Search

We review morphological features of the amniote egg and embryos in a comparative phylogenetic framework, including all major clades of extant vertebrates. We discuss 40 characters that are relevant for an analysis of the evolutionary history of the vertebrate egg. Special attention is given to the morphology of the cellular yolk sac, the eggshell, and extraembryonic membranes. Many features that are typically assigned to amniotes, such as a large yolk sac, delayed egg deposition, and terrestrial reproduction have evolved independently and convergently in numerous clades of vertebrates. We use phylogenetic character mapping and ancestral character state reconstruction as tools to recognize sequence, order, and patterns of morphological evolution and deduce a hypothesis of the evolutionary history of the amniote egg. Besides amnion and chorioallantois, amniotes ancestrally possess copulatory organs (secondarily reduced in most birds), internal fertilization, and delayed deposition of eggs that contain an embryo in the primitive streak or early somite stage. Except for the amnion, chorioallantois, and amniote type of eggshell, these features evolved convergently in almost all major clades of aquatic vertebrates possibly in response to selective factors such as egg predation, hostile environmental conditions for egg development, or to adjust hatching of young to favorable season. A functionally important feature of the amnion membrane is its myogenic contractility that moves the (early) embryo and prevents adhering of the growing embryo to extraembryonic materials. This function of the amnion membrane and the liquid-filled amnion cavity may have evolved under the requirements of delayed deposition of eggs that contain developing embryos. The chorioallantois is a temporary embryonic exchange organ that supports embryonic development. A possible evolutionary scenario is that the amniote egg presents an exaptation that paved the evolutionary pathway for reproduction on land. As shown by numerous examples from anamniotes, reproduction on land has occurred multiple times among vertebrates-the amniote egg presenting one "solution" that enabled the conquest of land for reproduction.

Amniote vertebrates share a suite of extra-embryonic membranes that distinguish them from anamniotes. Other than that, however, their reproductive characteristics could not be more different. They differ in basic ectothermic vs endothermic physiology, in that two clades evolved powered flight, and one clade evolved a protective shell. In terms of reproductive strategies, some produce eggs and others give birth to live young, at various degrees of development. Crucially, endotherms provide lengthy parental care, including thermal and food provisioning—whereas ectotherms seldom do. These differences could be expected to manifest themselves in major differences between clades in quantitative reproductive traits. We review the reproductive characteristics, and the distributions of brood sizes, breeding frequencies, offspring sizes and their derivatives (yearly fecundity and biomass production rates) of the four major amniote clades (mammals, birds, turtles and squamates), and several major subclades (birds: Palaeognathae, Galloanserae, Neoaves; mammals: Metatheria and Eutheria). While there are differences between these clades in some of these traits, they generally show similar ranges, distribution shapes and central tendencies across birds, placental mammals and squamates. Marsupials and turtles, however, differ in having smaller offspring, a strategy which subsequently influences other traits.

Epithelial-to-mesenchymal transition (EMT) is a dynamic process that produces migratory cells from epithelial precursors. However, EMT is not binary; rather it results in migratory cells which adopt diverse strategies including collective and individual cell migration to arrive at target destinations. Of the many embryonic cells that undergo EMT, the vertebrate neural crest is a particularly good example which has provided valuable insight into these processes. Neural crest cells from different species often adopt different migratory strategies with collective migration predominating in anamniotes, whereas individual cell migration is more prevalent in amniotes. Here, we will provide a perspective on recent work toward understanding the process of neural crest EMT focusing on how these cells undergo collective and individual cell migration.

This review elaborates the idea that organ regeneration derives from specific evolutionary histories of vertebrates. Regenerative ability depends on genomic regulation of genes specific to the life-cycles that have differentially evolved in anamniotes and amniotes. In aquatic environments, where fish and amphibians live, one or multiple metamorphic transitions occur before the adult stage is reached. Each transition involves the destruction and remodeling of larval organs that are replaced with adult organs. After organ injury or loss in adult anamniotes, regeneration uses similar genes and developmental process than those operating during larval growth and metamorphosis. Therefore, the broad presence of regenerative capability across anamniotes is possible because generating new organs is included in their life history at metamorphic stages. Soft hyaluronate-rich regenerative blastemas grow in submersed or in hydrated environments, that is, essential conditions for regeneration, like during development. In adult anamniotes, the ability to regenerate different organs decreases in comparison to larval stages and becomes limited during aging. Comparisons of genes activated during metamorphosis and regeneration in anamniotes identify key genes unique to these processes, and include thyroid, wnt and non-coding RNAs developmental pathways. In the terrestrial environment, some genes or developmental pathways for metamorphic transitions were lost during amniote evolution, determining loss of regeneration. Among amniotes, the formation of soft and hydrated blastemas only occurs in lizards, a morphogenetic process that evolved favoring their survival through tail autotomy, leading to a massive although imperfect regeneration of the tail. Deciphering genes activity during lizard tail regeneration would address future attempts to recreate in other amniotes regenerative blastemas that grow into variably completed organs.

Besides having medical applications, comparative studies on reproductive biology are very useful, providing, for instance, essential knowledge for basic, conservation and biotechnological research. In order to maintain the reproductive potential and the survival of all vertebrate species, both sperm and steroid production need to occur inside the testis. From the approximately fifty thousand vertebrate species still alive, very few species are already investigated; however, our knowledge regarding Sertoli cell biology is quite good. In this regard, it is already known that since testis differentiation the Sertoli cells are the somatic cells in charge of supporting and orchestrating germ cells during development and full spermatogenesis in adult animals. In the present review, we highlight key aspects related to Sertoli cell biology in vertebrates and show that this key testis somatic cell presents huge and intrinsic plasticity, particularly when cystic (fish and amphibians) and non-cystic (reptiles, birds and mammals) spermatogenesis is compared. In particular, we briefly discuss the main aspects related to Sertoli cells functions, interactions with germ cells, Sertoli cells proliferation and efficiency, as well as those regarding spermatogonial stem cell niche regulation, which are crucial aspects responsible for the magnitude of sperm production. Most importantly, we show that we could greatly benefit from investigations using different vertebrate experimental models, mainly now that there is a big concern regarding the decline in human sperm counts caused by a multitude of factors.

