Sex determination: why so many ways of doing it?

Energy as the cornerstone of environmentally driven sex allocation

In recent years, observations of distinct organisms have linked the quality of the environment experienced by a given individual and the sex it will develop. In most described cases, facing relatively harsh conditions resulted in masculinization, while thriving in favorable conditions promoted the development of an ovary. This was shown indistinctively in some species presenting a genetic sex determination (GSD), which were able to sex-reverse, and in species with an environmental sex determination (ESD) system. However, this pattern strongly depends on evolutionary constrains and is detected only when females need more energy for reproduction. Here, I describe the mechanisms involved in this environmentally driven sex allocation (EDSA), which involves two main energy pathways, lipid and carbohydrate metabolism. These pathways act through various enzymes and are not necessarily independent of the previously known transducers of environmental signals in species with ESD: calcium-redox, epigenetic, and stress regulation pathways. Overall, there is evidence of a link between energy level and the sexual fate of individuals of various species, including reptiles, fish, amphibians, insects, and nematodes. As energy pathways are evolutionarily conserved, this knowledge opens new avenues to advance our understanding of the mechanisms that allow animals to adapt their sex according to the local environment.

Strong variation in frequencies of male and female determiners between neighboring housefly populations

Sex-determination mechanisms evolve rapidly and vary between species. Occasionally, polymorphic systems are found, like in the housefly. Studying the dynamics and stability of such systems can provide a better understanding of the evolution of sex-determination systems. In the housefly, dominant male-determining loci (M) can lie not only on the Y chromosome (M^Y ), but also on autosomes (M^A ) or the X chromosome (M^X ). M enforces male development by inhibiting the female-determining gene transformer (tra). A mutant tra allele, tra^D , is insensitive to M and is a dominant female determiner. M^Y prevails at high latitudes and polymorphic M loci together with tra^D at low latitudes. To get more insight into the stability and frequencies of these sex determiners with mutually exclusive dominance, we investigated 5 regional Spanish populations. We found strong variation among populations. Two populations with hemizygous M^III were found, 2 contained homozygous M^X with additional hemizygous M^I and M^II in 1 population. One population contained homozygous and hemizygous M^X with additionally hemizygous M^II . All females in populations with homozygous M, had tra^D , whereas no tra^D was found in populations without homozygous M. Our results indicate locally stable systems may either harbor a single hemizygous M and no tra^D , corresponding to a male heterogametic system, or homozygous and/or multiple M and heterozygous tra^D , reminiscent of a female heterogametic system. They support that M loci can accumulate in the presence of a dominant female determiner. Limited migration between populations may contribute to the stability of these systems.

Extremely divergent COI sequences within an amphipod species complex: A possible role for endosymbionts?

Some heritable endosymbionts can affect host mtDNA evolution in various ways. Amphipods host diverse endosymbionts, but whether their mtDNA has been influenced by these endosymbionts has yet to be considered. Here, we investigated the role of endosymbionts (microsporidians and Rickettsia) in explaining highly divergent COI sequences in Paracalliope fluviatilis species complex, the most common freshwater amphipods in New Zealand. We first contrasted phylogeographic patterns using COI, ITS, and 28S sequences. While molecular species delimitation methods based on 28S sequences supported 3-4 potential species (N, C, SA, and SB) among freshwater lineages, COI sequences supported 17-27 putative species reflecting high inter-population divergence. The deep divergence between NC and S lineages (~20%; 28S) and the substitution saturation on the 3rd codon position of COI detected even within one lineage (SA) indicate a very high level of morphological stasis. Interestingly, individuals infected and uninfected by Rickettsia comprised divergent COI lineages in one of four populations tested, suggesting a potential influence of endosymbionts in mtDNA patterns. We propose several plausible explanations for divergent COI lineages, although they would need further testing with multiple lines of evidence. Lastly, due to common morphological stasis and the presence of endosymbionts, phylogeographic patterns of amphipods based on mtDNA should be interpreted with caution.

ZW sex-chromosome evolution and contagious parthenogenesis in Artemia brine shrimp

Eurasian brine shrimp (genus Artemia) have closely related sexual and asexual lineages of parthenogenetic females, which produce rare males at low frequencies. Although they are known to have ZW chromosomes, these are not well characterized, and it is unclear whether they are shared across the clade. Furthermore, the underlying genetic architecture of the transmission of asexuality, which can occur when rare males mate with closely related sexual females, is not well understood. We produced a chromosome-level assembly for the sexual Eurasian species Artemia sinica and characterized in detail the pair of sex chromosomes of this species. We combined this new assembly with short-read genomic data for the sexual species Artemia sp. Kazakhstan and several asexual lineages of Artemia parthenogenetica, allowing us to perform an in-depth characterization of sex-chromosome evolution across the genus. We identified a small differentiated region of the ZW pair that is shared by all sexual and asexual lineages, supporting the shared ancestry of the sex chromosomes. We also inferred that recombination suppression has spread to larger sections of the chromosome independently in the American and Eurasian lineages. Finally, we took advantage of a rare male, which we backcrossed to sexual females, to explore the genetic basis of asexuality. Our results suggest that parthenogenesis is likely partly controlled by a locus on the Z chromosome, highlighting the interplay between sex determination and asexuality.

A novel Dmrt gene of crustacean: functional analysis of idmrt-2 gene in the male reproductive system from Scylla paramamosain

The Dmrt (double-sex and mab-3 related transcription factor) gene family is considered to be a highly conserved gene family related to sex determination and sexual differentiation across species. In order to better understand the role of the idmrt-2 gene in gonad development in Scylla paramamosain, the idmrt-2 gene was cloned and analyzed. The cDNA contains a 1659 bp ORF region encoding 552 amino acids. The qRT-PCR results showed that idmrt-2 was significantly more expressed in the testis than in other tissues (p<0.05). The expression of idmrt-2 was highest in the spermatids stage (T2 stage), followed by the mature sperms stage (T3 stage) and significantly higher than in the spermatocytes stage (T1 stage) (p<0.05) during testicular development and the expression difference was not significant in different stages of ovarian development. RNAi studies revealed that after idmrt-2 was knocked down, the expression of Dmrt-like and foxl-2 genes in the testis decreased, as well as IAG expression in the androgenic gland. The findings suggest that idmrt-2 may be an IAG regulator and involved in testicular development.

Repeated turnovers keep sex chromosomes young in willows

Background

Salicaceae species have diverse sex determination systems and frequent sex chromosome turnovers. However, compared with poplars, the diversity of sex determination in willows is poorly understood, and little is known about the evolutionary forces driving their turnover. Here, we characterized the sex determination in two Salix species, S. chaenomeloides and S. arbutifolia, which have an XY system on chromosome 7 and 15, respectively.

Results

Based on the assemblies of their sex determination regions, we found that the sex determination mechanism of willows may have underlying similarities with poplars, both involving intact and/or partial homologs of a type A cytokinin response regulator (RR) gene. Comparative analyses suggested that at least two sex turnover events have occurred in Salix, one preserving the ancestral pattern of male heterogamety, and the other changing heterogametic sex from XY to ZW, which could be partly explained by the “deleterious mutation load” and “sexually antagonistic selection” theoretical models. We hypothesize that these repeated turnovers keep sex chromosomes of willow species in a perpetually young state, leading to limited degeneration.

