Unusual Diversity of Sex Chromosomes in African Cichlid Fishes

Rapid turnover and evolution of sex-determining regions in Sebastes rockfishes

Nature has evolved a wealth of sex determination (SD) mechanisms, driven by both genetic and environmental factors. Recent studies of SD in fishes have shown that not all taxa fit the classic paradigm of sex chromosome evolution and diverse SD methods can be found even among closely related species. Here, we apply a suite of genomic approaches to investigate sex-biased genomic variation in eight species of Sebastes rockfish found in the northeast Pacific Ocean. Using recently assembled chromosome-level rockfish genomes, we leverage published sequence data to identify disparate sex chromosomes and sex-biased loci in five species. We identify two putative male sex chromosomes in S. diaconus, a single putative sex chromosome in the sibling species S. carnatus and S. chrysomelas, and an unplaced sex determining contig in the sibling species S. miniatus and S. crocotulus. Our study provides evidence for disparate means of sex determination within a recently diverged set of species and sheds light on the diverse origins of sex determination mechanisms present in the animal kingdom.

Genetic basis of ecologically relevant body shape variation among four genera of cichlid fishes

Divergence in body shape is one of the most widespread and repeated patterns of morphological variation in fishes and is associated with habitat specification and swimming mechanics. Such ecological diversification is the first stage of the explosive adaptive radiation of cichlid fishes in the East African Rift Lakes. We use two hybrid crosses of cichlids (Metriaclima sp. × Aulonocara sp. and Labidochromis sp. × Labeotropheus sp., >975 animals total) to determine the genetic basis of body shape diversification that is similar to benthic-pelagic divergence across fishes. Using a series of both linear and geometric shape measurements, we identified 34 quantitative trait loci (QTL) that underlie various aspects of body shape variation. These QTL are spread throughout the genome, each explaining 3.2-8.6% of phenotypic variation, and are largely modular. Further, QTL are distinct both between these two crosses of Lake Malawi cichlids and compared to previously identified QTL for body shape in fishes such as sticklebacks. We find that body shape is controlled by many genes of small effect. In all, we find that convergent body shape phenotypes commonly observed across fish clades are most likely due to distinct genetic and molecular mechanisms.

No evidence for sex chromosomes in natural populations of the cichlid fish Astatotilapia burtoni

Sex determination (SD) is not conserved among teleost fishes and can even differ between populations of the same species. Across the outstandingly species-rich fish family Cichlidae, more and more SD systems are being discovered. Still, the picture of SD evolution in this group is far from being complete. Lake Tanganyika and its affluent rivers are home to Astatotilapia burtoni, which belongs to the extremely successful East African cichlid lineage Haplochromini. Previously, in different families of an A. burtoni laboratory strain, an XYW system and an XY system have been described. The latter was also found in a second laboratory strain. In a laboratory-reared family descending from a population of the species' southern distribution, a second XY system was discovered. Yet, an analysis of sex chromosomes for the whole species distribution is missing. Here, we examined the genomes of 11 natural populations of A. burtoni, encompassing a wide range of its distribution, for sex-linked regions. We did not detect signs of differentiated sex chromosomes and also not the previously described sex chromosomal systems present in laboratory lines, suggesting different SD systems in the same species under natural and (long-term) artificial conditions. We suggest that SD in A. burtoni is more labile than previously assumed and consists of a combination of non-genetic, polygenic, or poorly differentiated sex chromosomes.

Molecular parallelism in the evolution of a master sex-determining role for the anti-Mullerian hormone receptor 2 gene (amhr2) in Midas cichlids

