Absence of temperature-dependent sex determination in congeneric sexual and parthenogenetic Cnemidophorus lizards

Eggs from congeneric gonochoristic and parthenogenetic whiptail lizards were incubated at various temperatures. There was no significant deviation from a sex ratio of one-half in the sexual species at any temperature, whereas no males were produced in the parthenogenetic species. Temperature-dependent sex determination appears to be completely absent in these lizards.     

Phenotypic variation,sexual reproduction and evolutionary population dynamics

I studied the effects of introducing phenotypic variation into a well-known single species model for a population with discrete, non-overlapping generations. The phenotypes differed in their dynamic behaviour. The analysis was made under the assumption that the population was in an evolutionary stable state. Differences in the timing of the competitive impacts of the phenotypes on each other had a strong simplifying effect on the dynamics. This result could also be applied to competition between species. The effect of sexual reproduction on the dynamics of the population was analysed by assuming the simplest genetic model of one locus with two alleles. Sexual reproduction made the system much more stable in the (mathematical) sense that the number of attractors was reduced and their basins of attraction enlarged. In a dominant system sex tended to increase the frequency of the recessive allele, and in an overdominant system it induced gene frequencies of 1/2. Whether the attractors in the dominant system tended to be simpler or more complex than the attractors in the asexual system depended on the phenotype of the recessive homozygote. The overdominant sexual system tended to have simpler dynamics than the corresponding asexual population. A 2-locus model was used to study whether sexuals can invade an asexual population and vice versa. One locus coded for sexual and asexual reproduction, while the other coded for the dynamics. Enhanced stability through sexual reproduction seemed to be the reason why there was a clear asymmetry favouring sex in this evolutionary context.     

Gonadal sex differentiation in Alligator mississippiensis,a species with temperature-dependent sex determination

Temperature of egg incubation determines sex in Alligator mississippiensis hatchlings. To define the timing and morphology of sexual differentiation, alligator gonads were examined histologically and ultrastructurally throughout embryogenesis. At the male-producing temperature (33° C), the onset of testis differentiation occurred in most embryos during developmental stages 21–22, when a number of somatic cells in the medulla of the gonad became enlarged, forming presumptive Sertoli cells. Some enlarged somatic cells were also observed at the female-producing temperature (30° C) during gonadogenesis, but they were less widespread than at 33° C. Ovarian differentiation at 30° C began slighlty later, during stage 22–23, and was characterised by proliferation of germs cells in the cortex of the gonad. Testis formation in alligators may depend upon presumptive Sertoli cells differentiating prior to a critical event in embryogenesis, such as germ cell proliferation and meiosis. If follows that ovary formation occurs if this requirement is not met, as at lower incubation temperatures.     

Seasonal sex ratios and the evolution of temperature-dependent sex determination in oviparous lizards

Although the adaptive significance of temperature-dependent sex determination (TSD) remains a puzzle, recent models implicate a seasonal bias in offspring sex production that translates into sex-specific fitness benefits later in life. Sex-specific emergence has been linked to fitness gains in some fish, birds and reptiles, but field data supporting the occurrence of a seasonal pattern of sex ratios in oviparous lizards are lacking. We tested the hypothesis that patterns of nest site selection and seasonal temperature changes combine to inhibit the materialization of sex-biased hatching times in a population of water dragons (Intellagama lesueurii). As predicted, a seasonal increase in air and nest temperatures resulted in a sex bias by nesting date; male-producing clutches were laid 17.8 days sooner than female-producing clutches, on average. However, the seasonal ramping of nest temperatures also caused shorter relative incubation periods in the later, all-female clutches. As a consequence of this developmental ‘catch-up’, the mean hatching date for male-producing nests preceded the mean hatching date for female-producing nests by only 7.2 days. We suggest that a contracted distribution of hatching dates compared to the distribution of oviposition dates represents a general pattern for oviparous reptiles in seasonal climates, which in TSD species may largely offset the temporal disparity in nesting dates between the sexes. Although data are needed for other TSD species, such minor age differences between male and female hatchlings may not translate into fitness differences later in life, an assumption of some models for the evolution and maintenance of TSD.     

