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Sex determination refers to the developmental programme that commits the embryo to either the male or the female pathway. There are a plethora of mechanisms via which gender is decided. These mechanisms can be classified into three main categories depending on the origin of the primary, genetic, sex-determination signal, which can be zygotic, maternal or environmental. In the dipteran Sciara, the zygotic signal is a consequence of the maternal signal, and this in turn can be a consequence of the environmental signal. This makes Sciara a unique experimental model for studying the evolutionary relationships between the three primary signals triggering sexual development. [ABSTRACT FROM AUTHOR]

In this review, we describe the history of amniote sex determination as a classic example of Darwinian evolution. We suggest that evolutionary changes in sex determination provide a foundation for understanding important aspects of chromosome and genome organization that otherwise appear haphazard in their origins and contents. Species with genotypic sex determination often possess heteromorphic sex chromosomes, whereas species with environmental sex determination lack them. Through a series of mutations followed by selection at key genes, sex-determining mechanisms have turned over many times throughout the amniote lineage. As a consequence, amniote genomes have undergone gains or losses of sex chromosomes. We review the genomic and ecological contexts in which either temperature-dependent or genotypic sex determination has evolved. Once genotypic sex determination emerges in a lineage, viviparity and heteromorphic sex chromosomes become more likely to evolve. For example, in extinct marine reptiles, genotypic sex determination apparently led to viviparity, which in turn facilitated their pelagic radiation. Sex chromosomes comprise genome regions that differ from autosomes in recombination rate, mutation rate, levels of polymorphism, and the presence of sex-determining and sexually antagonistic genes. In short, many aspects of amniote genome complexity, life history, and adaptive radiation appear contingent on evolutionary changes in sex-determining mechanisms. Copyright © 2010 S. Karger AG, Basel [ABSTRACT FROM AUTHOR]

Stress experienced by a reproducing female can substantially affect the morphology, behavior, and physiology (and hence fitness) of her offspring. In addition, recent studies demonstrate that stress hormones (corticosterone) influence sex determination of embryos. To explore these issues, we manipulated corticosterone levels in eggs of two Australian lizard species (Amphibolurus muricatus and Bassiana duperreyi) that exhibit temperature-dependent sex determination (TSD). Elevated corticosterone levels during embryonic development affected body size, growth rates, and sex ratios of the resultant offspring, but the direction and magnitude of these effects differed between the species. Corticosterone enhanced growth rates of hatchling B. duperreyi but inhibited growth of A. muricatus. Eggs with elevated levels of corticosterone produced more daughters in A. muricatus and more sons in B. duperreyi. The sex-ratio effect in A. muricatus may have been due to sex-specific embryonic mortality, but it may represent a direct effect on sex determination in B. duperreyi (because embryonic mortality was not affected by corticosterone manipulation in this species). These results demonstrate the complexity of proximate mechanisms for sex determination among reptiles with TSD and illustrate the potential role of corticosterone in sex-determining systems. [ABSTRACT FROM AUTHOR]

Abstract: Flounder of the genus Paralichthys exhibit a unique mode of sex determination where both low and high temperatures induce male-skewed sex ratios, while intermediate temperatures produce a 1:1 sex ratio. Male differentiation is thus easily induced in genetic females creating a combination of genetic (GSD) and environmental sex determination (ESD). Since male flounder become reproductively fit at substantially smaller body sizes than females, temperature or other environmental variables that elicit lower growth rates may also influence sex differentiation toward male development. This review covers our current knowledge of sex determination and differentiation in flatfishes including possible adaptive significance of ESD and involvement of factors such as aromatase (cyp19). [Copyright &y& Elsevier]

