Pattern does not equal process: exactly when is sex environmentally determined?

Effects of temperature and host stage on the parasitization rate and offspring sex ratio of Aenasius bambawalei Hayat in Phenacoccus solenopsis Tinsley

Temperature and host stage are important factors that determine the successful development of parasitoids. Aenasius bambawalei Hayat (Hymenoptera: Encyrtidae) is a primary parasitoid of the newly invasive mealybug, Phenacoccus solenopsis Tinsley (Hemiptera: Pseudococcidae). The effects of temperature on the parasitic characteristics of A. bambawalei have seldom been investigated. In the study, we explored the effects of temperature, exposure time, and host stage on the parasitization rate and offspring sex ratio (female to male) of A. bambawalei under laboratory conditions. The laboratory results showed that the successful parasitization rate of A. bambawalei increased with higher temperatures and older host stages. When the parasitoids were exposed to 36 °C for 24 h, the parasitization rate of female adults (52%) was nearly two times that of 3rd instar nymphs. Additionally, heat stress duration and host stage resulted in an increase in the offspring sex ratio of A. bambawalei. When A. bambawalei was exposed to 36 °C for 24 h, the offspring sex ratio increased dramatically to 81.78% compared with those exposed for 12 h, and it increased to 45.34% compared with those exposed for 16 h. The offspring sex ratio was clearly higher when the host stage was an adult female mealybug Our findings provide important guidance for the mass rearing and field releases of A. bambawalei for the management of P. solenopsis in the future.

Influence of water availability on gender determination of gametophytes in a diploid-polyploid complex of a xerophytic fern genus

Environmental sex determination (ESD) is present in several animal and plant lineages. Diverse factors such as temperature, light or water availability have been described as sex determinants in these organisms. Among plants, ferns frequently display ESD. This work compares the effect of different levels of water availability in two diploid species of the xerophytic fern genus Cheilanthes and in their derived tetraploid, and if they are sensitive to antheridiogen (i.e. maleness-inducing pheromone). Different watering regimes were applied to isolated gametophyte cultures of the three study species. Gametophyte survival, size, gender and sporophyte production were assessed after 13, 18 and 23 weeks of culture. Cultures combining spores and adult gametophytes were established to test the effect of antheridiogen. Isolated gametophytes had an asexual to female to bisexual sequence that did not depend upon the degree of soil moisture. Both gender expression and growth reduction in response to water scarcity of the allotetraploid were more similar to those of one of the diploid parents. In all watering regimes, survival was higher in the allotetraploid, suggesting hybrid vigour, whereas automixis rate was similar in the three species and reached ∼50 % at high moisture. This breeding system can ensure reproduction in the absence of males. In the three species, female gametophytes produced antheridiogens that enhanced maleness. This promotes a mixed mating system that could be favourable for ferns growing in xeric habitats.

Genetics of sexual development: an evolutionary playground for fish

Teleost fishes are the most species-rich clade of vertebrates and feature an overwhelming diversity of sex-determining mechanisms, classically grouped into environmental and genetic systems. Here, we review the recent findings in the field of sex determination in fish. In the past few years, several new master regulators of sex determination and other factors involved in sexual development have been discovered in teleosts. These data point toward a greater genetic plasticity in generating the male and female sex than previously appreciated and implicate novel gene pathways in the initial regulation of the sexual fate. Overall, it seems that sex determination in fish does not resort to a single genetic cascade but is rather regulated along a continuum of environmental and heritable factors.

