained the greatest variability in the model and the distributions of otolith weights for each age class had considerable overlap. We conclude that substantial inter-annual variability in the age-class to otolith weight relationship, in addition to the relatively low precision when aging H. australis by counting annuli in sectioned otoliths, limits the predictive capacity of this model for future monitoring. Nevertheless, substantial cost savings could be made through recalibrating the model for new samples through direct aging of a subset of otoliths each year. The population of H. australis is continuing to rebuild from a previously overfished state, with an expectation that older fish will become more abundant in the fishery. Age estimation from counting annuli in sectioned otoliths is likely to be the most reliable method of identifying older individuals.

Teleost B cells producing neutralizing antibodies contribute to protection against salmonid alphavirus (SAV) infection, the etiological agent of pancreas disease, thereby reducing mortality and disease severity. Our previous studies show differences in B cell responses between the systemic immune tissues (head kidney (HK) and spleen) and the peritoneal cavity (PerC) after intraperitoneal SAV3 infection in Atlantic salmon (Salmo salar) where the response in PerC dominates at the late time points. By employing the same infection model, we aimed to further characterize these B cells. Immunophenotyping of teleost B cells is challenging due to limited availability of markers; however, RNA-seq opens an opportunity to explore differences in transcriptomic responses of these cells. Our analysis identified 334, 259 and 613 differentially expressed genes (DEGs) in Atlantic salmon IgM+IgD+ B cells from HK, spleen, and PerC, respectively, at 6 weeks post SAV3 infection. Of these, only 34 were common to all the three immune sites. Additionally, out of the top 100 genes with the highest fold change in expression, only four genes were common across B cells from the three sites. Functional enrichment analyses of DEGs using KEGG and GO databases demonstrated differences in enriched innate immune signaling and the cytokine-cytokine interaction pathways in B cells across the sites, with varying numbers of genes involved. Overall, these findings show the presence of transcriptionally distinct B cell subsets with innate immune functions in HK, spleen and PerC of SAV3-infected Atlantic salmon. Further, our data provide new insights into the immunoregulatory role of fish B cells through the differential expression of various cytokine ligands and receptors and will be a useful resource for further studies into B cell immune compartments.

The study aims to characterize the immune cell landscape in convalescent Asian seabass (Lates calcarifer) blood samples after exposure to scale-drop disease virus (SDDV). Traditional immunophenotyping approaches used in human and mouse studies are impractical for non-model organisms like the Asian seabass due to the lack of specific antibody-based reagents. To overcome this challenge, 10x Genomics single-cell RNA sequencing was employed. The analysis of blood samples revealed 24 distinct leukocyte clusters, with elevated proportions of B cells, granulocytes, and T cells in the convalescent group compared to the uninfected group. While distinguishing granulocyte and macrophage subsets was challenging, the analysis of differential gene expression in the macrophage population indicated that the upregulated genes were linked to inflammatory processes. Specific T cell clusters showed notable expressions of cd4-1, cd8a, perforin-1 and il-2rβ, suggesting the presence of CD4+ T helper (Th), CD8+ cytotoxic T (Tc) cells, immature T cells, and naive T cells. Attempts to categorize CD4+ T cells into Th subtypes lacked clear distinctions, while CD8+ T cells exhibited three clusters, predominantly Tc1 cells. Furthermore, comparisons between convalescent and uninfected groups revealed increased percentages of activated and antibody-secreting B cells in the convalescent group. This single-cell analysis provides vital insights into the immune cell dynamics in convalescent and uninfected Asian seabass, providing valuable information on potential immune responses to SDDV infection.