Squamate reptiles possess two general modes of sex determination: (1) genotypic sex determination (GSD), where the sex of an individual is determined by sex chromosomes, i.e. by sex-specific differences in genotype; and (2) temperature-dependent sex determination (TSD), where sex chromosomes are absent and sex is determined by nongenetic factors. After gathering information about sex-determining mechanisms for more than 400 species, we employed comparative phylogenetic analyses to reconstruct the evolution of sex determination in Squamata. Our results suggest relative uniformity in sex-determining mechanisms in the majority of the squamate lineages. Well-documented variability is found only in dragon lizards (Agamidae) and geckos (Gekkota). Polarity of the sex-determining mechanisms in outgroups identified TSD as the ancestral mode for Squamata. After extensive review of the literature, we concluded that to date there is no known well-documented transition from GSD to TSD in reptiles, although transitions in the opposite direction are plentiful and well corroborated by cytogenetic evidence. We postulate that the evolution of sex-determining mechanisms in Squamata was probably restricted to the transitions from ancestral TSD to GSD. In other words, transitions were from the absence of sex chromosomes to the emergence of sex chromosomes, which have never disappeared and constitute an evolutionary trap. This evolutionary trap hypothesis could change the understanding of phylogenetic conservatism of sex-determining systems in many large clades such as butterflies, snakes, birds, and mammals. © 2009 The Linnean Society of London, Zoological Journal of the Linnean Society, 2009, 156, 168–183. [ABSTRACT FROM AUTHOR]

Mollusks, especially bivalves, exhibit a great diversity of sex determining mechanisms, including both genetic and environmental sex determination. Some bivalve species can be gonochoristic (separate sexes), while others are hermaphroditic (sequential or simultaneous). Several models have been proposed for specific bivalve species, utilizing information gained from gene expression data, as well as limited RAD-seq data (e.g., from Crassostrea gigas). However, these mechanisms are not as well studied as those in model organisms (e.g., Mus musculus , Drosophila melanogaster , Caenorhabditis elegans) and many genes involved in sex differentiation are not well characterized. We used phylotranscriptomics to better understand which possible sex differentiating genes are in bivalves and how these genes relate to similar genes in diverse phyla. We collected RNAseq data from eight phylogenetically diverse bivalve species: Argopecten irradians , Ensis directus , Geukensia demissa , Macoma tenta , Mercenaria mercenaria , Mya arenaria , Mytilus edulis , and Solemya velum. Using these data, we assembled representative transcriptomes for each species. We then searched for candidate sex differentiating genes using BLAST and confirmed the identity of nine genes using phylogenetics analyses from nine phyla. To increase the confidence of identification, we included ten bivalve genomes in our analyses. From the analysis of doublesex and mab-3 related transcription factor (DMRT) genes, we confirmed the identify of a Mollusk-specific sex determining DMRT gene: DMRT1L. Based on gene expression data from M. edulis and previous research, DMRT1L and FoxL2 are key genes for male and female development, respectively. [Display omitted] • Phylotranscriptomics identified nine candidate sex differentiating genes • DMRT1L is a mollusk-specific gene, with a possible role in sex differentiation • We propose FoxL2 and DMRT1L are two important genes for bivalve sex differentiation [ABSTRACT FROM AUTHOR]

In several species of short-lived Australian agamid lizards, an individual's sex is determined by the nest temperatures encountered during incubation. The adaptive significance of such systems remains unclear. Here, we explore the hypothesis that (1) the optimal timing of hatching differs between the sexes, and thus (2) temperature-dependent sex determination (TSD) enhances maternal and offspring fitness by generating seasonal shifts in offspring sex ratios. Our model predicts that TSD can indeed enhance maternal fitness returns in short-lived lizards if (1) male-male competition is intense, thus reducing mating success of newly-matured males (but not females), and (2) the nesting season is prolonged, such that seasonal effects become significant. Available data on the distribution of TSD in Australian agamid lizards broadly support these predictions. Because both the level of male-male competition and the length of nesting season can vary at small spatial and temporal scales, selective forces on sex-determining mechanisms also should vary. Hence, our model predicts extensive small-scale (intraspecific) variation in sex-determining systems within agamid lizards, as well as among species. [ABSTRACT FROM AUTHOR]

• JH agonists have potential to disrupt the population decline in cladocerans. • Overviewed the current knowledge on JH systems in cladocerans. • AOPs addressing JH synthesis and signaling disruption were developed and evaluated. Juvenile hormone (JH) are a family of multifunctional hormones regulating larval development, molting, metamorphosis, reproduction, and phenotypic plasticity in arthropods. Based on its importance in arthropod life histories, many insect growth regulators (IGRs) mimicking JH have been designed to control harmful insects in agriculture and aquaculture. These JH analogs (JHAs) may also pose hazards to nontarget species by causing unexpected endocrine-disrupting (ED) effects such as molting and metamorphosis defects, larval lethality, and disruption of the sexual identity. This critical review summarizes the current knowledge of the JH-mediated effects in the freshwater cladoceran crustaceans such as Daphnia species on JHA-triggered endocrine disruptive outputs to establish a systematic understanding of JHA effects. Based on the current knowledge, adverse outcome pathways (AOPs) addressing the JHA-mediated ED effects in cladoceran leading to male offspring production and subsequent population decline were developed. The weight of evidence (WoE) of AOPs was assessed according to established guidelines. The review and AOP development aim to present the current scientific understanding of the JH pathway and provide a robust reference for the development of tiered testing strategies and new risk assessment approaches for JHAs in future ecotoxicological research and regulatory processes. [ABSTRACT FROM AUTHOR]

