Effects of testosterone on urogenital tract morphology and androgen receptor expression in immature Eastern Fence lizards (Sceloporus undulatus)

In non-avian reptiles, the onset of sexual dimorphism of the major structures of the urogenital tract varies temporally relative to gonadal differentiation, more so than in other amniote lineages. In the current study, we used tonic-release implants to investigate the effects of exogenous testosterone (T) on postnatal development of the urogenital tract in juvenile Eastern Fence Lizards (Sceloporus undulatus) to better understand the mechanisms underlying the ontogeny of sexual differentiation in reptiles. We examined gonads, mesonephric kidneys and ducts (male reproductive tract primordia), paramesonephric ducts (oviduct primordia), sexual segments of the kidneys (SSKs), and hemiphalluses to determine which structures were sexually dimorphic independent of T treatment and which structures exhibited sexually dimorphic responses to T. To better understand tissue-level responsiveness to T treatment, we also characterized androgen receptor (AR) expression by immunohistochemistry. At approximately 4 months after hatching in control animals, gonads were well differentiated but quiescent; paramesonephric ducts had fully degenerated in males; mesonephric kidneys, mesonephric ducts, and SSKs remained sexually undifferentiated; and hemiphalluses could not be everted in either sex. Exogenous T caused enlargement, regionalization, and secretory activity of the mesonephric ducts and SSKs in both sexes; enlargement and regionalization of the oviducts in females; and enlargement of male hemipenes. The most responsive tissues exhibited moderate but diffuse staining for AR in control lizards and intense nuclear staining in T-treated lizards, suggestive of autoregulation of AR. The similarity between sexes in the responsiveness of the mesonephric ducts and SSK to T indicates an absence of sexually dimorphic organizational effects in these structures prior to treatment, which was initiated approximately 2 months after hatching. In contrast, the sex-specific responses in oviducts and hemipenes indicate that significant organization and/or differentiation had taken place prior to treatment.

An extra-genital cell population contributes to urethra closure during mouse penis development

Hypospadias, or incomplete closure of the urethra along the penis, is the second most common birth defect in the United States. We discovered a population of extra-genital mesenchymal cells that are essential for proper penile urethra closure in mouse embryos. This extra-genital population first appeared in the mesenchyme posterior to the hindlimb of the fetus after the onset of penis formation. These extra-genital cells, which transiently express a lineage marker Nr5a1, migrated centrally and colonized the penis bilateral to the urethra epithelium. Removal of the Nr5a1+ extra-genital cells, using a cell-type specific ablation model, resulted in severe hypospadias. The absence of extra-genital cells had the most significant impacts on another mesenchymal cells, the peri-urethra that were immediately adjacent to the Nr5a1+ extra-genital cells. Single cell mRNA sequencing revealed that the extra-genital cells extensively interact with the peri-urethra, particularly through Neuregulin 1, an epidermal Growth Factor (EGF) ligand. Disruption of Neuregulin 1 signaling in the ex-vivo slice culture system led to failure of urethra closure, recapitulating the phenotypes of extra-genital cell ablation. These results demonstrate that the Nr5a1+ extra-genital mesenchymal cells from outside of the fetal penis are indispensable for urethra closure through their interaction with the peri-urethra mesenchymal cells. This discovery provides a new entry point to understand the biology of penis formation and potential causes of hypospadias in humans.

First evidence of hemiclitores in snakes

Female genitalia are conspicuously overlooked in comparison to their male counterparts, limiting our understanding of sexual reproduction across vertebrate lineages. This study is the first complete description of the clitoris (hemiclitores) in female snakes. We describe morphological variation in size and shape (n = 9 species, 4 families) that is potentially comparable to the male intromittent organs in squamate reptiles (hemipenes). Dissection, diffusible iodine contrast-enhanced micro-CT and histology revealed that, unlike lizard hemiclitores, the snake hemiclitores are non-eversible structures. The two individual hemiclitores are separated medially by connective tissue, forming a triangular structure that extends posteriorly. Histology of the hemiclitores in Australian death adders (Acanthophis antarcticus) showed erectile tissue and strands/bundles of nerves, but no spines (as is found in male hemipenes). These histological features suggest the snake hemiclitores have functional significance in mating and definitively show that the hemiclitores are not underdeveloped hemipenes or scent glands, which have been erroneously indicated in other studies. Our discovery supports that hemiclitores have been retained across squamates and provides preliminary evidence of differences in this structure among snake species, which can be used to further understand systematics, reproductive evolution and ecology across squamate reptiles.

