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The cladoceran crustacean Daphnia exhibits phenotypic plasticity, a phenomenon that leads to diverse phenotypes from one genome. Alternative usage of gene isoforms has been considered a key gene regulation mechanism for controlling different phenotypes. However, to understand the phenotypic plasticity of Daphnia, gene isoforms have not been comprehensively analyzed. Here we identified 25,654 transcripts derived from the 9710 genes expressed during environmental sex determination of Daphnia magna using the long-read RNA-Seq with PacBio Iso-Seq. We found that 14,924 transcripts were previously unidentified and 5713 genes produced two or more isoforms. By a combination of Illumina short-read RNA-Seq, we detected 824 genes that implemented switching of the highest expressed isoform between females and males. Among the 824 genes, we found isoform switching of an ortholog of CREB-regulated transcription coactivator, a major regulator of carbohydrate metabolism in animals, and a correlation of this switching event with the sexually dimorphic expression of carbohydrate metabolic genes. These results suggest that a comprehensive catalog of isoforms may lead to understanding the molecular basis for environmental sex determination of Daphnia. We also infer the applicability of the full-length isoform analyses to the elucidation of phenotypic plasticity in Daphnia. [ABSTRACT FROM AUTHOR]

We implement a newly developed framework, expressed as a mathematical model that we solve numerically, for understanding environmental sex determination in populations with consistent trends in abundance. Though broadly applicable, the analysis here focuses on the steadily declining North Atlantic eel populations. This enables us to show how the eco-evolutionary dynamics of eels reflect sex-specific and habitat-specific relationships among demographic features. When increasing stress levels resulting from Human-Induced Rapid Environmental Change are imposed on these populations, they become increasingly vulnerable to highly biased sex ratios. Our analysis is both prescriptive (identifying the key components and the priorities for deepening our understanding of them) and predictive (indicating expected qualitative patterns to be tested in future work). Priorities include establishing sex ratios and age/sex structure by habitat, measuring the magnitude and effects of social and individual stress levels, estimating effects of fishing pressure, and investigating reproduction in the Sargasso Sea. Key predictions address expected biases and trends in sex ratios, habitats more likely to be population sources (e.g. river basins) or sinks (e.g. estuary), expected trends in bimaturism (sex-specific maturation times), implications of the relative speed of adaptation and of environmental deterioration, and fitness responses to stress levels, early-stage mortality, and female demographic parameters. [ABSTRACT FROM AUTHOR]

Freshwater eels are of tropical marine origin and have spread worldwide. Of the 19 recognised species or subspecies, all 6 temperate species are listed within IUCN Threatened Species categories, together with 7 tropical species. Many reasons have been advanced to explain the significant declines of eels including habitat fragmentation and loss, spread of parasites and diseases, pollution, impacts of hydro dams, overfishing and oceanic changes. The present paper reviews some of the unique life history characteristics of Anguillids—fecundity and semelparous spawning, adaptability, resilience, and being energetically conservative—but despite such remarkable adaptations, according to the IUCN the genus is facing possible extinction. It is suggested that Anguillid life history strategies rely on two additional components—environmental determination of sex, and the need for surplus production (the Surplus Production Hypothesis, SPH). Surplus production is required to counteract the many hazards that eels face throughout an often long and complex life history. Recruitment of temperate species has fallen by over 90%, and resulting lower densities of juvenile eels in tidal and lower river reaches provides less incentive for upstream migration and dispersal. Female eels are generally found further inland than males, with development of sex associated with lower densities. With fewer juveniles migrating to inland areas, the numbers of females will become reduced, and further compound the reduced spawner biomass and consequent reduced recruitment of glass eels. The SPH hypothesis emphasises the importance of conservative harvest of glass eels, habitat connectivity, the need to ensure maximum female escapement, and the importance of stocking inland waterways with juvenile eels. [ABSTRACT FROM AUTHOR]