The study aims to characterize the immune cell landscape in convalescent Asian seabass (Lates calcarifer) blood samples after exposure to scale-drop disease virus (SDDV). Traditional immunophenotyping approaches used in human and mouse studies are impractical for non-model organisms like the Asian seabass due to the lack of specific antibody-based reagents. To overcome this challenge, 10x Genomics single-cell RNA sequencing was employed. The analysis of blood samples revealed 24 distinct leukocyte clusters, with elevated proportions of B cells, granulocytes, and T cells in the convalescent group compared to the uninfected group. While distinguishing granulocyte and macrophage subsets was challenging, the analysis of differential gene expression in the macrophage population indicated that the upregulated genes were linked to inflammatory processes. Specific T cell clusters showed notable expressions of cd4-1, cd8a, perforin-1 and il-2rβ, suggesting the presence of CD4+ T helper (Th), CD8+ cytotoxic T (Tc) cells, immature T cells, and naive T cells. Attempts to categorize CD4+ T cells into Th subtypes lacked clear distinctions, while CD8+ T cells exhibited three clusters, predominantly Tc1 cells. Furthermore, comparisons between convalescent and uninfected groups revealed increased percentages of activated and antibody-secreting B cells in the convalescent group. This single-cell analysis provides vital insights into the immune cell dynamics in convalescent and uninfected Asian seabass, providing valuable information on potential immune responses to SDDV infection. [ABSTRACT FROM AUTHOR]

Inflammatory bowel disease (IBD) is a nonspecific chronic inflammatory disease resulting from an immune disorder in the intestine that is prone to relapse and incurable. The understanding of the pathogenesis of IBD remains unclear. In this study, we found that ace (angiotensin-converting enzyme), expressed abundantly in the intestine, plays an important role in IBD. The deletion of ace in zebrafish caused intestinal inflammation with increased expression of the inflammatory marker genes interleukin 1 beta (il1b), matrix metallopeptidase 9 (mmp9), myeloid-specific peroxidase (mpx), leukocyte cell-derived chemotaxin-2-like (lect2l), and chemokine (C-X-C motif) ligand 8b (cxcl8b). Moreover, the secretion of mucus in the ace−/− mutants was significantly higher than that in the wild-type zebrafish, validating the phenotype of intestinal inflammation. This was further confirmed by the IBD model constructed using dextran sodium sulfate (DSS), in which the mutant zebrafish had a higher susceptibility to enteritis. Our study reveals the role of ace in intestinal homeostasis, providing a new target for potential therapeutic interventions. [ABSTRACT FROM AUTHOR]

Adult male animals typically court and attempt to mate with females, while attacking other males. Emerging evidence from mice indicates that neurons expressing the estrogen receptor ESR1 in behaviorally relevant brain regions play a central role in mediating these mutually exclusive behavioral responses to conspecifics. However, the findings in mice are unlikely to apply to vertebrates in general because, in many species other than rodents and some birds, androgens--rather than estrogens--have been implicated in male behaviors. Here, we report that male medaka (Oryzias latipes) lacking one of the two androgen receptor subtypes (Ara) are less aggressive toward other males and instead actively court them, while those lacking the other subtype (Arb) are less motivated to mate with females and conversely attack them. These findings indicate that, in male medaka, the Ara- and Arb-mediated androgen signaling pathways facilitate appropriate behavioral responses, while simultaneously suppressing inappropriate responses, to males and females, respectively. Notably, males lacking either receptor retain the ability to discriminate the sex of conspecifics, suggesting a defect in the subsequent decision-making process to mate or fight. We further show that Ara and Arb are expressed in intermingled but largely distinct populations of neurons, and stimulate the expression of different behaviorally relevant genes including galanin and vasotocin, respectively. Collectively, our results demonstrate that male teleosts make adaptive decisions to mate or fight as a result of the activation of one of two complementary androgen signaling pathways, depending on the sex of the conspecific that they encounter. [ABSTRACT FROM AUTHOR]