Morphological and structural differences between the hemipenes and hemiclitores of golden lancehead snakes, Bothrops insularis (Amaral, 1922), revealed by radiography

The golden lancehead (Bothrops insularis) is a critically endangered and island endemic snake. A remarkable characteristic of golden lanceheads is the high frequency of females showing hemiclitores. Here, we use radiography to identify morphological and structural differences between the hemipenes and hemiclitores in captive individuals. Sex was identified using the probing method and confirmed by visualizing the gonads by ultrasound. Hemipenes appeared as herringbone-shaped structures with bone radiopacity and hemiclitores as triangular structures with soft tissue radiopacity. These differences suggest that hemiclitores spines fail to develop the level of ossification observed in hemipenis spines.

The Squamate Clitoris: A Review and Directions for Future Research

The clitoris is a part of the genitalia of female amniotes that typically functions to stimulate sensory arousal. It usually consists of a small organ that is dimorphic and homologous to the penis. The developing amniote embryo forms a genital tubule, then sex hormones initiate a developmental cascade to form either a penis or clitoris. In squamates (lizards and snakes) the genital tubule develops into a paired hemiphallus structure called the "hemiclitores" in the female and the "hemipenes" in the male. The complex evolution of squamate hemipenes has been extensively researched since early discoveries in the 1800's, and this has uncovered huge diversity in hemipenis size, shape, and ornamentation (e.g., protrusions of spines, hooks, chalices, cups). In contrast, the squamate hemiclitoris has been conspicuously under investigated, and the studies that describe this anatomy are fraught with inconsistences. This paper aims to clarify the current state of knowledge of the squamate hemiclitoris, providing a foundation for further research on its morphology and functional role. We show that while several studies have described the gross anatomy of hemiclitores in lizards, comparative information is entirely lacking for snakes. Several papers cite earlier authors as having reported discoveries of the snake hemiclitores in vipers and colubrid snakes. However, our examination of this reveals only erroneous reports of hemiclitores in snakes and shows that these stem from misinterpretations of the true anatomy or species involved. An especially problematic source of confusion is the presence of intersex individuals in some snake populations; these form reproductively functional ovaries and a single hemipenis, with the latter sometimes mistaken for a hemiclitoris (the intersex hemipenis is usually smaller and less spinous than the male hemipenis). Further research is recommended to identify the defining anatomical features of the squamate hemiclitores. Such studies will form a vital basis of future comparative analyses of variation in female genitalia in squamates and other amniotes.

A staging table of embryonic development for a viviparous (live-bearing) lizard Eremias multiocellata (Squamata: Lacertidae)

As the only viviparous reptile in China that has both temperature-dependent sex determination (TSD) and genetic-dependent sex determination (GSD) mechanisms, Eremias multiocellata is considered as an ideal species for studying the sex determination mechanism in viviparous lizards. However, studies on embryonic stage of viviparous lizards and morphological characteristics of each stage are limited. In the present study, the embryonic development process of E. multiocellata is divided into 15 stages (stages 28-42) according to the morphology of embryos. Embryos sizes are measured and continuous dynamic variation of some key features, including limbs, genitals, eyes, pigments, and brain scales are color imaged by a stereoscopic microscope. Furthermore, based on these morphological characteristics, we compare the similarities and differences in the embryonic development of E. multiocellata with other squamate species. Our results not only identified the staging table of E. multiocellata with continuous changes of external morphological characteristics but also developed a staging scheme for an important model species that provides a necessary foundation for study of sex determination in a viviparous lizard.