Sexual reproduction is one of the most taxonomically conserved traits, yet sex-determining mechanisms (SDMs) are quite diverse. For instance, there are numerous forms of environmental sex determination (ESD), in which an organism’s sex is determined not by genotype, but by environmental factors during development. Important questions remain regarding transitions between SDMs, in part because the organisms exhibiting unique mechanisms often make difficult study organisms. One potential solution is to utilize mutant strains in model organisms better suited to answering these questions. We have characterized two such strains of the model nematode Caenorhabditis elegans. These strains harbour temperature-sensitive mutations in key sex-determining genes. We show that they display a sex ratio reaction norm in response to rearing temperature similar to other organisms with ESD. Next, we show that these mutations also cause deleterious pleiotropic effects on overall fitness. Finally, we show that these mutations are fundamentally different at the genetic sequence level. These strains will be a useful complement to naturally occurring taxa with ESD in future research examining the molecular basis of and the selective forces driving evolutionary transitions between sex determination mechanisms. [ABSTRACT FROM AUTHOR]

1. Understanding individual and population responses to climate change is emerging as an important challenge. Because many phenotypic traits are sensitive to environmental conditions, directional climate change could significantly alter trait distribution within populations and may generate an evolutionary response. 2. In species with environment-dependent sex determination, climate change may lead to skewed sex ratios at hatching or birth. However, there are virtually no empirical data on the putative link between climatic parameters and sex ratios from natural populations. 3. We monitored a natural population of viviparous lizards with temperature-dependent sex determination ( Niveoscincus ocellatus) over seven field seasons. Sex ratios at birth fluctuated significantly among years and closely tracked thermal conditions in the field, with the proportion of male offspring increasing in colder years. 4. This is the first study to demonstrate the effect of local climatic conditions (e.g. temperature) on offspring sex ratio fluctuations in a free-living population of a viviparous ectotherm. A succession of warmer-than-usual years (as predicted under many climate-change scenarios) likely would generate female-biased sex ratios at birth, while an increase in interannual variation (as also predicted under climate change scenarios) could lead to significant fluctuations in cohort sex ratios. If cohort sex ratio bias at birth leads to adult sex ratio bias, long-term directional changes in thermal conditions may have important effects on population dynamics in this species. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Maternal and environmental factors are important sources of phenotypic variation because both factors influence offspring traits in ways that impact offspring and maternal fitness. The present study explored the effects of maternal factors (maternal body size, egg size, yolk-steroid allocation, and oviposition-site choice) and seasonally-variable environmental factors on offspring phenotypes and sex ratios in a multi-clutching lizard with environmental sex determination ( Amphibolurus muricatus). Maternal identity had strong effects on offspring morphology, but the nature of maternal effects differed among successive clutches produced by females throughout the reproductive season (i.e. maternal identity by environment interactions). The among-female and among-clutch variation in offspring traits (including sex ratios) was not mediated through maternal body size, egg size, or variation in yolk steroid hormones. This lack of nongenetic maternal effects suggests that phenotypic variation may be generated by gene by environment interactions. These results demonstrate a significant genetic component to variation in offspring phenotypes, including sex ratios, even in species with environmental sex determination. © 2008 The Linnean Society of London, Biological Journal of the Linnean Society, 2008, 95, 256–266. [ABSTRACT FROM AUTHOR]

