Gene expression dynamics during temperature-dependent sex determination in a sea turtle

Fifty years ago, researchers discovered a link between ambient temperature and the sex of turtle embryos. More recently, significant progress has been made in understanding the influence of temperature on freshwater turtles. However, our understanding of the key genetic factors in other turtle groups, such as sea turtles, remains limited. To address this gap, we conducted RNA-seq analyses on embryonic tissues from the sea olive ridley turtle during the thermosensitive period (stages 21-26) at temperatures known to produce males (26 °C) and females (33 °C). Our findings revealed that incubation temperatures primarily influence genes with broad expression across tissues due to differential cell division rates and later have an effect regulating gonad-specific transcripts. This effect is mostly related to gene activation rather than transcription repression. We performed transcriptome analyses following shifts in incubation temperatures of bi-potential gonads. This approach allowed us to identify genes that respond rapidly and may be closer to the beginning of the temperature-sensing pathway. Notably, we observed swift adaptations in the expression levels of chromatin modifiers JARID2 and KDM6B, as well as the splicing factor SRSF5, and transcription regulators THOC2, DDX3X and CBX3, but little impact in the overall gonad-specific pathways, indicating that temperature-sensing genes may change rapidly but the rewiring of the gonad's developmental fate is complex and resilient. AUTHOR SUMMARY: Sea turtles, one of the most iconic creatures of our oceans, confront a troubling reality of endangerment, a peril magnified by the looming specter of climate change. This climatic shift is gradually increasing the temperature of the nesting beaches thus causing dramatic male/female population biases. Conservation efforts will need genetic and molecular information to reverse the negative effects of climate change on the populations. In this study, we conducted the first transcriptomic analysis of embryonic tissues, including gonads, brain, liver, and mesonephros, in the olive ridley sea turtle during the critical thermosensitive period spanning stages 21-26. We examined both male-producing (26 °C) and female-producing (33 °C) temperatures and found that incubation temperatures influence temperature-sensitive genes that are either expressed globally or specifically associated with the gonads. These findings indicate that incubation temperatures predominantly sway genes with broad expression patterns due to differential cell division rates. This natural process was opted in the gonads to drive sex determination. We also identified genes that are rapidly capable of sensing temperature changes and that could play a role in the activation of the sex determination pathway. Overall, our study sheds light on the intricate interplay between temperature and gene expression during sea turtle development, revealing dynamic changes in the transcriptome and highlighting the involvement of key genetic players in sex determination.

On the evolutionary origin of discrete phenotypic plasticity

Phenotypic plasticity provides an attractive strategy for adapting to various environments, but the evolutionary mechanism of the underlying genetic system is poorly understood. We use a simple gene regulatory network model to explore how a species acquires phenotypic plasticity, particularly focusing on discrete phenotypic plasticity, which has been difficult to explain by quantitative genetic models. Our approach employs a population genetic framework that integrates the developmental process, where each individual undergoes growth to develop its phenotype, which subsequently becomes subject to selection pressures. Our model considers two alternative types of environments, with the gene regulatory network including a sensor gene that turns on and off depending on the type of environment. With this assumption, we demonstrate that the system gradually adapts by acquiring the ability to produce two distinct optimum phenotypes under two types of environments without changing genotype, resulting in phenotypic plasticity. We find that the resulting plasticity is often discrete after a lengthy period of evolution. Our results suggest that gene regulatory networks have a notable capacity to flexibly produce various phenotypes in response to environmental changes. This study also shows that the evolutionary dynamics of phenotype may differ significantly between mechanistic-based developmental models and quantitative genetics models, suggesting the utility of incorporating gene regulatory networks into evolutionary models.

Temperature-Dependent Sex Determination in Crocodilians and Climate Challenges

The sex of crocodilians is determined by the temperature to which the eggs, and hence the developing embryo are exposed during critical periods of development. Temperature-dependent sex determination is a process that occurs in all crocodilians and numerous other reptile taxa. The study of artificial incubation temperatures in different species of crocodiles and alligators has determined the specific temperature ranges that result in altered sex ratios. It has also revealed the precise temperature thresholds at which an equal number of males and females are generated, as well as the specific developmental period during which the sex of the hatchlings may be shifted. This review will examine the molecular basis of the sex-determination mechanism in crocodilians elucidated during recent decades. It will focus on the many patterns and theories associated with this process. Additionally, we will examine the consequences that arise after hatching due to changes in incubation temperatures, as well as the potential benefits and dangers of a changing climate for crocodilians who display sex determination based on temperature.

Daphnia uses its circadian clock for short-day recognition in environmental sex determination

Some organisms have developed a mechanism called environmental sex determination (ESD), which allows environmental cues, rather than sex chromosomes or genes, to determine offspring sex.1,2,3,4 ESD is advantageous to optimize sex ratios according to environmental conditions, enhancing reproductive success.5,6 However, the process by which organisms perceive and translate diverse environmental signals into offspring sex remains unclear. Here, we analyzed the environmental perception mechanism in the crustacean, Daphnia pulex, a seasonal (photoperiodic) ESD arthropod, capable of producing females under long days and males under short days.7,8,9,10 Through breeding experiments, we found that their circadian clock likely contributes to perception of day length. To explore this further, we created a genetically modified daphnid by knocking out the clock gene, period, using genome editing. Knockout disrupted the daphnid's ability to sustain diel vertical migration (DVM) under constant darkness, driven by the circadian clock, and leading them to produce females regardless of day length. Additionally, when exposed to an analog of juvenile hormone (JH), an endocrine factor synthesized in mothers during male production, or subjected to unfavorable conditions of high density and low food availability, these knockout daphnids produced males regardless of day length, like wild-type daphnids. Based on these findings, we propose that recognizing short days via the circadian clock is the initial step in sex determination. This recognition subsequently triggers male production by signaling the endocrine system, specifically via the JH signal. Establishment of a connection between these two processes may be the crucial element in evolution of ESD in Daphnia.