The evolution of sex chromosomes and their differentiation from autosomes is a major event during genome evolution that happened many times in several lineages. The repeated evolution and lability of sex-determination mechanisms in fishes makes this a well-suited system to test for general patterns in evolution. According to current theory, differentiation is triggered by the suppression of recombination following the evolution of a new master sex-determining gene. However, the molecular mechanisms that establish recombination suppression are known from few examples, owing to the intrinsic difficulties of assembling sex-determining regions (SDRs). The development of forward-genetics and long-read sequencing have generated a wealth of data questioning central aspects of the current theory. Here, we demonstrate that sex in Midas cichlids is determined by an XY system, and identify and assemble the SDR by combining forward-genetics, long-read sequencing and optical mapping. We show how long-reads aid in the detection of artefacts in genotype-phenotype mapping that arise from incomplete genome assemblies. The male-specific region is restricted to a 100-kb segment on chromosome 4 that harbours transposable elements and a Y-specific duplicate of the anti-Mullerian receptor 2 gene, which has evolved master sex-determining functions repeatedly. Our data suggest that amhr2Y originated by an interchromosomal translocation from chromosome 20 to 4 pre-dating the split of Midas and Flier cichlids. In the latter, it is pseudogenized and translocated to another chromosome. Duplication of anti-Mullerian genes is a common route to establishing new sex determiners, highlighting the role of molecular parallelism in the evolution of sex determination.

Master-Key Regulators of Sex Determination in Fish and Other Vertebrates-A Review

In vertebrates, mainly single genes with an allele ratio of 1:1 trigger sex-determination (SD), leading to initial equal sex-ratios. Such genes are designated master-key regulators (MKRs) and are frequently associated with DNA structural variations, such as copy-number variation and null-alleles. Most MKR knowledge comes from fish, especially cichlids, which serve as a genetic model for SD. We list 14 MKRs, of which dmrt1 has been identified in taxonomically distant species such as birds and fish. The identification of MKRs with known involvement in SD, such as amh and fshr, indicates that a common network drives SD. We illustrate a network that affects estrogen/androgen equilibrium, suggesting that structural variation may exert over-expression of the gene and thus form an MKR. However, the reason why certain factors constitute MKRs, whereas others do not is unclear. The limited number of conserved MKRs suggests that their heterologous sequences could be used as targets in future searches for MKRs of additional species. Sex-specific mortality, sex reversal, the role of temperature in SD, and multigenic SD are examined, claiming that these phenomena are often consequences of artificial hybridization. We discuss the essentiality of taxonomic authentication of species to validate purebred origin before MKR searches.

East African cichlid fishes

Cichlid fishes are a very diverse and species-rich family of teleost fishes that inhabit lakes and rivers of India, Africa, and South and Central America. Research has largely focused on East African cichlids of the Rift Lakes Tanganyika, Malawi, and Victoria that constitute the biodiversity hotspots of cichlid fishes. Here, we give an overview of the study system, research questions, and methodologies. Research on cichlid fishes spans many disciplines including ecology, evolution, physiology, genetics, development, and behavioral biology. In this review, we focus on a range of organismal traits, including coloration phenotypes, trophic adaptations, appendages like fins and scales, sensory systems, sex, brains, and behaviors. Moreover, we discuss studies on cichlid phylogenies, plasticity, and general evolutionary patterns, ranging from convergence to speciation rates and the proximate and ultimate mechanisms underlying these processes. From a methodological viewpoint, the last decade has brought great advances in cichlid fish research, particularly through the advent of affordable deep sequencing and advances in genetic manipulations. The ability to integrate across traits and research disciplines, ranging from developmental biology to ecology and evolution, makes cichlid fishes a fascinating research system.

Genomic basis of Y-linked dwarfism in cichlids pursuing alternative reproductive tactics

Sexually antagonistic selection, which favours different optima in males and females, is predicted to play an important role in the evolution of sex chromosomes. Body size is a sexually antagonistic trait in the shell-brooding cichlid fish Lamprologous callipterus, as "bourgeois" males must be large enough to carry empty snail shells to build nests whereas females must be small enough to fit into shells for breeding. In this species, there is also a second male morph: smaller "dwarf" males employ an alternative reproductive strategy by wriggling past spawning females into shells to fertilize eggs. L. callipterus male morphology is passed strictly from father to son, suggesting Y-linkage. However, sex chromosomes had not been previously identified in this species, and the genomic basis of size dimorphism was unknown. Here we used whole-genome sequencing to identify a 2.4-Mb sex-linked region on scaffold_23 with reduced coverage and single nucleotide polymorphism density in both male morphs compared to females. Within this sex region, distinct Y-haplotypes delineate the two male morphs, and candidate genes for body size (GHRHR, a known dwarfism gene) and sex determination (ADCYAP1R1) are in high linkage disequilibrium. Because differences in body size between females and males are under strong selection in L. callipterus, we hypothesize that sexual antagonism over body size initiated early events in sex chromosome evolution, followed by Y divergence to give rise to bourgeois and dwarf male reproductive strategies. Our results are consistent with the hypothesis that sexually antagonistic traits should be linked to young sex chromosomes.