We studied the topography of retinal ganglion cells (GCs) and estimated spatial resolving power (SRP) in the pajama cardinalfish Sphaeramia nematoptera (Bleeker, 1856), a relatively small brightly colored fish inhabiting coral reefs and lagoons in the Western Pacific. S. nematoptera is an active night predator feeding on near‐bottom animal plankton and benthos. DAPI staining was used to label nuclei of GCs and non‐GCs in the inner plexiform and ganglion cell layers. Non‐GCs were distinguished from GCs in Nissl‐stained retinal wholemounts based on cell size, shape, and staining intensity. The proportion of displaced amacrine cells (DACs) varied from 15.46 ± 1.12 (visual streak [VS]) to 17.99 ± 1.06% (dorsal periphery) (mean ± S.E.M., N = 5); the respective proportions of glial cells were 6.61 ± 0.84 and 5.89 ± 0.76%. Thus, 76%–78% of cells in the ganglion cell layer and inner plexiform layer were GCs. The minimum spatial coverage of GCs (3600–4600 cells/mm2) was detected in the dorsal and ventral periphery. It gradually increased toward the central retina to form a moderate VS. The maximum GC density (11,400–12,400 cells/mm2) was registered in the central portion of the VS. No pronounced concentric retinal specializations were found. The total number of GCs ranged within 595.2–635.9 × 103. The anatomical spatial resolving power was minimum in the ventral periphery (4.91–5.53 cpd) and maximum in the central portion of the VS (8.47–9.07 cpd). The respective minimum separable angles were 0.18–0.20° and 0.11–0.12°. The relatively high spatial resolving power and presence of the VS in the pajama cardinalfish are in line with its highly visual behavior. [ABSTRACT FROM AUTHOR]

The natriuretic peptide (NP) family consists of cardiac NPs (ANP, BNP, and VNP) and brain NPs (CNPs) in teleosts. In addition to CNP1-4, a paralogue of CNP4 (named CNP4b) was recently discovered in basal teleosts including Japanese eel. Mammals have lost most Cnps during the evolution, but teleost cnps were conserved and diversified, suggesting that CNPs are important hormones for maintaining brain functions in teleost. The present study evaluated the potency of each Japanese eel CNP to their NP receptors (NPR-A, NPR-B, NPR-C, and NPR-D) overexpressed in CHO cells. A comprehensive brain map of cnps- and nprs-expressing neurons in Japanese eel was constructed by integrating the localization results obtained by in situ hybridization. The result showed that CHO cells expressing NPR-A and NPR-B induced strong cGMP productions after stimulation by cardiac and brain NPs, respectively. Regarding brain distribution of cnps, cnp1 is engaged in the ventral telencephalic area and periventricular area including the parvocellular preoptic nucleus (Pp), anterior/posterior tuberal nuclei, and periventricular gray zone of the optic tectum. cnp3 is found in the habenular nucleus and prolactin cells in the pituitary. cnp4 is expressed in the ventral telencephalic area, while cnp4b is expressed in the motoneurons in the medullary area. Such CNP isoform-specific localizations suggest that function of each CNP has diverged in the eel brain. Furthermore, the Pp lacking the blood-brain barrier expressed both npra and nprb, suggesting that endocrine and paracrine NPs interplay for regulating the Pp functions in Japanese eels. [ABSTRACT FROM AUTHOR]

The genomes of plant and animal species are influenced by ancestral whole-genome duplication (WGD) events, which have profound impacts on the regulation and function of gene networks. To gain insight into the consequences of WGD events, we characterized the sequence conservation and expression patterns of ohnologs in the highly duplicated activin receptor signaling pathway in rainbow trout (RBT). The RBT activin receptor signaling pathway is defined by tissue-specific expression of inhibitors and ligands and broad expression of receptors and Co-Smad signaling molecules. Signaling pathway ligands exhibited shared expression, while inhibitors and Smad signaling molecules primarily express a single dominant ohnolog. Our findings suggest that gene function influences ohnolog evolution following duplication of the activin signaling pathway in RBT. [ABSTRACT FROM AUTHOR]