Sex determination mechanisms evolve surprisingly rapidly, yet little is known in the large nematode phylum other than for Caenorhabditis elegans, which relies on chromosomal XX-XO sex determination and a dosage compensation mechanism. Here we analyze by sex-specific genome sequencing and genetic analysis sex determination in two fungal feeding/plant-parasitic Bursaphelenchus nematodes and find that their sex differentiation is more likely triggered by random, epigenetic regulation than by more well-known mechanisms of chromosomal or environmental sex determination. There is no detectable difference in male and female chromosomes, nor any linkage to sexual phenotype. Moreover, the protein sets of these nematodes lack genes involved in X chromosome dosage counting or compensation. By contrast, our genetic screen for sex differentiation mutants identifies a Bursaphelenchus ortholog of tra-1, the major output of the C. elegans sex determination cascade. Nematode sex determination pathways might have evolved by "bottom-up" accretion from the most downstream regulator, tra-1. In most species, sex is determined by genetic or environmental factors. Here, the authors present evidence that sex determination in Bursaphelenchus nematodes is instead likely to be regulated by a random, epigenetic mechanism. [ABSTRACT FROM AUTHOR]

The medaka Oryzias latipes is a teleost fish with an XX/XY sex determination system similar to that of mammals. However, under high-temperature conditions, XX medaka are masculinized by an elevation of the key teleost glucocorticoid, cortisol. Cortisol inhibits female-type proliferation of germ cells and induces masculinization of XX medaka during gonadal sex differentiation. To identify masculinization mechanisms downstream of cortisol, we analysed the functions of gonadal soma-derived growth factor (gsdf) and anti-Müllerian hormone receptor type 2 (amhr2); these genes are known to play important roles in the inhibition of germ cell proliferation and male differentiation. We investigated the impact of gsdf and amhr2 on the proliferation of germ cells using gsdf knockout (KO) and amhr2 KO medaka. At hatching stage, loss of gsdf or amhr2 function recovered female-type proliferation in germ cells under cortisol treatment. Moreover, cortisol treatment of gsdf KO or amhr2 KO medaka did not induce masculinization of XX medaka. These results suggest that cortisol inhibits female-type proliferation of germ cells and induces masculinization of XX medaka through GSDF and AMHR2. This study thereby provides the first evidence that GSDF and AMHR2 are involved in cortisol-induced masculinization. [ABSTRACT FROM AUTHOR]

In mammals, including humans, males and females differ not only in physical appearance but in every cell of their body: male cells have a tiny Y-chromosome which females lack. Females instead have two X-chromosomes while males have only one. It is not universally true though, as majority of fishes, frogs, lizards and turtles have no sex chromosomes. Their sex is generally determined based on the environment (e.g. temperature) in which the eggs grow. Though, the Y-chromosome gene, Sry, triggers male development, it alone is not enough to differentiate the two sexes; orderly expression of a number of genes, generally present on the autosomes, is required to ensure differentiation of a specific gonad - testis or ovary. Individuals bearing testes become male while those having ovaries become female. Excepting the Y-chromosomal Sry, almost all other genes in this cascade are evolutionarily conserved throughout vertebrates. Mutually antagonistic interactions of the male and female pathway genes lead to the formation of the gonads that eventually determine the sex of the individual. Most disorders of sexual development occur due to mutations in any of these genes. [ABSTRACT FROM AUTHOR]

Key message: In sexually plastic Acer pensylvanicum, determination of sex can occur extremely late, within three weeks of spring flowering. Physical damage causing complete vascular tissue severance results in increased female expression.Abstract: Species with environmental sex determination are rare amongst angiosperms but widely distributed across taxa. The timing of floral development in species that change sex based on environmental cues is unexplored. We investigated the timing of differentiation of sexual organs in buds of Acer pensylvanicum, an understory tree in eastern North America with environmental sex determination. We collected branches from individuals at three collection times in the early spring of 2016 and kept them in a warm greenhouse until anthesis. All individuals exhibited complete or partial female inflorescences in the greenhouse in one or more collection. However, none of these same individuals produced only female flowers in the field. Unlike many other woody species that differentiate bud sexual primordia 9-12 months prior to flowering, A. pensylvanicum may differentiate the sexual organs in its flower buds as late as three weeks prior to anthesis. In a separate series of branch collections in 2017, we found that the stress response to cutting leads to increased female sex expression in branches, while earlier warm temperatures (e.g., those caused by growing in a protected greenhouse environment) or increased carbohydrate availability does not. Given the labile sex determination system of A. pensylvanicum, the ability to delay differentiation of buds into male or female until shortly before spring flowering would allow individual trees to respond to sex-determining damage cues as late as mid-spring. This supports the hypothesis that A. pensylvanicum may not exhibit the lag-time characteristic of temperate spring and early-summer flowering woody species and may change sex expression in response to stress. [ABSTRACT FROM AUTHOR]

