Search

Sex determination (SD) is a crucial developmental process, but its molecular underpinnings are very diverse, both between and within species. SD mechanisms have traditionally been categorized as either genetic (GSD) or environmental (ESD), depending on the type of cue that triggers sexual differentiation. However, mixed systems, with both genetic and environmental components, are more prevalent than previously thought. Here, we show theoretically that environmental effects on expression levels of genes within SD regulatory mechanisms can easily trigger within-species evolutionary divergence of SD mechanisms. This may lead to the stable coexistence of multiple SD mechanisms and to spatial variation in the occurrence of different SD mechanisms along environmental gradients. We applied the model to the SD system of the housefly, a global species with world-wide latitudinal clines in the frequencies of different SD systems, and found that it correctly predicted these clines if specific genes in the housefly SD system were assumed to have temperature-dependent expression levels. We conclude that environmental sensitivity of gene regulatory networks may play an important role in diversification of SD mechanisms.

International audience; Species that have sex determined by environmental conditions during development (i.e., environmental sex determination [ESD]) are especially vulnerable to environmental change, including altered stress levels, habitat loss, and species translocations. These factors can produce multigenerational trends in population size and eco-evolutionary dynamics not captured by existing theory based on lifetime reproductive success (R0). Here, we extend ESD theory to use per capita growth rate r as a more appropriate measure of evolutionary success (fitness), and we demonstrate the importance of this change when males and females can differ in maturation times and when maturation times vary with local conditions (plasticity). In these cases, we show that primary and secondary sex ratios may be strongly biased; that optimal maturation times, when locally plastic, depend on the balance between mortality and growth effects; and that plasticity of maturation times can ameliorate fitness costs of increasing environmental stress

Sex reversal at high temperatures during embryonic development (e.g., ZZ females) provides the opportunity for new genotypic crosses (e.g., ZZ male × ZZ female). This raises the alarming possibility that climatic warming could lead to the loss of an entire chromosome-one member of the sex chromosome pair (the Y or W)-and the transition of populations to environmental sex determination (ESD). Here we examine the evolutionary dynamics of sex-determining systems exposed to climatic warming using theoretical models. We found that the loss of sex chromosomes is not an inevitable consequence of sex reversal. A large frequency of ZZ sex reversal (50% reversal from male to female) typically divides the outcome between loss of the ZW genotype and the stable persistence of ZZ males, ZW females and ZZ females. The amount of warming associated with sex chromosome loss depended on several features of wild populations-environmental fluctuation, immigration, heritable variation in temperature sensitivity and differential fecundity of sex-reversed individuals. Chromosome loss was partially or completely buffered when sex-reversed individuals suffered a reproductive fitness cost, when immigration occurred or when heritable variation for temperature sensitivity existed. Thus, under certain circumstances, sex chromosomes may persist cryptically in systems where the environment is the predominant influence on sex.

Amniotes possess variability in sex determination, ranging from environmental sex determination to genotypic sex determination with differentiated sex chromosomes. Differentiated sex chromosomes have emerged independently several times. Their noteworthy convergent characteristic is the evolutionary stability, documented among amniotes in mammals, birds, and some lineages of lizards, snakes and turtles. Combining the analysis of multiple partial transcriptomes with the comparison of copy gene numbers between male and female genomes, we uncovered partial gene content of the highly differentiated ZZ/ZW sex chromosomes in the gecko genus Paroedura. The differentiated ZZ/ZW sex chromosomes of these geckos share genes with the part of the chicken chromosome 4 homologous with the XX/XY sex chromosomes of viviparous mammals and the ZZ/ZW sex chromosomes of lacertid lizards, as well as with the chicken chromosome 15, homologous with the XX/XY sex chromosomes of iguanas and ZZ/ZW sex chromosomes of softshell turtles. Along with other analogous cases, this finding reinforces the observation that particular chromosomes are repeatedly coopted for the function of sex chromosomes in amniotes. Notably, according to the phylogenetic distribution, the subclade of the genus Paroedura represents a rare case of the reversal of the for a considerable evolutionary time highly differentiated ZZ/ZW sex chromosomes back to poorly differentiated state.

The mangrove rivulus fish (Kryptolebias marmoratus) is an emerging model for research in the biological sciences. The species is androdioecious, with populations consisting predominantly of self-fertilizing hermaphrodites and a low abundance of males, but no females. Males arise either through environmental sex determination at the embryonic stage or environmentally driven sex change from hermaphrodite at the adult stage. Marked sexual dimorphisms in external morphological characters accompany gonadal transitions, including the loss of a caudal ocellus (‘eyespot’) and the development of orange color in males. How well external morphological features predict internal gonadal morphology, however, has not been systematically examined across the geographical range of this species. Our goal was to identify reliable external morphological predictors of animals possessing an ovotestis (hermaphrodite) or a testis (male) and to see if the strength of prediction is consistent throughout rivulus’ geographical range. We demonstrate that the state of the ocellus and, to a greater extent, orange coloration are key predictors of internal gonadal state. However, the degree to which these characteristics are predictive of internal gonadal state was region-specific, with the ocellus and orange status of animals from Bahamas and Belize, respectively, having a lower probability of forecasting internal gonadal state than those from other regions. These results will help to streamline laboratory studies by allowing unambiguous assignment of sex, and will assist field studies by providing greater opportunity to determine sex ratios in wild populations.

The study of temperature-dependent sex determination (TSD) in vertebrates has attracted major scientific interest. Recently, concerns for species with TSD in a warming world have increased because imbalanced sex ratios could potentially threaten population viability. In contrast, relatively little attention has been given to the direct effects of increased temperatures on successful embryonic development. Using 6603 days of sand temperature data recorded across 6 years at a globally important loggerhead sea turtle rookery-the Cape Verde Islands-we show the effects of warming incubation temperatures on the survival of hatchlings in nests. Incorporating published data (n = 110 data points for three species across 12 sites globally), we show the generality of relationships between hatchling mortality and incubation temperature and hence the broad applicability of our findings to sea turtles in general. We use a mechanistic approach supplemented by empirical data to consider the linked effects of warming temperatures on hatchling output and on sex ratios for these species that exhibit TSD. Our results show that higher temperatures increase the natural growth rate of the population as more females are produced. As a result, we project that numbers of nests at this globally important site will increase by approximately 30% by the year 2100. However, as incubation temperatures near lethal levels, the natural growth rate of the population decreases and the long-term survival of this turtle population is threatened. Our results highlight concerns for species with TSD in a warming world and underline the need for research to extend from a focus on temperature-dependent sex determination to a focus on temperature-linked hatchling mortalities.