Mormyroidea is a superfamily of weakly electric African fishes with great potential as a model in a variety of biomedical research areas including systems neuroscience, muscle cell and craniofacial development, ion channel biophysics, and flagellar/ciliary biology. However, they are currently difficult to breed in the laboratory setting, which is essential for any tractable model organism. As such, there is a need to better understand the reproductive biology of mormyroids to breed them more reliably in the laboratory to effectively use them as a biomedical research model. This review seeks to (1) briefly highlight the biomedically relevant phenotypes of mormyroids and (2) compile information about mormyroid reproduction including sex differences, breeding season, sexual maturity, gonads, gametes, and courtship/spawning behaviors. We also highlight areas of mormyroid reproductive biology that are currently unexplored and/or have the potential for further investigation that may provide insights into more successful mormyroid laboratory breeding methods. Research Highlights: Mormyroids have great potential as a model in biomedical research, but are difficult to breed in the lab.Reviewing literature on mormyroid reproductive biology reveals common themes throughout the group and highlights areas where further research may inform lab breeding efforts. [ABSTRACT FROM AUTHOR]

Hatching corresponds to the moment an individual leaves its egg envelope. Yet, hatching has scarcely aroused the interest of biologists, and the question posed here, 'why do larvae hatch when they do?' appears to have been rarely asked. In this proof-of-concept study, we tested the hypothesis that fish larvae hatch when a specific ratio between egg surface area (ESA) and larval surface area (LSA) is reached, irrespective of egg diameter and abiotic factors. An estimated mean ratio of 1: 1.52 (CI = 1.39–1.63) was obtained from the eggs of 33 species of teleosts, with diameters ranging from 0.63 to 5.7 mm. We assume that this ratio has a value beyond which the oxygen supply through the egg surface cannot cope with the oxygen requirements of the embryo. The detailed ecological consequences are unknown but can be expected to impact fisheries and aquaculture negatively. [ABSTRACT FROM AUTHOR]

The origin of T cells in the teleost's brain is unclear. While viewing the central nervous system (CNS) as immune privileged has been widely accepted, previous studies suggest that T cells residing in the thymus but not in the spleen of the teleost play an essential role in communicating with the peripheral organs. Here, we identified nine T cell subpopulations in the thymus and spleen of orange-spotted grouper (Epinephelus coioices) through single-cell RNA-sequencing analysis. After viral CNS infection with red-spotted grouper nervous necrosis virus (RGNNV), the number of slc43a2+ T cells synchronously increased in the spleen and brain. During the infection tests in asplenic zebrafish (tlx1▲ zebrafish model), no increase in the number of slc43a2+ T cells was observed in the brain. Single-cell transcriptomic analysis indicated that slc43a2+ T cells mature and functionally differentiate within the spleen and then migrate into the brain to trigger an immune response. This study suggests a novel route for T cell migration from the spleen to the brain during viral infection in fish. [ABSTRACT FROM AUTHOR]

Chemokines are cytokines that mediate leukocyte traffic between the lymphoid organs, the bloodstream, and the site of tissue damage, which is essential for an efficient immune response. In particular, the gamma interferon (IFN- γ) inducible chemokines CXCL9, CXCL10, and CXCL11, and their receptor CXCR3, are involved in T cell and macrophage recruitment to the site of infection. The nature and function of these chemokines and their receptor are well-known in mammals, but further research is needed to achieve a similar level of understanding in fish immunity. Thus, in this study, we seek to identify the genes encoding the components of the Atlantic salmon (Salmo salar) CXCL9, CXCL10, CXCL11/CXCR3 axis (CXCL9-11/CXCR3), predict the protein structure from the amino acid sequence, and explore the regulation of gene expression as well as the response of these chemokines and their receptor to viral infections. The cxcl9, cxcl10, cxcl11, and cxcr3 gene sequences were retrieved from the databases, and the phylogenetic analysis was conducted to determine the evolutionary relationships. The study revealed an interesting pattern of clustering and conservation among fish and mammalian species. The salmon chemokine sequences clustered with orthologs from other fish species, while the mammalian sequences formed separate clades. This indicates a divergent evolution of chemokines between mammals and fish, possibly due to different evolutionary pressures. While the structural analysis of the chemokines and the CXCR3 receptor showed the conservation of critical motifs and domains, suggesting preserved functions and stability throughout evolution. Regarding the regulation of gene expression, some components of the CXCL9-11/CXCR3 axis are induced by recombinant gamma interferon (rIFN-γ) and by Infectious pancreatic necrosis virus (IPNV) infection in Atlantic salmon cells. Further studies are needed to explore the role of Atlantic salmon CXCL9-11 chemokines in regulating immune cell migration and endothelial activation, as seen in mammals. To the best of our knowledge, there have been no functional studies of chemokines to understand these effects in Atlantic salmon.