Independent Evolution of Sex Chromosomes and Male Pregnancy-Related Genes in Two Seahorse Species

Unlike birds and mammals, many teleosts have homomorphic sex chromosomes, and changes in the chromosome carrying the sex-determining locus, termed "turnovers", are common. Recent turnovers allow studies of several interesting questions. One question is whether the new sex-determining regions evolve to become completely non-recombining, and if so, how and why. Another is whether (as predicted) evolutionary changes that benefit one sex accumulate in the newly sex-linked region. To study these questions, we analyzed the genome sequences of two seahorse species of the Syngnathidae, a fish group in which many species evolved a unique structure, the male brood pouch. We find that both seahorse species have XY sex chromosome systems, but their sex chromosome pairs are not homologs, implying that at least one turnover event has occurred. The Y-linked regions occupy 63.9% and 95.1% of the entire sex chromosome of the two species and do not exhibit extensive sequence divergence with their X-linked homologs. We find evidence for occasional recombination between the extant sex chromosomes that may account for their homomorphism. We argue that these Y-linked regions did not evolve by recombination suppression after the turnover, but by the ancestral nature of the low crossover rates in these chromosome regions. With such an ancestral crossover landscape, a turnover can instantly create an extensive Y-linked region. Finally, we test for adaptive evolution of male pouch-related genes after they became Y-linked in the seahorse.

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.

Morphometric and Genetic Description of Trophic Adaptations in Cichlid Fishes

Since Darwin, biologists have sought to understand the evolution and origins of phenotypic adaptations. The skull is particularly diverse due to intense natural selection on feeding biomechanics. We investigated the genetic and molecular origins of trophic adaptation using Lake Malawi cichlids, which have undergone an exemplary evolutionary radiation. We analyzed morphological differences in the lateral and ventral head shape among an insectivore that eats by suction feeding, an obligate biting herbivore, and their F2 hybrids. We identified variation in a series of morphological traits—including mandible width, mandible length, and buccal length—that directly affect feeding kinematics and function. Using quantitative trait loci (QTL) mapping, we found that many genes of small effects influence these craniofacial adaptations. Intervals for some traits were enriched in genes related to potassium transport and sensory systems, the latter suggesting co-evolution of feeding structures and sensory adaptations for foraging. Despite these indications of co-evolution of structures, morphological traits did not show covariation. Furthermore, phenotypes largely mapped to distinct genetic intervals, suggesting that a common genetic basis does not generate coordinated changes in shape. Together, these suggest that craniofacial traits are mostly inherited as separate modules, which confers a high potential for the evolution of morphological diversity. Though these traits are not restricted by genetic pleiotropy, functional demands of feeding and sensory structures likely introduce constraints on variation. In all, we provide insights into the quantitative genetic basis of trophic adaptation, identify mechanisms that influence the direction of morphological evolution, and provide molecular inroads to craniofacial variation.

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'.