Conclusions

Our findings further improve the evolutionary trajectory of sex chromosomes in Salicaceae species, explore the evolutionary forces driving the repeated turnovers of their sex chromosomes, and provide a valuable reference for the study of sex chromosomes in other species.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13059-022-02769-w.

Does polyploidy inhibit sex chromosome evolution in angiosperms?

Dioecy is rare in flowering plants (5-6% of species), but is often controlled genetically by sex-linked regions (SLRs). It has so far been unclear whether, polyploidy affects sex chromosome evolution, as it does in animals, though polyploidy is quite common in angiosperms, including in dioecious species. Plants could be different, as, unlike many animal systems, degenerated sex chromosomes, are uncommon in plants. Here we consider sex determination in plants and plant-specific factors, and propose that constraints created at the origin of polyploids limit successful polyploidization of species with SLRs. We consider the most likely case of a polyploid of a dioecious diploid with an established SLR, and discuss the outcome in autopolyploids and allopolyploids. The most stable system possibly has an SLR on just one chromosome, with a strongly dominant genetic factor in the heterogametic sex (e.g., xxxY male in a tetraploid). If recombination occurs with its homolog, this will prevent Y chromosome degeneration. Polyploidy may also allow for reversibility of multiplied Z or X chromosomes into autosomes. Otherwise, low dosage of Y-linked SLRs compared to their multiple homologous x copies may cause loss of reliable sex-determination at higher ploidy levels. We discuss some questions that can be studied using genome sequencing, chromosome level-assemblies, gene expression studies and analysis of loci under selection.

Heterogeneous Evolution of Sex Chromosomes in the Torrent Frog Genus Amolops

In sharp contrast to birds and mammals, in numerous cold-blooded vertebrates, sex chromosomes have been described as homomorphic. This sex chromosome homomorphy has been suggested to result from the high turnovers often observed across deeply diverged clades. However, little is known about the tempo and mode of sex chromosome evolution among the most closely related species. Here, we examined the evolution of sex chromosome among nine species of the torrent frog genus Amolops. We analyzed male and female GBS and RAD-seq from 182 individuals and performed PCR verification for 176 individuals. We identified signatures of sex chromosomes involving two pairs of chromosomes. We found that sex-chromosome homomorphy results from both turnover and X–Y recombination in the Amolops species, which simultaneously exhibits heterogeneous evolution on homologous and non-homologous sex chromosomes. A low turnover rate of non-homologous sex chromosomes exists in these torrent frogs. The ongoing X–Y recombination in homologous sex chromosomes will act as an indispensable force in preventing sex chromosomes from differentiating.

Sex differences in number of X chromosomes and X-chromosome inactivation in females promote greater variability in hearing among males

BACKGROUND

For more than 150 years, research studies have documented greater variability across males than across females ("greater male variability"-GMV) over a broad range of behavioral and morphological measures. In placental mammals, an ancient difference between males and females that may make an important contribution to GMV is the different pattern of activation of X chromosomes across cells in females (mosaic inactivation of one the two X chromosomes across cells) vs males (consistent activation of a single X chromosome in all cells). In the current study, variability in hearing thresholds was examined for human listeners with thresholds within the normal range. Initial analyses compared variability in thresholds across males vs. across females. If greater across-male than across-female variability was present, and if these differences in variability related to the different patterns X-chromosome activation in males vs. females, it was expected that correlations between related measures within a given subject (e.g., hearing thresholds at given frequency in the two ears) would be greater in males than females.

METHODS

Hearing thresholds at audiometric test frequencies (500-6000 or 500-8000 Hz) were extracted from two datasets representing more than 8500 listeners with normal hearing (4590 males, 4376 females). Separate data analyses were carried out on each dataset to compare: (1) relative variability in hearing thresholds across males vs. across females at each test frequency; (2) correlations between both across-ear and within-ear hearing thresholds within  males vs. within  females, and (3) mean thresholds for females vs. males at each frequency.

RESULTS

A consistent pattern of GMV in hearing thresholds was seen across frequencies in both datasets. In addition, both across-ear and within-ear correlations between thresholds were consistently greater in males than females. Previous studies have frequently reported lower mean thresholds for females than males for listeners with normal hearing. One of the datasets replicated this result, showing a clear and consistent pattern of lower mean thresholds for females. The second data set did not show clear evidence of this female advantage.

CONCLUSIONS

Hearing thresholds showed clear evidence of greater variability across males than across females and higher correlations across related threshold measures within males than within females. The results support a link between the observed GMV and the mosaic pattern of X-activation for females that is not present in males.

Individual Genetic Heterogeneity

Genetic variation has been widely covered in literature, however, not from the perspective of an individual in any species. Here, a synthesis of genetic concepts and variations relevant for individual genetic constitution is provided. All the different levels of genetic information and variation are covered, ranging from whether an organism is unmixed or hybrid, has variations in genome, chromosomes, and more locally in DNA regions, to epigenetic variants or alterations in selfish genetic elements. Genetic constitution and heterogeneity of microbiota are highly relevant for health and wellbeing of an individual. Mutation rates vary widely for variation types, e.g., due to the sequence context. Genetic information guides numerous aspects in organisms. Types of inheritance, whether Mendelian or non-Mendelian, zygosity, sexual reproduction, and sex determination are covered. Functions of DNA and functional effects of variations are introduced, along with mechanism that reduce and modulate functional effects, including TARAR countermeasures and intraindividual genetic conflict. TARAR countermeasures for tolerance, avoidance, repair, attenuation, and resistance are essential for life, integrity of genetic information, and gene expression. The genetic composition, effects of variations, and their expression are considered also in diseases and personalized medicine. The text synthesizes knowledge and insight on individual genetic heterogeneity and organizes and systematizes the central concepts.

Polycomb-mediated repression of paternal chromosomes maintains haploid dosage in diploid embryos of Marchantia

Complex mechanisms regulate gene dosage throughout eukaryotic life cycles. Mechanisms controlling gene dosage have been extensively studied in animals, however it is unknown how generalizable these mechanisms are to diverse eukaryotes. Here, we use the haploid plant Marchantia polymorpha to assess gene dosage control in its short-lived diploid embryo. We show that throughout embryogenesis, paternal chromosomes are repressed resulting in functional haploidy. The paternal genome is targeted for genomic imprinting by the Polycomb mark H3K27me3 starting at fertilization, rendering the maternal genome in control of embryogenesis. Maintaining haploid gene dosage by this new form of imprinting is essential for embryonic development. Our findings illustrate how haploid-dominant species can regulate gene dosage through paternal chromosome inactivation and initiates the exploration of the link between life cycle history and gene dosage in a broader range of organisms.