The developmental environment can have lasting effects on posthatching phenotype in oviparous animals. Innate immune response is one important component of fitness in vertebrates because it provides a generalized defense against infection. In addition, because male vertebrates are at a higher risk of infection than females, males may benefit more from increased innate immunity than females. We determined the effects of incubation temperature on the innate immune response of hatchling map turtles (Grapternys) by incubating eggs at a range of male and female producing-temperatures and assessing plasma complement activity in the resulting hatchlings. We found a significant effect of incubation environment on circulating complement in hatchling Graptemys ouachitensis, with male-producing temperatures yielding the highest innate immune response. Most important, these results demonstrate that immune response is affected by developmental environment in a species with environmental sex determination, potentially resulting in sex differences in the ability to fend off pathogens. [ABSTRACT FROM AUTHOR]

The author focuses on the impact of the sex determining mechanism on sex ratio and its interaction with genetic and environmental factors. He points out that a stable sex ratio will evolve and be maintained regardless of whether sex chromosomes alone determine sex. Additionally, he explains that sex chromosomes evolution and genetics argues against coexistence.

Ecological genomics is the study of adaptation of natural populations to their environment, and therefore seeks to link organism and population level processes through an understanding of genome organization and function. The planktonic microcrustacean Daphnia, which has long been an important system for ecology, is now being used as a genomic model as well. Here we review recent progress in selected areas of Daphnia genomics research. Production of parthenogenetic male offspring occurs through environmental cues, which clearly involves endocrine regulation and has also been studied as a toxicological response to juvenoid hormone analog insecticides. Recent progress has uncovered a putative juvenoid cis-response element, which together with microarray analysis will stimulate further research into nuclear hormone receptors and their associated transcriptional regulatory networks. Ecotoxicological studies indicate that mRNA profiling is a sensitive and specific research tool with promising applications in environmental monitoring and for uncovering conserved cellular processes. Rapid progress is expected to continue in these and other areas, as genomic tools for Daphnia become widely available to investigators.Heredity (2008) 100, 184–190; doi:10.1038/sj.hdy.6800999; published online 23 May 2007 [ABSTRACT FROM AUTHOR]

Many species of fish produced in aquaculture or for the ornamental fish trade exhibit sexual dimorphism in growth, age at maturity, secondary sexual characters, or other traits of interest. This has led to a desire to produce populations of only one sex for commercial ongrowing. Although direct sex reversal via manipulation of sex differentiation is used commercially (e.g., in tilapia aquaculture), in most cases there is a need to understand the sex determination system to some extent and manipulate this to produce monosex fish. Sex determination is the genetic or environmental process that establishes the sex (gender) of an organism, whereas sex differentiation is the process by which an undifferentiated gonad is transformed into an ovary or a testis. Fish are the most diverse group of vertebrates in terms of sex determination, and the number of fish species of interest to aquaculture keeps increasing. Together, these aspects explain the growing interest to understand how sex determination and differentiation produce the sex ratio. This review concentrates on recent research using the tools of molecular biology to broaden our understanding of the different aspects related to fish sex determination, both in model fish species and in species of commercial importance. [ABSTRACT FROM AUTHOR]

Sex determination in many fish species can be influenced by environmental variables such as temperature, salinity, and/or pH. For example, Japanese flounder (Paralichthys olivaceus) has male heterogametic sex determination (XX female and XY male), but XX juveniles can develop to become functional males depending upon environmental conditions. When environmental variables reverse a genetically determined sex, it is called sex reversal. If such a species is involved in a stocking program, the sex reversal can occur in the hatchery-reared stock, and there is a risk that the gene for sex determination may be exterminated. We have evaluated the conditions for the extermination of the sex determination gene for Japanese flounder and discussed how we can reduce such a risk. In this article, we extend the analysis to more general situations with both one-locus two-allele and environmental sex determination. Consequently, we generalize the result of the previous paper and conclude that using wild individuals as parent fish for stock enhancement (i.e., as broodstock) would be effective in reducing the genetic risk of stocking fish subject to environmentally influenced sex determination. [ABSTRACT FROM AUTHOR]

Background: Understanding of sex ratio dynamics in a given species requires understanding its sex determination system, as well as access for reliable tools for sex identification at different life stages. As in the case of many other amphibians, the common frogs (Rana temporaria) do not have well differentiated sex chromosomes, and an identification of individuals' genetic sex may be complicated by sex reversals. Here, we report results of studies shedding light on the sex determination system and sex ratio variation in this species. Results: A microsatellite locus RtSB03 was found to be sex-linked in four geographically disparate populations, suggesting male heterogamy in common frogs. However, in three other populations examined, no or little evidence for sex-linkage was detected suggesting either ongoing/recent recombination events, and/or frequent sex-reversals. Comparison of inheritance patterns of alleles in RtSB03 and phenotypic sex within sibships revealed a mixed evidence for sex-linkage: all individuals with male phenotype carried a male specific allele in one population, whereas results were more mixed in another population. Conclusion: These results make sense only if we assume that the RtSB03 locus is linked to male sex determination factor in some, but not in all common frog populations, and if phenotypic sex-reversals -- for which there is earlier evidence from this species -- are frequently occurring. [ABSTRACT FROM AUTHOR]