Sex chromosomes and karyotype of the (nearly) mythical creature, the Gila monster, Heloderma suspectum (Squamata: Helodermatidae)

A wide variety of sex determination systems exist among squamate reptiles. They can therefore serve as an important model for studies of evolutionary transitions among particular sex determination systems. However, we still have only a limited knowledge of sex determination in certain important lineages of squamates. In this respect, one of the most understudied groups is the family Helodermatidae (Anguimorpha) encompassing the only two venomous species of lizards which are potentially lethal to human beings. We uncovered homomorphic ZZ/ZW sex chromosomes in the Gila monster (Heloderma suspectum) with a highly heterochromatic W chromosome. The sex chromosomes are morphologically similar to the ZZ/ZW sex chromosomes of monitor lizards (Varanidae). If the sex chromosomes of helodermatids and varanids are homologous, female heterogamety may be ancestral for the whole Anguimorpha group. Moreover, we found that the karyotype of the Gila monster consists of 2n = 36 chromosomes (14 larger metacentric chromosomes and 22 acrocentric microchromosomes). 2n = 36 is the widely distributed chromosomal number among squamates. In his pioneering works representing the only previous cytogenetic examination of the family Helodermatidae, Matthey reported the karyotype as 2n = 38 and suggested a different chromosomal morphology for this species. We believe that this was probably erroneously. We also discovered a strong accumulation of telomeric sequences on several pairs of microchromosomes in the Gila monster, which is a trait documented relatively rarely in vertebrates. These new data fill an important gap in our understanding of the sex determination and karyotype evolution of squamates.

The sex-specific transcriptome of the hermaphrodite sparid sharpsnout seabream (Diplodus puntazzo)

Background

Teleosts are characterized by a remarkable breadth of sexual mechanisms including various forms of hermaphroditism. Sparidae is a fish family exhibiting gonochorism or hermaphroditism even in closely related species. The sparid Diplodus puntazzo (sharpsnout seabream), exhibits rudimentary hermaphroditism characterized by intersexual immature gonads but single-sex mature ones. Apart from the intriguing reproductive biology, it is economically important with a continuously growing aquaculture in the Mediterranean Sea, but limited available genetic resources. Our aim was to characterize the expressed transcriptome of gonads and brains through RNA-Sequencing and explore the properties of genes that exhibit sex-biased expression profiles.

Results

Through RNA-Sequencing we obtained an assembled transcriptome of 82,331 loci. The expression analysis uncovered remarkable differences between male and female gonads, while male and female brains were almost identical. Focused search for known targets of sex determination and differentiation in vertebrates built the sex-specific expression profile of sharpsnout seabream. Finally, a thorough genetic marker discovery pipeline led to the retrieval of 85,189 SNPs and 29,076 microsatellites enriching the available genetic markers for this species.

Conclusions

We obtained a nearly complete source of transcriptomic sequence as well as marker information for sharpsnout seabream, laying the ground for understanding the complex process of sex differentiation of this economically valuable species. The genes involved include known candidates from other vertebrate species, suggesting a conservation of the toolkit between gonochorists and hermaphrodites.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-655) contains supplementary material, which is available to authorized users.

Early environment influences later performance in fishes

Conditions fish encounter during embryogenesis and early life history can leave lasting effects not only on morphology, but also on growth rate, life-history and behavioural traits. The ecology of offspring can be affected by conditions experienced by their parents and mother in particular. This review summarizes such early impacts and their ecological influences for a variety of teleost species, but with special reference to salmonids. Growth and adult body size, sex ratio, egg size, lifespan and tendency to migrate can all be affected by early influences. Mechanisms behind such phenotypically plastic impacts are not well known, but epigenetic change appears to be one central mechanism. The thermal regime during development and incubation is particularly important, but also early food consumption and intraspecific density can all be responsible for later life-history variation. For behavioural traits, early experiences with effects on brain, sensory development and cognition appear essential. This may also influence boldness and other social behaviours such as mate choice. At the end of the review, several issues and questions for future studies are given.