Retroviruses are widely distributed in all vertebrates, as are their endogenous form, endogenous retrovirus (ERV), which serves as “fossil” evidence to trace the ancient origins and history of virus-host interactions over millions of years. The retroviral envelope (Env) plays a significant role in host range determination, but major information on genetic diversification in anamniotes is lacking. Here, by incorporating multiple-roundin silicosimilarity search and phylogenomic analysis, 25498 ERVs with gamma-type Env (GTE), covalently associated Env, were discovered by searching against all 974 available fish and 19 amphibian genomes, but no beta-type Env (BTE), noncovalently associated Env, were found. Furthermore, a nine-type classification system of anamniote GTE was proposed by combining phylogenetic and domain/motif analyses. The elastic genomic organization and overall phylogenetic incongruence between anamniotic Env and its neighboring polymerase (Pol) implied that early retroviral diversification in anamniotic vertebrates was facilitated by frequent recombination. At last, host opioid growth factor receptor (OGFr) gene capturing by anamniotic ERVs with GTE was reported for the first time. Overall, our findings overturn traditional Pol genotyping and reveal a complex evolutionary history of anamniotic retroviruses inferred by Env evolution.Author summaryAlthough the retroviral envelope (Env) protein in amniotes has been well studied, its evolutionary history in anamniotic vertebrates is ambiguous. By analyzing more than 25000 ERVs with gamma-type Env (GTE) in anamniotes, several important evolutionary features were identified. First, GTE were found to be widely distributed among different amphibians and fish. Second, nine types of GTE were discovered, revealing the great genetic diversity. Third, GTE-containing ERVs have rampantly proliferated in certain amphibians such asAmbystoma mexicanum, and the copy number was found to be markedly higher than 10000. Fourth, the incongruence between the Env and Pol phylogenies suggested that frequent recombination shaped the early evolution of anamniote retroviruses. Fifth, an ancient horizontal gene transfer event was discovered from anamniotes to ERVs with GTE. These findings reveal a complex evolution pattern for retroviral Env in anamniotes.

Transcriptional quiescence of post-meiotic spermatozoa associated with protamine-mediated chromatin condensation is widely recognized in animals. How sperm acquire the extratesticular maturational competence to move and fertilize the egg is therefore thought to occur via non-transcriptional mechanisms. Here, using transcriptional profiling during spermatozoon differentiation in a fish that does not condense chromatin with protamines, we uncover spatially distinct roles of the GnRH receptor and PDGF signaling pathways between the somatic epithelia of the extratesticular ducts and the maturing spermatozoa. In vitro induction and inhibition experiments demonstrate that the endocrine signaling pathways are conserved in different lineages of fish and activate de novo transcription of spermatozoon genes required for the acquisition of full motility. These experiments further confirmed that mitochondrial translation is important for sperm maturation in anamniotes as in amniotes, but that transcriptional quiescence of post-meiotic spermatozoa is not a pan vertebrate phenomenon. On the contrary, the data show that the identified signal transduction pathways between the soma and the sperm upregulate effector genes essential for maturational competence and male fertility.

The cerebellum coordinates vestibular input into the hindbrain to control balance and movement, and its anatomical complexity is increasingly viewed as a high-throughput processing center for sensory and cognitive functions. Cerebellum development however is relatively simple, and arises from a specialized structure in the anterior hindbrain called the rhombic lip, which along with the ventricular zone of the rostral-most dorsal hindbrain region, give rise to the distinct cell types that constitute the cerebellum. Granule cells, being the most numerous cell types, arise from the rhombic lip and form a dense and distinct layer of the cerebellar cortex. In this short review, we describe the various strategies used by amniotes and anamniotes to generate and diversify granule cell types during cerebellar development.

An evolutionary hypothesis explaining failure of regeneration among vertebrates is presented. Regeneration derives from postembryonic processes present during the life cycles of fish and amphibians that include larval and metamorphic phases with broad organ reorganizations. Developmental programs imprinted in their genomes are re-utilized with variations also in adults for regeneration. When vertebrates colonized land adopting the amniotic egg, some genes driving larval changes, and metamorphosis were lost and new genes evolved, further limiting regeneration. These included neural inhibitors for maintaining complex nervous systems, behavior and various levels of intelligence, and adaptive immune cells. The latter, that in anamniotes are executioners of metamorphic reorganization, became intolerant to embryonic-oncofetal-antigens impeding organ regeneration, a process that requires de-differentiation of adult cells and/or expansion of stem cells where these early antigens are formed. The evolution of terrestrial lifecycles produced vertebrates with complex bodies but no longer capable to regenerate their organs, mainly repaired by regengrow. Efforts of regenerative medicine to improve healing in humans should determine the diverse developmental pathways evolved between anamniotes and amniotes before attempting genetic manipulations such as the introduction of "anamniote regenerative genes" in amniotes. This operation may determine alteration in amniote developmental programs leading to teratomes, cancer, or death.

The retina is a complex tissue responsible for both detection and primary processing of visual stimuli. Although all vertebrate retinas share a similar, multi-layered organization, the ability to regenerate individual retinal cells varies tremendously, being extremely limited in mammals and birds when compared to anamniotes such as fish and amphibians. However, little is yet known about damage response and regeneration of retinal tissues in "non-classical" squamate reptiles (lizards, snakes), which occupy a key phylogenetic position within amniotes and exhibit unique regenerative features in many tissues. Here, we address this gap by establishing and characterizing a model of excitotoxic retinal damage in bearded dragon lizard (Pogona vitticeps). We particularly focus on identifying, at the cellular and molecular level, a putative endogenous cellular source for retinal regeneration, as diverse self-repair strategies have been characterized in vertebrates using a variety of retinal injury and transgenic models. Our findings reveal for the first time that squamates hold the potential for postnatal retinal regeneration following acute injury. Although no changes occur in the activity of physiologically active progenitors recently identified at the peripheral retinal margin of bearded dragon, two distinct successive populations of proliferating cells at central retina respond to neurotoxin treatment. Following an initial microglia response, a second source of proliferating cells exhibit common hallmarks of vertebrate Muller glia (MG) activation, including cell cycle re-entry, dedifferentiation into a progenitor-like phenotype, and re-expression of proneural markers. The observed lizard glial responses, although not as substantial as in anamniotes, appear more robust than the absent or neonatal-limited regeneration reported without exogenous stimulation in other amniotes. Altogether, these results help to complete our evolutionary understanding of regenerative potential of the vertebrate retina, and further highlight the major importance of glial cells in retinal regeneration. Furthermore, our work offers a new powerful vertebrate model to elucidate the developmental and evolutionary bases of retinal regeneration within amniotes. Such new understanding of self-repair mechanisms in non-classical species endowed with regenerative properties may help designing therapeutic strategies for vertebrate retinal diseases.