Regain of sex determination system and sexual reproduction ability in a synthetic octoploid male fish

Polyploids in vertebrates are generally associated with unisexual reproduction, but the direct consequences of polyploidy on sex determination system and reproduction mode remain unknown. Here, we synthesized a group of artificial octoploids between unisexual gynogenetic hexaploid Carassius gibelio and sexual tetraploid Carassius auratus. The synthetic octoploids were revealed to have more than 200 chromosomes, in which 50 chromosomes including the X/Y sex determination system were identified to transfer from sexual tetraploid C. auratus into the unisexual gynogenetic hexaploid C. gibelio. Significantly, a few synthetic octoploid males were found to be fertile, and one octoploid male was confirmed to regain sexual reproduction ability,which exhibits characteristics that are the same to sexual reproduction tetraploid males, such as 1:1 sex ratio occurrence, meiosis completion and euploid sperm formation in spermatogenesis, as well as normal embryo development and gene expression pattern during embryogenesis. Therefore, the current finding provides a unique case to explore the effect of sex determination system incorporation on reproduction mode transition from unisexual gynogenesis to sexual reproduction along with genome synthesis of recurrent polyploidy in vertebrates.     

Molecular players involved in temperature-dependent sex determination and sex differentiation in Teleost fish

The molecular mechanisms that underlie sex determination and differentiation are conserved and diversified. In fish species, temperature-dependent sex determination and differentiation seem to be ubiquitous and molecular players involved in these mechanisms may be conserved. Although how the ambient temperature transduces signals to the undifferentiated gonads remains to be elucidated, the genes downstream in the sex differentiation pathway are shared between sex-determining mechanisms. In this paper, we review recent advances on the molecular players that participate in the sex determination and differentiation in fish species, by putting emphasis on temperature-dependent sex determination and differentiation, which include temperature-dependent sex determination and genetic sex determination plus temperature effects. Application of temperature-dependent sex differentiation in farmed fish and the consequences of temperature-induced sex reversal are discussed.     

Linking community and ecosystem dynamics through spatial ecology

Classical approaches to food webs focus on patterns and processes occurring at the community level rather than at the broader ecosystem scale, and often ignore spatial aspects of the dynamics. However, recent research suggests that spatial processes influence both food web and ecosystem dynamics, and has led to the idea of 'metaecosystems'. However, these processes have been tackled separately by 'food web metacommunity' ecology, which focuses on the movement of traits, and 'landscape ecosystem' ecology, which focuses on the movement of materials among ecosystems. Here, we argue that this conceptual gap must be bridged to fully understand ecosystem dynamics because many natural cases demonstrate the existence of interactions between the movements of traits and materials. This unification of concepts can be achieved under the metaecosystem framework, and we present two models that highlight how this framework yields novel insights. We then discuss patches, limiting factors and spatial explicitness as key issues to advance metaecosystem theory. We point out future avenues for research on metaecosystem theory and their potential for application to biological conservation.     

Sexual reproduction and diversity: Connection between sexual selection and biological communities via population dynamics

Sexual reproduction is a mysterious phenomenon. Most animals and plants invest in sexual reproduction, even though it is more costly than asexual reproduction. Theoretical studies suggest that occasional or conditional use of sexual reproduction, involving facultative switching between sexual and asexual reproduction, is the optimal reproductive strategy. However, obligate sexual reproduction is common in nature. Recent studies suggest that the evolution of facultative sexual reproduction is prevented by males that coerce females into sexual fertilization; thus, sexual reproduction has the potential to enforce costs on a given species. Here, the effect of sex on biodiversity is explored by evaluating the reproductive costs arising from sex. Sex provides atypical selection pressure that favors traits that increase fertilization success, even at the expense of population growth rates, that is, sexual selection. The strength of sexual selection depends on the density of a given species. Sexual selection often causes strong negative effects on the population growth rates of species that occur at high density. Conversely, a species that reduces its density is released from this negative effect, and so increases its growth rate. Thus, this negative density-dependent effect on population growth that arises from sexual selection could be used to rescue endangered species from extinction, prevent the overgrowth of common species and promote the coexistence of competitive species. Recent publications on sexual reproduction provide several predictions related to the evolution of reproductive strategies, which is an important step toward integrating evolutionary dynamics, demographic dynamics and community dynamics.     