Sex determination in annual fishes: Searching for the master sex-determining gene in Austrolebias charrua (Cyprinodontiformes, Rivulidae)

Evolution of sex determination and differentiation in fishes involves a broad range of sex strategies (hermaphroditism, gonochorism, unisexuality, environmental and genetic sex determination). Annual fishes inhabit temporary ponds that dry out during the dry season when adults die. The embryos exhibit an atypical developmental pattern and remain buried in the bottom mud until the next rainy season. To elucidate genomic factors involved in the sex determination in annual fish, we explored the presence of a candidate sex-specific gene related to the cascade network in Austrolebias charrua. All phylogenetic analyses showed a high posterior probability of occurrence for a clade integrated by nuclear sequences (aprox. 900 bp) from both adults (male and female), with partial cDNA fragments of A. charrua from juveniles (male) and the dsx D. melanogaster gene. The expressed fragment was detected from blastula to adulthood stages showing a sexually dimorphic expression pattern. The isolated cDNA sequence is clearly related to dsx D. melanogaster gene and might be located near the top of the sex determination cascade in this species.

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.

Environmental stress-induced testis differentiation: androgen as a by-product of cortisol inactivation

This review deals with the gonadal masculinization induced by thermal stress in fish with focus on the action of 11β-hydroxysteroid dehydrogenase (11β-HSD) as this mechanism key transducer. High temperatures have been reported to produce male-skewed sex ratios in several species with TSD (temperature-dependent sex determination), and in some of them, this process was reported to be associated with high levels of cortisol, the hormone-related stress in vertebrates, during early gonad development. In addition, in pejerrey larvae reared at high-masculinizing temperatures, 11-ketotestosterone (11-KT), the main and most potent androgen in fish, was also detected at high levels. In testicular explants, cortisol induced the synthesis of 11-KT, suggesting that its synthesis could be under the control of the stress axis at the time of gonadal fate determination. 11β-HSD is one of the enzymes shared by the glucocorticoid and androgen pathways; this enzyme converts cortisol to cortisone and also participates in the finals steps of the synthesis of the 11-oxigenated androgens. Based on these data and literature information, here we propose that the masculinization induced by thermal stress can be considered as a consequence of cortisol inactivation and the concomitant synthesis of 11-KT and discussing this as a possible mechanism of masculinization induced by different types of environmental stressors.

Oyster sex determination is influenced by temperature - first clues in spat during first gonadic differentiation and gametogenesis

The sex-determining system of Crassostrea gigas is still poorly known, especially regarding the potential influence of temperature. In order to address this question, mRNA expressions of actors of the molecular cascade (Cg-DMl, Cg-SoxE, Cg-β-catenin, Cg-Foxl2/Cg-Foxl2os) and of Oyvlg, a germ cell marker, were investigated by real-time PCR in spat grown at different temperatures (18, 22, 25 and 28°C). In parallel, gonadic differentiation, gametogenesis and sex ratios were assessed by histology at each of these temperatures. Whatever the temperature, Cg-DMl, Cg-SoxE, Cg-β-catenin and Oyvlg expressions peaked at the same developmental stage, always after Cg-Foxl2/Cg-Foxl2os (around 40-44dpf for spat grown at 18°C). Temperatures increased the kinetics of first gonadic differentiation and gametogenesis. At 25°C a significant switch occurred in sex ratio towards males and in the balance of expression between male and female genes, in favor of males. A slight gametogenesis disturbance was also observed. These results strengthen the hypotheses about the sex-determining time window and molecular cascade governing the development of C. gigas, with notably the involvement of Cg-Foxl2/Cg-Foxl2os in the very early steps. They also suggest an influence of temperature on the oyster's sex determination which, associated to genetic control, would induce a mixed sex determination system (GSD+TSD).

Transitions between sex-determining systems in reptiles and amphibians

Important technological advances in genomics are driving a new understanding of the evolution of sex determination in vertebrates. In particular, comparative chromosome mapping in reptiles has shown an intriguing distribution of homology in sex chromosomes across reptile groups. When this new understanding is combined with the widespread distribution of genetic and temperature-dependent sex-determination mechanisms among reptiles, it is apparent that transitions between modes have occurred many times, as they have for amphibians (particularly between male and female heterogamety). It is also likely that thermosensitivity in sex determination is a key factor in those transitions in reptiles, and possibly in amphibians too. New models of sex determination involving temperature thresholds are providing the framework for the investigation of transitions and making possible key predictions about the homologies and sex-determination patterns expected among taxa in these groups. Molecular cytogenetics and other genomic approaches are essential to providing the fundamental material necessary to make advances in this field.

