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Mitochondrial genomes (also known as mitogenomes) serve as valuable molecular markers and have found widespread applications in molecular biology and ecology. There is abundant sequence variation in vertebrate mitogenomes, and occasionally, they exhibit gene rearrangements. In this study, two Chinese endemic Rana species, Rana jiemuxiensis and Rana hanluica, were sequenced and analyzed to obtain their complete mitogenomes. The two species were sympatrically distributed in the Zhangjiajie National Forest Park, in Wulingyuan District, Zhangjiajie City, Hunan Province, China. The mitogenome of R. jiemuxiensis was 17,506 bp, while that of R. hanluica was 17,505 bp, each comprising 13 protein-coding genes (PCGs), 22 transfer RNA genes (tRNAs), two ribosomal RNA genes (rRNAs), and a non-coding control region (D-loop). The gene content, nucleotide composition, and evolutionary rates of each mitogenome were analyzed and compared with those of congeners. A phylogenetic analysis based on 22 mitogenomes in Rana revealed that the two sympatric species were in two different lineages, indicating that they were genetically separated to a certain extent. Three types of gene rearrangement patterns were identified when examining the gene orders of the 22 Rana mitogenomes. Most of the species shared a second and dominant gene rearrangement pattern that originated from the first ancient pattern. A “tandem duplication – multiple deletion” hypothesis was proposed to explain the evolution of these different gene rearrangement patterns. This study provided valuable data references and enhanced our understanding of the phylogenetic implications and gene rearrangements of Rana species.

Abstract Boulenophrys sangzhiensis and Boulenophrys tuberogranulata, two narrow‐distributed toad species within the Megophryidae family in southern China, are experiencing population declines due to habitat loss and degradation. Despite their critical conservation status, the two species remain largely overlooked in public and scientific spheres. This study presented the first sequencing, assembly, and annotation of the complete mitogenomes of both species using next‐generation sequencing. The mitogenome of B. sangzhiensis was 16,950 bp, while that of B. tuberogranulata was 16,841 bp, each comprising 13 protein‐coding genes (PCGs), 22 transfer RNA genes (tRNAs), two ribosomal RNA genes (rRNAs), and a noncoding control region (D‐loop). The gene content, nucleotide composition, and evolutionary rates of each mitogenome were analyzed. Both mitogenomes exhibited negative AT skew and GC skew with high A + T content. ATP8 exhibited the highest evolutionary rate, while COI had the lowest. A phylogenetic analysis based on 28 mitogenomes revealed two major clades of Megophryidae, supporting the classification of two subfamilies, Megophryinae and Leptobrachiinae. Within the subfamily Megophryinae, the genus Boulenophrys was divided into two species groups. Intriguingly, despite coexisting in Zhangjiajie City, B. sangzhiensis and B. tuberogranulata exhibited distinct origins from the two different species groups, underscoring the unique role of the coexisting area Zhangjiajie in driving their speciation and preserving their current populations. A parallel pattern was also identified in the Leptobrachiinae genus Leptobrachium within the same region. This study provided valuable data references and enhanced our understanding of the molecular characteristics of these threatened amphibian species.

Mitochondrial genomes (mitogenomes) are widely used in scientific studies on phylogenetic relationships, molecular evolution and population genetics. Here, we sequenced and analysed the mitogenome of Rectoris luxiensis, a Yangtze River drainage endemic, but threatened cyprinid fish of Labeoninae. The complete mitogenome of R. luxiensis was 16,592 bp in length, encoding 13 protein coding genes (PCGs), 22 transfer RNA genes (tRNAs), two ribosomal RNA genes (rRNAs) and a control region. The mitogenome showed a high A+T content (58.2%) and a positive AT-skew (0.10) and negative GC-skew (–0.25) base composition pattern. All the 13 PCGs were found to start with ATG codons, except for the COXI, in which GTG was the start codon. The ratio of non-synonymous and synonymous substitutions (Ka/Ks) of all the 13 PCGs were less than 1, indicating negative or purifying selection evolved in these genes. Comparatively speaking, the evolutionary rate of ATP8 was the fastest and ND4L was the slowest. All tRNAs could fold into a typical cloverleaf secondary structure, except tRNASer1 that lacked a dihydrouridine arm. Phylogenetic relationships, based on the PCGs dataset of 91 mitogenomes of Labeoninae, showed that R. luxiensis grouped with Rectoris posehensis and they formed a monophyletic Rectoris. However, many non-monophyletic genera were revealed in labeoninae fishes, such as Cirrhinus, Decorus, Garra, Labeo and Pseudocrossocheilus, which indicated that the validities of some traditional genera required a further check. This study reported the complete mitogenome of R. luxiensis for the first time, which provided valuable data for future molecular evolution and conservation related studies of Rectoris and other species in Labeoninae.