Sex-determining mechanisms are broadly categorised as being based on either genetic or environmental factors. Vertebrate sex determination exhibits remarkable diversity but displays distinct phylogenetic patterns. While all eutherian mammals possess XY male heterogamety and female heterogamety (ZW) is ubiquitous in birds, poikilothermic vertebrates (fish, amphibians and reptiles) exhibit multiple genetic sex-determination (GSD) systems as well as environmental sex determination (ESD). Temperature is the factor controlling ESD in reptiles and temperature-dependent sex determination (TSD) in reptiles has become a focal point in the study of this phenomenon. Current patterns of climate change may cause detrimental skews in the population sex ratios of reptiles exhibiting TSD. Understanding the patterns of variation, both within and among populations and linking such patterns with the selection processes they are associated with, is the central challenge of research aimed at predicting the capacity of populations to adapt to novel conditions. Here we present a conceptual model that innovates by defining an individual reaction norm for sex determination as a range of incubation temperatures. By deconstructing individual reaction norms for TSD and revealing their underlying interacting elements, we offer a conceptual solution that explains how variation among individual reaction norms can be inferred from the pattern of population reaction norms. The model also links environmental variation with the different patterns of TSD and describes the processes from which they may arise. Specific climate scenarios are singled out as eco-evolutionary traps that may lead to demographic extinction or a transition to either male or female heterogametic GSD. We describe how the conceptual principles can be applied to interpret TSD data and to explain the adaptive capacity of TSD to climate change as well as its limits and the potential applications for conservation and management programs.

Four decades ago, it was proposed that environmental sex determination (ESD) evolves when individual fitness depends on the environment in a sex-specific fashion--a form of condition-dependent sex allocation. Many biological processes have been hypothesized to drive this sex asymmetry, yet a general explanation for the evolution of sex-determining mechanisms remains elusive. Here, we develop a mathematical model for a novel hypothesis of the evolution of ESD, and provide a first empirical test using data across turtles. ESD is favored when the sex-determining environment affects annual survival rates equivalently in males and females, and males and females mature at different ages. We compare this hypothesis to alternative hypotheses, and demonstrate how it captures a crucially different process. This maturation process arises naturally from common life histories and applies more broadly to condition-dependent sex allocation. Therefore, it has widespread implications for animal taxa. Across turtle species, ESD is associated with greater sex differences in the age at maturity compared to species without ESD, as predicted by our hypothesis. However, the effect is not statistically significant and will require expanded empirical investigation. Given variation among taxa in sex-specific age at maturity, our survival-to-maturity hypothesis may capture common selective forces on sex-determining mechanisms.

Frequent independent origins of environmental sex determination (ESD) are assumed within amniotes. However, the phylogenetic distribution of sex-determining modes suggests that ESD is likely very ancient and may be homologous across ESD groups. Sex chromosomes are demonstrated to be old and stable in endothermic (mammals and birds) and many ectothermic (non-avian reptiles) lineages, but they are mostly non-homologous between individual amniote lineages. The phylogenetic pattern may be explained by ancestral ESD with multiple transitions to later evolutionary stable genotypic sex determination. It is pointed out here that amniote ESD shares several key aspects with sequential hermaphroditism of fishes such as a lack of sex differences in genomes, biased population sex ratios, and potentially also molecular mechanism related to general stress responses. Here, it is speculated that ESD evolves via a heterochronic shift of the sensitive period of sex change from the adult to the embryonic stage in a hermaphroditic amniote ancestor. Also see the video abstract here https://youtu.be/q2mjtlCefu4.

The Atlantic silverside, Menidia menidia (Pisces: Atherinidae), exhibits an exceptionally high level of clinal variation in sex determination across its geographic range. Previous work suggested linear changes in the level of temperature-dependent sex determination (TSD) with increasing latitude. Based on comparisons at 31 sites encompassing the entire species' range, we find that the change in level of TSD with latitude is instead highly nonlinear. The level of TSD is uniformly high in the south (Florida to New Jersey), then declines rapidly into the northern Gulf of Maine where genotypic sex determination (GSD) predominates and then rebounds to moderate levels of TSD in the northern-most populations of the Gulf of St. Lawrence. Major latitudinal breakpoints occur in central New Jersey (40(o)N) and the northern Gulf of Maine (44(o)N). No populations display pure TSD or GSD. Length of the growing season is the likely agent of selection driving variation in TSD with a threshold at 210 days. Because gene flow among populations is high, such distinct patterns of geographic variation in TSD/GSD are likely maintained by contemporary selection thereby demonstrating the adaptive fine tuning of sex determining mechanisms.

© 2015 John Wiley & Sons Ltd. Temperature-dependent sex determination (TSD) is the predominant form of environmental sex determination (ESD) in reptiles, but the adaptive significance of TSD in this group remains unclear. Additionally, the viability of species with TSD may be compromised as climate gets warmer. We simulated population responses in a turtle with TSD to increasing nest temperatures and compared the results to those of a virtual population with genotypic sex determination (GSD) and fixed sex ratios. Then, we assessed the effectiveness of TSD as a mechanism to maintain populations under climate change scenarios. TSD populations were more resilient to increased nest temperatures and mitigated the negative effects of high temperatures by increasing production of female offspring and therefore, future fecundity. That buffered the negative effect of temperature on the population growth. TSD provides an evolutionary advantage to sea turtles. However, this mechanism was only effective over a range of temperatures and will become inefficient as temperatures rise to levels projected by current climate change models. Projected global warming threatens survival of sea turtles, and the IPCC high gas concentration scenario may result in extirpation of the studied population in 50 years., Financial support was obtained from a Marie Curie International Incoming Fellowship within the 7th European Community Framework Programme, The Leatherback Trust, the Earthwatch Institute, The Betx Chair Endowment of Drexel University and the Schrey Distinguished Professorship of Indiana University-Purdue Universitu Fort Wayne