Nest-site selection in two eublepharid gecko species with temperature-dependent sex determination and one with genotypic sex determination

At present, most turtles, all crocodilians, and several lizards are known to have temperature-dependent sex determination (TSD). Due to the dependence of sex determination on incubation temperature, the long-term survival of TSD species may be jeopardized by global climate changes. The current study was designed to assess the degree to which this concern is justified by examining nest-site selection in two species of Pattern II TSD geckos (Eublepharis macularius and Hemitheconyx caudicinctus) and comparing these preferences with those of a species with genotypic sex determination (GSD) (Coleonyx mitratus). Temperature preferences for nest sites were found to be both species-specific and female-specific. While H. caudicinctus females selected a mean nest-site temperature (32.4°) very close to the upper pivotal temperature (32°C) for the species, E. macularius females selected a mean nest-site temperature (28.7°C) well below this species' lower pivotal temperature (30.5°C). Thus, the resultant sex ratios are expected to differ between these two TSD species. Additionally, nest-site temperatures for the GSD species were significantly more variable (SE=+0.37) than were temperatures for either of the TSD species (E. macularius SE=±0.10; H. caudicinctus SE =+ 0.17), diereby further demonstrating temperature preferences within the TSD species.     

Analyzing the coordinated gene network underlying temperature-dependent sex determination in reptiles

Although gonadogenesis has been extensively studied in vertebrates with genetic sex determination, investigations at the molecular level in nontraditional model organisms with temperature-dependent sex determination are relatively new areas of research. Results show that while the key players of the molecular network underlying gonad development appear to be retained, their functions range from conserved to novel roles. In this review, we summarize experiments investigating candidate molecular players underlying temperature-dependent sex determination. We discuss some of the problems encountered unraveling this network, pose potential solutions, and suggest rewarding future directions of research.     

Sexual reproduction and sex determination in green algae

The sexual reproductive processes of some representative freshwater green algae are reviewed. Chlamydomonas reinhardtii is a unicellular volvocine alga having two mating types: mating type plus (mt⁺) and mating type minus (mt^?), which are controlled by a single, complex mating-type locus. Sexual adhesion between the gametes is mediated by sex-specific agglutinin molecules on their flagellar membranes. Cell fusion is initiated by an adhesive interaction between the mt⁺ and mt^? mating structures, followed by localized membrane fusion. The loci of sex-limited genes and the conformation of sex-determining regions have been rearranged during the evolution of volvocine algae; however, the essential function of the sex-determining genes of the isogamous unicellular Chlamydomonas reinhardtii is conserved in the multicellular oogamous Volvox carteri. The sexual reproduction of the unicellular charophycean alga, Closterium peracerosum-strigosum-littorale complex, is also focused on here. The sexual reproductive processes of heterothallic strains are controlled by two multifunctional sex pheromones, PR-IP and PR-IP Inducer, which independently promote multiple steps in conjugation at the appropriate times through different induction mechanisms. The molecules involved in sexual reproduction and sex determination have also been characterized.     

Influence of spatial structure on the maintenance of sexual reproduction

We include spatial extension into a model for the maintenance of sexual reproduction introduced recently. The model is based on a broad spectrum of resources, which regrow slowly. Other key features of the model are that sexual reproduction sets in when resources become scarce and that only a few genotypes can coexist locally. The extension of the model to several patches in space is done in two different ways. Model A is based on central egg deposition and allows migration of juveniles into all patches. Model B has a one-dimensional array of patches with migration only between neighboring patches. The main findings are that wide dispersal favors asexuals, while for slower migration there is a wide range of parameter values for which sexually reproducing species always win against asexuals. These results are conform with major patterns for the distribution of parthenogenesis in animals and plants, i.e. the prevalence of parthenogenetic reproduction in minute species, which are easily dispersed by physical forces, such as protists and small metazoans including e.g. bdelloid rotifers, tardigrades and nematodes.     