Sex determination in fishes is often enigmatic, a situation that is often made even more complex by the fact that the process of sexual differentiation in many species may be influenced by environmental conditions. This situation is typified in zebrafish, a popular model organism. Despite the vast array of information available for the species, the genetic controls of sex are unknown. Further, environmental parameters, such as rearing densities, seem to exert an influence on the sex ratios of captive stocks. In an effort to dissect the genetic and environmental controls underlying the expression of sex in this species, we manipulated growth of pure-bred and out-crossed zebrafish by varying their food supply during development. Faster-growing zebrafish were more likely to be female than siblings that were fed less, and out-crossed broods had higher proportions of females than broods from pure-bred crosses. The dependence of sex ratio on feeding rate is readily understood in terms of adaptive sex allocation: zebrafish life history seems to confer the greater pay-off for large size on females. A similar male/female difference in the pay-off for hybrid vigor could similarly account for the female bias of out-crossed broods—and it could be a manifestation of Haldane’s rule. [ABSTRACT FROM AUTHOR]

Temperature-dependent sex determination (TSD) occurs in all major reptile lineages, but the selective forces and physiological mechanisms that link sex to incubation temperature may differ among and within those groups. Different models for TSD evolution make different predictions about when offspring sex will respond to environmental cues. Although TSD has evolved in several lizard lineages, there is less detailed information on these taxa than in turtles and crocodilians with TSD. We incubated eggs of an agamid lizard (AmphiboIurus muricatus) and a scincid lizard (Bassiana duperreyi), two species with TSD. Rather than manipulate incubation temperature to identify periods of sexual lability (as in most previous studies of this topic), we topically applied the aromatase inhibitor fadrozole to eggs at a variety of times through the incubation period. Fadrozole application sex-reversed the resultant hatchlings if applied from the time of oviposition until at least 60% of the way through incubation. In all of the TSD lizard species studied so far, offspring sex is determined either while the eggs are held inside the mother's body or soon after oviposition, providing substantial maternal control over incubation temperatures at this critical period. Hence, the hypothesis that TSD evolves because it enables offspring sex to be matched to conditions that are unpredictable at the time of laying is less likely to apply to squamates than to turtles, sphenodontians, and (especially) crocodiles, in which the period of sexual lability is delayed until long after oviposition. [ABSTRACT FROM AUTHOR]

Although variation in population sex ratios is predicted to increase the extinction rate of clades with environmental sex determination (ESD), ESD is still seen in a wide array of natural systems. It is unclear how this common sex-determining system has persisted despite this inherent disadvantage associated with ESD. We use simulation modelling to examine the effect of the sex ratio variance caused by ESD on population colonization and establishment. We find that an accelerating function of establishment success on initial population sex ratio favours a system that produces variance in sex ratios over one that consistently produces even sex ratios. This sex ratio variance causes ESD to be favoured over genetic sex determination, even when the mean global sex ratio under both sex-determining systems is the same. Data from ESD populations suggest that the increase in population establishment can more than offset the increased risk of extinction associated with temporal fluctuations in the sex ratio. These findings demonstrate that selection in natural systems can favour increased variance in a trait, irrespective of the mean trait value. Our results indicate that sex ratio variation may provide an advantage to species with ESD, and may help explain the widespread existence of this sex-determining system. [ABSTRACT FROM AUTHOR]

Changes in the numbers and size-class structure of European silver eels, Anguilla anguilla, in the River Frémur (France) were examined over a 9-year period after installation of downstream eel passes. The number of silver eels migrating downstream peaked in 1999, then decreased strongly and steadily after 2000, reaching relatively low levels. At the same time, a gradual shift in the silver eel sex ratio from a dominance of males (size from 270 to 442 mm, age from 3 to 6 years) to females (size from 366 to 1112 mm, age from 4 to 9 years) was recorded. Possible explanations for the escapement patterns observed are environmental sex determination and the installation of eel passes on the main hydraulic engineering structures in 1992 and 1996. [ABSTRACT FROM AUTHOR]