Arachidonic acid is 1 of only 2 unsaturated fatty acids retained in the ovaries of crustaceans and an inhibitor of HR97g, a nuclear receptor expressed in adult ovaries. The authors hypothesized that, as a key fatty acid, arachidonic acid may be associated with reproduction and potentially environmental sex determination in Daphnia. Reproduction assays with arachidonic acid indicate that it alters female:male sex ratios by increasing female production. This reproductive effect only occurred during a restricted Pseudokirchneriella subcapitata diet. Next, the authors tested whether enriching a poorer algal diet (Chlorella vulgaris) with arachidonic acid enhances overall reproduction and sex ratios. Arachidonic acid enrichment of a C. vulgaris diet also enhances fecundity at 1.0 µM and 4.0 µM by 30% to 40% in the presence and absence of pyriproxyfen. This indicates that arachidonic acid is crucial in reproduction regardless of environmental sex determination. Furthermore, the data indicate that P. subcapitata may provide a threshold concentration of arachidonic acid needed for reproduction. Diet-switch experiments from P. subcapitata to C. vulgaris mitigate some, but not all, of arachidonic acid's effects when compared with a C. vulgaris-only diet, suggesting that some arachidonic acid provided by P. subcapitata is retained. In summary, arachidonic acid supplementation increases reproduction and represses pyriproxyfen-induced environmental sex determination in D. magna in restricted diets. A diet rich in arachidonic acid may provide protection from some reproductive toxicants such as the juvenile hormone agonist pyriproxyfen. Environ Toxicol Chem 2015;34:527-535. © 2014 SETAC.

Amniote vertebrates, the group consisting of mammals and reptiles including birds, possess various mechanisms of sex determination. Under environmental sex determination (ESD), the sex of individuals depends on the environmental conditions occurring during their development and therefore there are no sexual differences present in their genotypes. Alternatively, through the mode of genotypic sex determination (GSD), sex is determined by a sex-specific genotype, i.e. by the combination of sex chromosomes at various stages of differentiation at conception. As well as influencing sex determination, sex-specific parts of genomes may, and often do, develop specific reproductive or ecological roles in their bearers. Accordingly, an individual with a mismatch between phenotypic (gonadal) and genotypic sex, for example an individual sex-reversed by environmental effects, should have a lower fitness due to the lack of specialized, sex-specific parts of their genome. In this case, evolutionary transitions from GSD to ESD should be less likely than transitions in the opposite direction. This prediction contrasts with the view that GSD was the ancestral sex-determining mechanism for amniote vertebrates. Ancestral GSD would require several transitions from GSD to ESD associated with an independent dedifferentiation of sex chromosomes, at least in the ancestors of crocodiles, turtles, and lepidosaurs (tuataras and squamate reptiles). In this review, we argue that the alternative theory postulating ESD as ancestral in amniotes is more parsimonious and is largely concordant with the theoretical expectations and current knowledge of the phylogenetic distribution and homology of sex-determining mechanisms.

Most models for dioecy in flowering plants assume that dioecy arises directly from hermaphroditism through a series of independent feminizing and masculinizing mutations that become chromosomally linked. However, dioecy appears to evolve most frequently through monoecious grades. The major genetic models do not explain the evolution of unisexual flowers in monoecious and submonoecious populations, nor do they account for environmentally induced sexual plasticity. In this review, we explore the roles of environmental stress and hormones on sex determination, and propose a model that can explain the evolution of dioecy through monoecy, and the mechanisms of environmental sex determination. Environmental stresses elicit hormones that allow plants to mediate the negative effects of the stresses. Many of these same hormones are involved in the regulation of floral developmental genes. Recent studies have elucidated the mechanisms whereby these hormones interact and can act as switchpoints in regulatory pathways. Consequently, differential concentrations of plant hormones can regulate whole developmental pathways, providing a mechanism for differential development within isogenic individuals such as seen in monoecious plants. Sex-determining genes in such systems will evolve to generate clusters of coexpressed suites. Coexpression rather than coinheritance of gender-specific genes will define the sexual developmental fate. Therefore, selection for gender type will drive evolution of the regulatory sequences of such genes rather than their synteny. Subsequent mutations to hyper- or hyposensitive alleles within the hormone response pathway can result in segregating dioecious populations. Simultaneously, such developmental systems will remain sensitive to external stimuli that modify hormone responses.

In many aquacultured fish species, the production of monosex stocks is highly desirable because usually one sex grows more than the other or produces valuable gametes (e.g., caviar). The sex ratio of a population depends largely on the combined result of the processes of sex determination and differentiation. Fish exhibit a great diversity of sex-determining (SD) mechanisms, and sex differentiation involves the expression of a considerable number of genes in a spatial and temporal order. For genetic selection programs and monosex production in aquaculture, knowing these complex processes is essential. Functional genomics involving the application of microarrays, next-generation sequencing, and other functional genomic approaches, as well as epigenetics, can play an increasingly relevant role in deciphering relevant genes and signaling pathways and how these can be influenced by the environment. Here, we present this chapter on sex determination and differentiation in fish and review the advances made to date using functional genomic analysis using various experimental approaches. We also illustrate some technical complications involving changes in gene expression and DNA methylation. Finally, we suggest some avenues for further research in both model fish species and fish species facing specific problems within an aquaculture context. © 2012 John Wiley & Sons, Inc.

Selection is expected to maintain primary sex ratios at an evolutionary equilibrium. In organisms with temperature-dependent sex determination (TSD), targets of sex-ratio selection include the thermal sensitivity of the sex-determining pathway (hereafter, sex determination threshold) and nest-site choice. However, offspring sex may be canalized for nests located in thermally extreme environments; thus, genetic variance for the sex determination threshold is not expressed and is invisible to direct selection. The concept of 'effective heritability' accounts for this dependence and provides a more realistic prediction of the expected evolutionary response to selection in the wild. Past estimates of effective heritability of the sex determination threshold, which were derived from laboratory data, suggested that the potential for the sex determination threshold to evolve in the wild was extremely low. We re-evaluated original estimates of this parameter by analysing field-collected measures of nest temperatures, vegetation cover and clutch sex ratios from nests in a population of painted turtles (Chrysemys picta). We coupled these data with measurements of broad-sense heritability of the sex determination threshold in C. picta, using an experiment that splits clutches of eggs between a constant temperature (i.e. typical laboratory incubation) and a daily fluctuating temperature (i.e. similar to natural nests) with the same mean. We found that (i) the effective heritability of the sex determination threshold appears to have been historically underestimated and the effective heritability of nest-site choice has been overestimated and (ii) significant family-by-incubation treatment interaction exists for sex for C. picta between constant- and fluctuating-temperature regimes. Our results suggest that the thermal sensitivity of the sex-determining pathway may play a larger, more complex role in the microevolution of TSD than traditionally thought.