Besides having medical applications, comparative studies on reproductive biology are very useful, providing, for instance, essential knowledge for basic, conservation and biotechnological research. In order to maintain the reproductive potential and the survival of all vertebrate species, both sperm and steroid production need to occur inside the testis. From the approximately fifty thousand vertebrate species still alive, very few species are already investigated; however, our knowledge regarding Sertoli cell biology is quite good. In this regard, it is already known that since testis differentiation the Sertoli cells are the somatic cells in charge of supporting and orchestrating germ cells during development and full spermatogenesis in adult animals. In the present review, we highlight key aspects related to Sertoli cell biology in vertebrates and show that this key testis somatic cell presents huge and intrinsic plasticity, particularly when cystic (fish and amphibians) and non-cystic (reptiles, birds and mammals) spermatogenesis is compared. In particular, we briefly discuss the main aspects related to Sertoli cells functions, interactions with germ cells, Sertoli cells proliferation and efficiency, as well as those regarding spermatogonial stem cell niche regulation, which are crucial aspects responsible for the magnitude of sperm production. Most importantly, we show that we could greatly benefit from investigations using different vertebrate experimental models, mainly now that there is a big concern regarding the decline in human sperm counts caused by a multitude of factors.

Amniotes, such as mammals and reptiles, have vision and other senses represented in the pallium, whereas anamniotes, such as amphibians, fish and cyclostomes (including lampreys), which diverged much earlier, were historically thought to process olfactory information predominantly or even exclusively in the pallium. Here, we show that there is a separate visual area with retinotopic representation, and that somatosensory information from the head and trunk is represented in an adjacent area in the lamprey pallial cortex (lateral pallium). These cortical sensory areas flank a non-primary-sensory motor area. Both vision and somatosensation are relayed via the thalamus. These findings suggest that the basic sensorimotor representation of the mammalian neocortex, as well as the sensory thalamocortical relay, had already evolved in the last common ancestor of cyclostomes and gnathostomes around 560 million years ago. This study uses isolated eye–brain preparations to show topographic visual and somatosensory representation in the lamprey pallium, which is similar to the basic sensorimotor representation of the mammalian neocortex.

Comprehensive reviews dealing with the embryology, morphology, vascularization, innervation, and cytology of avian pineal glands are available. Among species of birds, the morphology and cytology of the pineal gland are remarkably varied. According to Collin, the unique cell type in the avian pineal, the secretory pinealocyte, is evolutionarily derived from a cone-like receptor cell found among some reptiles and anamniotes. The rhythmic variation in melatonin content of the pineal gland of chickens persists as a free-running, circadian rhythm for at least 2 weeks in continuous darkness, but it disappears within 2 weeks in constant light. The autotransplanted pineal gland of chickens apparently released greater amounts of melatonin into the blood at night than during the day, since by bioassay melatonin could be detected there only when sampled at night. The injury occasioned by the surgical removal of the pineal gland can affect the measured response in various ways, including increasing the opacity of the skull and underlying tissues.

Evolution selects those adaptive features that increase reproductive probabilities and facilitate survival. Analysing the brain circuits mediating risk-avoidance (e.g. defense) and those allowing reward-seeking (motivated) behaviours in different vertebrates leads to several main conclusions. First, circuits mediating risk-avoidance are similar in all studied vertebrates, where they include amygdala homologues located in the posterior half of the cerebral hemispheres, in close relationship with the chemosensory systems. Second, in all vertebrates, reward-seeking behaviours involve the activity of tegmento-striatal dopaminergic pathways, plus other inputs to the ventral striatum, including amygdalo-striatal glutamatergic projections. Third, output structures in these forebrain circuits for both risk-avoidance and reward-seeking behaviours occupy the caudal and rostral poles of the ventral striato-pallidum, namely the central amygdala and nucleus accumbens-olfactory tubercle respectively. This brain configuration was already present in at least the ancestral amniote, likely also in anamniotes. Finally, social behaviours (sexual, agonistic-territorial, parental) are fundamental for reproduction and survival. Consequently, the so-called socio-sexual brain network that governs these conducts is closely related with brain centres mediating motivation (maybe also risk-avoidance). Central nonapeptidergic circuits are apparently required for endowing social stimuli with rewarding (attractive) properties. More studies in non-mammals are required to further test and expand these ideas.

FGIN-1-27 is an agonist at the translocator protein 18 kDa (TSPO), a cholesterol transporter that is associated with neurosteroidogenesis. This protein has been identified as a peripheral binding site for benzodiazepines; in anamniotes, however, a second TSPO isoform that is absent in amniotes has been implicated in erythropoiesis. Functional conservation of the central benzodiazepine-binding site located in the GABAA receptors has been demonstrated in anamniotes and amniotes alike; however, it was not previously demonstrated for TSPO. The present investigation explored the behavioral effects of FGIN-1-27 on an anxiety test in zebrafish (Danio rerio, Family: Cyprinide) and on a mixed anxiety/panic test on wall lizards (Tropidurus oreadicus, Family: Tropiduridae). Results showed that FGIN-1-27 reduced anxiety-like behavior in the zebrafish light/dark preference test similar to diazepam, but with fewer sedative effects. Similarly, FGIN-1-27 also reduced anxiety- and fear-like behaviors in the defense test battery in wall lizards, again producing fewer sedative-like effects than diazepam; the benzodiazepine was also unable to reduce fear-like behaviors in this species. These results A) underline the functional conservation of TSPO in defensive behavior in anamniotes; B) strengthen the proposal of using anamniote behavior as models in behavioral pharmacology; and C) suggest TSPO/neurosteroidogenesis as a target in treating anxiety disorders.

Spermatogenesis, the production of sperm, is a fundamental part of the reproductive cycle in vertebrates and warrants detailed study. Reptiles exhibit a hybrid pattern of spermatogenesis that has similarities with both anamniotes and amniotes. Researching this interesting transition will help us better understand vertebrate evolution. To this end, we histologically examined the spermatogenic and testicular cycle of Rough Greensnakes (Opheodrys aestivus) from Arkansas. We hypothesized that spermatogenesis in Rough Greensnakes would follow the different spermatogenic stages found in previously published studies on temperate snakes. In addition, we hypothesized that both season and seminiferous tubule diameter would have a significant effect on seminiferous tubule epithelial height. Finally, we hypothesized that Rough Greensnakes would exhibit postnuptial spermatogenesis. We constructed a cell wheel illustrating the chronological development of all germ cell stages for this species and measured semi...