Individual Genetic Heterogeneity

Genetic variation has been widely covered in literature, however, not from the perspective of an individual in any species. Here, a synthesis of genetic concepts and variations relevant for individual genetic constitution is provided. All the different levels of genetic information and variation are covered, ranging from whether an organism is unmixed or hybrid, has variations in genome, chromosomes, and more locally in DNA regions, to epigenetic variants or alterations in selfish genetic elements. Genetic constitution and heterogeneity of microbiota are highly relevant for health and wellbeing of an individual. Mutation rates vary widely for variation types, e.g., due to the sequence context. Genetic information guides numerous aspects in organisms. Types of inheritance, whether Mendelian or non-Mendelian, zygosity, sexual reproduction, and sex determination are covered. Functions of DNA and functional effects of variations are introduced, along with mechanism that reduce and modulate functional effects, including TARAR countermeasures and intraindividual genetic conflict. TARAR countermeasures for tolerance, avoidance, repair, attenuation, and resistance are essential for life, integrity of genetic information, and gene expression. The genetic composition, effects of variations, and their expression are considered also in diseases and personalized medicine. The text synthesizes knowledge and insight on individual genetic heterogeneity and organizes and systematizes the central concepts.

A review of the effects of incubation conditions on hatchling phenotypes in non-squamate reptiles

Developing embryos of oviparous reptiles show substantial plasticity in their responses to environmental conditions during incubation, which can include altered sex ratios, morphology, locomotor performance and hatching success. While recent research and reviews have focused on temperature during incubation, emerging evidence suggests other environmental variables are also important in determining hatchling phenotypes. Understanding how the external environment influences development is important for species management and requires identifying how environmental variables exert their effects individually, and how they interact to affect developing embryos. To address this knowledge gap, we review the literature on phenotypic responses in oviparous non-squamate (i.e., turtles, crocodilians and tuataras) reptile hatchlings to temperature, moisture, oxygen concentration and salinity. We examine how these variables influence one another and consider how changes in each variable alters incubation conditions and thus, hatchling phenotypes. We explore how incubation conditions drive variation in hatchling phenotypes and influence adult populations. Finally, we highlight knowledge gaps and suggest future research directions.

Characterization of a Sex-Determining Region and Its Genomic Context via Statistical Estimates of Haplotype Frequencies in Daughters and Sons Sequenced in Pools

Sex chromosomes are generally derived from a pair of autosomes that have acquired a locus controlling sex. Sex chromosomes may evolve reduced recombination around this locus and undergo a long process of molecular divergence. At that point, the original loci controlling sex may be difficult to pinpoint. This difficulty has affected many model species from mammals to birds to flies, which present highly diverged sex chromosomes. Identifying sex-controlling loci is easier in species with molecularly similar sex chromosomes. Here we aimed at pinpointing the sex-determining region (SDR) of Armadillidium vulgare, a terrestrial isopod with female heterogamety (ZW females and ZZ males) and whose sex chromosomes appear to show low genetic divergence. To locate the SDR, we assessed single-nucleotide polymorphism (SNP) allele frequencies in F1 daughters and sons sequenced in pools (pool-seq) in several families. We developed a Bayesian method that uses the SNP genotypes of individually sequenced parents and pool-seq data from F1 siblings to estimate the genetic distance between a given genomic region (contig) and the SDR. This allowed us to assign more than 43 Mb of contigs to sex chromosomes, and to demonstrate extensive recombination and very low divergence between these chromosomes. By taking advantage of multiple F1 families, we delineated a very short genomic region (∼65 kb) that presented no evidence of recombination with the SDR. In this short genomic region, the comparison of sequencing depths between sexes highlighted female-specific genes that have undergone recent duplication, and which may be involved in sex determination in A. vulgare.

Gonadal Sex Differentiation: Supporting Versus Steroidogenic Cell Lineage Specification in Mammals and Birds

The gonads of vertebrate embryos are unique among organs because they have a developmental choice; ovary or testis formation. Given the importance of proper gonad formation for sexual development and reproduction, considerable research has been conducted over the years to elucidate the genetic and cellular mechanisms of gonad formation and sexual differentiation. While the molecular trigger for gonadal sex differentiation into ovary of testis can vary among vertebrates, from egg temperature to sex-chromosome linked master genes, the downstream molecular pathways are largely conserved. The cell biology of gonadal formation and differentiation has long thought to also be conserved. However, recent discoveries point to divergent mechanisms of gonad formation, at least among birds and mammals. In this mini-review, we provide an overview of cell lineage allocation during gonadal sex differentiation in the mouse model, focusing on the key supporting and steroidogenic cells and drawing on recent insights provided by single cell RNA-sequencing. We compare this data with emerging information in the chicken model. We highlight surprising differences in cell lineage specification between species and identify gaps in our current understanding of the cell biology underlying gonadogenesis.

