Embryonic gonadal and sexual organ development in a small viviparous skink,Niveoscincus ocellatus

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.

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.

Transcriptomic thermal plasticity underlying gonadal development in a turtle with ZZ/ZW sex chromosomes despite canalized genotypic sex determination

Understanding genome-wide responses to environmental conditions during embryogenesis is essential for discerning the evolution of developmental plasticity and canalization, two processes generating phenotypic variation targeted by natural selection. Here, we present the first comparative trajectory analysis of matched transcriptomic developmental time series from two reptiles incubated under identical conditions, a turtle with a ZZ/ZW system of genotypic sex determination (GSD), Apalone spinifera, and a turtle with temperature-dependent sex determination (TSD), Chrysemys picta. Results from our genome-wide, hypervariate gene expression analysis of sexed embryos across five developmental stages revealed that substantial transcriptional plasticity in the developing gonads can persist for >145 Myr, long after the canalization of sex determination via the evolution of sex chromosomes, while some gene-specific thermal sensitivity drifts or evolves anew. Such standing thermosensitivity represents an underappreciated evolutionary potential harbored by GSD species that may be co-opted during future adaptive shifts in developmental programing, such as a GSD to TSD reversal, if favored by ecological conditions. Additionally, we identified novel candidate regulators of vertebrate sexual development in GSD reptiles, including sex-determining candidate genes in a ZZ/ZW turtle.

Genetic determination and JARID2 over-expression in a thermal incubation experiment in Casque-Headed Lizard

Non-avian reptiles, unlike mammals and birds, have undergone numerous sex determination changes. Casque-Headed Lizards have replaced the ancestral XY system shared across pleurodonts with a new pair of XY chromosomes. However, the evolutionary forces that triggered this transition have remained unclear. An interesting hypothesis suggests that species with intermediate states, with sex chromosomes but also thermal-induced sex reversal at specific incubation temperatures, could be more susceptible to sex determination turnovers. We contrasted genotypic data (presence/absence of the Y chromosome) against the histology of gonads of embryos from stages 35–37 incubated at various temperatures, including typical male-producing (26°C) and female-producing (32°C) temperatures. Our work apparently reports for the first time the histology of gonads, including morphological changes, from stages 35–37 of development in the family Corytophanidae. We also observed that all embryos developed hemipenes, suggesting sex-linked developmental heterochrony. We observed perfect concordance between genotype and phenotype at all temperatures. However, analysis of transcriptomic data from embryos incubated at 26°C and 32°C identified transcript variants of the chromatin modifiers JARID2 and KDM6B that have been linked to temperature-dependent sex determination in other reptiles. Our work tested the validity of a mixed sex determination system in the family Corytophanidae. We found that XY chromosomes are dominant; however, our work supports the hypothesis of a conserved transcriptional response to incubation temperatures across non-avian reptiles that could be a reminiscence of an ancestral sex determination system.

Sex reversal explains some, but not all, climate-mediated sex ratio variation within a viviparous reptile

Evolutionary transitions in sex-determining systems have occurred frequently yet understanding how they occur remains a major challenge. In reptiles, transitions from genetic to temperature-dependent sex determination can occur if the gene products that determine sex evolve thermal sensitivity, resulting in sex-reversed individuals. However, evidence of sex reversal is limited to oviparous reptiles. Here we used thermal experiments to test whether sex reversal is responsible for differences in sex determination in a viviparous reptile, Carinascincus ocellatus, a species with XY sex chromosomes and population-specific sex ratio response to temperature. We show that sex reversal is occurring and that its frequency is related to temperature. Sex reversal was unidirectional (phenotypic males with XX genotype) and observed in both high- and low-elevation populations. We propose that XX-biased genotypic sex ratios could produce either male- or female-biased phenotypic sex ratios as observed in low-elevation C. ocellatus under variable rates of XX sex reversal. We discuss reasons why sex reversal may not influence sex ratios at high elevation. Our results suggest that the mechanism responsible for evolutionary transitions from genotypic to temperature-dependent sex determination is more complex than can be explained by a single process such as sex reversal.

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.