Lungfishes are a group of sarcopterygian fishes currently considered the closest living relatives of tetrapods, and represent an interesting group for the study of evolutionary traits in the transition from fishes to tetrapods. Catecholaminergic systems in the brain are among the most carefully analyzed neurotransmitter systems in the brain of most vertebrate groups. Their organization shows major shared characteristics, although traits particular to each vertebrate class have also been found, primarily between anamniotes and amniotes. Given the relevance of lungfishes in evolutionary terms, the present study provides the first comprehensive and detailed map of the catecholaminergic structures in the brain of two representative species of lungfishes, an African lungfish (Protopterus dolloi) and the Australian lungfish (Neoceratodus forsteri), as revealed by immunohistochemistry. Distinct groups of catecholaminergic cells were observed in the olfactory bulb, pallium, and preoptic area of the telencephalon, and the subpallium is devoid of these cells. Hypothalamic and diencephalic groups were detected and, in particular, the dopaminergic nucleus of the periventricular organ was evidenced with dopamine antibodies but not with anti-tyrosine hydroxylase. A well developed mesostriatal system was revealed formed by conspicuous groups of dopamine cells in the midbrain tegmentum and profuse innervation of the subpallium. Comparison of these results with those from other classes of vertebrates shows numerous common traits shared by most groups and also highlights particular features in lungfishes different from actinopterygian fishes that resemble those of amphibians and amniotes.

Senescence represents a mechanism to avoid undesired cell proliferation that plays a role in tumor suppression, wound healing and embryonic development. In order to gain insight on the evolution of senescence, we looked at its presence in developing axolotls (urodele amphibians) and in zebrafish (teleost fish), which are both anamniotes. Our data indicate that cellular senescence is present in various developing structures in axolotls (pronephros, olfactory epithelium of nerve fascicles, lateral organs, gums) and in zebrafish (epithelium of the yolk sac and in the lower part of the gut). Senescence was particularly associated with transient structures (pronephros in axolotls and yolk sac in zebrafish) suggesting that it may play a role in the elimination of these tissues. Our data supports the notion that cellular senescence evolved early in vertebrate evolution to influence embryonic development., Summary: We report the presence of senescent cells in several transient structures in developing amphibian and teleost fish, suggesting novel mechanisms of morphogenesis that appeared early in vertebrate evolution.

In birds as in all amniotes, the site of gastrulation is a midline structure, the primitive streak. This appears as cells in the one cell-thick epiblast undergo epithelial-to-mesenchymal transition to ingress and form definitive mesoderm and endoderm. Global movements involving tens of thousands of cells in the embryonic epiblast precede gastrulation. They position the primitive streak precursors from a marginal position (equivalent to the situation in anamniotes) along the future antero-posterior axis (typical for amniotes). These epithelial movements continue in modified form during gastrulation, when they are accompanied by collective movements of different class in the forming mesoderm and endoderm. Here I discuss the nature of these collective cell movements shaping the embryo, their interplay with signaling events controlling fate specification and significance in an evolutionary perspective.

Amphibians occupy a position of great interest in terms of vertebrate evolution. Additionally, amphibians are known as a transitional group between amniotes and anamniotes. However, there are few studies on the gametogenesis of anamniotes vertebrates, especially anurans. Therefore, the purpose of this study was to analyze the histological feature of germ cells and their arrangement in the testis of Levantine frog,Pelophylax bedriagae(Camerano, 1882). Spermatogenic cells were organized in spermatocysts. Each spermatocyst contained cells at the same stage of the spermatogenic cycle. Identification of each cellular type in seminiferous tubule was carried out according to the size and morphology of cells and the degree of nuclear material compaction. Spermatogonia were large cells localized at the base of the seminiferous epithelium. Primary spermatocytes were examined in different phases of first meiotic division and distinguished from other cell types by their dark spherical nuclei or looser chromatin. Two types of spermatids, spherical and elongated cells, were observed. Seminiferous tubules were surrounded by peritubular myoid cells, and they contained no lumen. the lack of lumen in the seminiferous tubules and the cystic spermatogenesis probably provide synchronously production of a large number of sperms. the location of hyaluronic acid was also determined in interstitial tissue between seminiferous tubules to probably provide testicular integrity and viscoelasticity. Amphibians are major members of aquatic and terrestrial ecosystems. They serve as both predator and prey in both ecosystems. Therefore, they provide the transfer of energy between these systems. However, there are few studies on the gametogenesis of amphibian. Therefore, histological characteristics ofPelophylax bedriagaetestes were studied.

Amphibians have often been selected as experimental animals in brain research for comparative studies. They are tetrapod anamniotes that represent a key model for understanding the water-to-land transition. In amphibians the basic traits of brain organization in amniotes can be already recognized. In the present chapter we deal with the regional anatomy of the amphibian brain (in the three extant orders) focusing mainly on the chemoarchitecture and genoarchitecture, which serve to define anatomical subdivisions that are otherwise indistinct. The new data are interpreted within the framework of the current neuromeric model that helps comparisons across vertebrates.

The two somite compartments, dorso-lateral dermomyotome and medio-ventral sclerotome are major vertebrate novelties, but little is known about their evolutionary origin. We determined that sclerotome cells in Xenopus come from lateral somitic frontier (LSF) by lineage tracing, ablation experiments and histological analysis. We identified Twist1 as marker of migrating sclerotome progenitors in two amphibians, Xenopus and axolotl. From these results, three conclusions can be drawn. First, LSF is made up of multipotent somitic cells (MSCs) since LSF gives rise to sclerotome but also to dermomytome as already shown in Xenopus. Second, the basic scheme of somite compartmentalization is conserved from cephalochordates to anamniotes since in both cases, lateral cells envelop dorsally and ventrally the ancestral myotome, suggesting that lateral MSCs should already exist in cephalochordates. Third, the transition from anamniote to amniote vertebrates is characterized by extension of the MSCs domain to the entire somite at the expense of ancestral myotome since amniote somite is a naive tissue that subdivides into sclerotome and dermomyotome. Like neural crest pluripotent cells, MSCs are at the origin of major vertebrate novelties, namely hypaxial region of the somite, dermomyotome and sclerotome compartments. Hence, change in MSCs properties and location is involved in somite evolution.