Genome-wide molecular phylogenetic analyses and mating experiments which reveal the evolutionary history and an intermediate stage of speciation of a giant water bug

The intermediate stages of speciation are important for understanding the processes involved in the creation of biodiversity, and also comprise a number of interesting phenomena. However, difficulties are associated with dividing clear speciation stages because speciation is a continuous process. Therefore, the elucidation of speciation is an interesting and important task in evolutionary biology. We herein present an example of a species in an intermediate stage of speciation using the giant water bug Appasus japonicus (Heteroptera, Belostomatidae) that was investigated using mating experiments and phylogenetic analyses of the mtDNA COI (658 bp) and 16S rRNA (435 bp) regions, and nDNA SSR (13 loci) and its genome-wide SNPs (11,241 SNPs). The results of our phylogenetic analyses based on their mtDNA dataset and the genome-wide SNPs dataset strongly supported the paraphyly of the Japanese populations. Therefore, it is suggested that their ancestral lineage which being distributed in the Japanese Archipelago subsequently migrated to the Eurasian Continent (i.e., "back-dispersal" occurred). Furthermore, the results of the mating experiments suggested that among A. japonicus, even between closely related lineages, pre-mating reproductive isolation has been established by the differentiation of copulatory organ morphologies. In contrast, pre-mating reproductive isolation is not established in the absence of the differentiation of copulatory organ morphologies, even if genetic differentiation is prominent. These results suggested that their phylogenetic distance does not predict pre-mating reproductive isolation. Furthermore, in the present study, we present a clear example of pre-mating reproductive isolation driving speciation between closely related lineages.

Single cell transcriptomic analysis of external genitalia reveals complex and sexually dimorphic cell populations in the early genital tubercle

External genital organs are among the most recognizable sexually dimorphic characters. The penis and clitoris develop from the embryonic genital tubercle, an outgrowth at the anterior margin of the cloaca that undergoes an extensive period of development in male and female embryos prior to the onset of sexual differentiation. In mice, differentiation into the penis and clitoris begins around embryonic day (E)15.5. Current knowledge of cell types that comprise the genital tubercle is limited to a few studies that have fate mapped derivatives of endoderm, mesoderm, and ectoderm. Here we use single cell transcriptomics to characterize the cell populations in the genital tubercles of male and female mouse embryos at E14.5, approximately 24 ​h before the onset of sexual differentiation, and we present the first comprehensive atlas of single-cell gene expression during external genital development. Clustering analyses and annotation using marker genes shows 19 distinct cell populations in E14.5 genital tubercles. Mapping of cell clusters to anatomical locations using in situ gene expression patterns revealed granularity of cellular specializations and positional identities. Although E14.5 precedes sexually dimorphic morphogenesis of the genital tubercle, comparative analysis of males and females identified sexual dimorphisms at the single cell level, including male-specific cell clusters with transcriptional signatures of smooth muscle and bone progenitors, both of which are known to be sexually dimorphic in adult genitalia, as well as immune cells. These results provide a new resource for classification of external genital cell types based on gene expression profiles and reveal sex-specific cellular specializations in the early genital tubercle.

Ultrastructure of eggshell and embryological development of Salvator merianae (Squamata: Teiidae)

The objective of this study was to characterize the external morphology of Salvator meriane embryos in different stages of embryonic development and establish a relationship with the ultrastructure of the shell from oviductal transit to hatching. A total of 120 embryos were analyzed to describe their external morphology, and 78 eggs were used for the analysis of the shell. For embryonic development, the series was established according to the total length of the body. We established 40 embryonic stages from the primitive streak. In the early stages, the external morphological features are the C-shaped body, the maxillary, and mandibular fusion processes with the frontal process and the fusion of the forelimb with the digital plate. In the middle stages, the eyelid appears, and there are claws on the toes, cornification of fingers, and the onset of pigmentation. The last stage of embryonic development is characterized by the beginning of the formation of the scales, appear the toenails, and finalize the entire pigmentation. Regarding the relationship that exists with the ultrastructure of the egg during development, it was possible to observe a marked change in the composition of the shell and well-marked compaction during embryonic development, which may be related to the transport of calcium during embryonic ossification. Our results allowed us to show the complete sequence of embryonic development, determining the laying stage for this species. It was possible to establish a relationship with the ultrastructure of the eggshell from the oviductal transit to the moment of hatching.