Conditions experienced early in life can influence phenotypes in ecologically important ways, as exemplified by organisms with environmental sex determination. For organisms with temperature-dependent sex determination (TSD), variation in nest temperatures induces phenotypic variation that could impact population growth rates. In environments that vary over space and time, how does this variation influence key demographic parameters (cohort sex ratio and hatchling recruitment) in early life stages of populations exhibiting TSD? We leverage a 17-year data set on a population of painted turtles, Chrysemys picta, to investigate how spatial variation in nest vegetation cover and temporal variation in climate influence early life-history demography. We found that spatial variation in nest cover strongly influenced nest temperature and sex ratio, but was not correlated with clutch size, nest predation, total nest failure, or hatching success. Temporal variation in climate influenced percentage of total nest failure and cohort sex ratio, but not depredation rate, mean clutch size, or mean hatching success. Total hatchling recruitment in a year was influenced primarily by temporal variation in climate-independent factors, number of nests constructed, and depredation rate. Recruitment of female hatchlings was determined by stochastic variation in nest depredation and annual climate and also by the total nest production. Overall population demography depends more strongly on annual variation in climate and predation than it does on the intricacies of nest-specific biology. Finally, we demonstrate that recruitment of female hatchlings translates into recruitment of breeding females into the population, thus linking climate (and other) effects on early life stages to adult demographics.

The great diversity of sex determination mechanisms in animals and plants ranges from genetic sex determination (GSD, e.g. mammals, birds, and most dioecious plants) to environmental sex determination (ESD, e.g. many reptiles) and includes a mixture of both, for example when an individual’s genetically determined sex is environmentally reversed during ontogeny (ESR, environmental sex reversal, e.g. many fish and amphibia). ESD and ESR can lead to widely varying and unstable population sex ratios. Populations exposed to conditions such as endocrine-active substances or temperature shifts may decline over time due to skewed sex ratios, a scenario that may become increasingly relevant with greater anthropogenic interference on watercourses. Continuous exposure of populations to factors causing ESR could lead to the extinction of genetic sex factors and may render a population dependent on the environmental factors that induce the sex change. However, ESR also presents opportunities for population management, especially if the Y or W chromosome is not, or not severely, degenerated. This seems to be the case in many amphibians and fish. Population growth or decline in such species can potentially be controlled through the introduction of so-called Trojan sex genes carriers, individuals that possess sex chromosomes or genes opposite from what their phenotype predicts. Here, we review the conditions for ESR, its prevalence in natural populations, the resulting physiological and reproductive consequences, and how these may become instrumental for population management.

Summary 1. In environmental sex determination (ESD) gender is decided after conception, depending on the environment, rather than being genetically fixed. ESD in plants has been mainly studied in angiosperms, where the dominant form of ESD is sexual lability. Surprisingly, ESD has rarely been studied in homosporous ferns, the only plants in which ESD is the rule, rather than the exception. 2. Here, we address the mechanism underlying ESD for the fern Woodwardia radicans by experimentally manipulating nutrient availability and gametophyte density. 3. Stress (limited nutrient supply, crowding) affected sexual expression in W. radicans gametophytes in a way compatible with ESD. Under good growth conditions (low density or high nutrient), gametophytes matured sexually at a relatively large size and turned into females and subsequently into bisexuals. Under harsh growth conditions, gametophytes matured sexually at a smaller size and turned into males. 4. Interestingly, gametophyte sexual expression was consistent with the size-advantage model, because the number of archegonia increased with gametophyte size, but not the number of antheridia. 5. The sex switch threshold size was variable and decreased with increasing stress, as predicted by age and size to maturity models. 6. Synthesis. Sexual expression in fern gametophytes can be fruitfully studied within the ESD theoretical framework. Stress induced male expression in gametophytes in a way compatible with ESD. In addition, size-related patterns of sexual expression were consistent with the size-advantage model, because female function benefited more from a larger size than did male function. Finally, the sex switch threshold size was variable and decreased with increasing stress, a result predicted by age and size to maturity models but seldom empirically tested before.

Environmental sex determination has been documented in a variety of organisms for many decades and the adaptive significance of this unusual sex-determining mechanism has been clarified empirically in most cases. In contrast, temperature-dependent sex determination (TSD) in amniote vertebrates, first noted 40 years ago in a lizard, has defied a general satisfactory evolutionary explanation despite considerable research effort. After briefly reviewing relevant theory and prior empirical work, we draw attention to recent comparative analyses that illuminate the evolutionary history of TSD in amniote vertebrates and point to clear avenues for future research on this challenging topic. To that end, we then highlight the latest empirical findings in lizards and turtles, as well as promising experimental results from a model organism, that portend an exciting future of progress in finally elucidating the evolutionary cause(s) and significance of TSD.