From the origin of sex-determining factors to the evolution of sex-determining systems

Sex determination is typically classified as either genotypic or environmental. However, this dichotomy obscures the developmental origin and evolutionary modification of determinants of sex, and therefore hinders an understanding of the processes that generates diversity in sex-determining systems. Recent research on reptiles and fish emphasizes that sex determination is a multifactorial regulatory process that is best understood as a threshold dichotomy rather than as the result of genetically inherited triggers of development. Here we critically assess the relationship between the developmental origin of sex-determining factors and evolutionary transitions in sex-determining systems. Our perspective emphasizes the importance of both genetic and nongenetic causes in evolution of sex determination and may help to generate predictions with respect to the evolutionary patterns of sex-determining systems and the underlying diversity of developmental and genetic regulatory networks.

Fish with thermolabile sex determination (TSD) as models to study brain sex differentiation

As fish are ectothermic animals, water temperature can affect their basic biological processes such as larval development, growth and reproduction. Similar to reptiles, the incubation temperature during early phases of development is capable to modify sex ratios in a large number of fish species. This phenomenon, known as thermolabile sex determination (TSD) was first reported in Menidia menidia, a species belonging to the family Atherinopsidae. Since then, an increasing number of fish have also been found to exhibit TSD. Traditionally, likewise in reptiles, several TSD patterns have been described in fish, however it has been recently postulated that only one, females at low temperatures and males at high temperatures, may represent the "real" or "true" TSD. Many studies regarding the influence of temperature on the final sex ratios have been focused on the expression and activity of gonadal aromatase, the enzyme involved in the conversion of androgens into estrogens and encoded by the cyp19a1a gene. In this regard, teleost fish, may be due to a whole genome duplication event, produce another aromatase enzyme, commonly named brain aromatase, encoded by the cyp19a1b gene. Contrary to what has been described in other vertebrates, fish exhibit very high levels of aromatase activity in the brain and therefore they synthesize high amounts of neuroestrogens. However, its biological significance is still not understood. In addition, the mechanism whereby temperature can induce the development of a testis or an ovary still remains elusive. In this context the present review is aimed to discuss several theories about the possible role of brain aromatase using fish as models. The relevance of brain aromatase and therefore of neuroestrogens as the possible cue for gonadal differentiation is raised. In addition, the possible role of brain aromatase as the way to keep the high levels of neurogenesis in fish is also considered. Several key examples of how teleosts and aromatase regulation can offer more insight into basic mechanisms of TSD are also reviewed.

A review of sex determining mechanisms in geckos (Gekkota: Squamata)

Geckos are a species-rich clade of reptiles possessing diverse sex determining mechanisms. Some species possess genetic sex determination, with both male and female heterogamety, while other species have temperature-dependent sex determination. I compiled information from the literature on the taxonomic distribution of these sex determining mechanisms in geckos. Using phylogenetic data from the literature, I reconstructed the minimum number of transitions among these sex determining mechanisms with parsimony-based ancestral state reconstruction. While only a small number of gecko species have been characterized, numerous changes among sex determining mechanisms were inferred. This diversity, coupled with the high frequency of transitions, makes geckos excellent candidates as a model clade for the study of vertebrate sex determination and evolution.

Expression profiles of sex differentiation-related genes during ontogenesis in the European sea bass acclimated to two different temperatures