Sex determination, gonadal sex differentiation, and plasticity in vertebrate species

A diverse array of sex determination (SD) mechanisms, encompassing environmental to genetic, have been found to exist among vertebrates, covering a spectrum from fixed SD mechanisms (mammals) to functional sex change in fishes (sequential hermaphroditic fishes). A major landmark in vertebrate SD was the discovery of the SRY gene in 1990. Since that time, many attempts to clone an SRY ortholog from nonmammalian vertebrates remained unsuccessful, until 2002, when DMY/dmrt1by was discovered as the SD gene of a small fish, medaka. Surprisingly, however, DMY/dmrt1by was found in only 2 species among more than 20 species of medaka, suggesting a large diversity of SD genes among vertebrates. Considerable progress has been made over the last 3 decades, such that it is now possible to formulate reasonable paradigms of how SD and gonadal sex differentiation may work in some model vertebrate species. This review outlines our current understanding of vertebrate SD and gonadal sex differentiation, with a focus on the molecular and cellular mechanisms involved. An impressive number of genes and factors have been discovered that play important roles in testicular and ovarian differentiation. An antagonism between the male and female pathway genes exists in gonads during both sex differentiation and, surprisingly, even as adults, suggesting that, in addition to sex-changing fishes, gonochoristic vertebrates including mice maintain some degree of gonadal sexual plasticity into adulthood. Importantly, a review of various SD mechanisms among vertebrates suggests that this is the ideal biological event that can make us understand the evolutionary conundrums underlying speciation and species diversity.

Turtle Insights into the Evolution of the Reptilian Karyotype and the Genomic Architecture of Sex Determination

Sex chromosome evolution remains an evolutionary puzzle despite its importance in understanding sexual development and genome evolution. The seemingly random distribution of sex-determining systems in reptiles offers a unique opportunity to study sex chromosome evolution not afforded by mammals or birds. These reptilian systems derive from multiple transitions in sex determination, some independent, some convergent, that lead to the birth and death of sex chromosomes in various lineages. Here we focus on turtles, an emerging model group with growing genomic resources. We review karyotypic changes that accompanied the evolution of chromosomal systems of genotypic sex determination (GSD) in chelonians from systems under the control of environmental temperature (TSD). These transitions gave rise to 31 GSD species identified thus far (out of 101 turtles with known sex determination), 27 with a characterized sex chromosome system (13 of those karyotypically). These sex chromosomes are varied in terms of the ancestral autosome they co-opted and thus in their homology, as well as in their size (some are macro-, some are micro-chromosomes), heterogamety (some are XX/XY, some ZZ/ZW), dimorphism (some are virtually homomorphic, some heteromorphic with larger-X, larger W, or smaller-Y), age (the oldest system could be ~195 My old and the youngest < 25 My old). Combined, all data indicate that turtles follow some tenets of classic theoretical models of sex chromosome evolution while countering others. Finally, although the study of dosage compensation and molecular divergence of turtle sex chromosomes has lagged behind research on other aspects of their evolution, this gap is rapidly decreasing with the acceleration of ongoing research and growing genomic resources in this group.

Molecular characterization, computational analysis and expression profiling of Dmrt1 gene in Indian major carp, Labeo rohita (Hamilton 1822)

Sexual dimorphism of fish morphology, physiology and behavior is diverse and complex in nature. Doublesex and mab-3 related transcription factor (Dmrt) is a large protein family whose function is sexual development and differentiation in vertebrates. Here, we report a full-length cDNA sequence of Labeo rohita (rohu) Dmrt1 of 907 bp length having 798 bp of open reading frame encoding 265 amino acids. The molecular weight of rohu DMRT1 protein was found to be 28.74 KDa and isoelectric point was 7.53. DMRT1 protein contains 23 positively and 24 negatively charged amino acids with a GRAVY score of -0.618. A characteristic DM domain was found in DMRT1 protein, which is a novel DNA-binding domain. Phylogenetic analysis showed maximum similarity with Cyprinus carpio when compared with DMRT1 of other vertebrates. Molecular docking study identified active sites to be targeted for drug designing. Rohu DMRT1 was observed to interact with other proteins such as FOXL2, CYP19a1a, AMH and SOX9a. Differential expression study revealed higher expression in testis tissue implying its role in male sex differentiation and testicular development. The information generated in the present work could facilitate further research to resolve the issues related to gonadal maturation and reproduction of commercially important aquaculture species.

Hemimetabolous insects elucidate the origin of sexual development via alternative splicing

Insects are the only known animals in which sexual differentiation is controlled by sex-specific splicing. The doublesex transcription factor produces distinct male and female isoforms, which are both essential for sex-specific development. dsx splicing depends on transformer, which is also alternatively spliced such that functional Tra is only present in females. This pathway has evolved from an ancestral mechanism where dsx was independent of tra and expressed and required only in males. To reconstruct this transition, we examined three basal, hemimetabolous insect orders: Hemiptera, Phthiraptera, and Blattodea. We show that tra and dsx have distinct functions in these insects, reflecting different stages in the changeover from a transcription-based to a splicing-based mode of sexual differentiation. We propose that the canonical insect tra-dsx pathway evolved via merger between expanding dsx function (from males to both sexes) and narrowing tra function (from a general splicing factor to dedicated regulator of dsx).

Genomics of Developmental Plasticity in Animals

Developmental plasticity refers to the property by which the same genotype produces distinct phenotypes depending on the environmental conditions under which development takes place. By allowing organisms to produce phenotypes adjusted to the conditions that adults will experience, developmental plasticity can provide the means to cope with environmental heterogeneity. Developmental plasticity can be adaptive and its evolution can be shaped by natural selection. It has also been suggested that developmental plasticity can facilitate adaptation and promote diversification. Here, we summarize current knowledge on the evolution of plasticity and on the impact of plasticity on adaptive evolution, and we identify recent advances and important open questions about the genomics of developmental plasticity in animals. We give special attention to studies using transcriptomics to identify genes whose expression changes across developmental environments and studies using genetic mapping to identify loci that contribute to variation in plasticity and can fuel its evolution.