Higher temperatures associated with climate change have the potential to significantly alter the population sex ratio of species with temperature-dependent sex determination. Whether or not elevated temperature affects sex determination depends on both the absolute temperature experienced and the stage of development at which the thermal conditions occur. We explored the importance of exposure timing during early development in the coral reef fish, Acanthochromis polyacanthus, by increasing water temperature 1.5 or 3 °C above the summer average (28.5 °C) at different stages of development. We also measured the effect of treatment temperature on fish size and condition, in order to gauge how the thermal threshold for sex-ratio bias may compare with other commonly considered physiological metrics. Increasing grow-out temperature from 28.5 to 30 °C had no effect on the sex ratio of offspring, whereas an increase to 31.5 °C (+3 °C) produced a strong male bias (average ~90%). The thermosensitive period for this species lasted between 25 and 60 d post hatching, with the bias in sex ratio greater the earlier that fish experienced warm conditions. Temperatures high enough to bias the sex ratio are likely to be seen first during late summer (January and February) and would affect clutches produced late in the breeding season. There was no change to fish condition in response to temperature; however, the two higher temperature treatments produced significantly smaller fish at sampling. Clutches produced early in the season could buffer the population from a skewed sex ratio, as their development will remain below the thermal threshold; however, continued ocean warming could mean that clutches produced earlier in the breeding season would also be affected in the longer term. A skewed sex ratio could be detrimental to population viability by reducing the number of females in the breeding population. [ABSTRACT FROM AUTHOR]

In a heterogeneous environment, when the fitness of males and females are differently influenced by habitat quality, habitat-dependent sex ratios may evolve to favor the production of the sex that benefits more (or loses less) from the local habitat. Similarly, sex-biased dispersal favors the evolution of habitat-dependent sex ratios. The present study documents the convergence stable sex ratios expected in the presence of sex-specific fitness gains when dispersal is partial, sex-biased or costly, using a simple model with patches of two qualities. Results show that partial dispersal reduces the sex ratio bias expected with sex-specific fitness gains. The direction of the sex ratio bias can be reversed by sex-biased dispersal or the existence of sex-specific dispersal costs, provided that fitness gains for the two sexes are not too different. The reversal of the sex ratio bias is more readily observed when sex-specific dispersal rates are opposite and extreme. Both dispersal and fitness gains, especially when they are sex-specific, should thus be considered when making predictions about sex ratio evolution in a heterogeneous environment. [ABSTRACT FROM AUTHOR]

Labile sex expression is considered to play a key role in the evolution of breeding systems and in the transition from hermaphroditism to dioecy, according to the evolutionary models proposed for plants. While in hermaphrodites sex allocation within the individual can be plastically adjusted in response to social environment, in dioecious species it is predicted to be fixed. However, labile sex expression in the form of gender plasticity can still be present in dioecious species of animals with environmental sex determination. It is still unclear how gender plasticity is involved in the evolution of breeding systems and what its role is in the transition from hermaphroditism to dioecy. We assessed the degree of plasticity in gender expression in three dioecious species of polychaete worms of the genus Ophryotrocha. We found sexual polymorphism and plasticity in sex expression during the juvenile phase to be a response to social environment. The majority of juveniles reared with an adult female or male expressed the gender opposite of that of the partner, so as to form heterosexual pairs. On the basis of these findings we outline a possible evolutionary pathway of the transition from hermaphroditism to dioecy in the genus Ophryotrocha. [ABSTRACT FROM AUTHOR]