Microorganisms play as fundamental contributors to maintain hosts’ fitness, which can be shaped by external environment. Moreover, symbiotic microbiome also varied within species (e.g., between sexes and developmental stages). However, we still need more studies to quantify whether the intraspecific variation patterns of symbiotic microbes can be modified with the change of environment. The Chinese giant salamander (CGS; Andrias davidianus) is a Critically Endangered species. Despite quantitative captive bred individuals were released to rebuild wild populations, the effectiveness is limited. More importantly, no studies have revealed the adaptation of released CGSs to the complex field conditions. In the present study, we explored whether reintroduction can reshape the intraspecific variations of symbiotic microbiota in captive bred CGSs using high-throughput amplicon sequencing of the16S rRNA gene. We found no significant difference of symbiotic microbiome in captive bred males and females, but released males and females differed significantly in skin microbiome. Juveniles had higher diversity of microbial symbiont than adults in hatchery, but lower diversity in field. Moreover, dominant bacterial taxa differed between juveniles and adults in both hatchery and field. Importantly, this symbiotic microbiome variations within species can be modified (alpha and beta diversity, and community composition) when captive bred individuals were released to the field. Overall, we observed a lower alpha diversity and higher relative abundance of Chryseobacterium, Plesiomonas, and Acinetobacter in the bacterial community of captive bred individuals. Instead, higher alpha diversity of symbiotic microbiota and higher relative abundance of S24-7 and Lactobacillus was detected in released individuals. These modifications may associate with the change of living environment, as well as the specific behavior within CGSs (e.g., movement patterns and foraging activities). Future studies can incorporate other approaches (e.g., blood physiology) to better evaluate the growth and health of reintroduced CGSs.

An adult Sinocyclocheilus maitianheensis, a surface-dwelling golden-line barbel fish, was collected from Maitian river (Kunming City, Yunnan Province, China) for whole-genome sequencing, assembly, and annotation. We obtained a genome assembly of 1.7 Gb with a scaffold N50 of 1.4 Mb and a contig N50 of 24.7 kb. A total of 39,977 protein-coding genes were annotated. Based on a comparative phylogenetic analysis of five Sinocyclocheilus species and other five representative vertebrates with published genome sequences, we found that S. maitianheensis is close to Sinocyclocheilus anophthalmus (a cave-restricted species with similar locality). Moreover, the assembled genomes of S. maitianheensis and other four Sinocyclocheilus counterparts were used for a fourfold degenerative third-codon transversion (4dTv) analysis. The recent whole-genome duplication (WGD) event was therefore estimated to occur about 18.1 million years ago. Our results also revealed a decreased tendency of copy number in many important genes related to immunity and apoptosis in cave-restricted Sinocyclocheilus species. In summary, we report the first genome assembly of S. maitianheensis, which provides a valuable genetic resource for comparative studies on cavefish biology, species protection, and practical aquaculture of this potentially economical fish.

Abstract Intermuscular bones (IBs) are widely present in morphologically generalized teleost fishes and are commonly found in the Cyprinidae. Intermuscular bones are small, hard spicules of bone that are formed by ossification in the myosepta between neighboring myomeres. Why fish have IBs, and whether there is any evolutionary pattern to their occurrence, has been poorly understood. However, the presence of IBs does substantially affect the meat quality and commercial values of many cyprinid fishes in aquaculture. In this study, we sampled 592 individuals of cyprinid fishes to systematically investigate the evolution of IBs from a phylogenetic point of view. We found that the total number of IBs in the Cyprinidae ranged from 73 to 169, and we clarified that only two categories of IBs (epineural and epipleural) were present in all examined cyprinids. Most of the IBs were distributed in the posterior region of the fish, which might be an optimal target for selecting fewer IB strains in aquaculture. There was a positive correlation between IBs and the number of vertebrae, thus making it possible to predict the approximate number of IBs by counting the number of vertebrae. Although the IBs displayed some correlation with phylogenetic relationships in some lineages and to ecological factors such as diet (especially carnivore), in an overall view the variations of IBs in cyprinids were extremely diverse. The number and patterns of IBs in these fishes may reflect their phylogenetic history, but have been shaped by multiple environment factors. In this study, we also confirmed that X‐ray photography remains an optimal and reliable method for the study of IBs.