Absence of Figla-like Gene Is Concordant with Femaleness in Cichlids Harboring the LG1 Sex-Determination System

Oreochromis niloticus has been used as a reference genome for studies of tilapia sex determination (SD) revealing segregating genetic loci on linkage groups (LGs) 1, 3, and 23. The master key regulator genes (MKR) underlying the SD regions on LGs 3 and 23 have been already found. To identify the MKR in fish that segregate for the LG1 XX/XY SD-system, we applied short variant discovery within the sequence reads of the genomic libraries of the Amherst hybrid stock, Coptodon zillii and Sarotherodon galilaeus, which were aligned to a 3-Mbp-region of the O. aureus genome. We obtained 66,372 variants of which six were concordant with the XX/XY model of SD and were conserved across these species, disclosing the male specific figla-like gene. We further validated this observation in O. mossambicus and in the Chitralada hybrid stock. Genome alignment of the 1252-bp transcript showed that the figla-like gene’s size was 2664 bp, and that its three exons were capable of encoding 99 amino acids including a 45-amino-acid basic helix–loop–helix domain that is typical of the ovary development regulator—factor-in-the-germline-alpha (FIGLA). In Amherst gonads, the figla-like gene was exclusively expressed in testes. Thus, the figla-like genomic presence determines male fate by interrupting the female developmental program. This indicates that the figla-like gene is the long-sought SD MKR on LG1.

Insights into chromosomal evolution and sex determination of Pseudobagrus ussuriensis (Bagridae, Siluriformes) based on a chromosome-level genome

Pseudobagrus ussuriensis is an aquaculture catfish with significant sexual dimorphism. In this study, a chromosome-level genome with a size of 741.97 Mb was assembled for female P. ussuriensis. A total of 26 chromosome-level contigs covering 97.34% of the whole-genome assembly were obtained with an N50 of 28.53 Mb and an L50 of 11. A total of 24,075 protein-coding genes were identified, with 91.54% (22,039) genes being functionally annotated. Based on the genome assembly, four chromosome evolution clusters of catfishes were identified and the formation process of P. ussuriensis chromosomes was predicted. A total of 55 sex-related quantitative trait loci (QTLs) with a phenotypic variance explained value of 100% were located on chromosome 8 (chr08). The QTLs and other previously identified sex-specific markers were located in a sex-determining region of 16.83 Mb (from 6.90 to 23.73 Mb) on chr08, which was predicted as the X chromosome. The sex-determining region comprised 554 genes, with 135 of which being differently expressed between males and females/pseudofemales, and 16 candidate sex-determining genes were screened out. The results of this study provided a useful chromosome-level genome for genetic, genomic and evolutionary studies of P. ussuriensis, and also be useful for further studies on sex-determination mechanism analysis and sex-control breeding of this fish.

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.

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.

Something Fishy about Siamese Fighting Fish (Betta splendens) Sex: Polygenic Sex Determination or a Newly Emerged Sex-Determining Region?

Fishes provide a unique and intriguing model system for studying the genomic origin and evolutionary mechanisms underlying sex determination and high sex-chromosome turnover. In this study, the mode of sex determination was investigated in Siamese fighting fish, a species of commercial importance. Genome-wide SNP analyses were performed on 75 individuals (40 males and 35 females) across commercial populations to determine candidate sex-specific/sex-linked loci. In total, 73 male-specific loci were identified and mapped to a 5.6 kb region on chromosome 9, suggesting a putative male-determining region (pMDR) containing localized dmrt1 and znrf3 functional sex developmental genes. Repeat annotations of the pMDR revealed an abundance of transposable elements, particularly Ty3/Gypsy and novel repeats. Remarkably, two out of the 73 male-specific loci were located on chromosomes 7 and 19, implying the existence of polygenic sex determination. Besides male-specific loci, five female-specific loci on chromosome 9 were also observed in certain populations, indicating the possibility of a female-determining region and the polygenic nature of sex determination. An alternative explanation is that male-specific loci derived from other chromosomes or female-specific loci in Siamese fighting fish recently emerged as new sex-determining loci during domestication and repeated hybridization.

Polygenic sex determination produces modular sex polymorphism in an African cichlid fish

SignificanceSex differences in traits can occur when those traits are modified by genetic factors inherited on sex chromosomes. We investigated how sex differences emerge in a species with more than one set of sex chromosomes, measuring a variety of morphological, physiological, and behavioral traits. Rather than exhibiting sexual dimorphism associated with primary sex, the species has higher-order sexual polymorphism in secondary sexual characteristics or more than two phenotypic sexes. Variation in secondary sexual characteristics is modular, involving the interplay of sex-linked and sex-limited traits. Our findings provide implications for how sex determination systems and whole-organism fitness traits coevolve, including that significant creation or loss of variation in diverse traits can occur during transitions among sex chromosome systems.