The European sea bass is a teleost fish that lacks sex chromosomes and for which temperature influences sex ratios. However, correlation between temperature, developmental stage at a given age and sex-specific gene expression is hampered by the lack of sex markers. To study this correlation, fish were exposed to feminizing (15 degrees C) or masculinizing temperature (21 degrees C) from 0-120 days post fertilization, throughout the thermosensitive period (TSP). Aromatase (cyp19a1a), 11beta-hydroxylase (cyp11b), androgen receptor (arb) and estrogen receptors (era, erb1 and erb2) were assessed by qPCR prior and during sex differentiation. Canonical discriminant analysis (CDA), with length--as proxy for developmental stage--and cyp19a1a expression as predictors, was validated and used to reliably assign gonadal sex to fish sampled within and outside the TSP. Differences in cyp19a1a and cyp11b expression could be detected 1-month before the first signs of histological sex differentiation. Cyp19a1a and cyp11b were significantly higher in future females and males, respectively, and revealed as robust molecular markers to predict future ovarian and testicular differentiation. In contrast, no association between phenotypic sex and arb, era, erb1 and erb2 expression was found, suggesting that these genes do not contribute to the differentiation of a particular sex. The CDA-based approach implemented here could be used to sex undifferentiated animals in species where genetic sex cannot be known owing to the lack of simple sex determining systems, as it is the case of many fish and reptiles with or without temperature-dependent sex determination, and provide a useful tool to relate gene expression and phenotypic sex.

Identification of the major sex-determining region of turbot (Scophthalmus maximus)

Sex determination in fish is a labile character in evolutionary terms. The sex-determining (SD) master gene can differ even between closely related fish species. This group is an interesting model for studying the evolution of the SD region and the gonadal differentiation pathway. The turbot (Scophthalmus maximus) is a flatfish of great commercial value, where a strong sexual dimorphism exists for growth rate. Following a QTL and marker association approach in five families and a natural population, we identified the main SD region of turbot at the proximal end of linkage group (LG) 5, close to the SmaUSC-E30 marker. The refined map of this region suggested that this marker would be 2.6 cM and 1.4 Mb from the putative SD gene. This region appeared mostly undifferentiated between males and females, and no relevant recombination frequency differences were detected between sexes. Comparative genomics of LG5 marker sequences against five model species showed no similarity of this chromosome to the sex chromosomes of medaka, stickleback, and fugu, but suggested a similarity to a sex-associated QTL from Oreochromis spp. The segregation analysis of the closest markers to the SD region demonstrated a ZW/ZZ model of sex determination in turbot. A small proportion of families did not fit perfectly with this model, which suggests that other minor genetic and/or environmental factors are involved in sex determination in this species.

Isolation and development of a molecular sex marker for Bassiana duperreyi, a lizard with XX/XY sex chromosomes and temperature-induced sex reversal

Sex determination in the endemic Australian lizard Bassiana duperreyi (Scincidae) is influenced by sex chromosomes and incubation temperature, challenging the traditional dichotomy in reptilian sex determination. Analysis of those interactions requires sex chromosome markers to identify temperature-induced sex reversal. Here, we report the isolation of Y chromosome DNA sequence from B. duperreyi using amplified fragment length polymorphism PCR, the conversion of that sequence to a single-locus assay, and its combination with a single-copy nuclear gene (C-mos) to form a duplex PCR test for chromosomal sex. The accuracy of the assay was tested on an independent panel of individuals with known phenotypic sex. When used on offspring from field nests, our test identified the likely occurrence of a low rate of natural sex reversal in this species. This work represents the first report of Y chromosome sequence from a reptile and one of the few reptile sex tests.

Evolution of "determinants" in sex-determination: a novel hypothesis for the origin of environmental contingencies in avian sex-bias