Ovotestes suggest cryptic genetic influence in a reptile model for temperature-dependent sex determination

Sex determination and differentiation in reptiles is complex. Temperature-dependent sex determination (TSD), genetic sex determination (GSD) and the interaction of both environmental and genetic cues (sex reversal) can drive the development of sexual phenotypes. The jacky dragon (Amphibolurus muricatus) is an attractive model species for the study of gene-environment interactions because it displays a form of Type II TSD, where female-biased sex ratios are observed at extreme incubation temperatures and approximately 50 : 50 sex ratios occur at intermediate temperatures. This response to temperature has been proposed to occur due to underlying sex determining loci, the influence of which is overridden at extreme temperatures. Thus, sex reversal at extreme temperatures is predicted to produce the female-biased sex ratios observed in A. muricatus. The occurrence of ovotestes during development is a cellular marker of temperature sex reversal in a closely related species Pogona vitticeps. Here, we present the first developmental data for A. muricatus, and show that ovotestes occur at frequencies consistent with a mode of sex determination that is intermediate between GSD and TSD. This is the first evidence suggestive of underlying unidentified sex determining loci in a species that has long been used as a model for TSD.

Australian lizards are outstanding models for reproductive biology research

Australian lizards are a diverse group distributed across the continent and inhabiting a wide range of environments. Together, they exhibit a remarkable diversity of reproductive morphologies, physiologies, and behaviours that is broadly representative of vertebrates in general. Many reproductive traits exhibited by Australian lizards have evolved independently in multiple lizard lineages, including sociality, complex signalling and mating systems, viviparity, and temperature-dependent sex determination. Australian lizards are thus outstanding model organisms for testing hypotheses about how reproductive traits function and evolve, and they provide an important basis of comparison with other animals that exhibit similar traits. We review how research on Australian lizard reproduction has contributed to answering broader evolutionary and ecological questions that apply to animals in general. We focus on reproductive traits, processes, and strategies that are important areas of current research, including behaviours and signalling involved in courtship; mechanisms involved in mating, egg production, and sperm competition; nesting and gestation; sex determination; and finally, birth in viviparous species. We use our review to identify important questions that emerge from an understanding of this body of research when considered holistically. Finally, we identify additional research questions within each topic that Australian lizards are well suited for reproductive biologists to address.

Development and differentiation of the reproductive system of Tropidurus catalanensis (Squamata: Tropiduridae)

Development and differentiation of the reproductive system in lizards begin in the embryonic period, although the stage and time of their occurrence vary according to populations and species. In this study, the events of the development and differentiation of the reproductive system of males and females of Tropidurus catalanensis were characterized during the embryonic, neonatal, and juvenile periods. Embryos at Stages 27, 34, 37, 40, and 41, neonates and juveniles, from Corrientes, Argentina, were analyzed. At Stage 27, the genital ridge was not observed but primordial germ cells were recorded in the yolk sac as well as the mesenteric mesenchyme, indicating the beginning of germ cell migration. Gonadal differentiation commenced at Stage 34. In males from Stage 37, the testes possessed seminiferous cords with Sertoli cells and spermatogonia, while in hatchlings seminiferous tubules and interstitial tissue with mature Leydig cells were present. Spermatogenesis was observed in a specimen of 51.9 mm snout-vent length, corresponding to the minimum reproductive size. In females, from Stage 37 until hatching, the ovaries had a cavernous medulla and a cortex with somatic cells and abundant oogonia. The onset of meiosis and folliculogenesis occurred in the juvenile period.

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.

Disentangling sex allocation in a viviparous reptile with temperature-dependent sex determination: a multifactorial approach

Females are predicted to alter sex allocation when ecological, physiological and behavioural variables have different consequences on the fitness of male and female offspring. Traditionally, tests of sex allocation have examined single causative factors, often ignoring possible interactions between multiple factors. Here, we used a multifactorial approach to examine sex allocation in the viviparous skink, Niveoscincus ocellatus. We integrated a 16-year observational field study with a manipulative laboratory experiment to explore whether the effects of the maternal thermal environment interact with the resources available to females for reproduction to affect sex allocation decisions. We found strong effects of temperature on sex allocation in the field, with females born in warm conditions and males in cold conditions; however, this was not replicated in the laboratory. In contrast, we found no effect of female resource availability on sex allocation, either independently, or in interaction with temperature. These results corresponded with an overall lack of an effect of resource availability on any of the life history traits that we predicted would mediate the benefits of differential sex allocation in this system, suggesting that selection for sex allocation in response to resource availability may be relatively weak. Combined, these results suggest that temperature may be the predominant factor driving sex allocation in this system.