Are hemipenial traits under sexual selection in Tropidurus lizards? Hemipenial development, male and female genital morphology, allometry and coevolution in Tropidurus torquatus (Squamata: Tropiduridae)

Male genitalia show considerable morphological variation among animals with internal fertilization and exhibit a high level of evolvability in lizards. Studies have suggested that sexual selection may be driving hemipenial evolution against natural selection and pleiotropy. Given the direct interaction of male and female genitals, coevolution of the aforementioned is posited by several hypotheses of genital evolution. However, there are only a few studies on female genitalia morphology, resulting in a lack of coevolution description and understanding. Studies of allometric patterns have filled some gaps by answering questions about male genital evolution and could prove a powerful tool in clarifying coevolution between male and female genitals. Here, we studied the genital morphology of Tropidurus torquatus. This Tropidurus lizard species is an emerging Neotropical lizard model organism notable for having enlarged hemipenial lobes in contrast with other tropidurid species. In this study, we analyzed hemipenial development in early and late stages, describing both morphological variation and ontogenetic allometric pattern. We used quantitative traits to describe male and female genital morphology, examining their static allometric patterns and correspondence. Our study provides a quantitative discussion on the evolution of lizard genitals, suggesting that sexual selection plays an important role in genital evolution in Tropidurus lizards.

Developmental asynchrony and antagonism of sex determination pathways in a lizard with temperature-induced sex reversal

Vertebrate sex differentiation follows a conserved suite of developmental events: the bipotential gonads differentiate and shortly thereafter sex specific traits become dimorphic. However, this may not apply to squamates, a diverse vertebrate lineage comprising of many species with thermosensitive sexual development. Of the three species with data on the relative timing of gonad differentiation and genital dimorphism, the females of two (Niveoscincus ocellatus and Barisia imbricata) exhibit a phase of temporary pseudohermaphroditism or TPH (gonads have differentiated well before genital dimorphism). We report a third example of TPH in Pogona vitticeps, an agamid with temperature-induced male to female sex reversal. These findings suggest that for female squamates, genital and gonad development may not be closely synchronised, so that TPH may be common. We further observed a high frequency of ovotestes, a usually rare gonadal phenotype characterised by a mix of male and female structures, exclusively associated with temperature-induced sex reversal. We propose that ovotestes are evidence of a period of antagonism between male and female sex-determining pathways during sex reversal. Female sexual development in squamates is considerably more complex than has been appreciated, providing numerous avenues for future exploration of the genetic and hormonal cues that govern sexual development.

Sex determination mode does not affect body or genital development of the central bearded dragon (Pogona vitticeps)

Background

The development of male- or female-specific phenotypes in squamates is typically controlled by either temperature-dependent sex determination (TSD) or chromosome-based genetic sex determination (GSD). However, while sex determination is a major switch in individual phenotypic development, it is unknownhow evolutionary transitions between GSD and TSD might impact on the evolution of squamate phenotypes, particularly the fast-evolving and diverse genitalia. Here, we take the unique opportunity of studying the impact of both sex determination mechanisms on the embryological development of the central bearded dragon (Pogona vitticeps). This is possible because of the transitional sex determination system of this species, in which genetically male individuals reverse sex at high incubation temperatures. This can trigger the evolutionary transition of GSD to TSD in a single generation, making P. vitticeps an ideal model organism for comparing the effects of both sex determination processes in the same species.