In our review we have completed current knowledge on myotomal myogenesis in model and non-model vertebrate species (fishes, amphibians, reptiles, birds and mammals) at morphological and molecular levels. Data obtained from these studies reveal distinct similarities and differences between amniote and anamniote species. Based on the available data, we decided to present evolutionary implications in vertebrate trunk muscle development. Despite the fact that in all vertebrates muscle fibres are multinucleated, the pathways leading to them vary between vertebrate taxa. In fishes during early myogenesis myoblasts differentiate into multinucleated lamellae or multinucleate myotubes. In amphibians, myoblasts fuse to form multinucleated myotubes or, bypassing fusion, directly differentiate into mononucleated myotubes. Furthermore, mononucleated myotubes were also observed during primary myogenesis in amniotes. The mononucleated state of myogenic cells could be considered as an old phylogenetic, plesiomorphic feature, whereas direct multinuclearity of myotubes has a synapomorphic character. On the other hand, the explanation of this phenomenon could also be linked to the environmental conditions in which animals develop. The similarities observed in vertebrate myogenesis might result from a conservative myogenic programme governed by the Pax3/Pax7 and myogenic regulatory factor (MRF) network, whereas differences in anamniotes and amniotes are established by spatiotemporal pattern expression of MRFs during muscle differentiation and/or environmental conditions.

Whereas our understanding of the dopaminergic system in mammals allows for a distinction between ventral tegmental area (VTA) and substantia nigra pars compacta (SNc), no clear evidence for separate structures in anamniotes has been presented to date. To broaden the insight into the organization and regulation of neuromodulatory systems in anuran amphibians, tracing and immunohistochemical investigations were performed in the Oriental fire-bellied toad, Bombina orientalis. Topographically organized catecholaminergic "nigrostriatal," "mesolimbic," "mesocortical," and spinal cord projections arising from the posterior tubercle and mesencephalic tegmentum were identified. We compared these results with published data from lampreys, chondrichthyes, teleosts, amphibians, reptiles, birds, and mammals. Based on the pattern of organization, as well as the differential innervation by the habenular nuclei, domains gradually comparable to the mammalian paranigral VTA, ventral tier of the SNc, interfascicular nucleus of the VTA, and supramamillary/retromamillary area were identified. Additionally, we could demonstrate topographic separate populations of habenula neurons projecting via a direct excitatory or indirect GABAergic pathway onto the catecholaminergic VTA/SNc homologs and serotonergic raphe nuclei. The indirect GABAergic habenula pathway derives from neurons in the superficial mamillary area, which in terms of its connectivity and chemoarchitecture resembles the mammalian rostromedial tegmental nucleus. These results demonstrate a much more elaborate interconnection principle of the anuran dopaminergic system than previously assumed. Based on the data presented it seems that most features of the dopaminergic system of amniotes had already evolved when the amphibian line of evolution diverged from that leading up to mammals, reptiles, and birds.

The intricate neuronal architecture of the striatum plays a pivotal role in the functioning of the basal ganglia circuits involved in the control of various aspects of motor, cognitive, and emotional functions. Unlike the cerebral cortex, which has a laminar structure, the striatum is primarily composed of two functional subdivisions (i.e., the striosome and matrix compartments) arranged in a mosaic fashion. This review addresses whether striatal compartmentalization is present in non-mammalian vertebrates, in which simple cognitive and behavioral functions are executed by primitive sensori-motor systems. Studies show that neuronal subpopulations that share neurochemical and connective properties with striosomal and matrix neurons are present in the striata of not only anamniotes (fishes and amphibians), but also amniotes (reptiles and birds). However, these neurons do not form clearly segregated compartments in these vertebrates, suggesting that such compartmentalization is unique to mammals. In the ontogeny of the mammalian forebrain, the later-born matrix neurons disperse the early-born striosome neurons into clusters to form the compartments in tandem with the development of striatal afferents from the cortex. We propose that striatal compartmentalization in mammals emerged in parallel with the evolution of the cortex and possibly enhanced complex processing of sensory information and behavioral flexibility phylogenetically.

Genomic GC content can vary locally, and GC-rich regions are usually associated with increased DNA thermostability in thermophilic prokaryotes and warm-blooded eukaryotes. Among vertebrates, fish and amphibians appeared to possess a distinctly less heterogeneous AT/GC organization in their genomes, whereas cytogenetically detectable GC heterogeneity has so far only been documented in mammals and birds. The subject of our study is the gar, an ancient "living fossil" of a basal ray-finned fish lineage, known from the Cretaceous period. We carried out cytogenomic analysis in two gar genera (Atractosteus and Lepisosteus) uncovering a GC chromosomal pattern uncharacteristic for fish. Bioinformatic analysis of the spotted gar (Lepisosteus oculatus) confirmed a GC compartmentalization on GC profiles of linkage groups. This indicates a rather mammalian mode of compositional organization on gar chromosomes. Gars are thus the only analyzed extant ray-finned fishes with a GC compartmentalized genome. Since gars are cold-blooded anamniotes, our results contradict the generally accepted hypothesis that the phylogenomic onset of GC compartmentalization occurred near the origin of amniotes. Ecophysiological findings of other authors indicate a metabolic similarity of gars with mammals. We hypothesize that gars might have undergone convergent evolution with the tetrapod lineages leading to mammals on both metabolic and genomic levels. Their metabolic adaptations might have left footprints in their compositional genome evolution, as proposed by the metabolic rate hypothesis. The genome organization described here in gars sheds new light on the compositional genome evolution in vertebrates generally and contributes to better understanding of the complexities of the mechanisms involved in this process.

The evolution of the process of cornification in amniote epidermis from the general process of keratinization present in simple epithelia of anamniotes took place through the evolution of specialized intermediate filament (α) keratins, keratin-associated proteins (KAPs) and corneous proteins (CPs). The scanty information on the three-dimensional conformation of known KAPs and CPs indicate these proteins contain α-helix, random coiled, or beta sheets with different lengths and organizations. CP genes originated in a chromosome locus indicated as epidermal differentiation complex (EDC), and transformed the epidermal keratinization of anamniotes into the cornified epidermis and skin appendages of amniotes (claws, beaks, and feathers). In particular, peculiar genes encoding for small proteins with a central region of 34 amino acids conformed as beta sheets were originated in the EDC of sauropsids (reptiles and birds). These proteins were traditionally indicated as beta-keratins because they form filaments of 3-4 nm in diameter and show an X-ray beta pattern. Different from other proteins of the EDC, dimers of these corneous beta-proteins associate into long polymers of filamentous proteins utilized in sauropsids skin appendages, such as scales and feathers. Future challenges in this area of research will be the study on gene regulation and expression for these proteins, their origin and evolution in different lineages of sauropsids, and their role in determining the material properties of sauropsid scales and other skin appendages.