Experimental evidence of masculinization by continuous illumination in a temperature sex determination teleost (Atherinopsidae) model: is oxidative stress involved?

The present study evaluates the influence of continuous light on phenotypic sex ratios in Chirostoma estor, a temperature sex determination animal model. Relative gene expression levels of 5 day old larvae were performed on two early gonad differentiation genes (sox9 and foxl2), two stress axis activation genes (gcr1 and crf) and four reactive oxygen species (ROS) antagonist effector genes (sod2, ucp2, gsr and cat). Two light treatments were applied from fertilization; control (12L:12D) simulated natural photoperiod and a continuous illumination photoperiod. By the end of the trial (12 weeks after hatching), differentiated and normal gonads were clearly identifiable in both treatments by histological observations. Regarding sex ratio, 73% of phenotypic males were found in continuous illumination compared with 40% in controls. Consistently, the sox9 gene (involved in early testis differentiation) showed an over expression in 64% of the individual larvae analysed compared with foxl2 (ovarian differentiation) suggesting a masculinization tendency in continuous illumination. On the other hand, only 36% of individuals showed the same tendency in the control treatment consistent with phenotypic sex ratios found under normal culture conditions. Relative gene expression results did not show significant difference in sod2, ucp2 and gcr1 levels, but cat, gsr and crf showed significantly higher expression levels in the continuous illumination treatment suggesting that both, the stress axis and ROS response mechanisms were activated at this time. This study suggests, a link between continuous light, oxidative stress and environmental sex determination in vertebrates. However, further research is necessary to describe this possible upstream mechanism that may drive some aspects of sexual plasticity in vertebrates.

Past, present and future of epigenetics in brain sexual differentiation

Sexual differentiation has long been considered "epigenetic", although the meaning of that word has shifted over time. Here, we track the evolution of ideas about epigenetics in sexual differentiation, and identify principles that have emerged from recent studies. Experiments manipulating a particular epigenetic mechanism during neonatal life demonstrate a role for both histone acetylation and DNA methylation in the development of sex differences in the brain and behaviour of rodents. In addition, hormone-dependent sex differences in the number of neurones of a particular phenotype may be programmed by differences in DNA methylation early in life. Genome-wide studies suggest that many effects of neonatal testosterone on the brain methylome do not emerge until adulthood, and there may be sex biases in the use of epigenetic marks that do not correlate with differences in gene expression. In other words, even when the transcription of a gene does not differ between males and females, the epigenetic underpinnings of that expression may differ. Finally, recent evidence suggests that sex differences in epigenetic marks may primarily serve to make gene expression more similar in males and females. We discuss the implications of these findings for understanding sex differences in susceptibility to disease, and point to recent conceptual and technical advances likely to influence the field going forward.

Thyroid hormone modulates offspring sex ratio in a turtle with temperature-dependent sex determination

The adaptive significance of temperature-dependent sex determination (TSD) has attracted a great deal of research, but the underlying mechanisms by which temperature determines the sex of a developing embryo remain poorly understood. Here, we manipulated the level of a thyroid hormone (TH), triiodothyronine (T₃), during embryonic development (by adding excess T₃ to the eggs of the red-eared slider turtle Trachemys scripta, a reptile with TSD), to test two competing hypotheses on the proximate basis for TSD: the developmental rate hypothesis versus the hormone hypothesis Exogenous TH accelerated embryonic heart rate (and hence metabolic rate), developmental rate, and rates of early post-hatching growth. More importantly, hyperthyroid conditions depressed expression of Cyp19a1 (the gene encoding for aromatase) and levels of oestradiol, and induced more male offspring. This result is contrary to the direction of sex-ratio shift predicted by the developmental rate hypothesis, but consistent with that predicted by the hormone hypothesis Our results suggest an important role for THs in regulating sex steroid hormones, and therefore, in affecting gonadal sex differentiation in TSD reptiles. Our study has implications for the conservation of TSD reptiles in the context of global change because environmental contaminants may disrupt the activity of THs, and thereby affect offspring sex in TSD reptiles.

17β-Estradiol modulates cell proliferation of medullary cords during ovarian differentiation of the Lepidochelys olivacea sea turtle

In turtles undergoing temperature sex determination (TSD), bipotential gonads express Sox9 in medullary cords at both female- (FPT) and male-producing temperatures (MPT). Subsequently, when the sex fate of medullary cords becomes dimorphic, at FPT, Sox9 is downregulated, whereas at MPT, its expression is maintained. Medullary cords in the ovary turn into ovarian lacuna, whereas in the testis they differentiate as seminiferous cords. When embryos of Lepidochelys olivacea sea turtle are incubated at MPT and treated with estradiol, Sox9 expression persists in the medullary cords in the form of tiny ovotestis-like formations. The perturbed development of the treated gonads is due to a significant decrease in the number of proliferating cells. This suggests that the disturbed effect caused by exogenous estradiol may be due to a conflict between the gene networks regulated by temperature and the increased level of endogenous estrogens, induced by the treatment. Here, we decided to use fadrozole and fulvestrant, an aromatase inhibitor and an estrogen-receptor antagonist, respectively, to provide insights into the role played by endogenous estrogens in regulating the cell proliferation of the two main gonadal compartments: the medullary cords and the cortex. Comparing cell proliferation patterns, our current results suggest that the endogenous estrogens are involved in determining the sex fate of medullary cords, by repressing proliferation. Interestingly, our results showed that endogenous estradiol levels are unnecessary for the thickening of the ovarian cortex.