An epigenetic regulatory switch controlling temperature-dependent sex determination in vertebrates

正Most vertebrates present two sexes,and females and males are determined via two diverse strategies including genotypic sex determination(GSD)and environmental sex determination(ESD)(Mei and Gui,2015;Ma et al.,2016).The most common form of ESD is temperature-dependent sex determination(TSD).Although several master sex-determining genes and their molecular pathways have been elucidated in vertebrates with GSD,the molecular mechanism underlying TSD remains unclear(Bachtrog et al.,2014;     

Unexpected resilience of species with temperature-dependent sex determination at the Cretaceous-Palaeogene boundary

It has been suggested that climate change at the Cretaceous-Palaeogene (K-Pg) boundary, initiated by a bolide impact or volcanic eruptions, caused species with temperature-dependent sex determination (TSD), including dinosaurs, to go extinct because of a skewed sex ratio towards all males. To test this hypothesis, the sex-determining mechanisms (SDMs) of Cretaceous tetrapods of the Hell Creek Formation (Montana, USA) were inferred using parsimony optimizations of SDMs on a tree, including Hell Creek species and their extant relatives. Although the SDMs of non-avian dinosaurs could not be inferred, we were able to determine the SDMs of 62 species; 46 had genotypic sex determination (GSD) and 16 had TSD. The TSD hypothesis for extinctions performed poorly, predicting between 32 and 34 per cent of survivals and extinctions. Most surprisingly, of the 16 species with TSD, 14 of them survived into the Early Palaeocene. In contrast, 61 per cent of species with GSD went extinct. Possible explanations include minimal climate change at the K-Pg, or if climate change did occur, TSD species that survived had egg-laying behaviour that prevented the skewing of sex ratios, or had a sex ratio skewed towards female rather than male preponderance. Application of molecular clocks may allow the SDMs of non-avian dinosaurs to be inferred, which would be an important test of the pattern discovered here.     

Lymphoid aggregates in gonads of embryos, hatchlings, and young of turtles with temperature-dependent sex determination

Cellular infiltrations forming lymphoid-like aggregates were previously observed in gonads of two turtle species exhibiting temperature-dependent sex determination (TSD): at hatching in Chelydra serpentina; at and after hatching in Emys orbicularis. We show here that such aggregates are also present in gonads of Testudo graeca by the end of embryonic development, suggesting that their occurrence is general in turtles. Since in C. serpentina, infiltrations were observed mainly in testes exhibiting remnants of the germinal epithelium, it was assumed that their occurrence was an expression of maleness leading to rejection of this epithelium. The generality of this hypothesis was tested in E. orbicularis by looking for lymphoid-like aggregates in three types of gonads (testes, ovotestes, and ovaries) and for the stages at which they occur. Gonads were from embryos, hatchlings, and young incubated at various temperatures. Ovotestes obtained by treatment with an aromatase inhibitor of eggs incubated at female-producing temperature were also examined. In these gonads, the differentiation of Sertoli cells in testicular cords/tubes was ascertained by expression of SOX9. Moreover, the cell composition of aggregates was determined on electron micrographs. Aggregates appear in ovaries and ovotestes by the end of embryonic development and are present in the majority of these gonads at hatching, and at least up to one year after hatching. They are composed mainly of lymphocytes and fibroblasts. Aggregates are not present in typical testes. Since they occur in most ovaries, they cannot be seen as an expression of maleness. Rather, lymphocytic infiltration and formation of lymphoid aggregates in turtle gonads can be seen as components of the immune system, and can be under the control of gonadal endogenous sex steroids.