Genomic consequences of domestication of the Siamese fighting fish

Siamese fighting (betta) fish are among the most popular and morphologically diverse pet fish, but the genetic bases of their domestication and phenotypic diversification are largely unknown. We assembled de novo the genome of a wild Betta splendens and whole-genome sequenced 98 individuals across five closely related species. We find evidence of bidirectional hybridization between domesticated ornamental betta and other wild Betta species. We discover dmrt1 as the main sex determination gene in ornamental betta and that it has lower penetrance in wild B. splendens. Furthermore, we find genes with signatures of recent, strong selection that have large effects on color in specific parts of the body or on the shape of individual fins and that most are unlinked. Our results demonstrate how simple genetic architectures paired with anatomical modularity can lead to vast phenotypic diversity generated during animal domestication and launch betta as a powerful new system for evolutionary genetics.

Genome-Wide SNP Analysis of Hybrid Clariid Fish Reflects the Existence of Polygenic Sex-Determination in the Lineage

The African catfish (Clarias gariepinus) may exhibit the co-existence of XX/XY and ZZ/ZW sex-determination systems (SDSs). However, the SDS of African catfish might be influenced by a polygenic sex-determination (PSD) system, comprising multiple independently segregating sex “switch” loci to determine sex within a species. Here, we aimed to detect the existence of PSD using hybrid. The hybrid produced by crossing male African catfish with female bighead catfish (C. macrocephalus, XX/XY) is a good animal model to study SDSs. Determining the SDS of hybrid catfish can help in understanding the interactions between these two complex SDS systems. Using the genotyping-by-sequencing “DART-seq” approach, we detected seven moderately male-linked loci and seventeen female-linked loci across all the examined hybrid specimens. Most of these loci were not sex-linked in the parental species, suggesting that the hybrid exhibits a combination of different alleles. Annotation of the identified sex-linked loci revealed the presence of one female-linked locus homologous with the B4GALNT1 gene, which is involved in the spermatogenesis pathway and hatchability. However, this locus was not sex-linked in the parental species, and the African catfish might also exhibit PSD.

Sex-specific aging in animals: Perspective and future directions

Sex differences in aging occur in many animal species, and they include sex differences in lifespan, in the onset and progression of age-associated decline, and in physiological and molecular markers of aging. Sex differences in aging vary greatly across the animal kingdom. For example, there are species with longer-lived females, species where males live longer, and species lacking sex differences in lifespan. The underlying causes of sex differences in aging remain mostly unknown. Currently, we do not understand the molecular drivers of sex differences in aging, or whether they are related to the accepted hallmarks or pillars of aging or linked to other well-characterized processes. In particular, understanding the role of sex-determination mechanisms and sex differences in aging is relatively understudied. Here, we take a comparative, interdisciplinary approach to explore various hypotheses about how sex differences in aging arise. We discuss genomic, morphological, and environmental differences between the sexes and how these relate to sex differences in aging. Finally, we present some suggestions for future research in this area and provide recommendations for promising experimental designs.

Roles of anti-Müllerian hormone and its duplicates in sex determination and germ cell proliferation of Nile tilapia