Demographic and genetic consequences of disturbed sex determination

During sex determination, genetic and/or environmental factors determine the cascade of processes of gonad development. Many organisms, therefore, have a developmental window in which their sex determination can be sensitive to, for example, unusual temperatures or chemical pollutants. Disturbed environments can distort population sex ratios and may even cause sex reversal in species with genetic sex determination. The resulting genotype-phenotype mismatches can have long-lasting effects on population demography and genetics. I review the theoretical and empirical work in this context and explore in a simple population model the role of the fitness vyy of chromosomally aberrant YY genotypes that are a consequence of environmentally induced feminization. Low vyy is mostly beneficial for population growth. During feminization, low vyy reduces the proportion of genetic males and hence accelerates population growth, especially at low rates of feminization and at high fitness costs of the feminization itself (i.e. when feminization would otherwise not affect population dynamics much). When sex reversal ceases, low vyy mitigates the negative effects of feminization and can even prevent population extinction. Little is known about vyy in natural populations. The available models now need to be parametrized in order to better predict the long-term consequences of disturbed sex determination.This article is part of the themed issue 'Adult sex ratios and reproductive decisions: a critical re-examination of sex differences in human and animal societies'.

Neuroendocrine disruption of organizational and activational hormone programming in poikilothermic vertebrates

In vertebrates, sexual differentiation of the reproductive system and brain is tightly orchestrated by organizational and activational effects of endogenous hormones. In mammals and birds, the organizational period is typified by a surge of sex hormones during differentiation of specific neural circuits; whereas activational effects are dependent upon later increases in these same hormones at sexual maturation. Depending on the reproductive organ or brain region, initial programming events may be modulated by androgens or require conversion of androgens to estrogens. The prevailing notion based upon findings in mammalian models is that male brain is sculpted to undergo masculinization and defeminization. In absence of these responses, the female brain develops. While timing of organizational and activational events vary across taxa, there are shared features. Further, exposure of different animal models to environmental chemicals such as xenoestrogens such as bisphenol A-BPA and ethinylestradiol-EE2, gestagens, and thyroid hormone disruptors, broadly classified as neuroendocrine disrupting chemicals (NED), during these critical periods may result in similar alterations in brain structure, function, and consequently, behaviors. Organizational effects of neuroendocrine systems in mammals and birds appear to be permanent, whereas teleost fish neuroendocrine systems exhibit plasticity. While there are fewer NED studies in amphibians and reptiles, data suggest that NED disrupt normal organizational-activational effects of endogenous hormones, although it remains to be determined if these disturbances are reversible. The aim of this review is to examine how various environmental chemicals may interrupt normal organizational and activational events in poikilothermic vertebrates. By altering such processes, these chemicals may affect reproductive health of an animal and result in compromised populations and ecosystem-level effects.

Dimorphic DNA methylation during temperature-dependent sex determination in the sea turtle Lepidochelys olivacea

Sex determination in vertebrates depends on the expression of a conserved network of genes. Sea turtles such as Lepidochelys olivacea have temperature-dependent sex determination. The present work analyses some of the epigenetic processes involved in this. We describe sexual dimorphism in global DNA methylation patterns between ovaries and testes of L. olivacea and show that the differences may arise from a combination of DNA methylation and demethylation events that occur during sex determination. Irrespective of incubation temperature, 5-hydroxymethylcytosine was abundant in the bipotential gonad; however, following sex determination, this modification was no longer found in pre-Sertoli cells in the testes. These changes correlate with the establishment of the sexually dimorphic DNA methylation patterns, down regulation of Sox9 gene expression in ovaries and irreversible gonadal commitment towards a male or female differentiation pathway. Thus, DNA methylation changes may be necessary for the stabilization of the gene expression networks that drive the differentiation of the bipotential gonad to form either an ovary or a testis in L. olivacea and probably among other species that manifest temperature-dependent sex determination.

The Increasing Prevalence in Intersex Variation from Toxicological Dysregulation in Fetal Reproductive Tissue Differentiation and Development by Endocrine-Disrupting Chemicals

An increasing number of children are born with intersex variation (IV; ambiguous genitalia/hermaphrodite, pseudohermaphroditism, etc.). Evidence shows that endocrine-disrupting chemicals (EDCs) in the environment can cause reproductive variation through dysregulation of normal reproductive tissue differentiation, growth, and maturation if the fetus is exposed to EDCs during critical developmental times in utero. Animal studies support fish and reptile embryos exhibited IV and sex reversal when exposed to EDCs. Occupational studies verified higher prevalence of offspring with IV in chemically exposed workers (male and female). Chemicals associated with endocrine-disrupting ability in humans include organochlorine pesticides, poly-chlorinated biphenyls, bisphenol A, phthalates, dioxins, and furans. Intersex individuals may have concurrent physical disorders requiring lifelong medical intervention and experience gender dysphoria. An urgent need exists to determine which chemicals possess the greatest risk for IV and the mechanisms by which these chemicals are capable of interfering with normal physiological development in children.