Sex chromosomes are believed to be stable in endotherms, but young and evolutionary unstable in most ectothermic vertebrates. Within lacertids, the widely radiated lizard group, sex chromosomes have been reported to vary in morphology and heterochromatinization, which may suggest turnovers during the evolution of the group. We compared the partial gene content of the Z-specific part of sex chromosomes across major lineages of lacertids and discovered a strong evolutionary stability of sex chromosomes. We can conclude that the common ancestor of lacertids, living around 70 million years ago (Mya), already had the same highly differentiated sex chromosomes. Molecular data demonstrating an evolutionary conservation of sex chromosomes have also been documented for iguanas and caenophidian snakes. It seems that differences in the evolutionary conservation of sex chromosomes in vertebrates do not reflect the distinction between endotherms and ectotherms, but rather between amniotes and anamniotes, or generally, the differences in the life history of particular lineages.

Background: The pharyngeal arches are a series of bulges found on the lateral surface of the head of vertebrate embryos, and it is within these segments that components of the later anatomy are laid down. In most vertebrates, the post-otic pharyngeal arches will form the branchial apparatus, while in amniotes these segments are believed to generate the larynx. It has been unclear how the development of these segments has been altered with the emergence of the amniotes. Results: In this study, we examined the development of pharyngeal arches in amniotes and show that the post-otic pharyngeal arches in this clade are greatly diminished. We find that the post-otic segments do not undergo myogenesis or skeletogenesis, but are remodelled before these processes occur. We also find that nested DLX expression, which is a feature of all the pharyngeal arches in anamniotes, is associated with the anterior segments but less so with the posterior arches in amniotes. We further show that the posterior arches of the mouse embryo fail to properly delineate, which demonstrates the lack of function of these posterior segments in later development. Conclusion: In amniotes, there has been a loss of the ancestral "branchial" developmental programme that is a general feature of gnathostomes; myogenesis and skeletogenesis This is likely to have facilitated the emergence of the larynx as a new structure not constrained by the segmental organisation of the posterior pharyngeal region.

The hypothesis here presented tries to explain why organ regeneration is present in fish and amphibians (anamniotes) but is absent in reptiles, birds and mammals (amniotes). Anamniotes possess complex life cycles including larvae and metamorphosis stages, the latter representing a physiological form of organ destruction and regeneration coded in their genome that can be reactivated in adults in the form of regeneration. Part of the genome for larvae and metamorphosis phases was likely lost in amniotes with the evolution of direct development, the potentiation of the immune system and the increase in complexity of the nervous system. These events consequently determined incapability for organ regeneration in extant amniotes with the exception of the lizard tail. This likely derives from the evolution of a mechanism of immunosuppression that allows the regeneration of the tail although the complete morphogenetic plane of tail embryogenesis is lost. The lizard model of imperfect but outstanding organ regeneration indicates the possibility to improve organ regeneration also in other amniotes. In fact, the induction of a blastema in the amputated lizard limb has stimulated the formation of short limbs containing cartilaginous bones of the femur, tibia and fibula, and these experiments foster some hope for future attempts to induce limb and digit regeneration also in mammals.

In contrast to amniotes (reptiles, birds and mammals), anamniotes (fishes and amphibians) can effectively regenerate body appendages such as fins, limbs and tails. Why such a useful capability was progressively lost in amniotes remains unknown. As we have hypothesized recently, one of the reasons for this could be loss of some genes regulating the regeneration in evolution of amniotes. Here, we demonstrate the validity of this hypothesis by showing that genes of small GTPases Ras-dva1 and Ras-dva2, that had been lost in a stepwise manner during evolution of amniotes and disappeared completely in placental mammals, are important for regeneration in anamniotes. Both Ras-dva genes are quickly activated in regenerative wound epithelium and blastema forming in the amputated adult Danio rerio fins and Xenopus laevis tadpoles’ tails and hindlimb buds. Down-regulation of any of two Ras-dva genes in fish and frog resulted in a retardation of regeneration accompanied by down-regulation of the regeneration marker genes. On the other hand, Ras-dva over-expression in tadpoles’ tails restores regeneration capacity during the refractory period when regeneration is blocked due to natural reasons. Thus our data on Ras-dva genes, which were eliminated in amniotes but play role in anamniotes regeneration regulation, satisfy our hypothesis.

The dorsal part of the developing telencephalon is one of the brain areas that has suffered most drastic changes throughout vertebrate evolution. Its evolutionary increase in complexity was thought to be partly achieved by the appearance of a new neurogenic niche in the embryonic subventricular zone (SVZ). Here, a new kind of amplifying progenitors (basal progenitors) expressing Tbr2, undergo a second round of divisions, which is believed to have contributed to the expansion of the neocortex. Accordingly, the existence of a pallial SVZ has been classically considered exclusive of mammals. However, the lack of studies in ancient vertebrates precludes any clear conclusion about the evolutionary origin of the SVZ and the neurogenic mechanisms that rule pallial development. In this work, we explore pallial neurogenesis in a basal vertebrate, the shark Scyliorhinus canicula, through the study of the expression patterns of several neurogenic markers. We found that apical progenitors and radial migration are present in sharks, and therefore, their presence must be highly conserved throughout evolution. Surprisingly, we detected a subventricular band of ScTbr2-expressing cells, some of which also expressed mitotic markers, indicating that the existence of basal progenitors should be considered an ancestral condition rather than a novelty of mammals or amniotes. Finally, we report that the transcriptional program for the specification of glutamatergic pallial cells (Pax6, Tbr2, NeuroD, Tbr1) is also present in sharks. However, the segregation of these markers into different cell types is not clear yet, which may be linked to the lack of layering in anamniotes This work was supported by the Spanish Ministerio de Economía y Competitividad-FEDER (BFU2014-5863-1P) SI

Oxygen limitation and surface area to volume relationships of the egg were long thought to constrain egg size in aquatic environments, but more recent evidence indicates that egg size per se does not influence oxygen availability to embryos. Here, we suggest that investment per offspring is nevertheless constrained in aquatic anamniotes by virtue of oxygen transport in free-living larvae. Drawing on the well-supported assumption that oxygen limitation is relatively pronounced in aquatic versus terrestrial environments and that oxygen limitation is particularly severe in warm aquatic environments, we employ comparative methods in the Amphibia to investigate this problem. Across hundreds of species and two major amphibian clades, the slope of species mean egg diameter over habitat temperature is negative for species with aquatic larvae but is positive or neutral for species featuring terrestrial eggs and no larvae. Yet across species with aquatic larvae, the negative slope of egg diameter over temperature is similar whether eggs are laid terrestrially or aquatically, consistent with an oxygen constraint arising at the larval stage. Finally, egg size declines more strongly with temperature for species that cannot breathe aerially before metamorphosis compared with those that can. Our results suggest that oxygen transport in larvae (not eggs) constrains investment per offspring. This study further extends the generality of temperature-dependent oxygen limitation as a mechanism driving the temperature-size rule in aquatic systems.