Transcriptome Profiling and Expression Localization of Key Sex-Related Genes in a Socially-Controlled Hermaphroditic Clownfish, Amphiprion clarkii

Clownfish can be an excellent research model for investigating the socially-controlled sexual development of sequential hermaphrodite teleosts. However, the molecular cascades underlying the social cues that orchestrate the sexual development process remain poorly understood. Here, we performed a comparative transcriptomic analysis of gonads from females, males, and nonbreeders of Amphiprion clarkii, which constitute a complete social group, allowing us to investigate the molecular regulatory network under social control. Our analysis highlighted that the gonads of nonbreeders and males exhibited high similarities but were far from females, both in global transcriptomic profiles and histological characteristics, and identified numerous candidate genes involved in sexual development, some well-known and some novel. Significant upregulation of cyp19a1a, foxl2, nr5a1a, wnt4a, hsd3b7, and pgr in females provides strong evidence for the importance of steroidogenesis in ovarian development and maintenance, with cyp19a1a playing a central role. Amh and sox8 are two potential key factors that may regulate testicular tissue development in early and late stages, respectively, as they are expressed at higher levels in males than in females, but with slightly different expression timings. Unlike previous descriptions in other fishes, the unique expression pattern of dmrt1 in A. clarkii implied its potential function in both male and female gonads, and we speculated that it might play promoting roles in the early development of both testicular and ovarian tissues.

Ecology and the evolution of sex chromosomes

Sex chromosomes are common features of animal genomes, often carrying a sex determination gene responsible for initiating the development of sexually dimorphic traits. The specific chromosome that serves as the sex chromosome differs across taxa as a result of fusions between sex chromosomes and autosomes, along with sex chromosome turnover-autosomes becoming sex chromosomes and sex chromosomes 'reverting' back to autosomes. In addition, the types of genes on sex chromosomes frequently differ from the autosomes, and genes on sex chromosomes often evolve faster than autosomal genes. Sex-specific selection pressures, such as sexual antagonism and sexual selection, are hypothesized to be responsible for sex chromosome turnovers, the unique gene content of sex chromosomes and the accelerated evolutionary rates of genes on sex chromosomes. Sex-specific selection has pronounced effects on sex chromosomes because their sex-biased inheritance can tilt the balance of selection in favour of one sex. Despite the general consensus that sex-specific selection affects sex chromosome evolution, most population genetic models are agnostic as to the specific sources of these sex-specific selection pressures, and many of the details about the effects of sex-specific selection remain unresolved. Here, I review the evidence that ecological factors, including variable selection across heterogeneous environments and conflicts between sexual and natural selection, can be important determinants of sex-specific selection pressures that shape sex chromosome evolution. I also explain how studying the ecology of sex chromosome evolution can help us understand important and unresolved aspects of both sex chromosome evolution and sex-specific selection.

Genomic evidence of paternal genome elimination in the globular springtail Allacma fusca

Paternal genome elimination (PGE) - a type of reproduction in which males inherit but fail to pass on their father's genome - evolved independently in six to eight arthropod clades. Thousands of species, including several important for agriculture, reproduce via this mode of reproduction. While PGE is well established in some of the clades, the evidence in globular springtails (Symphypleona) remains elusive, even though they represent the oldest and most species rich clade putatively reproducing via PGE. We sequenced genomic DNA from whole bodies of Allacma fusca males with high fractions (>27.5%) of sperm to conclusively confirm that all the sperm carry one parental haplotype only. Although it is suggestive that the single haplotype present in sperm is maternally inherited, definitive genetic proof of the parent of origin is still needed. The genomic approach we developed allows for detection of genotypic differences between germline and soma in all species with sufficiently high fraction of germline in their bodies. This opens new opportunities for scans of reproductive modes in small organisms.

Genomic architecture of supergenes: connecting form and function

Supergenes are tightly linked sets of loci that are inherited together and control complex phenotypes. While classical supergenes-governing traits such as wing patterns in Heliconius butterflies or heterostyly in Primula-have been studied since the Modern Synthesis, we still understand very little about how they evolve and persist in nature. The genetic architecture of supergenes is a critical factor affecting their evolutionary fate, as it can change key parameters such as recombination rate and effective population size, potentially redirecting molecular evolution of the supergene in addition to the surrounding genomic region. To understand supergene evolution, we must link genomic architecture with evolutionary patterns and processes. This is now becoming possible with recent advances in sequencing technology and powerful forward computer simulations. The present theme issue brings together theoretical and empirical papers, as well as opinion and synthesis papers, which showcase the architectural diversity of supergenes and connect this to critical processes in supergene evolution, such as polymorphism maintenance and mutation accumulation. Here, we summarize those insights to highlight new ideas and methods that illuminate the path forward for the study of supergenes in nature. This article is part of the theme issue 'Genomic architecture of supergenes: causes and evolutionary consequences'.

Highly rapid and diverse sex chromosome evolution in the Odorrana frog species complex

Sex chromosomes in poikilothermal vertebrates are characterized by rapid and diverse evolution at the species or population level. Our previous study revealed that the Taiwanese frog Odorrana swinhoana (2n = 26) has a unique system of multiple sex chromosomes created by three sequential translocations among chromosomes 1, 3, and 7. To reveal the evolutionary history of sex chromosomes in the Odorrana species complex, we first identified the original, homomorphic sex chromosomes, prior to the occurrence of translocations, in the ancestral-type population of O. swinhoana. Then, we extended the investigation to a closely related Japanese species, Odorrana utsunomiyaorum, which is distributed on two small islands. We used a high-throughput nuclear genomic approach to analyze single-nucleotide polymorphisms and identify the sex-linked markers. Those isolated from the O. swinhoana ancestral-type population were found to be aligned to chromosome 1 and showed male heterogamety. In contrast, almost all the sex-linked markers isolated from O. utsunomiyaorum were heterozygous in females and homozygous in males and were aligned to chromosome 9. Morphologically, we confirmed chromosome 9 to be heteromorphic in females, showing a ZZ-ZW sex determination system, in which the W chromosomes were heterochromatinized in a stripe pattern along the chromosome axis. These results indicated that after divergence of the two species, the ancestral homomorphic sex chromosome 1 underwent highly rapid and diverse evolution, i.e., sequential translocations with two autosomes in O. swinhoana, and turnover to chromosome 9 in O. utsunomiyaorum, with a transition from XY to ZW heterogamety and change to heteromorphy.

Sex chromosomes as supergenes of speciation: why amphibians defy the rules?

As reflected by the two rules of speciation (Haldane's rule and the large X-/Z-effect), sex chromosomes are expected to behave like supergenes of speciation: they recombine only in one sex (XX females or ZZ males), supposedly recruit sexually antagonistic genes and evolve faster than autosomes, which can all contribute to pre-zygotic and post-zygotic isolation. While this has been mainly studied in organisms with conserved sex-determining systems and highly differentiated (heteromorphic) sex chromosomes like mammals, birds and some insects, these expectations are less clear in organismal groups where sex chromosomes repeatedly change and remain mostly homomorphic, like amphibians. In this article, we review the proposed roles of sex-linked genes in isolating nascent lineages throughout the speciation continuum and discuss their support in amphibians given current knowledge of sex chromosome evolution and speciation modes. Given their frequent recombination and lack of differentiation, we argue that amphibian sex chromosomes are not expected to become supergenes of speciation, which is reflected by the rarity of empirical studies consistent with a 'large sex chromosome effect' in frogs and toads. The diversity of sex chromosome systems in amphibians has a high potential to disentangle the evolutionary mechanisms responsible for the emergence of sex-linked speciation genes in other organisms. This article is part of the theme issue 'Genomic architecture of supergenes: causes and evolutionary consequences'.