Why is the sex of many reptiles determined by the temperatures that these animals experience during embryogenesis, rather than by their genes? The Charnov-Bull model suggests that temperature-dependent sex determination (TSD) can enhance maternal fitness relative to genotypic sex determination (GSD) if offspring traits affect fitness differently for sons versus daughters and nest temperatures either determine or predict those offspring traits. Although potential pathways for such effects have attracted much speculation, empirical tests largely have been precluded by logistical constraints (i.e., long life spans and late maturation of most TSD reptiles). We experimentally tested four differential fitness models within the Charnov-Bull framework, using a short-lived, early-maturing Australian lizard (Amphibolurus muricatus) with TSD. Eggs from wild-caught females were incubated at a range of thermal regimes, and the resultant hatchlings raised in large outdoor enclosures. We applied an aromatase inhibitor to half the eggs to override thermal effects on sex determination, thus decoupling sex and incubation temperature. Based on relationships between incubation temperatures, hatching dates, morphology, growth, and survival of hatchlings in their first season, we were able to reject three of the four differential fitness models. First, matching offspring sex to egg size was not plausible because the relationship between egg (offspring) size and fitness was similar in the two sexes. Second, sex differences in optimal incubation temperatures were not evident, because (1) although incubation temperature influenced offspring phenotypes and growth, it did so in similar ways in sons versus daughters, and (2) the relationship between phenotypic traits and fitness was similar in the two sexes, at least during preadult life. We were unable to reject a fourth model, in which TSD enhances offspring fitness by generating seasonal shifts in offspring sex ratio: that is, TSD allows overproduction of daughters (the sex likely to benefit most from early hatching) early in the nesting season. In keeping with this model, hatching early in the season massively enhanced body size at the beginning of the first winter, albeit with a significant decline in probability of survival. Thus, the timing of hatching is likely to influence reproductive success in this short-lived, early maturing species; and this effect may well differ between the sexes.

Genetic and environmental factors may interact to control sex determination in fishes. Acommonpatternofinitialfemaledifferentiationandsubsequentmaletransformation before maturation in non-hermaphroditic fishes and after maturation in sequentially hermaphroditic fishes has suggested that changes in developmental timing may be responsible for the evolution of various expressions of sexual lability. Sequential hermaphroditism is rare in freshwater fishes, but investigators report degrees of sexual lability at four distinct life stages in cichlid fishes. Some cichlids undergo genetic sex determination and are not labile. Lability at the larval stage allows temperature or pH to determine sex. Social interactions apparently determine sex at the juvenile stage in the Midas cichlid (Amphilophus citrinellus). Most reports of post-maturational sex change in cichlids are anecdotal or unsubstantiated. The common occurrence of samesex spawning suggests that many species are incapable of sex change. Sequential hermaphroditism is concluded not to be typical, except for the checkerboard cichlid (Crenicara punctulata), which regularly undergoes functional female-to-male transformation. Expression of sexual lability at four life stages in one family of fishes corroborates a role for developmental timing in the evolution of sequential hermaphroditism as well as environmentally controlled sex determination. The broad phylogenetic distribution of sexual lability in cichlids indicates that processes capable of producing sex change are generally present. The rarity of sequential hermaphroditism in cichlids and possibly other freshwater fishes is likely due to unpredictability of food and therefore potential mate distributions compared with coral reef habitats.

Sex ratios of the American snapping turtleChelydraserpentina are female-biased at cool temperatures, male-biased at moderate temperatures, and only females are produced at warm temperatures. The laboratory incubation of eggs at several constant temperatures yields patterns of skewed sex ratios. In this study, incubated eggs represented regional samples of C. serpentina from six different latitudes. The sample from the highest latitude yielded malebiased sex ratios across the broadest range of cool temperatures. This male-biased zone and those of the other five samples collectively contributed to a nested arrangement that is approximately symmetrical around a temperature of 24.5 ◦ C. Two indices of temperature-dependent sex determination (TSD) patterns (MF pivotal temperature, width of the male-biased zone) were significantly associated with latitude and with published data on several climatic variables (e.g. sunshine and rainfall). At high latitudes, females nest in exposed locations, locally the warmest places. At low latitudes females place their nests in shaded locations. Such nest site selection renders high latitude nests warmer than low latitude nests, even if only intermittently. Given that constant conditions show faster embryogenesis in high vs low latitude eggs, exposed nest sites seem to be an additional adaptation to hasten embryogenesis in high latitude nests. At low latitudes, shading prevents nests from overheating. We propose that a trade-off between evolution, (1) to balance sex ratio and (2) to maintain male-biased sex ratios in a temperature range that favours male fitness, leads to the observed arrangement of concentric TSD patterns centred around male production at 24.5 ◦ C.

In the crustacean Gammarus duebeni, sex is determined by the interaction of a number of environmental, genetic and parasitic factors, which may, in turn, influence sex ratios and population dynamics. We produce novel evidence that environmental sex determination (ESD) in G. duebeni depends primarily on the interaction of two environmental cues: day length and temperature. Whereas previous work found that male-biased sex ratios were produced under long day conditions and female biases under short days, we show that, at the lower temperatures normally experienced by this species at northern latitudes, the reverse pattern can occur. We measured ESD in four U.K. populations and found among-population variation in the level of ESD and in the cues that determined sex. In the light of these findings, we conclude that patterns of ESD across the four populations may reflect selection based on differences in breeding season and discrete/overlapping generations.

The size advantage hypothesis suggests that natural selection will favor size-dependent sex expression when one sex gains more than the other by being large. But members of a minority sex will also have a higher reproductive value, on average. Thus, an individual's reproductive success depends on the reproductive decisions made by neighboring individuals. As a result, the optimal relationship between size and sex may differ among populations. In Arisaema triphyllum, the probability for an individual to be female increases with size, regardless of the character measured. A reciprocal transplant experiment showed the relationship between size and sexual expression is environmentally plastic. Plants originating from our two study sites became female at a larger average size when grown at one site than when grown at the other. In addition to environmental influence on sex expression, the experiment demonstrated genetic differences in the relationship between size and sex. Plants collected from one site became female at a larger size than those from the other, regardless of where they were grown. Thus, while the environment in which an individual was grown had a substantial influence on its sex expression, populations only a few kilometers apart have genetically different relationships between size and sex.