The sex ratios of the offspring of males from broods of honmoroko Gnathopogon caerulescens that displayed thermolabile sex determination (TSD), and those from eggs collected from the wild, strongly suggest that the combination of parents plays an important supplementary role in TSD in this species. The proportion of females in all broods from eight pairings of fish captured in the field decreased significantly at 30°C compared to 20°C, and one brood was entirely female at the lower temperature. These results suggest that phenotypic males (XX-males) exist in nature, probably as a result of sex change from genetic females caused by TSD. Sex ratios in relation to water temperature fall into five patterns, two of which seem to result from normal pairings of XX-females and XY- males, and from pairings of XX-males and XX-females with non-thermosensitivity. Two other patterns are thought to result from the same types of pairing, but with thermosensitivity. The last pattern, which shows male bias at both low and high temperature, is not explained only by the combination of genotypic sex determination (XX/XY) and TSD. Fluctuating temperatures close to natural conditions showed little potential to masculinize broods laid in the field between April and early June. However, the sex ratios of fish spawned in early June showed male bias at low and high temperatures. The sex determination mechanism in G. caerulescens may involve the interaction between temperature and sex-determining genetic factors, and the relative importance of each component differs with breeding season. [ABSTRACT FROM AUTHOR]

Because pollen disperses and ovules do not, a basic difference in dispersal abilities of male and female gametes exists in plants. With an analytical model, we show that the combination of such sex-biased dispersal of gametes and variation of habitat quality results in two opposite selective forces acting on the evolution of sex allocation in plants: (i) a plant should overproduce pollen in good patches and overproduce ovules in poor patches in order to equilibrate secondary sex ratios of gametes after pollen dispersal; (ii) a plant should overproduce ovules in good patches and overproduce pollen in poor patches in order to increase the likelihood that its progeny establishes in good patches. Our theoretical results indicate that the evolution of habitat-dependent sex allocation should be favoured in plants, in a direction that depends on the relative dispersal ability of pollen and seeds. We also show that superficially similar predictions obtained for habitat-dependent evolutionarily stable sex allocation in animals actually result from a completely different balance between the two underlying evolutionary forces. [ABSTRACT FROM AUTHOR]

We compiled information from the literature on the taxonomic distributions in extant teleost fishes of alternative sex-determination systems: male-heterogametic (XY) gonochorism, female-heterogametic (ZW) gonochorism, hermaphroditism, unisexuality, and environmental dependency. Then, using recently published molecular phylogenies based on whole-genomic or partial mitochondrial DNA sequences, we inferred the histories and evolutionary transitions between these reproductive modes by employing maximum parsimony and maximum likelihood methods of phylogenetic character mapping. Across a broad teleost phylogeny involving 25 taxonomic orders, a highly patchy distribution of different sex-determination mechanisms was uncovered, implying numerous transitions between alternative modes, but this heterogeneity also precluded definitive statements about ancestral states for most clades. Closer inspection of family-level and genus-level phylogenies within each of four orders further bolstered the conclusion that shifts in sex-determining modes are evolutionarily frequent and involve a variety of distinct ancestral-descendant pathways. For possible reasons discussed herein, the evolutionary lability of sex-determining modes in fishes contrasts strikingly with the evolutionary conservatism of sex determination within both mammals and birds. © 2006 The Linnean Society of London, Biological Journal of the Linnean Society, 2006, 87, 83–93. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Charnov and Bull suggested that environmentally induced variation in adult body size coupled with sex-specific differences in fitness can select for the evolution of adaptive environmental sex determination (ESD). In this study we determine whether larval density affects sex determination in the copepod Tigriopus californicus (Baker, 1912), as predicted by Charnov and Bull. Individuals reared at low densities developed faster and were significantly larger than siblings reared at high densities. For these laboratory-reared individuals, sexual selection on male body size was stronger than fecundity selection on female body size, but this sex-specific pattern of selection was reversed in the field. Differences in food availability (for females) and the mode of competition (for males) may account for the conflicting results between laboratory and field. We found a weak effect of larval density on sex determination in a pilot experiment but no effect in a second, more powerful experiment. While larval density did not affect the sex ratio of T. californicus, our sex-specific estimates of selection on adult body size will inform future models of adaptive ESD in this species and other copepods. [ABSTRACT FROM AUTHOR]

