Characterisation and expression of Sox9 in the Leopard gecko, Eublepharis macularius

Histological Analysis of Gonadal Ridge Development and Sex Differentiation of Gonads in Three Gecko Species

Reptiles constitute a highly diverse group of vertebrates, with their evolutionary lineages having diverged relatively early. The types of sex determination exemplify the diversity of reptiles; however, there are limited data regarding the gonadal development in squamate reptiles. Geckos constitute a group that is increasingly used in research and that serves as a potential reptilian model organism. The aim of this study was to trace the changes in the structure of developing gonads in the embryos of three gecko species: the crested gecko, leopard gecko, and mourning gecko. These species represent different families of the Gekkota infraorder and exhibit different types of sex determination. Gonadal development was examined from the formation of the earliest gonadal ridges through the development of undifferentiated gonadal structures, sex differentiation of gonads, and the formation of testicular and ovarian structures. The study showed that the gonadal primordia of these three gecko species formed on the most dorsally located surface of the dorsal mesentery, and both the coelomic epithelium and the nephric mesenchyme contributed to their development. As in other reptile species, primordial germ cells settled in the gonadal ridges, and the undifferentiated gonad was composed of a cortex and a medulla. Ovarian differentiation started with the thickening of the gonadal cortex and proliferation of germ cells in this region. A characteristic feature of the developing gecko ovaries was the thickened crescent-shaped cortex on the medial and ventral surfaces of the ovaries. The ovarian medulla also grew and exhibited diverse tendencies to form cords. In the leopard gecko, advanced cord-like structures with lumens were observed in the ovaries, which were not seen in the crested gecko. Testicular differentiation was characterized by cortical thinning and the disappearance of germ cells in this region. In the medulla, the development of distinct cords with early lumen formation was noted. A characteristic feature of embryonic gonads was their growth in a horizontal plane. In this study, gonadal development was characterized by several features that are shared by geckos and other reptiles, along with features that are specific only to geckos.

How do maternal androgens and estrogens affect sex determination in reptiles with temperature-dependent sex?

Temperature sex determination (TSD) in reptiles has been studied to elucidate the mechanisms by which temperature is transformed into a biological signal that determines the sex of the embryo. Temperature is thought to trigger signals that alter gene expression and hormone metabolism, which will determine the development of female or male gonads. In this review, we focus on collecting and discussing important and recent information on the role of maternal steroid hormones in sex determination in oviparous reptiles such as crocodiles, turtles, and lizards that possess TSD. In particular, we focus on maternal androgens and estrogens deposited in the egg yolk and their metabolites that could also influence the sex of offspring. Finally, we suggest guidelines for future research to help clarify the link between maternal steroid hormones and offspring sex.

Genetics and biology of coloration in reptiles: the curious case of the Lemon Frost geckos

Although there are more than 10,000 reptile species, and reptiles have historically contributed to our understanding of biology, genetics research into class Reptilia has lagged compared with other animals. Here, we summarize recent progress in genetics of coloration in reptiles, with a focus on the leopard gecko, Eublepharis macularius. We highlight genetic approaches that have been used to examine variation in color and pattern formation in this species as well as to provide insights into mechanisms underlying skin cancer. We propose that their long breeding history in captivity makes leopard geckos one of the most promising emerging reptilian models for genetic studies. More broadly, technological advances in genetics, genomics, and gene editing may herald a golden era for studies of reptile biology.

Temperature Incubation Influences Gonadal Gene Expression during Leopard Gecko Development

During development, sexual differentiation results in physiological, anatomical and metabolic differences that implicate not only the gonads but also other body structures. Sex in Leopard geckos is determined by egg incubation temperature. Based on the premise that the developmental decision of gender does not depend on a single gene, we performed an analysis on E. macularius to gain insights into the genes that may be involved in gonads' sexual differentiation during the thermosensitive period. All the genes were identified as differentially expressed at stage 30 during the labile phase of sex differentiation. In this way, the expression of genes known to be involved in gonadal sexual differentiation, such as WNT4, SOX9, DMRT1, Erα, Erβ, GnRH, P450 aromatase, PRLand PRL-R, was investigated. Other genes putatively involved in sex differentiation were sought by differential display. Our findings indicate that embryo exposure to a sex-determining temperature induces differential expression of several genes that are involved not only in gonadal differentiation, but also in several biological pathways (ALDOC, FREM1, BBIP1, CA5A, NADH5, L1 non-LTR retrotransposons, PKM). Our data perfectly fit within the new studies conducted in developmental biology, which indicate that in the developing embryo, in addition to gonadal differentiation, sex-specific tissue and metabolic polarization take place in all organisms.