Twenty years ago, Bulmer and Bull suggested that disruptive selection, produced by environmental fluctuations, can result in an evolutionary transition from environmental sex determination (ESD) to genetic sex determination (GSD). We investigated the feasibility of such a process, using mutation-limited adaptive dynamics and individual-based computer simulations. Our model describes the evolution of a reaction norm for sex determination in a metapopulation setting with partial migration and variation in an environmental variable both within and between local patches. The reaction norm represents the probability of becoming a female as a function of environmental state and was modeled as a sigmoid function with two parameters, one giving the location (i.e., the value of the environmental variable for which an individual has equal chance of becoming either sex) and the other giving the slope of the reaction norm for that environment. The slope can be interpreted as being set by the level of developmental noise in morph determination, with less noise giving a steeper slope and a more switchlike reaction norm. We found convergence stable reaction norms with intermediate to large amounts of developmental noise for conditions characterized by low migration rates, small differential competitive advantages between the sexes over environments, and little variation between individual environments within patches compared to variation between patches. We also considered reaction norms with the slope parameter constrained to a high value, corresponding to little developmental noise. For these we found evolutionary branching in the location parameter and a transition from ESD toward GSD, analogous to the original analysis by Bulmer and Bull. Further evolutionary change, including dominance evolution, produced a polymorphism acting as a GSD system with heterogamety. Our results point to the role of developmental noise in the evolution of sex determination.

Summary 1 Sexual differentiation of Equisetum (horsetails) gametophytes is influenced by environmental conditions, and wild populations tend to be female biased. We show that photon irradiance has a very significant and reproducible effect on sex determination in three species of Equisetum grown in vitro. 2 Under our experimental conditions, high light levels (PAR = 24–48 µmol m−2 s−1) induce most gametophytes to differentiate as female, whereas only males are observed at PAR

Van Valen (1973) characterized evolution as the control of development by ecology. Sex determination in fishes provides some clear examples of this “control” in operation. Teleost fishes show a remarkable variety of sex determination and differentiation patterns. These range from systems in which sex is determined by sex chromosomes, as in birds and mammals, to simultaneous hermaphrodites that alternate spawning as a female and male on a second to second basis. This extraordinary flexibility may result from a combined lack of developmental constraint on reproductive structures in many lineages and selection for sexual lability in the face of environmental unpredictability. This review addresses environmental influences on sex determination and differentiation in fishes. There is a variety of documented environmental influences on sex determination (ESD) in fishes. We focus here on two classes of examples where the key environmental cues are of clear ecological relevance, the effects appear especially likely to be important as a normal part of the life history, and where there is evidence suggesting the sexual patterns observed represent adaptations that increase individual fitness. These classes are sex determination that is controlled by social interactions (behavioral sex determination [BSD]) (Crews 1993) and temperaturedependent sex determination (TSD). Sex determination controlled by social influences can occur before or after sexual maturation but appears to maximize the expected reproductive success of individuals in both cases. Here we first address BSD and then TSD in fishes. For each pattern of sex determination, we discuss selection pressures that appear to favor these patterns, examples of each, and what is known regarding the underlying physiological mechanisms. For more comprehensive and general reviews of patterns and mechanisms of sex determination in fishes, the reader is referred to several excellent reviews (Nakamura et al. 1998; Baroiller et al. 1999; Baroiller and D’Cotta 2001; Piferrer 2001). The major focus in studies of physiological mediation of teleost sex determination is what is referred to by endocrinologists as the hypothalamo-pituitary-gonadal (HPG) axis (Fig. 1). This axis consists primarily of hypothalamic neurosecretory neurons producing gonadotropin-releasing hormone (GnRH), gonadotropins produced in and released from the pituitary gland (GtH I and GtH II), and the gonad as the major site of steroid biosynthesis with its steroid metabolizing enzymes, steroid hormone receptors, and a variety of other proteins that mediate steroid hormone action. One steroid biosynthetic enzyme that has been a particularly fruitful focus in correlative and manipulative studies of vertebrate sex determination is cytochrome P-450 aromatase. This enzyme catalyzes the conversion of androgens to estrogens (primarily testosterone to estradiol-17 ). Aromatase expression correlates with female determination in a variety of vertebrates, and aromatasespecific antagonists can block female development in fishes, amphibians, reptiles, and birds (Elbrecht and Smith 1992; Lance and Bogart 1992; Crews et al. 1994; Wennstrom and Crews 1995; Kitano et al. 1999; D’Cotta et al. 2001). Estradiol-17 plays a central role in female reproductive physiology in fishes, whereas the androgen 11-ketotestosterone (11-KT) is crucial to gamete maturation and the expression of secondary sexual characteristics in males (Borg 1994; Brantley et al. 1993). Importantly, testosterone levels often do not differ between male and female fishes or are higher in females (Borg 1994). Because of the central role of aromatase in the biosynthesis of estrogens, it will be a focus in consideration of mechanisms by which environmental information leads to sex determination responses. More generally, our understanding of vertebrate sexual function indicates the HPG axis plays the key role in transducing environmental information into gonadal determination, differentiation, and maturation events. A general theme of this review is where and how this transduction may occur in the HPG axis.

Environmental sex determination is reported in various fish species, including some fishes subject to stock enhancement. We studied the influence of stock enhancement on the sex-determining system of fish with both genotypic and environmental sex determination. We constructed and analyzed recurrence formula models for the dynamics of gene frequency in the male heterogametic (XX female and XY male) sex-determining system of the Japanese flounder (Paralichthys olivaceus). In this species, an XX individual can develop as a phenotypic male (sex-reversed male) depending on the conditions experienced by a juvenile. We show that the release of sex-reversed males may result in the extinction of the Y gene that determines sex. We also studied how the risk depends on the kind of hatchery broodstock used (e.g., fish collected from the wild or fish from a lineage established in a hatchery), sex-reversal rates in the hatchery and in the wild, and the relative contribution of released fish to reproduction in the wild. We propose some ways to reduce the risk.