Duplicates of amh are crucial for fish sex determination and differentiation. In Nile tilapia, unlike in other teleosts, amh is located on X chromosome. The Y chromosome amh (amhΔ-y) is mutated with 5 bp insertion and 233 bp deletion in the coding sequence, and tandem duplicate of amh on Y chromosome (amhy) has been identified as the sex determiner. However, the expression of amh, amhΔ-y, and amhy, their roles in germ cell proliferation and the molecular mechanism of how amhy determines sex is still unclear. In this study, expression and functions of each duplicate were analyzed. Sex reversal occurred only when amhy was mutated as revealed by single, double, and triple mutation of the 3 duplicates in XY fish. Homozygous mutation of amhy in YY fish also resulted in sex reversal. Earlier and higher expression of amhy/Amhy was observed in XY gonads compared with amh/Amh during sex determination. Amhy could inhibit the transcription of cyp19a1a through Amhr2/Smads signaling. Loss of cyp19a1a rescued the sex reversal phenotype in XY fish with amhy mutation. Interestingly, mutation of both amh and amhy in XY fish or homozygous mutation of amhy in YY fish resulted in infertile females with significantly increased germ cell proliferation. Taken together, these results indicated that up-regulation of amhy during the critical period of sex determination makes it the sex-determining gene, and it functions through repressing cyp19a1a expression via Amhr2/Smads signaling pathway. Amh retained its function in controlling germ cell proliferation as reported in other teleosts, while amhΔ-y was nonfunctionalized.

A Chromosome-Level Genome Assembly of Mozambique Tilapia (Oreochromis mossambicus) Reveals the Structure of Sex Determining Regions

The Mozambique tilapia (Oreochromis mossambicus) is a fascinating taxon for evolutionary and ecological research. It is an important food fish and one of the most widely distributed tilapias. Because males grow faster than females, genetically male tilapia are preferred in aquaculture. However, studies of sex determination and sex control in O. mossambicus have been hindered by the limited characterization of the genome. To address this gap, we assembled a high-quality genome of O. mossambicus, using a combination of high coverage of Illumina and Nanopore reads, coupled with Hi-C and RNA-Seq data. Our genome assembly spans 1,007 Mb with a scaffold N50 of 11.38 Mb. We successfully anchored and oriented 98.6% of the genome on 22 linkage groups (LGs). Based on re-sequencing data for male and female fishes from three families, O. mossambicus segregates both an XY system on LG14 and a ZW system on LG3. The sex-patterned SNPs shared by two XY families narrowed the sex determining regions to ∼3 Mb on LG14. The shared sex-patterned SNPs included two deleterious missense mutations in ahnak and rhbdd1, indicating the possible roles of these two genes in sex determination. This annotated chromosome-level genome assembly and identification of sex determining regions represents a valuable resource to help understand the evolution of genetic sex determination in tilapias.

A dense linkage map for a large repetitive genome: discovery of the sex-determining region in hybridizing fire-bellied toads (Bombina bombina and Bombina variegata)

Genomic analysis of hybrid zones offers unique insights into emerging reproductive isolation and the dynamics of introgression. Because hybrid genomes consist of blocks inherited from one or the other parental taxon, linkage information is essential. In most cases, the spectrum of local ancestry tracts can be efficiently uncovered from dense linkage maps. Here, we report the development of such a map for the hybridizing toads, Bombina bombina and Bombina variegata (Anura: Bombinatoridae). Faced with the challenge of a large (7–10 Gb), repetitive genome, we set out to identify a large number of Mendelian markers in the nonrepetitive portion of the genome that report B. bombina vs B. variegata ancestry with appropriately quantified statistical support. Bait sequences for targeted enrichment were selected from a draft genome assembly, after filtering highly repetitive sequences. We developed a novel approach to infer the most likely diplotype per sample and locus from the raw read mapping data, which is robust to over-merging and obviates arbitrary filtering thresholds. Validation of the resulting map with 4755 markers underscored the large-scale synteny between Bombina and Xenopus tropicalis. By assessing the sex of late-stage F2 tadpoles from histological sections, we identified the sex-determining region in the Bombina genome to 7 cM on LG5, which is homologous to X. tropicalis chromosome 5, and inferred male heterogamety. Interestingly, chromosome 5 has been repeatedly recruited as a sex chromosome in anurans with XY sex determination.