Indirect evidence suggests that blastopore closure during gastrulation of anamniotes, including amphibians such asXenopus laevis,depends on circumblastoporal convergence forces generated by the marginal zone (MZ), but direct evidence is lacking. We show that explanted MZs generate tensile convergence forces up to 1.5 μN during gastrulation and over 4 μN thereafter. These forces are generated by convergent thickening (CT) until the midgastrula and increasingly by convergent extension (CE) thereafter. Explants from ventralized embryos, which lack tissues expressing CE but close their blastopores, produce up to 2 μN of tensile force, showing that CT alone generates forces sufficient to close the blastopore. Uniaxial tensile stress relaxation assays show stiffening of mesodermal and ectodermal tissues around the onset of neurulation, potentially enhancing long-range transmission of convergence forces. These results illuminate the mechanobiology of early vertebrate morphogenic mechanisms, aid interpretation of phenotypes, and give insight into the evolution of blastopore closure mechanisms.

Although bats (Chiroptera) belong to the most diverse mammalian orders, study of diversity of their natural predators has been seriously neglected for a long time. While some recent reviews contain comprehensive overviews of our recent knowledge on these phenomena for some animal groups, such work is still lacking for anamniotes - fishes and amphibians. Here, I am summarising bat predation incidences by anamniotes that were published in scientific journals and public web domains. I found out that at least 14 species of fishes and 14 species of frogs were observed as feeding on bats. Moreover, 7 and 16 species of bats were recorded as victims of hunting activity of fishes and frogs, respectively. Majority of localised incidences came from the New World; frog (between latitude 49°N to 28°S) attacks were distributed mainly in tropical or subtropical areas < 30° from equator (70% of all records), fish (44°N to 20°S) incidences reach same proportion in colder areas (> 30° from equator) and warmer areas. In some cases, these predators can regularly feed on bats, especially when hunting near roosting places of bats; however, with respect to number of recorded cases (21 for fishes and 37 for amphibians), bat predation by anamniotes seems to be very rare and opportunistic with only very limited influence on bat populations or their behaviour.

Although vertebrates share many common traits, their germline development and function exhibit significant divergence. In particular, this article focuses on their spermatogenesis. The fundamental elements that constitute vertebrate spermatogenesis and the evolutionary changes that occurred upon transition from water to land will be discussed. The life-long continuity of spermatogenesis is supported by the function of stem cells. Series of mitotic and meiotic germ cell divisions are 'incomplete' due to incomplete cytokinesis, forming syncytia interconnected via intercellular bridges (ICBs). Throughout this process, germ cells are supported by appropriate microenvironments established primarily by somatic Sertoli cells. In anamniotes (fish and amphibians) spermatogenesis progresses in cysts, in which developing germ cell syncytia are individually encapsulated by Sertoli cells. Accordingly, Sertoli cells undergo turnover with germ cells that they nourish. This mode of cystic spermatogenesis is also observed in nonvertebrates as insects. In amniotes (reptiles, birds, and mammals), however, Sertoli cells do not turn over but comprise a persistent structure of seminiferous tubules. Sertoli cells nourish different stages of germ cells simultaneously in distinct regions of their surface. This function of Sertoli cells is spatiotemporally orchestrated, and the seminiferous epithelial cycle and spermatogenic wave make the seminiferous tubules a high-throughput factory for sperm production. Furthermore, contrary to the organized differentiating cells, undifferentiated spermatogonia that comprise the stem cell compartment exhibit active motion over the basal layer of seminiferous tubules and the frequent breakdown of ICBs. Thus, amniote seminiferous tubules represent a typical facultative (or open) niche environment without a stem cell tethering anatomically defined niche. WIREs Dev Biol 2016, 5:119-131. doi: 10.1002/wdev.204 For further resources related to this article, please visit the WIREs website.

The hippocampus in humans and other mammals is essential for episodic and relational memories. Comparative evidence indicates that a hippocampal pallium homologue is present in birds, reptiles, amphibians, ray-finned fishes, cartilaginous fishes and agnathans. Some of their characteristics, such as the topological position and the pattern of connectivity, appear remarkably well conserved. We review here substantial data showing that in all the vertebrate groups studied up to date, from fish to mammals, the hippocampus plays a fundamental role in spatial memory. In these vertebrates groups, the hippocampal pallium homologue is involved in the use of map-like, relational representations of the objective space that provide stable allocentric frames of reference, thus allowing flexible navigation. These similarities suggest a common evolutionary ancestry and indicate that the functional properties of the hippocampus appear early in the vertebrate phylogenesis and are retained through the independent evolution of the vertebrate lineages.

Appendage regeneration occurs by a sequence of events resembling those that take place during development in the embryo. This requires embryonic conditions such as hydration and hyaluronate content where Wnt and other signaling pathways, together with non- coding RNAs, can be re-expressed. These conditions among vertebrates are fully met only in amputated limbs of amphibians, likely because they are neotenic and maintain larval characteristics, including immaturity of their immune system and permanence of numerous stem cells. Although some key genes orchestrating limb regeneration are also present in amniotes, including humans, these genes are not expressed after injury. In amniotes a key problem for regeneration derives from the efficient immune system, largely deficient in anamniotes. As a consequence, wounds and appendage loss tend to scar instead of regenerating. Efforts of regenerative medicine in the attempt to induce the regrowth of limbs in humans must produce outgrowths with high hydration and hyaluronate content in order to create the immune-suppressed conditions similar to those present during development. The induced blastema must be manipulated for long periods of time in order to maintain the same regions present during limb development, an apical epidermal ridge and a polarizing region that forms gradients of expression of Wnt, Shh, FGF, BMP and Hox-genes. Pharmacological treatments to direct the regenerating limb into normal growth without risk of inducing abnormal or tumorigenic growth must be monitored during the course of the regeneration process - a medical treatment lasting years to fully regain the size of the lost appendage.