Environmental sex determination may yield biased sex ratios, as is the case in many reptilian species with temperature-dependent sex determination (TSD). Determin- ing TSD thermal ecology is important to understanding sex ratio evolution in those species, as TSD systems may pose a challenge to Fisherian models that predict the production of 1:1 sex ratios through frequency-dependent selection. Unfortunately, the relationship be- tween fluctuating temperatures and sex in nature remains poorly understood. Here I present a novel statistical model of TSD in the endangered Amazonian freshwater turtle, Podocnemis expansa, designed to account for heterogeneous daily fluctuations of natural nest temper- atures by comparing laboratory and field data on sex ratios, developmental stages, cu- mulative temperature units (CTUs), and time spent below the survival threshold (288C). Experiments encompassed constant, shift-once, and shift-twice temperatures in the labo- ratory, plus depth and shading treatments in the field. A constant temperature of 32.68C produced 1:1 males : females, the highest pivotal temperature ever reported for a turtle species. The thermosensitive period for sex determination (TSP) roughly occurs during developmental stages 17-24 as determined from shift experiments. The CTUs during the TSP plus the number of hours below 288C prior to the TSP significantly explained sex ratio production in the combined laboratory and field data set (positive and negative effects, respectively). However, the relationship for field nests only was not significant. Higher temperatures had a disproportionately larger effect on female production in the combined field and laboratory data set. Depth and shading had drastic effects on nest temperature, embryonic development, and sex ratio. Seasonal climate changes can override the effect of nest depth via differences in oviposition time. Two of the shaded nests experienced the coldest temperatures of all field nests but yielded the highest female proportion, according to a TSDII pattern (when females are produced at low and high temperatures and males are produced at intermediate temperatures). Thus Podocnemis expansa may be a TSDII species, but this intrinsic system seems to be realized as TSDIa in nature (males produced at low temperatures and females produced at high temperatures), because of the general lethality of cold temperatures which precludes the production of low-temperature females.

Gonadal biopsies of sea lamprey Petromyzon marinus larvae (118.5-142.5 mm) were examined quantitatively and categorized as ovaries, presumptive testes, or atypical gonads. Subsets of larvae were then killed 1, 2, 4, 8, 16 and 52 weeks following surgery at which time each larva was matched with its biopsy, and its gonad re-examined for quantifiable changes in composition. During 52 weeks, oocyte diameters increased significantly while numbers of oocytes per unit area significantly declined. Germ cell numbers remained within the range of typical presumptive testes. Overall abundance of typical ovaries decreased over the course of the experiment, whereas the number of presumptive testes increased. This was a direct result of presumptive testis development from atypical gonads, oocyte atresia and sex reversal. The results suggest an argument for environmental sex determination in lampreys, while indicating that they remain sexually labile throughout most of the larval period, despite primary differentiation.

At Arapaho Prairie, in the sandhills of western Nebraska, the dioecious annual Croton texensis (Euphorbiaceae) exhibits biased sex ratios. Moreover, the direction of bias changes from year to year: in 1994 the study population was significantly female biased, in 1995 and 1996 it was significantly male biased, and in 1997 and 1998 the sex ratio did not differ from 1 : 1. Such variation in the observed sex ratio in plants is frequently attributed to environmental sex determination (ESD), which is favored by natural selection if the rate of fitness gain across an environmental gradient is greater for one sex than the other. We performed experiments to determine: (1) whether variation in the sex ratio is correlated with environmental conditions, as would be expected if ESD is operating, and (2) whether ESD, if present, would be favored by natural selection. In a common garden experiment in which water and fertilizer were manipulated the sex ratio was marginally male biased in treatments in which water was added, but not different from 1 : 1 in other treatments. In field plots into which seeds were planted none of several soil characteristics, nor overall plot quality for C. texensis (measured as average plant biomass) were correlated with plot sex ratio. However, plots in which a large number of planted seeds emerged tended to be female biased. These results provide very weak evidence for sex ratio bias across an environmental gradient, and thus provide little evidence for ESD. Moreover, sex-by-environment interactions for fitness, which are required for the evolution of ESD, were absent for all measured variables. Thus, ESD does not appear to be favored by natural selection in this population. Instead, these biases may have been caused by differences between the sexes in germination and/or early mortality.

Unlike birds and mammals, in many reptiles the temperature experienced by a developing embryo determines its gonadal sex. To understand how temperature-dependent sex determination (TSD) evolves, we must first determine the nature of genetic variation for sex ratio. Here, we analyze among-family variation for sex ratio in three TSD species: the American alligator (Alligator mississipiensis), the common snapping turtle (Chelydra serpentina) and the painted turtle (Chrysemys picta). Significant family effects and significant temperature effects were detected in all three species. In addition, family-by-temperature interactions were evident in the alligator and the snapping turtle, but not in the painted turtle. Overall, the among-family variation detected in this study indicates potential for sex-ratio evolution in at least three reptiles with TSD. Consequently, climate change scenarios that are posited on the presumption that sex-ratio evolution in TSD reptiles is genetically constrained may require reevaluation.

Environmental sex determination (ESD) permits adaptive sex choice under patchy environmental conditions, where the environment affects sex-specific fitness and where offspring can predict their likely adult status by monitoring an appropriate environmental cue. For Gammarus duebeni, an amphipod with ESD, it has been proposed that this flexible sex determination system is adaptive because males gain more from large size. Under ESD, young which are born earlier in the season become mostly males and, experiencing longer to grow, are therefore larger at breeding than females which are born later in the season. In order to test the hypothesis that ESD is adaptive for this species we investigated the relationship between size and fitness for both males and females, in a population of G. duebeni known to have ESD. We measured size related pairing success and fecundity, and used these two measures to calculate the relative fitness gains achieved through an increase in size for either sex. The fitness of both males and females increased with size, but males gained more from an increase in size than did females, throughout the breeding season. The data support the adaptive explanation for the evolution and maintenance of ESD in this species.

In many cladoceran zooplankton, females divide their reproductive effort among females, males, and resting eggs depending on a suite of environmental conditions. Having alternative life-history strategies allows cladocerans such as Daphnia to achieve high reproductive rates asexually when conditions are favorable and to produce offspring sexually for survival when the environment becomes unsuitable. Because of their rapid reproduction, Daphnia and other cladocerans are useful model organisms in aquatic toxicology. Bioassays using Daphnia serve two functions, to estimate the ecosystem level risk posed by specific chemicals and to quickly detect contaminated waters. Bioassays examine effects in terms of life history features such as growth, survival, and asexual fecundity. We studied the effects of toxic stress on Daphnia galeata mendotae grown under conditions in which it produces female, male, and ephippial offspring. Fecundities in terms of the three types of offspring showed different sensitivities to chemical stress. The stressors were a toxic strain of the cyanobacterium Microcystis aeruginosa and nonylphenol, a plasticizer, which is a surfactant that interferes with endocrine function in vertebrates. Production of resting eggs and female offspring were affected in both experiments, but the experiments showed different dose responses. Production of males was less sensitive. Exposure to nonylphenol also produced a characteristic developmental abnormality at environmentally relevant concentrations. Life table analysis showed that stress during the sexual phase of the life cycle of Daphnia reduces short-term population growth and as well as affecting the stock of dormant individuals.