The recent development of single cell sequencing technologies has revolutionized the state-of-art of cell biology, allowing the simultaneous measurement of thousands of genes in single cells. This technology has been applied to study the transcriptome of single cells in homeostasis and also in response to pathogenic exposure, greatly increasing our knowledge of the immune response to infectious agents. Yet the number of these studies performed in aquacultured fish species is still very limited. Thus, in the current study, we have used the 10x Genomics single cell RNA sequencing technology to study the response of rainbow trout (Oncorhynchus mykiss) peripheral blood leukocytes (PBLs) to infectious pancreatic necrosis virus (IPNV), an important trout pathogen. The study allowed us to obtain a transcriptomic profile of 12 transcriptionally distinct leukocyte cell subpopulations that included four different subsets of B cells, T cells, monocytes, two populations of dendritic-like cells (DCs), hematopoietic progenitor cells, non-specific cytotoxic cells (NCC), neutrophils and thrombocytes. The transcriptional pattern of these leukocyte subpopulations was compared in PBL cultures that had been exposed in vitro to IPNV for 24 h and mock-infected cultures. Our results revealed that monocytes and neutrophils showed the highest number of upregulated protein-coding genes in response to IPNV. Interestingly, IgM+IgD+ and IgT+ B cells also upregulated an important number of genes to the virus, but a much fainter response was observed in ccl4+ or plasma-like cells (irf4+ cells). A substantial number of protein-coding genes and genes coding for ribosomal proteins were also transcriptionally upregulated in response to IPNV in T cells and thrombocytes. Interestingly, although genes coding for ribosomal proteins were regulated in all affected PBL subpopulations, the number of such genes transcriptionally regulated was higher in IgM+IgD+ and IgT+ B cells. A further analysis dissected which of the regulated genes were common and which were specific to the different cell clusters, identifying eight genes that were transcriptionally upregulated in all the affected groups. The data provided constitutes a comprehensive transcriptional perspective of how the different leukocyte populations present in blood respond to an early viral encounter in fish.

Steroid hormones bind to specific receptors that act as transcription factors to modify gene expression in the brain to regulate physiological and behavioural processes. The specific genes controlled by steroid hormones in the brain are not fully known. Identifying these genes is integral to establishing a comprehensive understanding of how hormones impact physiology and behaviour. A popular organism for answering this question is the cichlid fish Astatotilapia burtoni. Recently, CRISPR/Cas9 was used to engineer A. burtoni that lack functional androgen receptor (AR) genes encoding ARα. ARα mutant male A. burtoni produced fewer aggressive displays and possessed reduced expression of the gene encoding brain-specific aromatase, cyp19a1, in the ventromedial hypothalamus (VMH), an aggression locus. As a follow-up, we investigated whether ARα deficiency affected cyp19a1 expression in female A. burtoni using the same genetic line. We find that female A. burtoni possessing one or two non-functional ARα alleles had much higher expression of cyp19a1 in the preoptic area (POA), while females with one non-functional ARα allele possessed lower expression of cyp19a1 in the putative fish homologue of the bed nucleus of the stria terminalis (BNST). Thus, ARα may have a sex-specific role in modifying cyp19a1 expression in the teleost POA and BNST, regions that underlie sex differences across vertebrates.