Mechanisms related to sexual determination by temperature in reptiles

A number of strategies have emerged that appear to relate to the evolution of mechanisms for sexual determination in vertebrates, among which are genetic sex determination caused by sex chromosomes and environmental sex determination, where environmental factors influence the phenotype of the sex of an individual. Within the reptile group, some orders such as: Chelonia, Crocodylia, Squamata and Rhynchocephalia, manifest one of the most intriguing and exciting environmental sexual determination mechanisms that exists, comprising temperature-dependent sex determination (TSD), where the temperature of incubation that the embryo experiences during its development is fundamental to establishing the sex of the individual. This makes them an excellent model for the study of sexual determination at the molecular, cellular and physiological level, as well as in terms of their implications at an evolutionary and ecological level. There are different hypotheses concerning how this process is triggered and this review aims to describe any new contributions to particular TSD hypotheses, analyzing them from the "eco-evo-devo" perspective.

Localization and distribution of gonadal proteins in the oviparous lizard Sceloporus aeneus (Squamata: Phrynosomatidae)

Among vertebrates, several specific proteins are involved in the function and development of gonads. Several genes such as SOX9, FOXL2, DDX4, IFITM3, and DPPA3, are active during embryonic differentiation and maintain their expression in adult tissues, playing important roles in the function and development of the line cell, where these are produced. Among reptiles, molecular mechanisms for sex differentiation have been analyzed in turtles, crocodiles, and some lizards, while in adult stages such studies are scarce. The aim of this study was to locate and analyze the distribution of important gonadal proteins in adult and embryonic ovaries and testes of the oviparous lizard Sceloporus aeneus (Squamata: Phrynosomatidae). Adult specimens and embryos of the lizard S. aeneus were collected in Milpa Alta, a suburb located Southwest of Mexico City. Expression of gonadal proteins was analyzed using immunofluorescent staining and confocal microscopy. Our results showed that SOX9 is located in Sertoli cells of embryonic and adult testes. FOXL2 is expressed in follicular cells of adult ovaries. DDX4 and IFITM3 are located in germ line cells as well as in follicular cells of adult ovaries. DPPA3 was observed in somatic and germ line cells of adult and embryonic gonads. Our observations show that important molecules of vertebrate ovaries and testes are conserved in S. aeneus and it is suggested that these may have a similar role during gonadal development and function.

Yolk-albumen testosterone in a lizard with temperature-dependent sex determination: relation with development

The leopard gecko (Eublepharis macularius) exhibits temperature-dependent sex determination as well as temperature-influenced polymorphisms. Research suggests that in oviparous reptiles with temperature-dependent sex determination, steroid hormones in the yolk might influence sex determination and sexual differentiation. From captive leopard geckos that were all from the same incubation temperature regime, we gathered freshly laid eggs, incubated them at one of two female-biased incubation temperatures (26 or 34°C), and measured testosterone content in the yolk-albumen at early or late development. No differences in the concentration of testosterone were detected in eggs from different incubation temperatures. We report testosterone concentrations in the yolk-albumen were higher in eggs of late development than early development at 26°C incubation temperatures, a finding opposite that reported in other TSD reptiles studied to date.

Segregating variation for temperature-dependent sex determination in a lizard

Temperature-dependent sex determination (TSD) was first reported in 1966 in an African lizard. It has since been shown that TSD occurs in some fish, several lizards, tuataras, numerous turtles and all crocodilians. Extreme temperatures can also cause sex reversal in several amphibians and lizards with genotypic sex determination. Research in TSD species indicates that estrogen signaling is important for ovary development and that orthologs of mammalian genes have a function in gonad differentiation. Nevertheless, the mechanism that actually transduces temperature into a biological signal for ovary versus testis development is not known in any species. Classical genetics could be used to identify the loci underlying TSD, but only if there is segregating variation for TSD. Here, we use the 'animal model' to analyze inheritance of sexual phenotype in a 13-generation pedigree of captive leopard geckos, Eublepharis macularius, a TSD reptile. We directly show genetic variance and genotype-by-temperature interactions for sex determination. Additive genetic variation was significant at a temperature that produces a female-biased sex ratio (30°C), but not at a temperature that produces a male-biased sex ratio (32.5°C). Conversely, dominance variance was significant at the male-biased temperature (32.5°C), but not at the female-biased temperature (30°C). Non-genetic maternal effects on sex determination were negligible in comparison with additive genetic variance, dominance variance and the primary effect of temperature. These data show for the first time that there is segregating variation for TSD in a reptile and consequently that a quantitative trait locus analysis would be practicable for identifying the genes underlying TSD.