Tenuous Transcriptional Threshold of Human Sex Determination. I. SRY and Swyer Syndrome at the Edge of Ambiguity

Male sex determination in mammals is initiated by SRY, a Y-encoded transcription factor. The protein contains a high-mobility-group (HMG) box mediating sequence-specific DNA bending. Mutations causing XY gonadal dysgenesis (Swyer syndrome) cluster in the box and ordinarily arise de novo. Rare inherited variants lead to male development in one genetic background (the father) but not another (his sterile XY daughter). De novo and inherited mutations occur at an invariant Tyr adjoining the motif's basic tail (box position 72; Y127 in SRY). In SRY-responsive cell lines CH34 and LNCaP, de novo mutations Y127H and Y127C reduced SRY activity (as assessed by transcriptional activation of principal target gene Sox9) by 5- and 8-fold, respectively. Whereas Y127H impaired testis-specific enhancer assembly, Y127C caused accelerated proteasomal proteolysis; activity was in part rescued by proteasome inhibition. Inherited variant Y127F was better tolerated: its expression was unperturbed, and activity was reduced by only twofold, a threshold similar to other inherited variants. Biochemical studies of wild-type (WT) and variant HMG boxes demonstrated similar specific DNA affinities (within a twofold range), with only subtle differences in sharp DNA bending as probed by permutation gel electrophoresis and fluorescence resonance-energy transfer (FRET); thermodynamic stabilities of the free boxes were essentially identical. Such modest perturbations are within the range of species variation. Whereas our cell-based findings rationalize the de novo genotype-phenotype relationships, a molecular understanding of inherited mutation Y127F remains elusive. Our companion study uncovers cryptic biophysical perturbations suggesting that the para-OH group of Y127 anchors a novel water-mediated DNA clamp.

Cadmium Ecotoxic Effects on Embryonic Dmrt1 and Aromatase Expression in Chrysemys picta Turtles May Implicate Changes in DNA Methylation

Temperature-dependent sex determination (TSD) decides the sex fate of an individual based on incubation temperature. However, other environmental factors, such as pollutants, could derail TSD sexual development. Cadmium is one such contaminant of soils and water bodies known to affect DNA methylation, an epigenetic DNA modification with a key role in sexual development of TSD vertebrate embryos. Yet, whether cadmium alters DNA methylation of genes underlying gonadal formation in turtles remains unknown. Here, we investigated the effects of cadmium on the expression of two gene regulators of TSD in the painted turtle, Chrysemys picta, incubated at male-producing and female-producing temperatures using qPCR. Results revealed that cadmium alters transcription of Dmrt1 and aromatase, overriding the normal thermal effects during embryogenesis, which could potentially disrupt the sexual development of TSD turtles. Results from a preliminary DNA methylation-sensitive PCR assay implicate changes in DNA methylation of Dmrt1 as a potential cause that requires further testing (aromatase methylation assays were precluded).

Temporal variation in DNA methylation during gonadal development in a reptile with temperature-dependent sex determination

DNA methylation plays a significant role in transducing external environmental signals to a cellular response in reptiles; however, whether the methylation patterns are conserved across species remains unclear. Here, we examined the genome-wide DNA methylation differentiation between male and female hatchling gonads of the temperature-dependent sex determination (TSD) Mauremys mutica (M. mutica) using methylation-dependent restriction-site associated DNA sequencing (MethylRAD-seq) to test differentially methylated genes underlying sexual development. Several categories, including heat shock genes (HSP90A, HSP30C), histone- (KDM8) and ubiquitin-related genes (TRIM39), kinases (WNK3) and gonad differentiation or gonadal development related genes (HSD17B8, HSD17B12), were identified as candidates for future study. Additionally, we identified several regulatory pathways potentially mediating TSD thermosensitivity such as the GnRH signaling pathway and calcium signaling pathway. These findings provide evidence that sexually dimorphic DNA methylation may be associated with sex determination or sex differentiation in TSD M. mutica.

Recent expansion of the non-recombining sex-linked region on Silene latifolia sex chromosomes

Evolution of a non-recombining sex-specific region on the Y (or W) chromosome (NRY) is a key step in sex chromosome evolution, but how recombination suppression evolves is not well understood. Studies in many different organisms indicated that NRY evolution often involves several expansion steps. Why such NRY expansions occur remains unclear, although it is though that they are likely driven by sexually antagonistic selection. This paper describes a recent NRY expansion due to shift of the pseudoautosomal boundary on the sex chromosomes of a dioecious plant Silene latifolia. The shift resulted in inclusion of at least 16 pseudoautosomal genes into the NRY. This region is pseudoautosomal in closely related Silene dioica and Silene diclinis, indicating that the NRY expansion occurred in S. latifolia after it speciated from the other species ~120 thousand years ago. As S. latifolia and S. dioica actively hybridise across Europe, interspecific gene flow could blur the PAR boundary in these species. The pseudoautosomal genes have significantly elevated genetic diversity (π ~ 3% at synonymous sites), which is consistent with balancing selection maintaining diversity in this region. The recent shift of the PAR boundary in S. latifolia offers an opportunity to study the process of on-going NRY expansion.

Identification of a candidate sex determination gene in Culaea inconstans suggests convergent recruitment of an Amh duplicate in two lineages of stickleback

Sex chromosomes vary greatly in their age and levels of differentiation across the tree of life. This variation is largely due to the rates of sex chromosome turnover in different lineages; however, we still lack an explanation for why sex chromosomes are so conserved in some lineages (e.g. mammals, birds) but so labile in others (e.g. teleosts, amphibians). To identify general mechanisms driving transitions in sex determination systems or forces which favour their conservation, we first require empirical data on sex chromosome systems from multiple lineages. Stickleback fishes are a valuable model lineage for the study of sex chromosome evolution due to variation in sex chromosome systems between closely-related species. Here, we identify the sex chromosome and a strong candidate for the master sex determination gene in the brook stickleback, Culaea inconstans. Using whole-genome sequencing of wild-caught samples and a lab cross, we identify AmhY, a male specific duplication of the gene Amh, as the candidate master sex determination gene. AmhY resides on Chromosome 20 in C. inconstans and is likely a recent duplication, as both AmhY and the sex-linked region of Chromosome 20 show little sequence divergence. Importantly, this duplicate AmhY represents the second independent duplication and recruitment of Amh as the sex determination gene in stickleback and the eighth example known across teleosts. We discuss this convergence in the context of sex chromosome turnovers and the role that the Amh/AmhrII pathway, which is crucial for sex determination, may play in the evolution of sex chromosomes in teleosts.