Phenotypic plasticity within yeast colonies: differential partitioning of cell fates

Across many phyla, a common aspect of multicellularity is the organization of different cell types into spatial patterns. In the budding yeast Saccharomyces cerevisiae, after diploid colonies have completed growth, they differentiate to form alternating layers of sporulating cells and feeder cells. In the current study, we found that as yeast colonies developed, the feeder cell layer was initially separated from the sporulating cell layer. Furthermore, the spatial pattern of sporulation in colonies depended on the colony's nutrient environment; in two environments in which overall colony sporulation efficiency was very similar, the pattern of feeder and sporulating cells within the colony was very different. As noted previously, under moderately suboptimal conditions for sporulation-low acetate concentration or high temperature-the number of feeder cells increases as does the dependence of sporulation on the feeder-cell transcription factor, Rlm1. Here we report that even under a condition that is completely blocked sporulation, the number of feeder cells still increased. These results suggest broader implications to our recently proposed "Differential Partitioning provides Environmental Buffering" or DPEB hypothesis.

A Novel Candidate Gene for Temperature-Dependent Sex Determination in the Common Snapping Turtle

Temperature-dependent sex determination (TSD) was described nearly 50 years ago. Researchers have since identified many genes that display differential expression at male- vs. female-producing temperatures. Yet, it is unclear whether these genes (1) are involved in sex determination per se, (2) are downstream effectors involved in differentiation of ovaries and testes, or (3) are thermo-sensitive but unrelated to gonad development. Here we present multiple lines of evidence linking CIRBP to sex determination in the snapping turtle, Chelydra serpentina We demonstrate significant associations between a single nucleotide polymorphism (SNP) (c63A > C) in CIRBP, transcript levels in embryonic gonads during specification of gonad fate, and sex in hatchlings from a thermal regime that produces mixed sex ratios. The A allele was induced in embryos exposed to a female-producing temperature, while expression of the C allele did not differ between female- and male-producing temperatures. In accord with this pattern of temperature-dependent, allele-specific expression, AA homozygotes were more likely to develop ovaries than AC heterozygotes, which, in turn, were more likely to develop ovaries than CC homozygotes. Multiple regression using SNPs in CIRBP and adjacent loci suggests that c63A > C may be the causal variant or closely linked to it. Differences in CIRBP allele frequencies among turtles from northern Minnesota, southern Minnesota, and Texas reflect small and large-scale latitudinal differences in TSD pattern. Finally, analysis of CIRBP protein localization reveals that CIRBP is in a position to mediate temperature effects on the developing gonads. Together, these studies strongly suggest that CIRBP is involved in determining the fate of the bipotential gonad.

Epigenetic mechanisms in sexual differentiation of the brain and behaviour

Circumstantial evidence alone argues that the establishment and maintenance of sex differences in the brain depend on epigenetic modifications of chromatin structure. More direct evidence has recently been obtained from two types of studies: those manipulating a particular epigenetic mechanism, and those examining the genome-wide distribution of specific epigenetic marks. The manipulation of histone acetylation or DNA methylation disrupts the development of several neural sex differences in rodents. Taken together, however, the evidence suggests there is unlikely to be a simple formula for masculine or feminine development of the brain and behaviour; instead, underlying epigenetic mechanisms may vary by brain region or even by dependent variable within a region. Whole-genome studies related to sex differences in the brain have only very recently been reported, but suggest that males and females may use different combinations of epigenetic modifications to control gene expression, even in cases where gene expression does not differ between the sexes. Finally, recent findings are discussed that are likely to direct future studies on the role of epigenetic mechanisms in sexual differentiation of the brain and behaviour.

Methyl farnesoate synthesis is necessary for the environmental sex determination in the water flea Daphnia pulex

Sex-determination systems can be divided into two groups: genotypic sex determination (GSD) and environmental sex determination (ESD). ESD is an adaptive life-history strategy that allows control of sex in response to environmental cues in order to optimize fitness. However, the molecular basis of ESD remains largely unknown. The micro crustacean Daphnia pulex exhibits ESD in response to various external stimuli. Although methyl farnesoate (MF: putative juvenile hormone, JH, in daphnids) has been reported to induce male production in daphnids, the role of MF as a sex-determining factor remains elusive due to the lack of a suitable model system for its study. Here, we establish such a system for ESD studies in D. pulex. The WTN6 strain switches from producing females to producing males in response to the shortened day condition, while the MFP strain only produces females, irrespective of day-length. To clarify whether MF has a novel physiological role as a sex-determining factor in D. pulex, we demonstrate that a MF/JH biosynthesis inhibitor suppressed male production in WTN6 strain reared under the male-inducible condition, shortened day-length. Moreover, we show that juvenile hormone acid O-methyltransferase (JHAMT), a critical enzyme of MF/JH biosynthesis, displays MF-generating activity by catalyzing farnesoic acid. Expression of the JHAMT gene increased significantly just before the MF-sensitive period for male production in the WTN6 strain, but not in the MFP strain, when maintained under male-inducible conditions. These results suggest that MF synthesis regulated by JHAMT is necessary for male offspring production in D. pulex. Our findings provide novel insights into the genetic underpinnings of ESD and they begin to shed light on the physiological function of MF as a male-fate determiner in D. pulex.

