Genomic Anatomy of Homozygous XX Females and YY Males Reveals Early Evolutionary Trajectory of Sex-determining Gene and Sex Chromosomes in Silurus Fishes.

Sex chromosomes display remarkable diversity and variability among vertebrates. Compared with research on the X/Y and Z/W chromosomes, which have long evolutionary histories in mammals and birds, studies on the sex chromosomes at early evolutionary stages are limited. Here, we precisely assembled the genomes of homozygous XX female and YY male Lanzhou catfish (Silurus lanzhouensis) derived from an artificial gynogenetic family and a self-fertilized family, respectively. Chromosome 24 (Chr24) was identified as the sex chromosome based on resequencing data. Comparative analysis of the X and Y chromosomes showed an approximate 320 kb Y-specific region with a Y-specific duplicate of anti-Mullerian hormone type II receptor (amhr2y), which is consistent with findings in 2 other Silurus species but on different chromosomes (Chr24 of Silurus meridionalis and Chr5 of Silurus asotus). Deficiency of amhr2y resulted in male-to-female sex reversal, indicating that amhr2y plays a male-determining role in S. lanzhouensis. Phylogenetic analysis and comparative genomics revealed that the common sex-determining gene amhr2y was initially translocated to Chr24 of the Silurus ancestor along with the expansion of transposable elements. Chr24 was maintained as the sex chromosome in S. meridionalis and S. lanzhouensis, whereas a sex-determining region transition triggered sex chromosome turnover from Chr24 to Chr5 in S. asotus. Additionally, gene duplication, translocation, and degeneration were observed in the Y-specific regions of Silurus species. These findings present a clear case for the early evolutionary trajectory of sex chromosomes, including sex-determining gene origin, repeat sequence expansion, gene gathering and degeneration in sex-determining region, and sex chromosome turnover.

Evolution of sex-determination in dioecious plants: From active Y to X/A balance?

Sex chromosomes in plants have been known for a century, but only recently have we begun to understand the mechanisms behind sex determination in dioecious plants. Here, we discuss evolution of sex determination, focusing on Silene latifolia, where evolution of separate sexes is consistent with the classic "two mutations" model-a loss of function male sterility mutation and a gain of function gynoecium suppression mutation, which turned an ancestral hermaphroditic population into separate males and females. Interestingly, the gynoecium suppression function in S. latifolia evolved via loss of function in at least two sex-linked genes and works via gene dosage balance between sex-linked, and autosomal genes. This system resembles X/A-ratio-based sex determination systems in Drosophila and Rumex, and could represent a steppingstone in the evolution of X/A-ratio-based sex determination from an active Y system.

Repeated translocation of a supergene underlying rapid sex chromosome turnover in Takifugu pufferfish.

Recent studies have revealed a surprising diversity of sex chromosomes in vertebrates. However, the detailed mechanism of their turnover is still elusive. To understand this process, it is necessary to compare closely related species in terms of sex-determining genes and the chromosomes harboring them. Here, we explored the genus Takifugu, in which one strong candidate sex-determining gene, Amhr2, has been identified. To trace the processes involved in transitions in the sex-determination system in this genus, we studied 12 species and found that while the Amhr2 locus likely determines sex in the majority of Takifugu species, three species have acquired sex-determining loci at different chromosomal locations. Nevertheless, the generation of genome assemblies for the three species revealed that they share a portion of the male-specific supergene that contains a candidate sex-determining gene, GsdfY, along with genes that potentially play a role in male fitness. The shared supergene spans ∼100 kb and is flanked by two duplicated regions characterized by CACTA transposable elements. These results suggest that the shared supergene has taken over the role of sex-determining locus from Amhr2 in lineages leading to the three species, and repeated translocations of the supergene underlie the turnover of sex chromosomes in these lineages. These findings highlight the underestimated role of a mobile supergene in the turnover of sex chromosomes in vertebrates.

Dmrt1 induces the male pathway in a turtle species with temperature-dependent sex determination.

The molecular mechanism underlying temperature-dependent sex determination (TSD) has been a long-standing mystery; in particular, the thermosensitive genetic triggers for gonadal sex differentiation are largely unknown. Here, we have characterized a conserved DM domain gene, Dmrt1, in the red-eared slider turtle Trachemys scripta (T. scripta), which exhibits TSD. We found that Dmrt1 has a temperature-dependent, sexually dimorphic expression pattern, preceding gonadal sex differentiation, and is capable of responding rapidly to temperature shifts and aromatase inhibitor treatment. Most importantly, loss- and gain-of-function analyses provide solid evidence that Dmrt1 is both necessary and sufficient to initiate male development in T. scripta Furthermore, the DNA methylation dynamics of the Dmrt1 promoter are tightly correlated with temperature and could mediate the impact of temperature on sex determination. Collectively, our findings demonstrate that Dmrt1 is a candidate master male sex-determining gene in this TSD species, consistent with the idea that DM domain genes are conserved during the evolution of sex determination mechanisms.