Masculinizer and Doublesex as Key Factors Regulate Sexual Dimorphism in Ostrinia furnacalis

Simple Summary

In animals, sexually dimorphic traits are ubiquitous and play vital roles in reproduction, courtship, and environmental adaptation, especially in insects. In this study, we used the CRISPR/Cas9 genome editing system to generate somatic mutations of the Masculinizer (Masc) and doublesex (dsx) genes in the sex determination pathway of Ostrinia furnacalis. The OfMasc and Ofdsx genes are structural orthologs of the key sex regulation factors in Bombyx mori. Mutation of the OfMasc and Ofdsx genes induced abnormal external genitalia, adult sterility, and sex reversal of sexually dimorphic traits including wing pigmentation, gene expression patterns, and dsx sex-specific splicing. These results demonstrate that the Masc and dsx genes are conserved factors in sexually dimorphic traits, and therefore represent potential target genes in the effort to control O. furnacalis and other lepidopteran pests.

Abstract

Sex determination is an important and traditional biological process. In Lepidoptera, Masculinizer (Masc) and doublesex (dsx) are the essential genes for sex determination and play critical roles in sexual differentiation and development. The functions of Masc and dsx have been characterized in several model insect species. However, the molecular mechanism and sex determination functions of Masc and dsx in Ostrinia furnacalis, an agricultural pest, are still unknown. Here, we successfully used the CRISPR/Cas9 genome editing system to knock out OfMasc and Ofdsx. Mutation of OfMasc induced male external genital defects and sterility. Disruptions of the Ofdsx common region caused sex-specific defects in the external genitals and adult sterility. In addition, we found that OfMasc and Ofdsx can regulate the pigmentation genes that control wing pigmentation patterns. These results demonstrate that OfMasc and Ofdsx play key roles in the sex determination of O. furnacalis, and suggest novel genetic control approaches for the management of pests, including O. furnacalis.

Genetic determination and JARID2 over-expression in a thermal incubation experiment in Casque-Headed Lizard

Non-avian reptiles, unlike mammals and birds, have undergone numerous sex determination changes. Casque-Headed Lizards have replaced the ancestral XY system shared across pleurodonts with a new pair of XY chromosomes. However, the evolutionary forces that triggered this transition have remained unclear. An interesting hypothesis suggests that species with intermediate states, with sex chromosomes but also thermal-induced sex reversal at specific incubation temperatures, could be more susceptible to sex determination turnovers. We contrasted genotypic data (presence/absence of the Y chromosome) against the histology of gonads of embryos from stages 35–37 incubated at various temperatures, including typical male-producing (26°C) and female-producing (32°C) temperatures. Our work apparently reports for the first time the histology of gonads, including morphological changes, from stages 35–37 of development in the family Corytophanidae. We also observed that all embryos developed hemipenes, suggesting sex-linked developmental heterochrony. We observed perfect concordance between genotype and phenotype at all temperatures. However, analysis of transcriptomic data from embryos incubated at 26°C and 32°C identified transcript variants of the chromatin modifiers JARID2 and KDM6B that have been linked to temperature-dependent sex determination in other reptiles. Our work tested the validity of a mixed sex determination system in the family Corytophanidae. We found that XY chromosomes are dominant; however, our work supports the hypothesis of a conserved transcriptional response to incubation temperatures across non-avian reptiles that could be a reminiscence of an ancestral sex determination system.

Induction and inhibition of Drosophila X chromosome gene expression are both impeded by the dosage compensation complex

Sex chromosomes frequently differ from the autosomes in the frequencies of genes with sexually dimorphic or tissue-specific expression. Multiple hypotheses have been put forth to explain the unique gene content of the X chromosome, including selection against male-beneficial X-linked alleles, expression limits imposed by the haploid dosage of the X in males, and interference by the dosage compensation complex on expression in males. Here, we investigate these hypotheses by examining differential gene expression in Drosophila melanogaster following several treatments that have widespread transcriptomic effects: bacterial infection, viral infection, and abiotic stress. We found that genes that are induced (upregulated) by these biotic and abiotic treatments are frequently under-represented on the X chromosome, but so are those that are repressed (downregulated) following treatment. We further show that whether a gene is bound by the dosage compensation complex in males can largely explain the paucity of both up- and downregulated genes on the X chromosome. Specifically, genes that are bound by the dosage compensation complex, or close to a dosage compensation complex high-affinity site, are unlikely to be up- or downregulated after treatment. This relationship, however, could partially be explained by a correlation between differential expression and breadth of expression across tissues. Nonetheless, our results suggest that dosage compensation complex binding, or the associated chromatin modifications, inhibit both up- and downregulation of X chromosome gene expression within specific contexts, including tissue-specific expression. We propose multiple possible mechanisms of action for the effect, including a role of Males absent on the first, a component of the dosage compensation complex, as a dampener of gene expression variance in both males and females. This effect could explain why the Drosophila X chromosome is depauperate in genes with tissue-specific or induced expression, while the mammalian X has an excess of genes with tissue-specific expression.

Sex-limited chromosomes and non-reproductive traits

Sex chromosomes are typically viewed as having originated from a pair of autosomes, and differentiated as the sex-limited chromosome (e.g. Y) has degenerated by losing most genes through cessation of recombination. While often thought that degenerated sex-limited chromosomes primarily affect traits involved in sex determination and sex cell production, accumulating evidence suggests they also influence traits not sex-limited or directly involved in reproduction. Here, we provide an overview of the effects of sex-limited chromosomes on non-reproductive traits in XY, ZW or UV sex determination systems, and discuss evolutionary processes maintaining variation at sex-limited chromosomes and molecular mechanisms affecting non-reproductive traits.

Evolutionary History of Sexual Differentiation Mechanism in Insects

Alternative splicing underpins functional diversity in proteins and the complexity and diversity of eukaryotes. An example is the doublesex gene, the key transcriptional factor in arthropod sexual differentiation. doublesex is controlled by sex-specific splicing and promotes both male and female differentiation in holometabolan insects, whereas in hemimetabolan species, doublesex has sex-specific isoforms but is not required for female differentiation. How doublesex evolved to be essential for female development remains largely unknown. Here, we investigate ancestral states of doublesex using Thermobia domestica belonging to Zygentoma, the sister group of Pterygota, that is, winged insects. We find that, in T. domestica, doublesex expresses sex-specific isoforms but is only necessary for male differentiation of sexual morphology. This result supports the hypothesis that doublesex initially promoted male differentiation during insect evolution. However, T. domestica doublesex has a short female-specific region and upregulates the expression of vitellogenin homologs in females, suggesting that doublesex may already play some role in female morphogenesis of the common ancestor of Pterygota. Reconstruction of the ancestral sequence and prediction of protein structures show that the female-specific isoform of doublesex has an extended C-terminal disordered region in holometabolan insects but not in nonholometabolan species. We propose that doublesex acquired its function in female morphogenesis through a change in the protein motif structure rather than the emergence of the female-specific exon.