Genetic and environmental factors may interact to control sex determination in fishes. A common pattern of initial female differentiation and subsequent male transformation 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 same-sex 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. [ABSTRACT FROM AUTHOR]

Abstract: Heterosaccus lunatus parasitizes the portunid crab, Charybdis callianassa, in Moreton bay, Queensland, Australia. With the host crabs maintained at 25 °C this sacculinid rhizocephalan released larval broods every 5–6 days. This study examines the effect of photoperiod on the sex proportions of successive larval broods of individual parasites. Parasitized host crabs were maintained individually in circulating 25 °C seawater within two light–tight boxes, each with a controlled light–dark cycle operating. Box A had an LD 16:8 cycle (summer) and Box B an LD 8:16 cycle (winter). Both boxes had five host crabs and when parasites released their larval broods the larvae were flushed out and retained in a filter. Each captured brood then had its larvae sized to determine the proportions of the two sexes present. H. lunatus is ideal for such study because the sex(es) of all the larval stages (four naupliar stages and the cyprid) can be unequivocally identified by size (length). The experiment was run over a 6-week period during March and April 2004 when a total of 86 larval broods were released and their sex proportion(s) determined. It is concluded that larval sex determination is effected by photoperiod, with females becoming dominant under the winter condition (LD 8:16) and males dominant under the summer condition (LD 16:8). All initial broods were male dominated, so for Box A broods the male preponderance was maintained, whilst for Box B broods there was the progressive changeover to female preponderance. Such environmental sex determination is unusual because it is oogenesis which is being controlled, whereby two ovum sizes can be produced, either singly or together, the larger being the male egg. Further work is now necessary to verify the likely hormonal control processes involved. [Copyright &y& Elsevier]

Theory suggests that maternal effects are especially important in organisms with environmentally-sensitive sex-determining mechanisms. However, there is no substantive body of empirical evidence to confirm this conjecture. We integrated field and laboratory studies to jointly evaluate the significance of behavioral (nest-site choice) and physiological (yolk hormone allocation) maternal effects on offspring sex ratio in the common snapping turtle (Chelydra serpentina), a species with temperature-dependent sex determination (TSD). Of the 16 microhabitat variables measured, only three (south, east, and total overstory vegetation cover) were significantly correlated with nest temperature: cooler nests were located under more vegetation cover. In turn, these microhabitat predictors of nest temperature, and nest temperature itself, may influence nest sex ratio: shadier, cooler nests were more likely to produce a higher proportion of male offspring than less shady, warmer nests. Analysis of eggs from these same nests incubated in a common garden design in the laboratory revealed that clutch sex ratio was unaffected by levels of yolk estradiol, yolk testosterone, or their interaction. Examination of both behavioral and physiological maternal effects revealed no concordant impact on offspring sex ratio. However, eggs from nests that produced male-biased sex ratios in the field yielded higher proportions of males under constant-temperature conditions in the laboratory. Our study confirms the importance of behavioral maternal effects in nature on offspring sex ratios in species with TSD, while also revealing the potential presence of a predisposition for sex-ratio production underlying TSD in this system. [ABSTRACT FROM AUTHOR]

Despite the importance of maternal effects in evolution, and knowledge of links among nest site choice, timing of nesting, offspring sex, and reproductive success in animals with environmental sex determination, these attributes have not been rigorously studied in a combined and natural context. To address this need we studied the relationships between three maternal traits (nest site choice, lay date, and nest depth) and two fitness-related attributes of offspring (hatchling sex and embryonic survival) in the riverine turtle Carettochelys insculpta, a species with temperature-dependent sex determination, for four years. Predation and flooding were the major sources of embryonic mortality in 191 nests. Embryonic survival was influenced by both lay date and nest site choice: in one year when nesting began later than average, nests laid later and at lower elevations were destroyed by early wet season river rises. In other years early nesting precluded flood mortality. However, turtles did not nest at the highest available elevations, and a field experiment confirmed that turtles were constrained to nest at lower elevations where they could construct a nest chamber. The principal determinant of hatchling sex in 140 nests was lay date, which in turn was apparently related to the magnitude of the previous wet season(s). Clutches laid earlier in the season (a female's first clutch) produced mainly males, while later clutches (her second clutch) yielded mostly females, due to seasonal increases in air temperatures. Accordingly, later nesting produced female-biased hatchling sex ratios in 1996, while earlier nesting resulted in sex ratios near unity in the other years. However, all-female nests were more likely to be flooded than mixed-sex or all-male nests in years when nesting was late. In conclusion, we found evidence that the position of two maternal trait distributions (elevation of the nest site and lay date), associated with the reproductive strategy of C. insculpta, reflect a combination of natural selection, physical constraints, and phenotypic plasticity. © 2004 The Linnean Society of London, Biological Journal of the Linnean Society, 2004, 81, 1–16. [ABSTRACT FROM AUTHOR]