Sox5 is involved in germ-cell regulation and sex determination in medaka following co-option of nested transposable elements.

BACKGROUND: Sex determination relies on a hierarchically structured network of genes, and is one of the most plastic processes in evolution. The evolution of sex-determining genes within a network, by neo- or sub-functionalization, also requires the regulatory landscape to be rewired to accommodate these novel gene functions. We previously showed that in medaka fish, the regulatory landscape of the master male-determining gene dmrt1bY underwent a profound rearrangement, concomitantly with acquiring a dominant position within the sex-determining network. This rewiring was brought about by the exaptation of a transposable element (TE) called Izanagi, which is co-opted to act as a silencer to turn off the dmrt1bY gene after it performed its function in sex determination. RESULTS: We now show that a second TE, Rex1, has been incorporated into Izanagi. The insertion of Rex1 brought in a preformed regulatory element for the transcription factor Sox5, which here functions in establishing the temporal and cell-type-specific expression pattern of dmrt1bY. Mutant analysis demonstrates the importance of Sox5 in the gonadal development of medaka, and possibly in mice, in a dmrt1bY-independent manner. Moreover, Sox5 medaka mutants have complete female-to-male sex reversal. CONCLUSIONS: Our work reveals an unexpected complexity in TE-mediated transcriptional rewiring, with the exaptation of a second TE into a network already rewired by a TE. We also show a dual role for Sox5 during sex determination: first, as an evolutionarily conserved regulator of germ-cell number in medaka, and second, by de novo regulation of dmrt1 transcriptional activity during primary sex determination due to exaptation of the Rex1 transposable element.

Sex chromosome differentiation and the W- and Z-specific loci in Xenopus laevis.

Genetic sex-determining systems in vertebrates include two basic types of heterogamety; XX (female)/XY (male) and ZZ (male)/ZW (female) types. The African clawed frog Xenopus laevis has a ZZ/ZW-type sex-determining system. In this species, we previously identified a W-specific sex (female)-determining gene dmw, and specified W and Z chromosomes, which could be morphologically indistinguishable (homomorphic). In addition to dmw, we most recently discovered two genes, named scanw and ccdc69w, and one gene, named capn5z in the W- and Z-specific regions, respectively. In this study, we revealed the detail structures of the W/Z-specific loci and genes. Sequence analysis indicated that there is almost no sequence similarity between 278kb W-specific and 83kb Z-specific sequences on chromosome 2Lq32-33, where both the transposable elements are abundant. Synteny and phylogenic analyses indicated that all the W/Z-specific genes might have emerged independently. Expression analysis demonstrated that scanw and ccdc69w or capn5z are expressed in early differentiating ZW gonads or testes, thereby suggesting possible roles in female or male development, respectively. Importantly, the sex-determining gene (SDG) dmw might have been generated after allotetraploidization, thereby indicating the construction of the new sex-determining system by dmw after species hybridization. Furthermore, by direct genotyping, we confirmed that diploid WW embryos developed into normal female frogs, which indicate that the Z-specific region is not essential for female development. Overall, these findings indicate that sex chromosome differentiation has started, although no heteromorphic sex chromosomes are evident yet, in X. laevis. Homologous recombination suppression might have promoted the accumulation of mutations and transposable elements, and enlarged the W/Z-specific regions, thereby resulting in differentiation of the W/Z chromosomes.

Construction of a high-density genetic map and the X/Y sex-determining gene mapping in spinach based on large-scale markers developed by specific-locus amplified fragment sequencing (SLAF-seq).