Climate and sex ratio variation in a viviparous lizard

The extent to which key biological processes, such as sex determination, respond to environmental fluctuations is fundamental for assessing species' susceptibility to ongoing climate change. Few studies, however, address how climate affects offspring sex in the wild. We monitored two climatically distinct populations of the viviparous skink Niveoscincus ocellatus for 16 years, recording environmental temperatures, offspring sex and date of birth. We found strong population-specific effects of temperature on offspring sex, with female offspring more common in warm years at the lowland site but no effect at the highland site. In contrast, date of birth advanced similarly in response to temperature at both sites. These results suggest strong population-specific effects of temperature on offspring sex that are independent of climatic effects on other physiological processes. These results have significant implications for our understanding of the ecological and evolutionary consequences of variation in sex ratios under climate change.

Potential targets for selection during the evolution of viviparity in cold-climate reptiles

Viviparity (live-bearing) has evolved from oviparity (egg-laying) in more than 100 lineages of squamate reptiles (lizards and snakes). This transition generally has occurred in cool climates, where thermal differentials between eggs in the (cool) nest versus the (warm) maternal oviduct influence embryonic development, in ways that may enhance offspring fitness. To identify specific traits potentially under selection, we incubated eggs of a montane scincid lizard at conditions simulating natural nests, maternal body temperatures, and an intermediate stage (2-week uterine retention of eggs prior to laying). Incubation at maternal temperatures throughout incubation affected the hatchling lizard's activity level and boldness, as well as its developmental rate, morphology, and locomotor ability. A treatment that mimicked the initial stages of the transition toward viviparity had a major effect on some hatchling traits (locomotor speeds), a minor effect on others (tail length, total incubation period) and no effect on yet others (offspring behaviors). More generally, different aspects of the phenotype are sensitive to incubation conditions at different stages of development; thus, the evolution of reptilian viviparity may have been driven by a succession of advantages that accrued at different stages of embryogenesis.

Placental and embryonic tissues exhibit aromatase activity in the viviparous lizard Niveoscincus metallicus

Aromatase is a key regulator of circulating testosterone (T) and 17-β-oestradiol (E2), two steroids which are critical to the development, maintenance and function of reproductive tissues. The role of aromatase in sexual differentiation in oviparous (egg-laying) reptiles is well understood, yet has never been explored in viviparous (live-bearing) reptiles. As a first step towards understanding the functions of aromatase during gestation in viviparous reptiles, we measured aromatase activity in maternal and embryonic tissues at three stages of gestation in the viviparous skink, Niveoscincus metallicus. Maternal ovaries and adrenals maintained high aromatase activity throughout gestation. During the early phases of embryonic development, placental aromatase activity was comparable to that in maternal ovaries, but declined significantly at progressive stages of gestation. Aromatase activity in the developing brains and gonads of embryos was comparable with measurements in oviparous reptiles. Aromatase activity in the developing brains peaked mid development, and declined to low levels in late stage embryos. Aromatase activity in the embryonic gonads was low at embryonic stage 29-34, but increased significantly at mid-development and then remained high in late stage embryos. We conclude that ovarian estrogen synthesis is supplemented by placental aromatase activity and that maternal adrenals provide an auxiliary source of sex steroid. The pattern of change in aromatase activity in embryonic brains and gonads suggests that brain aromatase is important during sexual differentiation, and that embryonic gonads are increasingly steroidogenic as development progresses. Our data indicate vital roles of aromatase in gestation and development in viviparous lizards.

Altitudinal divergence in maternal thermoregulatory behaviour may be driven by differences in selection on offspring survival in a viviparous lizard

Plastic responses to temperature during embryonic development are common in ectotherms, but their evolutionary relevance is poorly understood. Using a combination of field and laboratory approaches, we demonstrate altitudinal divergence in the strength of effects of maternal thermal opportunity on offspring birth date and body mass in a live-bearing lizard (Niveoscincus ocellatus). Poor thermal opportunity decreased birth weight at low altitudes where selection on body mass was negligible. In contrast, there was no effect of maternal thermal opportunity on body mass at high altitudes where natural selection favored heavy offspring. The weaker effect of poor maternal thermal opportunity on offspring development at high altitude was accompanied by a more active thermoregulation and higher body temperature in highland females. This may suggest that passive effects of temperature on embryonic development have resulted in evolution of adaptive behavioral compensation for poor thermal opportunity at high altitudes, but that direct effects of maternal thermal environment are maintained at low altitudes because they are not selected against. More generally, we suggest that phenotypic effects of maternal thermal opportunity or incubation temperature in reptiles will most commonly reflect weak selection for canalization or selection on maternal strategies rather than adaptive plasticity to match postnatal environments.