High-temperature stress will put the thermo-sensitive teleost yellow catfish (Tachysurus fulvidraco) in danger through reducing reproductivity

Recently, concerns for species that sex differentiation is influenced by temperature in the context of global warming have increased because disrupted operational sex ratios could threaten population maintenance. In contrast, little attention has been given to the reproductive ability of populations that experienced elevated temperatures. In this study, we demonstrated that high temperature (HT) would decrease population size via three different aspects of reproductive ability for the first time. We show that, in a thermo-sensitive teleost yellow catfish, a short period of HT (+3 °C) exposure during the critical period of sex differentiation leads to a different percentage of masculinization of XX genotypic females (1-23%) in wet-lab and natural water bodies. Combining the results of gonadal appearance, histology, sperm parameters, and fertilization rate, we found that XX pseudo-males induced by HT display significantly discounted fertility and reproductive performance compared to XY normal males. We demonstrate that the survival of the XY genotype is lower than XX genotype under environmental stress, including HT, hypoxia, and parasite infection, and the differential survival seems unrelated to male-biased sexual size dimorphism. The mathematical model predicts that the phenotypic female percent will be stabilized at 50% and the population will be sustainably maintained when masculinizing force is less than 0.5, while HT will put the population in danger when the masculinizing force exceeds 0.5. However, when we combine the real-world data of reproductive ability and mathematic model, our results suggest the population size decreases and the long-term survival of the studied species are threatened under the projected pace of increasing temperature. These findings will be useful for understanding the long-term effects of increasing temperature on sex ratio, reproduction and population maintenance in teleost.

Extreme variation in recombination rate and genetic diversity along the Sylvioidea neo-sex chromosome

Recombination strongly impacts sequence evolution by affecting the extent of linkage and the efficiency of selection. Here, we study recombination over the Z chromosome in great reed warblers (Acrocephalus arundinaceus) using pedigree-based linkage mapping. This species has extended Z and W chromosomes ("neo-sex chromosomes") formed by a fusion between a part of chromosome 4A and the ancestral sex chromosomes, which provides a unique opportunity to assess recombination and sequence evolution in sex-linked regions of different ages. We assembled an 87.54 Mbp and 90.19 cM large Z with a small pseudoautosomal region (0.89 Mbp) at one end and the fused Chr4A-part at the other end of the chromosome. A prominent feature in our data was an extreme variation in male recombination rate along Z with high values at both chromosome ends, but an apparent lack of recombination over a substantial central section, covering 78% of the chromosome. The nonrecombining region showed a drastic loss of genetic diversity and accumulation of repeats compared to the recombining parts. Thus, our data emphasize a key role of recombination in affecting local levels of polymorphism. Nonetheless, the evolutionary rate of genes (dN/dS) did not differ between high and low recombining regions, suggesting that the efficiency of selection on protein-coding sequences can be maintained also at very low levels of recombination. Finally, the Chr4A-derived part showed a similar recombination rate as the part of the ancestral Z that did recombine, but its sequence characteristics reflected both its previous autosomal, and current Z-linked, recombination patterns.

Fish female-biased gene cyp19a1a leads to female antiviral response attenuation between sexes by autophagic degradation of MITA

From insects to mammals, both innate and adaptive immune response are usually higher in females than in males, with the sex chromosome and hormonal differences considered the main reasons. Here, we report that zebrafish cyp19a1a (cytochrome P450, family 19, subfamily A, polypeptide 1a), an autosomal gene with female-biased expression, causes female fish to exhibit a lower antiviral response. First, we successfully constructed an infection model by intraperitoneal injection of spring viremia of carp virus (SVCV) into zebrafish (Danio rerio) and Carassius auratus herpesvirus (CaHV) in gibel carp (Carassius gibelio). Specifically, female fish were more vulnerable to viral infection than males, accompanied by a significantly weaker interferon (IFN) expression. After screening several candidates, cyp19a1a, which was highly expressed in female fish tissues, was selected for further analysis. The IFN expression and antiviral response were significantly higher in cyp19a1a-/- than in cyp19a1a+/+. Further investigation of the molecular mechanism revealed that Cyp19a1a targets mediator of IRF3 activation (MITA) for autophagic degradation. Interestingly, in the absence of MITA, Cyp19a1a alone could not elicit an autophagic response. Furthermore, the autophagy factor ATG14 (autophagy-related 14) was found interacted with Cyp19a1a to either promote or attenuate Cyp19a1a-mediated MITA degradation by either being overexpressed or knocked down, respectively. At the cellular level, both the normal and MITA-enhanced cellular antiviral responses were diminished by Cyp19a1a. These findings demonstrated a sex difference in the antiviral response based on a regulation mechanism controlled by a female-biased gene besides sex chromosome and hormonal differences, supplying the current understanding of sex differences in fish.

Identification of ancestral sex chromosomes in the frog Glandirana rugosa bearing XX-XY and ZZ-ZW sex-determining systems

Sex chromosomes constantly exist in a dynamic state of evolution: rapid turnover and change of heterogametic sex during homomorphic state, and often stepping out to a heteromorphic state followed by chromosomal decaying. However, the forces driving these different trajectories of sex chromosome evolution are still unclear. The Japanese frog Glandirana rugosa is one taxon well suited to the study on these driving forces. The species has two different heteromorphic sex chromosome systems, XX-XY and ZZ-ZW, which are separated in different geographic populations. Both XX-XY and ZZ-ZW sex chromosomes are represented by chromosome 7 (2n = 26). Phylogenetically, these two systems arose via hybridization between two ancestral lineages of West Japan and East Japan populations, of which sex chromosomes are homomorphic in both sexes and to date have not yet been identified. Identification of the sex chromosomes will give us important insight into the mechanisms of sex chromosome evolution in this species. Here, we used a high-throughput genomic approach to identify the homomorphic XX-XY sex chromosomes in both ancestral populations. Sex-linked DNA markers of West Japan were aligned to chromosome 1, whereas those of East Japan were aligned to chromosome 3. These results reveal that at least two turnovers across three different sex chromosomes 1, 3 and 7 occurred during evolution of this species. This finding raises the possibility that cohabitation of the two different sex chromosomes from ancestral lineages induced turnover to another new one in their hybrids, involving transition of heterogametic sex and evolution from homomorphy to heteromorphy.

Sex reversal explains some, but not all, climate-mediated sex ratio variation within a viviparous reptile

Evolutionary transitions in sex-determining systems have occurred frequently yet understanding how they occur remains a major challenge. In reptiles, transitions from genetic to temperature-dependent sex determination can occur if the gene products that determine sex evolve thermal sensitivity, resulting in sex-reversed individuals. However, evidence of sex reversal is limited to oviparous reptiles. Here we used thermal experiments to test whether sex reversal is responsible for differences in sex determination in a viviparous reptile, Carinascincus ocellatus, a species with XY sex chromosomes and population-specific sex ratio response to temperature. We show that sex reversal is occurring and that its frequency is related to temperature. Sex reversal was unidirectional (phenotypic males with XX genotype) and observed in both high- and low-elevation populations. We propose that XX-biased genotypic sex ratios could produce either male- or female-biased phenotypic sex ratios as observed in low-elevation C. ocellatus under variable rates of XX sex reversal. We discuss reasons why sex reversal may not influence sex ratios at high elevation. Our results suggest that the mechanism responsible for evolutionary transitions from genotypic to temperature-dependent sex determination is more complex than can be explained by a single process such as sex reversal.