BACKGROUND: Cultivated spinach (Spinacia oleracea L.) is one of the most widely cultivated types of leafy vegetable in the world, and it has a high nutritional value. Spinach is also an ideal plant for investigating the mechanism of sex determination because it is a dioecious species with separate male and female plants. Some reports on the sex labeling and localization of spinach in the study of molecular markers have surfaced. However, there have only been two reports completed on the genetic map of spinach. The lack of rich and reliable molecular markers and the shortage of high-density linkage maps are important constraints in spinach research work. In this study, a high-density genetic map of spinach based on the Specific-locus Amplified Fragment Sequencing (SLAF-seq) technique was constructed; the sex-determining gene was also finely mapped. RESULTS: Through bio-information analysis, 50.75 Gb of data in total was obtained, including 207.58 million paired-end reads. Finally, 145,456 high-quality SLAF markers were obtained, with 27,800 polymorphic markers and 4080 SLAF markers were finally mapped onto the genetic map after linkage analysis. The map spanned 1,125.97 cM with an average distance of 0.31 cM between the adjacent marker loci. It was divided into 6 linkage groups corresponding to the number of spinach chromosomes. Besides, the combination of Bulked Segregation Analysis (BSA) with SLAF-seq technology(super-BSA) was employed to generate the linkage markers with the sex-determining gene. Combined with the high-density genetic map of spinach, the sex-determining gene X/Y was located at the position of the linkage group (LG) 4 (66.98 cM-69.72 cM and 75.48 cM-92.96 cM), which may be the ideal region for the sex-determining gene. CONCLUSIONS: A high-density genetic map of spinach based on the SLAF-seq technique was constructed with a backcross (BC1) population (which is the highest density genetic map of spinach reported at present). At the same time, the sex-determining gene X/Y was mapped to LG4 with super-BSA. This map will offer a suitable basis for further study of spinach, such as gene mapping, map-based cloning of Specific genes, quantitative trait locus (QTL) mapping and marker-assisted selection (MAS). It will also provide an efficient reference for studies on the mechanism of sex determination in other dioecious plants.

Unstable Linkage of Molecular Markers with Sex Determination Gene in Pacific Salmon (Oncorhynchus spp.).

In the present study, we tested the congruence between the sdY sex-specific marker and other commonly used male markers, located on the Y-chromosome, with the sex phenotypes in 5 species of Pacific salmon in Asian waters, including Chinook, chum, sockeye, masu, and pink salmon. We found that the localization of the sex-specific marker of both males and females of these species is not consistent with the phenotypic sex. Also, no linkage was found between noncoding markers and the sdY gene in the same species samples. Possible genetic and physiological mechanisms underlying this discrepancy are discussed.

Evolution of the sex-determining gene in the teleostean genus Oryzias.

In the genetic sex determination of vertebrates, the gonadal sex depends on the combination of sex chromosomes that a zygote possesses. Despite the discovery of the sex-determining gene (SRY/Sry) in mammals in 1990s, the sex-determining gene in non-mammalian vertebrates remained an enigma for over a decade. In most mammals, the male-inducing master sex-determining gene is located on the Y chromosome and is therefore absent from XX females. A second sex-determining gene, Dmy, was described in the Oryzias latipes in 2002 and has a DNA-binding motif that is different from the motif in the mammalian sex-determining gene SRY or Sry. Dmy is also located on the Y chromosome and is therefore absent in XX females. Seven other sex-determining genes, including candidate genes, are now known in birds, a frog species, and 5 fish species. These findings over the past twenty years have increased our knowledge of sex-determining genes and sex chromosomes among vertebrates. Here, we review recent advances in our understanding of sex-determining genes and genetic sex determination systems in fish, especially those of the Oryzias species, which are described in detail. The facts suggest some patterns of how new sex-determining genes emerged and evolved. We believe that these facts are common not only in Oryzias but also in other fish species. This knowledge will help to elucidate the conserved mechanisms from which various sex-determining mechanisms have evolved.

Identification and Genomic Localization of Autosomal sdY Locus in a Population of Atlantic Salmon (Salmo salar).

The determination of sex in salmonid fishes is controlled by genetic mechanisms, with males being the heterogametic sex. The master sex-determining gene, the sexually dimorphic gene on the Y chromosome (sdY), is a conserved gene across various salmonid species. Nevertheless, variations in the genomic location of sdY have been observed both within and between species. Furthermore, different studies have reported discordances in the association between the sdY and the phenotypic gender. While some males seem to lack this locus, there have been reports of females carrying sdY. Although the exact reasons behind this discordance remain under investigation, some recent studies have proposed the existence of an autosomal, non-functional copy of sdY as a potential cause. In this study, we confirmed the presence of this autosomal sdY in the SalmoBreed strain of Atlantic salmon using a genotyping platform through a novel approach that allows for high-throughput screening of a large number of individuals. We further characterized the segregation profile of this locus across families and found the ratio of genetically assigned female-to-male progeny to be in accordance with the expected profile of a single autosomal sdY locus. Additionally, our mapping efforts localized this locus to chromosome 3 and suggested a putative copy on chromosome 6.