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 < 6 µmol m-2 s-1 . Addition of sugar to the culture medium, however, promotes male development, suggesting that the effect of increasing photon irradiance is not related to the accumulation of photosynthates. 3 According to evolutionary theory, environmental sex determination can cause population sex ratios to deviate from unity, and is favoured when the environment consists of patches that differentially affect fitness as a male or a female. However, in two out of three species, we found that the fitness of males increases faster than the fitness of females with increasing light levels, contrary to the predictions of the Charnov and Bull model. 4 We conclude that environmental sex determination in response to light cannot alone account for the female bias in natural populations of Equisetum gametophytes. [ABSTRACT FROM AUTHOR]

Focuses on a variety of documented environmental influences on sex determination in fishes. Behavioral sex determination; Temperature-dependent sex determination; Physiological mediation of teleost sex determination.

Horsetails (Equisetum, Sphenophyta) are homosporous, and sexual differentiation of Equisetum gametophytes is under the influence of environmental conditions. Still, the environmental cues responsible for sex determination of Equisetum gametophytes in vitro and in the wild have remained elusive. Here, we show that significantly different sex ratios are obtained when gametophytes are grown on media with or without sugar. In our experimental conditions, male gametophytes outnumber females in the presence of 60-120 mM sucrose and 120 mM glucose, whereas in the absence of sugar, most gametophytes differentiate as female. A similar effect is also observed on already differentiated female gametophytes, which become hermaphroditic sooner when cultured in the presence of sucrose in vitro. Interestingly, these results are reproducible within and across species representative of the two subgenera Equisetum and Hippochaete, indicating that the entire genus may share an identical pattern of environmental sex determination. [ABSTRACT FROM AUTHOR]

Eggs of wild caught Trachemys scripta and Graptemys pseudogeographica kohnii, two emydine turtles known to have temperature-dependent sex determination, were incubated at 28.5°C in boxes aerated with one of four gas mixtures (two in G. p. kohnii). Across all eggs, elevated levels of carbon dioxide significantly feminized sex ratios in both species and also reduced hatching success. When eggs were grouped into clutches, all comparisons between 0% and elevated carbon dioxide showed feminization in the portions of the clutches at elevated carbon dioxide, a statistically significant result for T. scripta. Accumulation of biogenic carbon dioxide from embryonic respiration and other sources is thus likely to affect sex ratios in natural nests under some conditions, perhaps through an effect on some aspects of embryonic pH. [ABSTRACT FROM AUTHOR]

When the environment varies spatially, so that some habitats are more favorable to reproduction than others, an individual should attempt to increase the number of offspring establishing in high-quality habitats. Hence, if male and female dispersal behavior differ, it may be adaptive to produce more offspring of the more dispersing sex in low-quality habitats, since these offspring are likely to disperse to another patch, and more offspring of the most philopatric sex in high-quality habitats, since these offspring are likely to remain in that patch. Such a strategy is shown to be evolutionarily stable provided that male and female dispersal rates are different and that reproductive success varies between habitats (lack of ideal free distribution). Highly biased sex ratios are predicted (1) in rare habitats, (2) in poor habitats, (3) when difference between habitat quality is large, (4) when at least one sex disperses at a rate close to random with respect to habitat availability, (5) when both sexes disperse at a high rate, (6) when individuals are unable to select their reproducing habitat, and, presumably, (7) with moderate temporal variation of habitat quality. The model appears to be a good candidate to explain the pattern of sex ratio variation in a variety of species : phytophagous arthropods, species with environmental sex determination, and territorial passerines. [ABSTRACT FROM PUBLISHER]