The notorious parasite Ichthyophthirius multifiliis (Ich) has been recorded worldwide in fish species and causes white spot disease, posing major threats and resulting in severe losses to international fish production. Extensively effective strategies for treating Ich are not available yet, and genetic mechanisms of hosts in response to the parasite are still largely unknown. In this study, we selected Kanglang white minnow (KWM, Anabarilius grahami) to examine its liver transcriptional changes after Ich infection, as white spot disease is one bottleneck problem in exploring this economically important species. We divided the experimental fishes into three groups (control, early-infected, and late-infected) to examine differentially expressed genes (DEGs). A total of 831 DEGs were identified and classified into 128 significantly enriched GO (Gene Ontology) terms and 71 significantly enriched KEGG (Kyoto Encyclopedia of Genes and Genomes) pathways. Most of these terms or pathways were functionally enriched in immunity, inflammatory response, and apoptosis, such as nucleotide-binding oligomerization domain-like (NOD-like) receptor signaling, tumor necrosis factor (TNF) signaling, interleukin-17 (IL-17) signaling, and apoptosis pathways. We also identified 178 putative antimicrobial peptides (AMPs) and AMP precursors based on our previously reported genome assembly of KWM, and revealed that the expressional patterns varied according to different types. In summary, our work reported the first comprehensive transcriptional changes in KWM in response to the exogenous infection of Ich, which would lay a solid foundation for in-depth studies on disease defense or resistant strains selection in this valuable fish.

A Yunnan-Guizhou Plateau fish, the Kanglang white minnow (Anabarilius grahami), is a typical “3E” (Endangered, Endemic, and Economic) species in China. Its distribution is limited to Fuxian Lake, the nation’s second deepest lake, with a significant local economic value but a drastically declining wild population. This species has been evaluated as VU (Vulnerable) in the China Species Red List. As one of the “Four Famous Fish” in Yunnan province, the artificial breeding has been achieved since 2003. It has not only re-established its wild natural populations by reintroduction of the artificial breeding stocks, but also brought a wide and popular utilization of this species to the local fish farms. A. grahami has become one of the main native aquaculture species in Yunnan province, and the artificial production has been emerging in steady growth each year. To promote the conservation and sustainable utilization of this fish, we initiated its whole genome sequencing project using an Illumina Hiseq2500 platform. The assembled genome size of A. grahami is 1.006 Gb, accounting for 98.63% of the estimated genome size (1.020 Gb), with contig N50 and scaffold N50 values of 26.4 kb and 4.41 Mb, respectively. Approximately about 50.38% of the genome was repetitive. A total of 25,520 protein-coding genes were subsequently predicted. A phylogenetic tree based on 4,580 single-copy genes from A. grahami and 18 other cyprinids revealed three well-supported subclades within the Cyprinidae. This is the first inter-subfamily relationship of cyprinids at genome level, providing a simple yet useful framework for understanding the traditional but popular subfamily classification systems. Interestingly, a further population demography of A. grahami uncovered a historical relationship between this fish and Fuxian Lake, suggesting that range expansion or shrinkage of the habitat has had a remarkable impact on the population size of endemic plateau fishes. Additionally, a total of 33,836 simple sequence repeats (SSR) markers were identified, and 11 loci were evaluated for a preliminary genetic diversity analysis in this study, thus providing another useful genetic resource for studying this “3E” species.

Catfish is one of the major aquaculture species in the United States. However, the catfish industry is threatened by several bacterial diseases such as enteric septicemia of catfish (ESC), columnaris disease and Aeromonas disease, as well as by abiotic stresses such as high temperature and low oxygen. Research has been conducted for several decades to understand the host responses to these diseases and abiotic stresses. With the development of sequencing technologies, and the application of genome-wide association studies in aquaculture species, significant progress has been made. This review article summarizes recent progress in understanding the molecular responses of catfish after bacterial infection and stress challenges, and in understanding of genomic and genetic basis for disease resistance and stress tolerance.

In our previous work, we reported comparative genomics studies on 3 Sinocyclocheilus fishes, including the surface-dwelling S. graham (Sg), the semi-cave-dewlling S. rhinocerous (Sr) and the cave-restricted S. anshuiensis (Sa). Here, we performed a genomic survey on the immunity differences among the 3 fishes by combination of the genome and transcriptome data. Our results demonstrate that immune-related genetic pathways in Sa and Sr present more similarities than Sg. The innate immune activity in Sa seems to be higher than Sg and Sr, while Sr owns more copy numbers of MHC-related genes (related with the acquired immune system) than Sg and Sa. These differential immune activities in the 3 Sinocyclocheilus species may be due to their differential habitats. Generally speaking, most immunity genes transcribe the lowest levels in Sa when compared with Sg and Sr, which may be associate with a less various microbial environment in the cave-restricted habitat. However, Sr has more members of MHC-related genes than those in Sg and Sa, suggesting a great contribution from the semi-cave-dwelling condition.