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

SignificanceAlthough turnover of sex chromosomes is very common in many vertebrate lineages, the transition process is still elusive. We studied the sex-determining region (SDR) of 12 congeneric fish species of Takifugu fish that compose an important model for the study of genomics and sex determination. We found that while nine species retained their ancestral SDR, three species had acquired derived SDRs. Although the derived SDRs resided in three different chromosomes, they harbored a shared supergene flanked by two putative transposable elements. The results highlight the underestimated role of a mobile supergene in turnover of sex chromosomes in vertebrates.

Identification of a cis-sex chromosome transition in banded geckos (Coleonyx, Eublepharidae, Gekkota)

Sex-determination systems are highly variable amongst vertebrate groups, and the prevalence of genomic data has greatly expanded our knowledge of how diverse some groups truly are. Gecko lizards are known to possess a variety of sex-determination systems, and each new study increases our knowledge of this diversity. Here, we used RADseq to identify male-specific markers in the banded gecko Coleonyx brevis, indicating this species has a XX/XY sex-determination system. Furthermore, we show that these sex-linked regions are not homologous to the XX/XY sex chromosomes of two related Coleonyx species, C. elegans and C. mitratus, suggesting that a cis-sex chromosome turnover-a change in sex chromosomes without a concomitant change in heterogamety-has occurred within the genus. These findings demonstrate the utility of genome-scale data to uncover novel sex chromosomes and further highlight the diversity of gecko sex chromosomes.

Polymorphism of Sex Determination Amongst Wild Populations Suggests its Rapid Turnover Within the Nile Tilapia Species

Sex-determining regions have been identified in the Nile tilapia on linkage groups (LG) 1, 20 and 23, depending on the domesticated strains used. Sex determining studies on wild populations of this species are scarce. Previous work on two wild populations, from Lake Volta (Ghana) and from Lake Koka (Ethiopia), found the sex-determining region on LG23. These populations have a Y-specific tandem duplication containing two copies of the Anti-Müllerian Hormone amh gene (named amhY and amhΔY). Here, we performed a whole-genome short-reads analysis using male and female pools on a third wild population from Lake Hora (Ethiopia). We found no association of sex with LG23, and no duplication of the amh gene. Furthermore, we found no evidence of sex linkage on LG1 or on any other LGs. Long read whole genome sequencing of a male from each population confirmed the absence of a duplicated region on LG23 in the Lake Hora male. In contrast, long reads established the structure of the Y haplotype in Koka and Kpandu males and the order of the genes in the duplicated region. Phylogenies constructed on the nuclear and mitochondrial genomes, showed a closer relationship between the two Ethiopian populations compared to the Ghanaian population, implying an absence of the LG23Y sex-determination region in Lake Hora males. Our study supports the hypothesis that the amh region is not the sex-determining region in Hora males. The absence of the Y amh duplication in the Lake Hora population reflects a rapid change in sex determination within Nile tilapia populations. The genetic basis of sex determination in the Lake Hora population remains unknown.

Two duplicated gsdf homeologs cooperatively regulate male differentiation by inhibiting cyp19a1a transcription in a hexaploid fish

Although evolutionary fates and expression patterns of duplicated genes have been extensively investigated, how duplicated genes co-regulate a biological process in polyploids remains largely unknown. Here, we identified two gsdf (gonadal somatic cell-derived factor) homeologous genes (gsdf-A and gsdf-B) in hexaploid gibel carp (Carassius gibelio), wherein each homeolog contained three highly conserved alleles. Interestingly, gsdf-A and gsdf-B transcription were mainly activated by dmrt1-A (dsx- and mab-3-related transcription factor 1) and dmrt1-B, respectively. Loss of either gsdf-A or gsdf-B alone resulted in partial male-to-female sex reversal and loss of both caused complete sex reversal, which could be rescued by a nonsteroidal aromatase inhibitor. Compensatory expression of gsdf-A and gsdf-B was observed in gsdf-B and gsdf-A mutants, respectively. Subsequently, we determined that in tissue culture cells, Gsdf-A and Gsdf-B both interacted with Ncoa5 (nuclear receptor coactivator 5) and blocked Ncoa5 interaction with Rora (retinoic acid-related orphan receptor-alpha) to repress Rora/Ncoa5-induced activation of cyp19a1a (cytochrome P450, family 19, subfamily A, polypeptide 1a). These findings illustrate that Gsdf-A and Gsdf-B can regulate male differentiation by inhibiting cyp19a1a transcription in hexaploid gibel carp and also reveal that Gsdf-A and Gsdf-B can interact with Ncoa5 to suppress cyp19a1a transcription in vitro. This study provides a typical case of cooperative mechanism of duplicated genes in polyploids and also sheds light on the conserved evolution of sex differentiation.

Consequences of partially recessive deleterious genetic variation for the evolution of inversions suppressing recombination between sex chromosomes

The evolution of suppressed recombination between sex chromosomes is widely hypothesized to be driven by sexually antagonistic selection (SA), where tighter linkage between the sex-determining gene(s) and nearby SA loci is favored when it couples male-beneficial alleles to the proto-Y chromosome, and female-beneficial alleles to the proto-X. Despite limited empirical evidence, the SA selection hypothesis overshadows several alternatives, including an incomplete but often-repeated "sheltering hypothesis" that suggests that expansion of the sex-linked region (SLR) reduces homozygous expression of partially recessive deleterious mutations at selected loci. Here, we use population genetic models to evaluate the consequences of deleterious mutational variation for the evolution of neutral chromosomal inversions expanding the SLR on proto-Y chromosomes. We find that SLR-expanding inversions face a race against time: lightly loaded inversions are initially beneficial, but eventually become deleterious as they accumulate new mutations, and must fix before this window of opportunity closes. The outcome of this race is strongly influenced by inversion size, the mutation rate, and the dominance coefficient of deleterious mutations. Yet, small inversions have elevated fixation probabilities relative to neutral expectations for biologically plausible parameter values. Our results demonstrate that deleterious genetic variation can plausibly drive recombination suppression in small steps and would be most consistent with empirical patterns of small evolutionary strata or gradual recombination arrest.

Natural and Engineered Sex Ratio Distortion in Insects

Insects have evolved highly diverse genetic sex-determination mechanisms and a relatively balanced male to female sex ratio is generally expected. However, selection may shift the optimal sex ratio while meiotic drive and endosymbiont manipulation can result in sex ratio distortion (SRD). Recent advances in sex chromosome genomics and CRISPR/Cas9-mediated genome editing brought significant insights into the molecular regulators of sex determination in an increasing number of insects and provided new ways to engineer SRD. We review these advances and discuss both naturally occurring and engineered SRD in the context of the Anthropocene. We emphasize SRD-mediated biological control of insects to help improve One Health, sustain agriculture, and conserve endangered species.