Exosome delivery to the testes for dmrt1 suppression: A powerful tool for sex-determining gene studies.

Exosomes are endosome-derived extracellular vesicles about 100 nm in diameter. They are emerging as promising delivery platforms due to their advantages in biocompatibility and engineerability. However, research into and applications for engineered exosomes are still limited to a few areas of medicine in mammals. Here, we expanded the scope of their applications to sex-determining gene studies in early vertebrates. An integrated strategy for constructing the exosome-based delivery system was developed for efficient regulation of dmrt1, which is one of the most widely used sex-determining genes in metazoans. By combining classical methods in molecular biology and the latest technology in bioinformatics, isomiR-124a was identified as a dmrt1 inhibitor and was loaded into exosomes and a testis-targeting peptide was used to modify exosomal surface for efficient delivery. Results showed that isomiR-124a was efficiently delivered to the testes by engineered exosomes and revealed that dmrt1 played important roles in maintaining the regular structure and function of testis in juvenile fish. This is the first de novo development of an exosome-based delivery system applied in the study of sex-determining gene, which indicates an attractive prospect for the future applications of engineered exosomes in exploring more extensive biological conundrums.

The identification and expression pattern of the sex determination genes and their sex-specific variants in the egg parasitoid Trichogramma dendrolimi Matsumura (Hymenoptera: Trichogrammatidae).

Introduction: Trichogramma wasps are egg parasitoids of agricultural lepidopteran pests. The sex of Trichogramma is determined by its ploidy as well as certain sex ratio distorters, such as the endosymbiotic bacteria Wolbachia spp. and the paternal sex ratio (PSR) chromosome. The sex determination systems of hymenopterans, such as Trichogramma spp., involve cascades of the genes transformer (tra), transformer-2 (tra2), and doublesex (dsx) and are associated with sex-specific tra and dsx splicing. First, these genes and their sex-specific variants must be identified to elucidate the interactions between the sex ratio disorders and the sex determination mechanism of Trichogramma. Methods: Here, we characterized the sex determination genes tra, tra2, and dsx in Trichogramma dendrolimi. Sex-specific tra and dsx variants were detected in cDNA samples obtained from both male and female Trichogramma wasps. They were observed in the early embryos (1-10 h), late embryos (12-20 h), larvae (32 h and 48 h), pre-pupae (96 h), and pupae (144 h, 168 h, 192 h, and 216 h) of both male and female T. dendrolimi offspring. Results: We detected female-specific tra variants throughout the entire early female offspring stage. The male-specific variant began to express at 9-10 h as the egg was not fertilized. However, we did not find any maternally derived, female-specific tra variant in the early male embryo. This observation suggests that the female-specific tra variant expressed in the female embryo at 1-9 h may not have originated from the maternal female wasp. Discussion: The present study might be the first to identify the sex determination genes and sex-specific gene splicing in Trichogramma wasps. The findings of this study lay the foundation for investigating the sex determination mechanisms of Trichogramma and other wasps. They also facilitate sex identification in immature T. dendrolimi and the application of this important egg parasitoid in biological insect pest control programs.

Characterization of the male-specific region containing the candidate sex-determining gene in Amur catfish (Silurus asotus) using third-generation- and pool-sequencing data.

Amur catfish (Silurus asotus) is an ecologically and economically important fish species in Asia. Here, we assembled the female and male Amur catfish genomes, with genome sizes of 757.15 and 755.44 Mb, respectively, at the chromosome level using nanopore and Hi-C technologies. Consistent with the known diploid chromosome count, both genomes contained 29 chromosome-size scaffolds covering 98.80 and 98.73 % of the complete haplotypic assembly with scaffold N50 of 28.87 and 27.29 Mb, respectively. The female (n = 40) and male (n = 40) pools were re-sequenced. Comparative analysis of sequencing and re-sequencing data from both sexes confirmed the presence of an XX/XY sex determination system in Amur catfish and revealed Chr5 as the sex chromosome containing an approximately 400 kb Y-specific region (MSY). Gene annotation revealed a male-specific duplicate of amhr2, namely amhr2y, in MSY, which is male-specific in different wild populations and expressed only in the testes. Amur catfish shared partially syntenic MSY and amhr2y genes with the southern catfish (S. meridionalis, Chr24), which were located on different chromosomes. High sequence divergence between amhr2y and amhr2 and high sequence similarity with amhr2y were observed in both species. These results indicate the common origin of the sex-determining (SD) gene and transition of amhr2y in the two Silurus species. Accumulation of repetitive elements in the MSY of both species may be the main driver of the transition of amhr2y. Overall, our study provides valuable catfish genomic resources. Moreover, determination of amhr2y as the candidate SD gene in Amur catfish provides another example of amhr2 as the SD gene in fish.