The causes of sex determination in amphipods are believed to be multi-factorial. Sex determination in Echinogammarus marinus (Leach, 1815) has been reported to be linked with feminising parasites. To date, however, no such studies have linked this species with environmental sex determination (ESD). A field study and laboratory breeding experiments were conducted to determine the influence of photoperiod on sex determination. Over the two-year field study, males dominated during August to November, whilst female-biased populations were observed during April to July. A significant linear relationship was observed between photoperiods and sex ratios from the field data. Under laboratory conditions, photoperiod was also shown to be an influential factor in sex determination, with a male bias over a long-day photo regime (61.5 % male broods) and a female biased over a short-day photoperiod regime (43.5 % male broods). Findings suggest that there is some level of ESD present within the species, suggesting considerable plasticity in the sex differentiation pathway. [ABSTRACT FROM AUTHOR]

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 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]

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]

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]

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]

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]

Mercurialis annua is a ruderal of pan-European distribution. In southern Spain, southern Portugal and northern Morocco, males coexist with monoecious (cosex) individuals at frequencies which vary between zero and about 30 per cent. Here, I report the results of two glasshouse experiments which aimed to characterize the mode of sex determination in one such androdioecious population. In a breeding experiment, cosexes isolated from males produced only cosexual progeny, whereas those allowed to mate with males produced both cosexual and male offspring. The results are consistent with a single-locus model of sex inheritance, with maleness determined by a dominant allele. In a density experiment, the frequency of males correlated positively with density, confirming field observations. Differential mortality or germination between the sexes was excluded by experimental design, so that the result supports the hypothesis of density-dependent gender choice. A negative correlation between density and the proportion of 'late cosexes' (males which turned cosexual late in their development), and the lack of any differences in the proportion of pure (unchanged) cosexes between density, suggest that only males were capable of sex change. This interpretation is consistent with the results of the breeding experiment, in which late cosexes occurred only in the offspring of cosexes allowed to mate with males. I argue that these findings help to explain the maintenance of androdioecy in M. annua. [ABSTRACT FROM AUTHOR]

An adaptive explanation for environmental sex determination is that it promotes sexual size dimorphism when larger size benefits one sex more than the other. That is, if growth rates are determined by environment during development, then it is beneficial to match developmental environment to the sex that benefits more from larger size. However, larger size may also be a consequence of larger size at hatching or growing for a longer time, i.e., delayed age at first reproduction. Therefore, the adaptive significance of sexual size dimorphism and environmental sex determination can only be interpreted within the context of both growth and maturation. In addition, in those animals that continue to grow after maturation, sexual size dimorphism at age of first reproduction could differ from sexual size dimorphism at later ages as growth competes for energy with reproduction and maintenance. I compared growth using annuli on carapace scales in two species of box turtles ( Terrapene carolina and T. ornata) that have similar patterns of environmental sex determination but, reportedly, have different patterns of sexual size dimorphism. In the populations I studied, sexual size dimorphism was in the same direction in both species; adult females were, on average, larger than adult males. This was due in part to males maturing earlier and therefore at smaller sizes than females. In spite of similar patterns of environmental sex determination, patterns of growth differed between the species. In T. carolina, males grew faster than females as juveniles but females had the larger asymptotic size. In T. ornata, males and females grew at similar rates and had similar asymptotic sizes. Sexual size dimorphism was greatest at maturation because, although males matured younger and smaller, they grew more as adults. There was, therefore, no consistent pattern of faster growth for females that may be ascribed to developmental temperature. [ABSTRACT FROM AUTHOR]

An introduction is presented to an article on the environmental and genotypic sex determination of the egg-laying Australian lizard Pogona vitticeps.

The reason for the frequent occurrence of environmental sex determination (ESD) in reptiles is still not well understood, although much effort has been devoted to solving the issue. Stimulated by the occurrence of nest-site philopatry in some species, this paper examines a diploid model of the influence of nest-site philopatry on the evolution of ESD. Analysis shows that nest-site philopatry can lead to ESD because the fitnesses of sons and daughters are hot influenced in the same way by nest-site quality. Daughters inherit the nest site and thus benefit more than sons from a high-quality nest site. Conversely, the fitness of daughters at low-quality nest sites is lower compared to the fitness of sons. Therefore, genes causing ESD can spread by causing the production of more sons at low-quality nest sites and more daughters at high-quality nest sites. Suggestions are made to test empirically whether nest-site philopatry led to the evolution of ESD. [ABSTRACT FROM AUTHOR]