A review of sex determining mechanisms in geckos (Gekkota: Squamata)

Geckos are a species-rich clade of reptiles possessing diverse sex determining mechanisms. Some species possess genetic sex determination, with both male and female heterogamety, while other species have temperature-dependent sex determination. I compiled information from the literature on the taxonomic distribution of these sex determining mechanisms in geckos. Using phylogenetic data from the literature, I reconstructed the minimum number of transitions among these sex determining mechanisms with parsimony-based ancestral state reconstruction. While only a small number of gecko species have been characterized, numerous changes among sex determining mechanisms were inferred. This diversity, coupled with the high frequency of transitions, makes geckos excellent candidates as a model clade for the study of vertebrate sex determination and evolution.

Molecular mechanisms of sex determination in reptiles

Charles Darwin first provided a lucid explanation of how gender differences evolve nearly 140 years ago. Yet, a disconnect remains between his theory of sexual selection and the mechanisms that underlie the development of males and females. In particular, comparisons between representatives of different phyla (i.e., flies and mice) reveal distinct genetic mechanisms for sexual differentiation. Such differences are hard to comprehend unless we study organisms that bridge the phylogenetic gap. Analysis of variation within monophyletic groups (i.e., amniotes) is just as important if we hope to elucidate the evolution of mechanisms underlying sexual differentiation. Here we review the molecular, cellular, morphological, and physiological changes associated with sex determination in reptiles. Most research on the molecular biology of sex determination in reptiles describes expression patterns for orthologs of mammalian sex-determining genes. Many of these genes have evolutionarily conserved expression profiles (i.e., DMRT1 and SOX9 are expressed at a higher level in developing testes vs. developing ovaries in all species), which suggests functional conservation. However, expression profiling alone does not test gene function and will not identify novel sex-determining genes or gene interactions. For that reason, we provide a prospectus on various techniques that promise to reveal new sex-determining genes and regulatory interactions among these genes. We offer specific examples of novel candidate genes and a new signaling pathway in support of these techniques.

The platelet-derived growth factor signaling system in snapping turtle embryos, Chelydra serpentina: potential role in temperature-dependent sex determination and testis development

The platelet-derived growth factor (Pdgf) signaling system is known to play a significant role during embryonic and postnatal development of testes in mammals and birds. In contrast, genes that comprise the Pdgf system in reptiles have never been cloned or studied in any tissue, let alone developing gonads. To explore the potential role of PDGF ligands and their receptors during embryogenesis, we cloned cDNA fragments of Pdgf-A, Pdgf-B, and receptors PdgfR-alpha and PdgfR-beta in the snapping turtle, a reptile with temperature-dependent sex determination (TSD). We then compared gene expression profiles in gonads from embryos incubated at a male-producing temperature to those from embryos at a female-producing temperature, as well as between hatchling testes and ovaries. Expression of Pdgf-B mRNA in embryonic gonads was significantly higher at a male temperature than at a female temperature, but there was no difference between hatchling testes and ovaries. This developmental pattern was reversed for Pdgf-A and PdgfR-alpha mRNA: expression of these genes did not differ in embryos, but diverged in hatchling testes and ovaries. Levels of PdgfR-beta mRNA in embryonic gonads were not affected by temperature and did not differ between testes and ovaries. However, expression of both receptors increased at least an order of magnitude from the embryonic to the post-hatching period. Finally, we characterized expression of these genes in several other embryonic tissues. The brain, heart, and liver displayed unique expression patterns that distinguished these tissues from each other and from intestine, lung, and muscle. Incubation temperature had a significant effect on expression of PdgfR-alpha and PdgfR-beta in the heart but not other tissues. Together, these findings demonstrate that temperature has tissue specific effects on the Pdgf system and suggest that Pdgf signaling is involved in sex determination and the ensuing differentiation of testes in the snapping turtle.

Analyzing the coordinated gene network underlying temperature-dependent sex determination in reptiles

Although gonadogenesis has been extensively studied in vertebrates with genetic sex determination, investigations at the molecular level in nontraditional model organisms with temperature-dependent sex determination are relatively new areas of research. Results show that while the key players of the molecular network underlying gonad development appear to be retained, their functions range from conserved to novel roles. In this review, we summarize experiments investigating candidate molecular players underlying temperature-dependent sex determination. We discuss some of the problems encountered unraveling this network, pose potential solutions, and suggest rewarding future directions of research.