Some thoughts about the words we use for thinking about sex chromosome evolution.

Sex chromosomes are familiar to most biologists since they first learned about genetics. However, research over the past 100 years has revealed that different organisms have evolved sex-determining systems independently. The differences in the ages of systems, and in how they evolved, both affect whether sex chromosomes have evolved. However, the diversity means that the terminology used tends to emphasize either the similarities or the differences, sometimes causing misunderstandings. In this article, I discuss some concepts where special care is needed with terminology. The following four terms regularly create problems: 'sex chromosome', 'master sex-determining gene', 'evolutionary strata' and 'genetic degeneration'. There is no generally correct or wrong use of these words, but efforts are necessary to make clear how they are to be understood in specific situations. I briefly outline some widely accepted ideas about sex chromosomes, and then discuss these 'problem terms', highlighting some examples where careful use of the words helps bring to light current uncertainties and interesting questions for future work. This article is part of the theme issue 'Sex determination and sex chromosome evolution in land plants'.

Generation of a chromosome-level genome assembly for Pacific halibut (Hippoglossus stenolepis) and characterization of its sex-determining genomic region.

The Pacific halibut (Hippoglossus stenolepis) is a key species in the North Pacific Ocean and Bering Sea ecosystems, where it also supports important fisheries. However, the lack of genomic resources limits our understanding of evolutionary, environmental and anthropogenic forces affecting key life history characteristics of Pacific halibut and prevents the application of genomic tools in fisheries management and conservation efforts. In the present study, we report on the first generation of a high-quality chromosome-level assembly of the Pacific halibut genome, with an estimated size of 602 Mb, 24 chromosome-length scaffolds that contain 99.8% of the assembly and a N50 scaffold length of 27.3 Mb. In the first application of this important resource, we conducted genome-wide analyses of sex-specific genetic variation by pool sequencing and characterized a potential sex-determining region in chromosome 9 with a high density of female-specific SNPs. Within this region, we identified the bmpr1ba gene as a potential candidate for master sex-determining (MSD) gene. bmpr1ba is a member of the TGF-ß family that in teleosts has provided the largest number of MSD genes, including a paralogue of this gene in Atlantic herring. The genome assembly constitutes an essential resource for future studies on Pacific halibut population structure and dynamics, evolutionary history and responses to environmental and anthropogenic influences. Furthermore, the genomic location of the sex-determining region in Pacific halibut has been identified and a putative candidate MSD gene has been proposed, providing further support for the rapid evolution of sex-determining mechanisms in teleost fish.

Identification of the sex-determining factor in the liverwort Marchantia polymorpha reveals unique evolution of sex chromosomes in a haploid system.

Sex determination is a central process for sexual reproduction and is often regulated by a sex determinant encoded on a sex chromosome. Rules that govern the evolution of sex chromosomes via specialization and degeneration following the evolution of a sex determinant have been well studied in diploid organisms. However, distinct predictions apply to sex chromosomes in organisms where sex is determined in the haploid phase of the life cycle: both sex chromosomes, female U and male V, are expected to maintain their gene functions, even though both are non-recombining. This is in contrast to the X-Y (or Z-W) asymmetry and Y (W) chromosome degeneration in XY (ZW) systems of diploids. Here, we provide evidence that sex chromosomes diverged early during the evolution of haploid liverworts and identify the sex determinant on the Marchantia polymorpha U chromosome. This gene, Feminizer, encodes a member of the plant-specific BASIC PENTACYSTEINE transcription factor family. It triggers female differentiation via regulation of the autosomal sex-determining locus of FEMALE GAMETOPHYTE MYB and SUPPRESSOR OF FEMINIZATION. Phylogenetic analyses of Feminizer and other sex chromosome genes indicate dimorphic sex chromosomes had already been established 430 mya in the ancestral liverwort. Feminizer also plays a role in reproductive induction that is shared with its gametolog on the V chromosome, suggesting an ancestral function, distinct from sex determination, was retained by the gametologs. This implies ancestral functions can be preserved after the acquisition of a sex determination mechanism during the evolution of a dominant haploid sex chromosome system.