In mammals, chemokines are a superfamily of cytokines that regulate cell migration or exert direct antimicrobial activity. The presence and organization of four invariant cysteine residues within the mature protein sequence can classify chemokines into four major subfamilies: CXC, CC, CX3C, and XC. In teleost, a novel subfamily of chemokines named CX has been identified. Since the first report of a CC chemokine (CK1) in rainbow trout in 1998, the number of identified chemokine genes in different fish species has expanded dramatically. In this paper, we reviewed the chemokine genes that have been identified so far in teleost fish and their tissue expressions and responses post-stimulation. We also discussed the functions of fish chemokines based on currently available information and compared them to human chemokines. Additionally, we predicted the structures of trout chemokines for the first time, and we found that the structures of trout chemokines share significant similarities. Finally, the adjuvant effects of fish chemokines in aquaculture were also reviewed. Overall, this review will give us a better understanding of the chemokines of teleost fish.

Influenced by two mechanisms of genetic sex determination and environmental sex determination, the sex control of fish is of great significance for aquaculture. Compared with other vertebrates, fishes exhibit great plasticity in sex differentiation and gonadal development. There are significant differences in the size and growth rate of male and female teleosts. Cultivating and breeding large and fast-growing unisexual populations is beneficial to improve economic benefits. Mastacembelus armatus is a kind of freshwater aquaculture fish with high nutritional and economic values. Male individuals grow faster and have a larger body size than females, therefore, cultivating all-male M. armatus has higher economic benefits.In the wild environment, the male to female ratio of the M. armatus is approximately 1∶1, however, there is a high female rate under artificial breeding conditions. At present, it has not been clear what factors cause the feminization of M. armatus, and there have been no successful reports of inducing physiological female sex reversal in the M. armatus, which greatly restricts the development of the M. armatus breeding industry and the improvement of economic benefits. At present, researches on the M. armatus mainly focus on morphology, physiology, reproductive biology, genetic diversity, and other aspects. The research on sex determination and differentiation of the M. armatus is relatively weak, and it is necessary to explore ideas from the sex determination mechanisms, sex regulation methods and sex control breeding techniques of other teleosts. Based on the mechanism of sex determination in teleosts, and the review summarizes several ways of sex control in teleosts, mainly including genetic factors regulation like sex determining genes on sex chromosomes and autosomes, as well as environmental factors regulation, such as temperature, exogenous hormones and fish feed. It aims to provide ideas for analyzing the causes and potential mechanism forhigh feminization of artificial cultivation of the M. armatus, and to provide important theoretical basis for the reproduction and maintenance of unisexual populations, the selection of excellent varieties, and the protection of germplasm resources of the M. armatus, to better understand the gonadal development and reproductive mechanism of the M. armatus, solve the bottleneck of low male rate in currently cultivated M. armatus and provide guarantees for the sustainable development of the M. armatus aquaculture industry.

Streptococcus agalactiae (Group B Lancefield) has emerged as a significant pathogen affecting both humans and animals, including aquatic species. Infections caused by S. agalactiae are becoming a growing concern in aquaculture and have been reported globally in various freshwater and marine fish species, particularly those inhabiting warm water environments. This has led to numerous outbreaks with high morbidity and mortality in fish. Nile tilapia (Oreochromis niloticus), a member of the Cichlid family, is one of the severely affected fish species by S. agalactiae. The current study aims to focus on S. agalactiae infection in cultured O. niloticus with reference to its transmission and sources of infection; risk factors influencing GBS infection, disease clinical signs, lesions, and pathogenesis; S. agalactiae virulence factors; and how to diagnose, treat, control, and prevent infection including vaccination and herbal extract medication.