Teaching About Sex and Gender in Neuroscience: More Than Meets the "XY"

Offering courses on the neuroscience of sex and gender can help support an inclusive curriculum in neuroscience. At the same time, developing and teaching such courses can be daunting to even the most enthusiastic educators, given the subject's complexities, nuances, and the difficult conversations that it invites. The authors of this article have all developed and taught such courses from different perspectives. Our aim is to provide educators with an overview of important conceptual topics as well as a comprehensive, but non-exhaustive, guide to resources for teaching about sex/gender in neuroscience based on our collective experience teaching courses on the topic. After defining vital terminology and briefly reviewing the biology of sex and sex determination, we describe some common topics within the field and contrast our current nuanced understandings from outdated misconceptions in the field. We review how (mis)representation of the neuroscience of sex/gender serves as a case study for how scientific results are communicated and disseminated. We consider how contextualization of sex/gender neuroscience research within a broader historical and societal framework can give students a wider perspective on the enterprise of science. Finally, we conclude with a brief discussion on how to choose learning goals for your course and implementation notes.

Lacking of sex-lethal gene lowers the fertility of male reproduction in Spodoptera litura (Lepidoptera)

Sex-lethal (Sxl) encodes an RNA-binding protein that acts as the switch of sex determination in Drosophila and influences the genitalia formation and gonadal development. However, its sex-determination roles are not conserved in all insects and its role in the gonadal development of Lepidoptera is not well documented. In this study, three splicing variants of Sxl mRNA were identified in Spodoptera litura and they highly expressed in gonads, particularly in the testis. The mRNA levels of SlSxl exhibited higher expression in the spermatid than the testis sheaths, and gradually increased with the spermiogenesis. Sex-lethal protein (SlSXL) is mainly distributed in the cytoplasm of spermatocytes and the head of spermatid. Knockout of SlSxl resulted in fewer eupyrene sperm bundles and apyrene sperm bundles in the testes of moth and a large number of undeveloped spermatocysts retained in the moth of mutant testis, and leading to the reduction of oviposition and hatch rate in the offsprings after mating with female. These results suggest that SlSxl is a critical player in the spermiogenesis of S. litura.

Impact of temperature-induced sex reversal on behavior and sound production in Nile tilapia (Oreochromis niloticus)

In some fish species, sex is determined by the combination of genetic and environmental factors. In most species concerned, extreme temperatures during the sensitive period of sex differentiation drives masculinization, independently of the female sex chromosomes. In Nile tilapia (XY male heterogamety), XX juveniles exposed to high temperatures (>32 °C) can masculinize and become phenotypical males (neomales). Whether these neomales exhibit a different behavior than XY males remains however unclear. Sex reversal being naturally relevant, we investigated the agonistic behavior of neomales during dyadic fights and the preference of gravid females for one of the two male types. We quantified the behavior, size of the nest, hormone circulating levels (testosterone, 11-ketotestosterone and cortisol) and sound production of the two male types in both contexts. Independently of the individual they face, neomales seem to display more aggressive behaviors than XY males but often fail to become dominant. Agonistic interactions were mainly silent, suggesting that sounds are unnecessary for the establishment of social hierarchy. Although males and neomales produce different kinds of sounds when facing a gravid female, the female does not exhibit a preference. Overall, no differences were observed for hormone circulating concentrations between the two male types. We suggest that the sex chromosomes and/or the sex reversal procedure may have differently shaped the brain of neomales, resulting in differences in the expression of behavior.

Sex determination mechanisms and sex control approaches in aquaculture animals

Aquaculture is one of the most efficient modes of animal protein production and plays an important role in global food security. Aquaculture animals exhibit extraordinarily diverse sexual phenotypes and underlying mechanisms, providing an ideal system to perform sex determination research, one of the important areas in life science. Moreover, sex is also one of the most valuable traits because sexual dimorphism in growth, size, and other economic characteristics commonly exist in aquaculture animals. Here, we synthesize current knowledge of sex determination mechanisms, sex chromosome evolution, reproduction strategies, and sexual dimorphism, and also review several approaches for sex control in aquaculture animals, including artificial gynogenesis, application of sex-specific or sex chromosome-linked markers, artificial sex reversal, as well as gene editing. We anticipate that better understanding of sex determination mechanisms and innovation of sex control approaches will facilitate sustainable development of aquaculture.

The Evolution of Widespread Recombination Suppression on the Dwarf Hamster (Phodopus) X Chromosome

The X chromosome of therian mammals shows strong conservation among distantly related species, limiting insights into the distinct selective processes that have shaped sex chromosome evolution. We constructed a chromosome-scale de novo genome assembly for the Siberian dwarf hamster (Phodopus sungorus), a species reported to show extensive recombination suppression across an entire arm of the X chromosome. Combining a physical genome assembly based on shotgun and long-range proximity ligation sequencing with a dense genetic map, we detected widespread suppression of female recombination across ∼65% of the Phodopus X chromosome. This region of suppressed recombination likely corresponds to the Xp arm, which has previously been shown to be highly heterochromatic. Using additional sequencing data from two closely related species (P. campbelli and P. roborovskii), we show that recombination suppression on Xp appears to be independent of major structural rearrangements. The suppressed Xp arm was enriched for several transposable element families and de-enriched for genes primarily expressed in placenta, but otherwise showed similar gene densities, expression patterns, and rates of molecular evolution when compared to the recombinant Xq arm. Phodopus Xp gene content and order was also broadly conserved relative to the more distantly related rat X chromosome. These data suggest that widespread suppression of recombination has likely evolved through the transient induction of facultative heterochromatin on the Phodopus Xp arm without major changes in chromosome structure or genetic content. Thus, substantial changes in the recombination landscape have so far had relatively subtle influences on patterns of X-linked molecular evolution in these species.

Switches, stability and reversals in the evolutionary history of sexual systems in fish

Sexual systems are highly diverse and have profound consequences for population dynamics and resilience. Yet, little is known about how they evolved. Using phylogenetic Bayesian modelling and a sample of 4614 species, we show that gonochorism is the likely ancestral condition in teleost fish. While all hermaphroditic forms revert quickly to gonochorism, protogyny and simultaneous hermaphroditism are evolutionarily more stable than protandry. In line with theoretical expectations, simultaneous hermaphroditism does not evolve directly from gonochorism but can evolve slowly from sequential hermaphroditism, particularly protandry. We find support for the predictions from life history theory that protogynous, but not protandrous, species live longer than gonochoristic species and invest the least in male gonad mass. The distribution of teleosts' sexual systems on the tree of life does not seem to reflect just adaptive predictions, suggesting that adaptations alone may not fully explain why some sexual forms evolve in some taxa but not others (Williams' paradox). We propose that future studies should incorporate mating systems, spawning behaviours, and the diversity of sex determining mechanisms. Some of the latter might constrain the evolution of hermaphroditism, while the non-duality of the embryological origin of teleost gonads might explain why protogyny predominates over protandry in teleosts.