Oryzias curvinotus in Sanya Does Not Contain the Male Sex-Determining Gene dmy.

Hainan medaka (Oryzias curvinotus) is distributed in the coastal waters of the South China Sea and is able to adapt to a wide range of salinities. In this study, we characterized O. curvinotus in Sanya River (SY-medaka), which lacks dmy (a male sex-determining gene in O. latipes and O. curvinotus). In a comparison of SY-medaka and Gaoqiao medaka (GQ-medaka), the morphological difference between the two populations does not reach the subspecies level and they can be considered two geographic populations of O. curvinotus. A mitochondrial cytochrome oxidase subunit I (CoI) sequence alignment showed that the sequence identities between SY-medaka and other geographic populations of O. curvinotus are as high as 95%. A phylogenetic analysis of the mitochondrial genome also indicated that SY-medaka belongs to O. curvinotus. Molecular marker-based genetic sex assays and whole genome re-sequencing showed that SY-medaka does not contain dmy. Further, in RNA-Seq analyses of the testis and ovaries of sexually mature SY-medaka, dmy expression was not detected. We speculate that high temperatures resulted in the loss of dmy in SY-medaka during evolution, or the lineage has another sex-determining gene. This study provides a valuable dataset for elucidating the mechanism underlying sex determination in Oryzias genus and advances research on functional genomics or reproduction biology in O. curvinotus.

The rise and fall of the ancient northern pike master sex-determining gene.

The understanding of the evolution of variable sex determination mechanisms across taxa requires comparative studies among closely related species. Following the fate of a known master sex-determining gene, we traced the evolution of sex determination in an entire teleost order (Esociformes). We discovered that the northern pike (Esox lucius) master sex-determining gene originated from a 65 to 90 million-year-old gene duplication event and that it remained sex linked on undifferentiated sex chromosomes for at least 56 million years in multiple species. We identified several independent species- or population-specific sex determination transitions, including a recent loss of a Y chromosome. These findings highlight the diversity of evolutionary fates of master sex-determining genes and the importance of population demographic history in sex determination studies. We hypothesize that occasional sex reversals and genetic bottlenecks provide a non-adaptive explanation for sex determination transitions.

A SNP in a Steroidogenic Enzyme Is Associated with Phenotypic Sex in Seriola Fishes.

Vertebrate sex development consists largely of two processes: "sex determination," the initial bifurcation of sexual identity, and "sex differentiation," which subsequently facilitates maleness or femaleness according to the sex determination signal. Steroid hormones promote multiple types of sexual dimorphism in eutherian mammals and avians [1-3], in which they are indispensable for proper sex differentiation. By contrast, in many poikilothermic vertebrates, steroid hormones have been proposed to be key players in sex determination as well as sex differentiation [4-8]. This hypothesis was introduced more than 50 years ago but has never been rigorously tested due to difficulties in discriminating the roles of steroids in sex determination and differentiation. We found that a missense SNP in the gene encoding the steroidogenic enzyme 17ß-hydroxysteroid dehydrogenase 1 (Hsd17b1) was perfectly associated with ZZ/ZW sex determination in Seriola fishes. Biochemical analyses revealed that a glutamate residue present specifically in Z-type HSD17B1 attenuated interconversion between 17-keto and 17ß-hydroxy steroids relative to the allelic product from the W chromosome, which harbors glycine at that position, by disrupting the hydrogen bond network between the steroid and the enzyme's catalytic residues. Hsd17b1 mRNA is constitutively expressed in undifferentiated and differentiating gonads of both genotypic sexes, whereas W-type mRNA is expressed only in genotypic females. Meanwhile, Cyp19a1 is predominantly expressed in differentiating ovary. We conclude that the combination of Hsd17b1 alleles determines sex by modulating endogenous estrogen levels in Seriola species. These findings strongly support the long-standing hypothesis on steroids in sex determination.

Environmental regulation of sex determination in fishes: Insights from Atheriniformes.

Sex determination is the first step toward the establishment of phenotypic sex in most vertebrates. Aquatic poikilotherms such as teleost fishes exhibit a high diversity of sex-determination mechanisms and gonadal phenotypes that are remarkably plastic and responsive to a variety of environmental factors (e.g., water temperature, pH, salinity, photoperiod, population density). This chapter reviews current knowledge of genotypic and environmental sex determination systems in fishes with special reference to Atheriniformes-one of the best-characterized taxa in this field-and offers perspectives to guide and stimulate further research.