Climate-driven population divergence in sex-determining systems

Sex determination is a fundamental biological process, yet its mechanisms are remarkably diverse. In vertebrates, sex can be determined by inherited genetic factors or by the temperature experienced during embryonic development. However, the evolutionary causes of this diversity remain unknown. Here we show that live-bearing lizards at different climatic extremes of the species' distribution differ in their sex-determining mechanisms, with temperature-dependent sex determination in lowlands and genotypic sex determination in highlands. A theoretical model parameterized with field data accurately predicts this divergence in sex-determining systems and the consequence thereof for variation in cohort sex ratios among years. Furthermore, we show that divergent natural selection on sex determination across altitudes is caused by climatic effects on lizard life history and variation in the magnitude of between-year temperature fluctuations. Our results establish an adaptive explanation for intra-specific divergence in sex-determining systems driven by phenotypic plasticity and ecological selection, thereby providing a unifying framework for integrating the developmental, ecological and evolutionary basis for variation in vertebrate sex determination.

Giving offspring a head start in life: field and experimental evidence for selection on maternal basking behaviour in lizards

The timing of birth is often correlated with offspring fitness in animals, but experimental studies that disentangle direct effects of parturition date and indirect effects mediated via variation in female traits are rare. In viviparous ectotherms, parturition date is largely driven by female thermal conditions, particularly maternal basking strategies. Our field and laboratory studies of a viviparous lizard (Niveoscincus ocellatus) show that earlier-born offspring are more likely to survive through their first winter and are larger following that winter, than are later-born conspecifics. Thus, the association between parturition date and offspring fitness is causal, rather than reflecting an underlying correlation between parturition date and maternal attributes. Survival selection on offspring confers a significant advantage for increased maternal basking in this species, mediated through fitness advantages of earlier parturition. We discuss the roles of environmentally imposed constraints and parent-offspring conflict in the evolution of maternal effects on parturition date.

Evolution of "determinants" in sex-determination: a novel hypothesis for the origin of environmental contingencies in avian sex-bias

Sex-determination is commonly categorized as either "genetic" or "environmental"-a classification that obscures the origin of this dichotomy and the evolution of sex-determining factors. The current focus on static outcomes of sex-determination provides little insight into the dynamic developmental processes by which some mechanisms acquire the role of sex determinants. Systems that combine "genetic" pathways of sex-determination (i.e., sex chromosomes) with "environmental" pathways (e.g., epigenetically induced segregation distortion) provide an opportunity to examine the evolutionary relationships between the two classes of processes and, ultimately, illuminate the evolution of sex-determining systems. Taxa with sex chromosomes typically undergo an evolutionary reduction in size of one of the sex chromosomes due to suppressed recombination, resulting in pronounced dimorphism of the sex chromosomes, and setting the stage for emergence of epigenetic compensatory mechanisms regulating meiotic segregation of heteromorphic sex chromosomes. Here we propose that these dispersed and redundant regulatory mechanisms enable environmental contingency in genetic sex-determination in birds and account for frequently documented context-dependence in avian sex-determination. We examine the evolution of directionality in such sex-determination as a result of exposure of epigenetic regulators of meiosis to natural selection and identify a central role of hormones in integrating female reproductive homeostasis, resource allocation to oocytes, and offspring sex. This approach clarifies the evolutionary relationship between sex-specific molecular genetic mechanisms of sex-determination and non-sex-specific epigenetic regulators of meiosis and demonstrates that both can determine sex. Our perspective shows how non-sex-specific mechanisms can acquire sex-determining function and, by establishing the explicit link between physiological integration of oogenesis and sex-determination, opens new avenues to the studies of adaptive sex-bias and sex-specific resource allocation in species with genetic sex-determination.