Environmental sex determination by temperature could be revealed significantly in 33 Apistogramma-species and in Poecilia melanogaster. In some, but not all, Apistogramma-species pH also influences the sex ratio, whereas neither temperature nor pH affect the sex ratio of Pseudocrenilabrus multicolor victoriae. The sex in offspring of A. trifasciata is determined within a sensitive period of about 30 to at least 40 days after spawning.

We tested the hypothesis that larger egg and hatchling sizes in the snapping turtle confer posthatching advantages, by comparing the growth rates and survival of hatchlings from different thermal and hydric environments during embryonic development. We also tested for sex-specific differences in survival and growth, two indices of hatchling fitness, as predicted by the Charnov & Bull (1977) model of environmental sex determination (ESD). In addition, we examined interclutch and interpopulation variability in the responses of embryos and hatchlings to incubation conditions. Eggs of 21 clutches from four Ontario populations were incubated at 22·0, 25·5 or 29·3°C in a dry (–350 kPa) or wet (–150 kPa) vermiculite substrate, and hatchling survival and growth in the laboratory were monitored for 11 months. Initial egg mass and substrate moisture were positively correlated with mass at hatching, but did not affect hatching success, posthatching growth or posthatching survival. Initially heavier hatchlings were more likely to survive, but did not grow more quickly. The effects of incubation environment and hatchling mass were not consistent among clutches or populations. Intermediate incubation temperatures produced mostly males, with higher embryo and hatchling survival, as well as posthatching growth, than the mostly females from the extreme temperatures. These results support the Charnov—Bull model. Embryo and hatchling mortality and deformities were higher in clutches from two populations which are contaminated with organochlorine toxins. Interclutch differences were significant for all variables, even when initial egg or hatchling sizes were considered. Hatchlings from the population near the northern limit of the species' range grew the slowest. These results indicate that hatchling size cannot be used as an index of hatchling quality or posthatching success, unless interclutch and interpopulation variation are taken into account.

Parental sex ratio control was investigated in Gammarus duebeni, an amphipod with an environmentally mediated sex determining system. The effect on the F2 generation sex ratio of the photoperiodic conditions experienced by a) the P generation during and after copulation, b) the F1 generation before and after sex determination, and c) the F2 generation themselves during the period of sex determination, was tested. The photoperiodic conditions the F2 generation experienced during the period of sex determination had a significant effect on their sex ratio (more males were produced under long-day than under short-day conditions), but the photoperiodic conditions experienced by the F1 generation males and females or the P generation on the F1 male's side had no effect on the F2 sex ratio. However, the photoperiodic conditions the P generation on the F1 female's side experienced significantly affected the F2 sex ratio. When these animals experienced long-day conditions the F2 generation became female biased and when they experienced short-day conditions, male biased. It is proposed that the mechanism of control operates through the F1 generation mothers whilst in an embryonic stage of development in the P generation mother's brood pouch. The photoperiodically mediated effects of the embryonic F1 generation mother and the F2 generation on sex determination are additive. A mechanism by which both F1 generation maternal and F2 generation sex ratio control could operate in the field is proposed.

In some animals sex is determined after conception by environmental factors (environmental sex determination. ESD). In the amphipod Gammarus duebeni sex is reportedly determined by photoperiod: there is a higher proportion of males in broods reared under long-day than under short-day photoperiods. It has been proposed that this is an adaptive response to seasonal population dynamics. A test of the hypothesis would be to demonstrate changes in the degree to which sex is determined by the environment in populations from different latitudes with different dynamics. This study reports such a test. Environmental response is significantly less strong in a southern population with a long breeding season than in a northern one in which breeding is seasonally restricted. Moreover, the threshold of the ESD cue for male or female determination is not defined when the breeding season is weakly seasonal. There is a broad correlation between latitude (and hence breeding seasonality) and the strength of ESD response across a series of population studies. Similarities between the Gammarus system of sex determination and that of the Atlantic silverside Menidia menidia, a fish with thermal ESD, are discussed.

Three possible causes of intersexuality in Gunznzurus duehmi, a crustacean with environmental sex determination, were investigated. Intersexuality appears to be a cost of the flexible sexdetermining mechanism of this species: the occurrence of intersexes is influenced by photoperiod, which also cues sex determination. Intersexuality is heritable: intersex mothers produce more intersex offspring than do true females. Parasitism by a feminizing microsporidian is ruled out as a significant cause of intersexuality.

Models of environmental sex determination (ESD) usually assume that genetic influences on sex are polygenic, but the validity of this (or any other) form of genotype-environment interaction is virtually unknown. In the Atlantic silverside, Menidia menidia, sex is determined by an interaction between temperature and genotype and the response of sex ratio to temperature differs among populations from different latitudes. We examined the genetic basis of this pattern by measuring among family variation in the proportion of females, F/(F + M), within and among high (21°C) and low (15°C) temperatures for two populations: one from Nova Scotia (NS) where the level of ESD is low, and another from South Carolina (SC) where the level of ESD is high. In NS fish, temperature had a significant influence on sex ratio in only 1 of 23 families. The distribution of the fraction of females within temperatures for families from NS was highly heterogeneous and tended to fall into distinct classes (0.0, 0.25, 0.5, 1.0) like that expected from Mendelian segregation of a major sex factor(s). In contrast, temperature had a highly significant influence on sex ratio in all SC families examined (N = 24). Family sex ratios within temperatures were highly heterogeneous and, at least at 15°C, did not conform to simple Mendelian ratios. At 21°C, the proportion of females in most SC families was near zero and so the underlying sex tendencies of different families could not be discerned. Based on a previous study, mid-latitude fish appear to have an intermediate form of sex determination: simple Mendelian sex-ratio patterns exist and there is a moderate thermal influence on sex ratio in most but not all families. We suggest that sex determination in M. menidia is controlled by an interaction between major genetic factors, polygenic factors, and temperature and that the relative importance of each component differs with latitude. High latitude populations appear to have evolved a major sex-determining factor(s) that overrides the effect of temperature, and this factor(s) is lacking in low latitude populations.