Sex hormones alter sex ratios in the Indian skipper frog, Euphlyctis cyanophlyctis: Determining sensitive stages for gonadal sex reversal

In amphibians, although genetic factors are involved in sex determination, gonadal sex differentiation can be modified by exogenous steroid hormones suggesting a possible role of sex steroids in regulating the process. We studied the effect of testosterone propionate (TP) and estradiol-17β (E2) on gonadal differentiation and sex ratio at metamorphosis in the Indian skipper frog, Euphlyctis cyanophlyctis with undifferentiated type of gonadal differentiation. A series of experiments were carried out to determine the optimum dose and sensitive stages for gonadal sex reversal. Our results clearly indicate the importance of sex hormones in controlling gonadal differentiation of E. cyanophlyctis. Treatment of tadpoles with 10, 20, 40, and 80μg/L TP throughout larval period resulted in the development of 100% males at metamorphosis at all concentrations. Similarly, treatment of tadpoles with 40μg/L TP during ovarian and testicular differentiation resulted in the development of 90% males, 10% intersexes and 100% males respectively. Treatment of tadpoles with 10, 20, 40, and 80μg/L E2 throughout larval period likewise produced 100% females at all concentrations. Furthermore, exposure to 40μg/L E2 during ovarian and testicular differentiation produced 95% females, 5% intersexes and 91% females, 9% intersexes respectively. Both TP and E2 were also effective in advancing the stages of gonadal development. Present study shows the effectiveness of both T and E2 in inducing complete sex reversal in E. cyanophlyctis. Generally, exposure to E2 increased the larval period resulting in significantly larger females than control group while the larval period of control and TP treated groups was comparable.

Molecular and morphological differentiation of testes and ovaries in relation to the thermosensitive period of gonad development in the snapping turtle, Chelydra serpentina

Ambient temperatures during embryonic development determine gonadal sex in many reptiles. The temperature sensitive period for sex determination has been defined by shifting eggs between female- and male-producing temperatures in a few species. This phase spans 20-35% of embryogenesis in most species, which makes it difficult to define the mechanisms that transduce temperature into a signal for ovarian versus testicular development. We present an extensive set of studies that define a brief period when high temperature specifies, and then determines, ovarian fate in a northern population of snapping turtles, Chelydra serpentina. We shifted embryos from male to female temperatures, or vice versa, at various stages of development. Gonads in embryos incubated at female temperatures commit to ovarian fate earlier (by stage 18) than gonads in embryos incubated at male temperatures commit to testicular fate (by stages 19-21). In double shift studies, embryos were incubated at a female temperature, exposed to a male temperature for set times, and shifted back to the original temperature, or vice versa. The time required to induce ovarian development (≤6 days at female temperatures) was much shorter than the time required to induce testicular formation (>20 days at male temperatures). Differentiation of the gonads at the histological level occurred after the sex-determining period. Nevertheless, we found that a change in temperature rapidly (within 24h) influenced expression and splicing of WT1 mRNA: the absolute abundance of WT1 mRNA, the relative abundance of +KTS versus -KTS isoforms, as well as the ratio of +KTS:-KTS isoforms was higher in gonads at a male versus a female temperature. In conclusion, ovarian fate is more readily determined than testicular fate in snapping turtle embryos. The short sex-determining period in this species (6-8% of embryogenesis) will facilitate studies of molecular mechanisms for specification and determination of gonad fate by temperature.

How is FLC repression initiated by cold?

Vernalization is the promotion of flowering in response to prolonged exposure to low temperatures. In Arabidopsis, FLOWERING LOCUS C (FLC), a suppressor of flowering, is repressed by low temperatures but the mechanism leading to the initial decrease in FLC transcription remains a mystery. No mutants that block the repression of FLC at low temperatures have been identified to date. If the failure to identify such a mutant is assumed to imply that no such mutant exists, then it follows that the first response to the drop in temperature is physical, not genetic. In this Opinion article we propose that the drop in temperature first causes a simple change in the topology of the chromatin polymer, which in turn initiates the repression of FLC transcription.

Wild sex in zebrafish: loss of the natural sex determinant in domesticated strains

Sex determination can be robustly genetic, strongly environmental, or genetic subject to environmental perturbation. The genetic basis of sex determination is unknown for zebrafish (Danio rerio), a model for development and human health. We used RAD-tag population genomics to identify sex-linked polymorphisms. After verifying this "RAD-sex" method on medaka (Oryzias latipes), we studied two domesticated zebrafish strains (AB and TU), two natural laboratory strains (WIK and EKW), and two recent isolates from nature (NA and CB). All four natural strains had a single sex-linked region at the right tip of chromosome 4, enabling sex genotyping by PCR. Genotypes for the single nucleotide polymorphism (SNP) with the strongest statistical association to sex suggested that wild zebrafish have WZ/ZZ sex chromosomes. In natural strains, "male genotypes" became males and some "female genotypes" also became males, suggesting that the environment or genetic background can cause female-to-male sex reversal. Surprisingly, TU and AB lacked detectable sex-linked loci. Phylogenomics rooted on D. nigrofasciatus verified that all strains are monophyletic. Because AB and TU branched as a monophyletic clade, we could not rule out shared loss of the wild sex locus in a common ancestor despite their independent domestication. Mitochondrial DNA sequences showed that investigated strains represent only one of the three identified zebrafish haplogroups. Results suggest that zebrafish in nature possess a WZ/ZZ sex-determination mechanism with a major determinant lying near the right telomere of chromosome 4 that was modified during domestication. Strains providing the zebrafish reference genome lack key components of the natural sex-determination system but may have evolved variant sex-determining mechanisms during two decades in laboratory culture.