Results

We conducted four incubation experiments on 265 P. vitticeps eggs, covering two temperature regimes ("normal" at 28 °C and "sex reversing" at 36 °C) and the two maternal sexual genotypes (concordant ZW females or sex-reversed ZZ females). From this, we provide the first detailed staging system for the species, with a focus on genital and limb development. This was augmented by a new sex chromosome identification methodology for P. vitticeps that is non-destructive to the embryo. We found a strong correlation between embryo age and embryo stage. Aside from faster growth in 36 °C treatments, body and external genital development was entirely unperturbed by temperature, sex reversal or maternal sexual genotype. Unexpectedly, all females developed hemipenes (the genital phenotype of adult male P. vitticeps), which regress close to hatching.

Conclusions

The tight correlation between embryo age and embryo stage allows the precise targeting of specific developmental periods in the emerging field of molecular research on P. vitticeps. The stability of genital development in all treatments suggests that the two sex-determining mechanisms have little impact on genital evolution, despite their known role in triggering genital development. Hemipenis retention in developing female P. vitticeps, together with frequent occurrences of hemipenis-like structures during development in other squamate species, raises the possibility of a bias towards hemipenis formation in the ancestral developmental programme for squamate genitalia.

Model Clades Versus Model Species: Anolis Lizards as an Integrative Model of Anatomical Evolution

Anolis lizards , known for their replicated patterns of morphological diversification, are widely studied in the fields of evolution and ecology. As a textbook example of adaptive radiation, this genus has supported decades of intense study in natural history, behavior, morphological evolution, and systematics. Following the publication of the A. carolinensis genome, research on Anolis lizards has expanded into new areas, toward obtaining an understanding the developmental and genetic bases of anole diversity. Here, we discuss recent progress in these areas and the burgeoning methodological toolkit that has been used to elucidate the genetic mechanisms underlying anatomical variation in this group. We also highlight the growing number of studies that have used A. carolinensis as the representative squamate in large-scale comparison of amniote evolution and development . Finally, we address one of the largest technical challenges biologists are facing in making Anolis a model for integrative studies of ecology, evolution, development , and genetics, the development of ex-ovo culturing techniques that have broad utility. Ultimately, with the power to ask questions across all biological scales in this diverse genus full, anoles are rapidly becoming a uniquely integrative and powerful biological system.

Resurrecting embryos of the tuatara, Sphenodon punctatus, to resolve vertebrate phallus evolution

The breadth of anatomical and functional diversity among amniote external genitalia has led to uncertainty about the evolutionary origins of the phallus. In several lineages, including the tuatara, Sphenodon punctatus, adults lack an intromittent phallus, raising the possibility that the amniote ancestor lacked external genitalia and reproduced using cloacal apposition. Accordingly, a phallus may have evolved multiple times in amniotes. However, similarities in development across amniote external genitalia suggest that the phallus may have a single evolutionary origin. To resolve the evolutionary history of amniote genitalia, we performed three-dimensional reconstruction of Victorian era tuatara embryos to look for embryological evidence of external genital initiation. Despite the absence of an intromittent phallus in adult tuataras, our observations show that tuatara embryos develop genital anlagen. This illustrates that there is a conserved developmental stage of external genital development among all amniotes and suggests a single evolutionary origin of amniote external genitalia.

Embryonic origin and compartmental organization of the external genitalia

Genital malformations occur at a high frequency in humans, affecting ~1:250 live births. The molecular mechanisms of external genital development are beginning to be identified; however, the origin of cells that give rise to external genitalia is unknown. Here we use cell lineage analysis to show that the genital tubercle, the precursor of the penis and clitoris, arises from two populations of progenitor cells that originate at the lateral edges of the embryo, at the level of the posterior hindlimb buds and anterior tail. During body wall closure, the left and right external genital progenitor pools are brought together at the ventral midline, where they form the paired genital swellings that give rise to the genital tubercle. Unexpectedly, the left and right external genital progenitor pools form two lineage-restricted compartments in the phallus. Together with previous lineage studies of limb buds, our results indicate that, at the pelvic level, the early lateral mesoderm is regionalized from medial to lateral into dorsal limb, ventral limb, and external genital progenitor fields. These findings have implications for the evolutionary diversification of external genitalia and for the association between external genital defects and disruption of body wall closure, as seen in the epispadias-extrophy complex.