Sex-determination is commonly categorized as either "genetic" or "environmental"-a classification that obscures the origin of this dichotomy and the evolution of sex-determining factors. The current focus on static outcomes of sex-determination provides little insight into the dynamic developmental processes by which some mechanisms acquire the role of sex determinants. Systems that combine "genetic" pathways of sex-determination (i.e., sex chromosomes) with "environmental" pathways (e.g., epigenetically induced segregation distortion) provide an opportunity to examine the evolutionary relationships between the two classes of processes and, ultimately, illuminate the evolution of sex-determining systems. Taxa with sex chromosomes typically undergo an evolutionary reduction in size of one of the sex chromosomes due to suppressed recombination, resulting in pronounced dimorphism of the sex chromosomes, and setting the stage for emergence of epigenetic compensatory mechanisms regulating meiotic segregation of heteromorphic sex chromosomes. Here we propose that these dispersed and redundant regulatory mechanisms enable environmental contingency in genetic sex-determination in birds and account for frequently documented context-dependence in avian sex-determination. We examine the evolution of directionality in such sex-determination as a result of exposure of epigenetic regulators of meiosis to natural selection and identify a central role of hormones in integrating female reproductive homeostasis, resource allocation to oocytes, and offspring sex. This approach clarifies the evolutionary relationship between sex-specific molecular genetic mechanisms of sex-determination and non-sex-specific epigenetic regulators of meiosis and demonstrates that both can determine sex. Our perspective shows how non-sex-specific mechanisms can acquire sex-determining function and, by establishing the explicit link between physiological integration of oogenesis and sex-determination, opens new avenues to the studies of adaptive sex-bias and sex-specific resource allocation in species with genetic sex-determination.

Temperature-dependent sex determination in fish revisited: prevalence, a single sex ratio response pattern, and possible effects of climate change

Background

In gonochoristic vertebrates, sex determination mechanisms can be classified as genotypic (GSD) or temperature-dependent (TSD). Some cases of TSD in fish have been questioned, but the prevalent view is that TSD is very common in this group of animals, with three different response patterns to temperature.

Methodology/Principal Findings

We analyzed field and laboratory data for the 59 fish species where TSD has been explicitly or implicitly claimed so far. For each species, we compiled data on the presence or absence of sex chromosomes and determined if the sex ratio response was obtained within temperatures that the species experiences in the wild. If so, we studied whether this response was statistically significant. We found evidence that many cases of observed sex ratio shifts in response to temperature reveal thermal alterations of an otherwise predominately GSD mechanism rather than the presence of TSD. We also show that in those fish species that actually have TSD, sex ratio response to increasing temperatures invariably results in highly male-biased sex ratios, and that even small changes of just 1–2°C can significantly alter the sex ratio from 1∶1 (males∶females) up to 3∶1 in both freshwater and marine species.

Conclusions/Significance

We demonstrate that TSD in fish is far less widespread than currently believed, suggesting that TSD is clearly the exception in fish sex determination. Further, species with TSD exhibit only one general sex ratio response pattern to temperature. However, the viability of some fish populations with TSD can be compromised through alterations in their sex ratios as a response to temperature fluctuations of the magnitude predicted by climate change.

New resources inform study of genome size, content, and organization in nonavian reptiles

Genomic resources for studies of nonavian reptiles have recently improved and will reach a new level of access once the genomes of the painted turtle (Chrysemys picta) and the green anole (Anolis carolinensis) have been published. Eleven speakers gathered for a symposium on reptilian genomics and evolutionary genetics at the 2008 meeting of the Society for Integrative and Comparative Biology in San Antonio, Texas. Presentations described results of reptilian genetic studies concerning molecular evolution, chromosomal evolution, genomic architecture, population dynamics, endocrinology and endocrine disruption, and the evolution of developmental mechanisms. The presented studies took advantage of the recent generation of genetic and genomic tools and resources. Novel findings demonstrated the positive impact made by the improved availability of resources like genome annotations and bacterial artificial chromosomes (BACs). The symposium was timely and important because it provided a vehicle for the dissemination of novel findings that advance the field. Moreover, this meeting fostered the synergistic interaction of the participants as a group, which is anticipated to encourage the funding and creation of further resources such as additional BAC libraries and genomic projects. Novel data have already been collected and studies like those presented in this symposium promise to shape and improve our understanding of overall amniote evolution. Additional reptilian taxa such as the American alligator (Alligator mississippiensis), tuatara (Sphenodon punctatus), and garter snake (Thamnophis sirtalis) should be the foci of future genomic projects. We hope that the following articles in this volume will help promote these efforts by describing the conclusions and the potential that the improvement of genomic resources for nonavian reptiles can continue having in this important area of integrative and comparative biology.