Genomic analysis of the Pacific oyster (Crassostrea gigas) reveals possible conservation of vertebrate sex determination in a mollusc

Despite the prevalence of sex in animal kingdom, we have only limited understanding of how sex is determined and evolved in many taxa. The mollusc Pacific oyster Crassostrea gigas exhibits complex modes of sexual reproduction that consists of protandric dioecy, sex change, and occasional hermaphroditism. This complex system is controlled by both environmental and genetic factors through unknown molecular mechanisms. In this study, we investigated genes related to sex-determining pathways in C. gigas through transcriptome sequencing and analysis of female and male gonads. Our analysis identified or confirmed novel homologs in the oyster of key sex-determining genes (SoxH or Sry-like and FoxL2) that were thought to be vertebrate-specific. Their expression profile in C. gigas is consistent with conserved roles in sex determination, under a proposed model where a novel testis-determining CgSoxH may serve as a primary regulator, directly or indirectly interacting with a testis-promoting CgDsx and an ovary-promoting CgFoxL2. Our findings plus previous results suggest that key vertebrate sex-determining genes such as Sry and FoxL2 may not be inventions of vertebrates. The presence of such genes in a mollusc with expression profiles consistent with expected roles in sex determination suggest that sex determination may be deeply conserved in animals, despite rapid evolution of the regulatory pathways that in C. gigas may involve both genetic and environmental factors.

Sex determination: why so many ways of doing it?

Sexual reproduction is an ancient feature of life on earth, and the familiar X and Y chromosomes in humans and other model species have led to the impression that sex determination mechanisms are old and conserved. In fact, males and females are determined by diverse mechanisms that evolve rapidly in many taxa. Yet this diversity in primary sex-determining signals is coupled with conserved molecular pathways that trigger male or female development. Conflicting selection on different parts of the genome and on the two sexes may drive many of these transitions, but few systems with rapid turnover of sex determination mechanisms have been rigorously studied. Here we survey our current understanding of how and why sex determination evolves in animals and plants and identify important gaps in our knowledge that present exciting research opportunities to characterize the evolutionary forces and molecular pathways underlying the evolution of sex determination.

Evolution of sex determination and sexually dimorphic larval sizes in parasitic barnacles

The parasitic (rhizocephalan) barnacles include species of which larval sex is determined by the mother (genetic sex determination, GSD), male larvae are larger than female larvae, and a female accepts only two dwarf males who sire all the eggs laid by her. In contrast, other species of parasitic barnacles exhibit monomorphic larvae that choose to become male or female depending on the condition of the host they settle (environmental sex determination, or ESD), and a female accepts numerous dwarf males. Here, we ask why these set of traits are observed together, by examining the evolution of sex determination and the larval size. ESD has an advantage over GSD because each larva has a higher chance of encountering a suitable host. On the other hand, GSD has two advantages over ESD: the larval size can be chosen differently between sexes, and their larvae can avoid spending time for sex determination on the host. We conclude that, in species whose female accepts only two males, the male larvae engage in intense contest competition for reproductive opportunities, and male's success-size relation is very different from female's. Then, larvae with predetermined sex (GSD) with sexually dimorphic larvae is more advantageous than ESD. In contrast, in species whose females accept many dwarf males, the competition among males is less intense, and producing larvae with undetermined sex should evolve. We also discuss the condition for females to evolve receptacles to limit the number of males she accepts.

The variety of vertebrate mechanisms of sex determination

The review deals with features of sex determination in vertebrates. The mechanisms of sex determination are compared between fishes, amphibians, reptilians, birds, and mammals. We focus on structural and functional differences in the role of sex-determining genes in different vertebrates. Special attention is paid to the role of estrogens in sex determination in nonmammalian vertebrates.

Expression and sequence evolution of aromatase cyp19a1 and other sexual development genes in East African cichlid fishes

Sex determination mechanisms are highly variable across teleost fishes and sexual development is often plastic. Nevertheless, downstream factors establishing the two sexes are presumably conserved. Here, we study sequence evolution and gene expression of core genes of sexual development in a prime model system in evolutionary biology, the East African cichlid fishes. Using the available five cichlid genomes, we test for signs of positive selection in 28 genes including duplicates from the teleost whole-genome duplication, and examine the expression of these candidate genes in three cichlid species. We then focus on a particularly striking case, the A- and B-copies of the aromatase cyp19a1, and detect different evolutionary trajectories: cyp19a1A evolved under strong positive selection, whereas cyp19a1B remained conserved at the protein level, yet is subject to regulatory changes at its transcription start sites. Importantly, we find shifts in gene expression in both copies. Cyp19a1 is considered the most conserved ovary-factor in vertebrates, and in all teleosts investigated so far, cyp19a1A and cyp19a1B are expressed in ovaries and the brain, respectively. This is not the case in cichlids, where we find new expression patterns in two derived lineages: the A-copy gained a novel testis-function in the Ectodine lineage, whereas the B-copy is overexpressed in the testis of the speciest-richest cichlid group, the Haplochromini. This suggests that even key factors of sexual development, including the sex steroid pathway, are not conserved in fish, supporting the idea that flexibility in sexual determination and differentiation may be a driving force of speciation.

Sex determination

How the sex of offspring is determined has puzzled philosophers and scientists for millennia. Modern science has identified both genetic and environmental factors, but the question is still not yet fully answered.

Both cell-autonomous mechanisms and hormones contribute to sexual development in vertebrates and insects

The differentiation of male and female characteristics in vertebrates and insects has long been thought to proceed via different mechanisms. Traditionally, vertebrate sexual development was thought to occur in two phases: a primary and a secondary phase, the primary phase involving the differentiation of the gonads, and the secondary phase involving the differentiation of other sexual traits via the influence of sex hormones secreted by the gonads. In contrast, insect sexual development was thought to depend exclusively on cell-autonomous expression of sex-specific genes. Recently, however, new evidence indicates that both vertebrates and insects rely on sex hormones as well as cell-autonomous mechanisms to develop sexual traits. Collectively, these new data challenge the traditional vertebrate definitions of primary and secondary sexual development, call for a redefinition of these terms, and indicate the need for research aimed at explaining the relative dependence on cell-autonomous versus hormonally guided sexual development in animals.