Ectothermal reptiles have internal pigmentation, which is not seen in endothermal birds and mammals. Here we show that the development of the dorsal neural tube-derived melanoblasts in turtle Trachemys scripta is regulated by similar mechanisms as in other amniotes, but significantly later in development, during the second phase of turtle trunk neural crest emigration. The development of melanoblasts coincided with a morphological change in the dorsal neural tube between stages mature G15 and G16. The melanoblasts delaminated and gathered in the carapacial staging area above the neural tube at G16, and differentiated into pigment-forming melanocytes during in vitro culture. The Mitf-positive melanoblasts were not restricted to the dorsolateral pathway as in birds and mammals but were also present medially through the somites similarly to ectothermal anamniotes. This matched a lack of environmental barrier dorsal and lateral to neural tube and the somites that is normally formed by PNA-binding proteins that block entry to medial pathways. PNA-binding proteins may also participate in the patterning of the carapacial pigmentation as both the migratory neural crest cells and pigment localized only to PNA-free areas.

Adult neurogenesis, an essential mechanism of brain plasticity, enables brain development along postnatal life, constant addition of new neurons, neuronal turnover, and/or regeneration. It is amply distributed but negatively modulated during development and along evolution. Widespread cell proliferation, high neurogenic, and regenerative capacities are considered characteristics of teleost brains during adulthood. These anamniotes are promising models to depict factors that modulate cell proliferation, migration, and neurogenesis, and might be intervened to promote brain plasticity in mammals. Nevertheless, the migration path of derived cells to their final destination was not studied in various teleosts, including most weakly electric fish. In this group adult brain morphology is attributed to sensory specialization, involving the concerted evolution of peripheral electroreceptors and electric organs, encompassed by the evolution of neural networks involved in electrosensory information processing. In wave type gymnotids adult brain morphology is proposed to result from lifelong region specific cell proliferation and neurogenesis. Consistently, pulse type weakly electric gymnotids and mormyrids show widespread distribution of proliferation zones that persists in adulthood, but their neurogenic potential is still unknown. Here we studied the migration process and differentiation of newborn cells into the neuronal phenotype in the pulse type gymnotid Gymnotus omarorum. Pulse labeling of S-phase cells with 5-Chloro-2'-deoxyuridine thymidine followed by 1 to 180 day survivals evidenced long distance migration of newborn cells from the rostralmost telencephalic ventricle to the olfactory bulb, and between layers of all cerebellar divisions. Shorter migration appeared in the tectum opticum and torus semicircularis. In many brain regions, derived cells expressed early neuronal markers doublecortin (chase: 1-30 days) and HuC/HuD (chase: 7-180 days). Some newborn cells expressed the mature neuronal marker tyrosine hydroxylase in the subpallium (chase: 90 days) and olfactory bulb (chase: 180 days), indicating the acquisition of a mature neuronal phenotype. Long term CldU labeled newborn cells of the granular layer of the corpus cerebelli were also retrogradely labeled "in vivo," suggesting their insertion into the neural networks. These findings evidence the neurogenic capacity of telencephalic, mesencephalic, and rhombencephalic brain proliferation zones in G. omarorum, supporting the phylogenetic conserved feature of adult neurogenesis and its functional significance.

The telencephalic basal ganglia (BG) of amniotes consist of two subdivisions, striatum and pallidum, which share many features, including development, cell types, neurotransmitter organization and hodology. In particular, these two subdivisions during development are defined on the basis of discrete gene expression patterns (genoarchitecture or genoarchitectonics). The characterization of the BG in the subpallium of representatives of the different classes of anamniote vertebrates was first approached in studies dealing with their localization, hodology and main neurochemical characteristics. Thus, it was proposed that an impressive degree of conservation exists across species. New insights can be gained by the comparative analysis of the expression of conserved transcription factors that distinctly define the striatal and pallidal components of the BG in all vertebrates. In addition, the expression of other genes that characterize neighboring regions of the BG is also useful to define the boundaries of each subdivision. Following this approach, we have analyzed the BG in the brain of juvenile representatives of amphibians, lungfishes, holosteans, Polypteriformes and Chondrichthyes. In addition, we briefly review previous data in teleosts and agnathans. The markers used include islet 1 and Dlx as striatal markers, whereas Nkx2.1 is essential for the identification of the pallidum. In turn, Pax6 and in particular Tbr1 are expressed in the pallium. These markers have been systematically analyzed in combination with neuronal markers of specific subpallial territories and cell populations, such as tyrosine hydroxylase, γ-aminobutyric acid, nitric oxide synthase, substance P and enkephalin. The results highlight that many genes share common distribution patterns and are arranged in conserved combinations whose identification has served to define homologies between components of the BG in distant species.

Cladistians (Polypteriformes) are currently considered basal to other living ray-finned fishes (actinopterygians), and their brain organization is therefore critical to providing information about the primitive neural characters that existed in the earliest ray-finned fishes. The organization of the serotonergic system in the brain has been carefully analyzed in most vertebrate groups, and in the present study we provide the first detailed information on the distribution of serotonergic cell bodies and fibers in the central nervous system of representative species of the two extant genera of cladistians, i.e. Polypterus senegalus and Erpetoichthys calabaricus, by means of immunohistochemistry against serotonin (5-HT). Distinct groups of immunoreactive cells were detected in the preoptic area, the hypothalamic paraventricular organ, the pineal organ, the pretectal region, the long column of the raphe in the rhombencephalic midline, the spinal cord, and amacrine cells in the inner nuclear layer of the retina. Fiber labeling was widely distributed in all main brain subdivisions but was more abundant in distinct pallial and subpallial areas, the preoptic area, the thalamus, the optic tectum, the tori semicircularis and lateralis, the rhombencephalic reticular formation, the nucleus of the solitary tract, and the dorsal aspect of the spinal cord. Our analysis makes it possible to establish which serotonergic structures characterized the earliest ray-finned fishes, and a comparison of these results with those from other classes of vertebrates, including a segmental analysis to correlate cell populations, reveals that most characteristics, such as the presence of serotonergic cells in the preoptic area and the basal hypothalamus, are preserved in all anamniotes. However, this system seems to be reduced in amniotes, mainly mammals, although important features are shared, such as the presence of serotonergic cells in the pineal organ, the retina, and the raphe nuclei.