Sexual dimorphic expression of DMRT1 and Sox9a during gonadal differentiation and hormone-induced sex reversal in the teleost fish Nile tilapia (Oreochromis niloticus)

We examined the expression profiles of tDMRT1 and Sox9a during gonadal sex differentiation and hormone-induced sex reversal. tDMRT1 was detected in the gonial germ-cell-surrounding cells in XY fry specifically before the appearance of any signs of morphological sex differentiation, that is, sex differences in germ cell number and histogenesis, such as differentiation into intratesticular efferent duct or ovarian cavity. The signals became localized in the Sertoli and epithelial cells comprising the efferent duct during gonadal differentiation. After the induction of XY sex reversal with estrogen, tDMRT1 decreased and then disappeared completely. In contrast, tDMRT1 was expressed in the germ-cell-surrounding cells in XX sex reversal with androgen. On the other hand, Sox9a did not show sexual dimorphism before the appearance of sex differences in histogenesis and was not expressed in the efferent duct in the testis. These results suggest that tDMRT1 is a superior testicular differentiation marker in tilapia.

Expression ofSox9,Mis, andDmrt1 in the gonad of a species with temperature-dependent sex determination

Sex determination in vertebrates, the process of forming an ovary or testis from a bipotential gonad, can be initiated by genetic or environmental factors. Elements of the downstream molecular pathways underlying these different sex-determining mechanisms have been evolutionarily conserved. We find the first evidence that Sox9 expression is preferentially organized in the testis early in the temperature-sensitive period in a species with temperature-dependent sex determination (Trachemys scripta). This pattern occurs before sexually dimorphic Mis expression and in a temporal hierarchy that is similar to mammals. Furthermore, we extend previous findings that Dmrt1 expression at early stages of sex determination has a dimorphic pattern consistent with a possible upstream role in determining the fate of the bipotential gonad.

Isolation and Sequence Analysis of the Sox-1, -2, -3 Homologs in Trionyx sinensis and Alligator sinensis Having Temperature-Dependent Sex Determination

Members of the Sox gene family are characterized by an HMG-box that shows sequence similarity with that of the mouse testis-determining gene Sry. Using degenerate primers PCR, seven and eight HMG-box motifs of Sry-related genes were cloned and sequenced from genomic DNA of Trionyx sinensis (termed TS41-47) and Alligator sinensis (AS41-48) with TSD (temperature-dependent sex determination). Among 15 Sry-related genes, TS41, TS42, AS41, and AS42 shared 80, 72, 81, and 79% amino acid identity, respectively, with each HMG-box domain of the mouse Sox-1, -2, and -3 genes by Blast analysis. Molecular phylogenetic analysis showed that the clustering of TS41-42 and AS41-42 was distant to the clustering of the nonreptilian vertebrate Sox-1, -2, -3 homologs, including fish, amphibian, bird, and mammals. The amino acid identity among TS41-42, AS41-42, and the nonreptilian vertebrate Sox-1, -2, -3 homologs is lower than identities among the Sox-1, -2, -3 homologs, suggesting that the sequence changes in TS41-42 of Trionyx sinensis and AS41-42 of Alligator sinensis might have occurred after the diversification of amniotes.

Sox8 is expressed at similar levels in gonads of both sexes during the sex determining period in turtles

A critical gene involved in mammalian sex determination and differentiation is the Sry-related gene Sox9. In reptiles, Sox9 resembles that of mammals in both structure and expression pattern in the developing gonad, but a causal role in male sex determination has not been established. A closely related gene, Sox8, is conserved in human, mouse, and trout and is expressed in developing testes and not developing ovaries in mouse. In this study, we tested the possibility of Sox8 being important for sex determination or sex differentiation in the red-eared slider turtle Trachemys scripta, in which sex is determined by egg incubation temperature between stages 15 and 20. We cloned partial turtle Sox8 and anti-Müllerian hormone (Amh) cDNAs, and analyzed the expression patterns of these genes in developing gonads by reverse transcriptase-polymerase chain reaction and whole-mount in situ hybridization. While Amh is expressed more strongly in males than in females at stage 17, Sox8 is expressed at similar levels in males and females throughout the sex-determining period. These observations suggest that differential transcription of Sox8 is not responsible for regulation of Amh, nor responsible for sex determination in turtle.

Yolk steroid hormones and sex determination in reptiles with TSD

In reptiles with temperature-dependent sex determination (TSD), the temperature at which the eggs are incubated determines the sex of the offspring. The molecular switch responsible for determining sex in these species has not yet been elucidated. We have examined the dynamics of yolk steroid hormones during embryonic development in the snapping turtle, Chelydra serpentina, and the alligator, Alligator mississippiensis, and have found that yolk estradiol (E(2)) responds differentially to incubation temperature in both of these reptiles. Based upon recently reported roles for E(2) in modulation of steroidogenic factor 1, a transcription factor known to be significant in the sex differentiation process, we hypothesize that yolk E(2) is a link between temperature and the gene expression pathway responsible for sex determination and differentiation in at least some of these species. Here we review the evidence that supports our hypothesis.