Sex Chromosomes and Master Sex-Determining Genes in Turtles and Other Reptiles

Among tetrapods, the well differentiated heteromorphic sex chromosomes of birds and mammals have been highly investigated and their master sex-determining (MSD) gene, Dmrt1 and SRY, respectively, have been identified. The homomorphic sex chromosomes of reptiles have been the least studied, but the gap with birds and mammals has begun to fill. This review describes our current knowledge of reptilian sex chromosomes at the cytogenetic and molecular level. Most of it arose recently from various studies comparing male to female gene content. This includes restriction site-associated DNA sequencing (RAD-Seq) experiments in several male and female samples, RNA sequencing and identification of Z- or X-linked genes by male/female comparative transcriptome coverage, and male/female transcriptomic or transcriptome/genome substraction approaches allowing the identification of Y- or W-linked transcripts. A few putative master sex-determining (MSD) genes have been proposed, but none has been demonstrated yet. Lastly, future directions in the field of reptilian sex chromosomes and their MSD gene studies are considered.

A new linkage group involved in sex determination in haplotilapiine cichlids

The high diversity of sex chromosomes and sex determination systems among haplotilapiines suggests that this large cichlid clade is a good model for investigating the evolution of genetics of sex determination. Nonetheless, information about sex determination in this clade remains sparse. The present study reports a microsatellite marker that is closely associated with sex in Xenotilapia rotundiventralis from Lake Tanganyika, Africa. This study is the first to suggest the role of linkage group 17 in sex determination in haplotilapiine cichlids.

Against the mainstream: exceptional evolutionary stability of ZW sex chromosomes across the fish families Triportheidae and Gasteropelecidae (Teleostei: Characiformes)

Teleost fishes exhibit a breath-taking diversity of sex determination and differentiation mechanisms. They encompass at least nine sex chromosome systems with often low degree of differentiation, high rate of inter- and intra-specific variability, and frequent turnovers. Nevertheless, several mainly female heterogametic systems at an advanced stage of genetic differentiation and high evolutionary stability have been also found across teleosts, especially among Neotropical characiforms. In this study, we aim to characterize the ZZ/ZW sex chromosome system in representatives of the Triportheidae family (Triportheus auritus, Agoniates halecinus, and the basal-most species Lignobrycon myersi) and its sister clade Gasteropelecidae (Carnegiella strigata, Gasteropelecus levis, and Thoracocharax stellatus). We applied both conventional and molecular cytogenetic approaches including chromosomal mapping of 5S and 18S ribosomal DNA clusters, cross-species chromosome painting (Zoo-FISH) with sex chromosome-derived probes and comparative genomic hybridization (CGH). We identified the ZW sex chromosome system for the first time in A. halecinus and G. levis and also in C. strigata formerly reported to lack sex chromosomes. We also brought evidence for possible mechanisms underlying the sex chromosome differentiation, including inversions, repetitive DNA accumulation, and exchange of genetic material. Our Zoo-FISH experiments further strongly indicated that the ZW sex chromosomes of Triportheidae and Gasteropelecidae are homeologous, suggesting their origin before the split of these lineages (approx. 40-70 million years ago). Such extent of sex chromosome stability is almost exceptional in teleosts, and hence, these lineages afford a special opportunity to scrutinize unique evolutionary forces and pressures shaping sex chromosome evolution in fishes and vertebrates in general.

Sex-biased gene expression and recent sex chromosome turnover

Cichlids are well known for their propensity to radiate generating arrays of morphologically and ecologically diverse species in short evolutionary time. Following this rapid evolutionary pace, cichlids show high rates of sex chromosome turnover. We here studied the evolution of sex-biased gene (SBG) expression in 14 recently diverged taxa of the Lake Tanganyika Tropheini cichlids, which show different XY sex chromosomes. Across species, sex chromosome sequence divergence predates divergence in expression between the sexes. Only one sex chromosome, the oldest, showed signs of demasculinization in gene expression and potentially contribution to the resolution of sexual conflict. SBGs in general showed high rates of turnovers and evolved mostly under drift. Sexual selection did not shape the rapid evolutionary changes of SBGs. Male-biased genes evolved faster than female-biased genes, which seem to be under more phylogenetic constraint. We found a relationship between the degree of sex bias and sequence evolution driven by sequence differences among the sexes. Consistent with other species, strong sex bias towards sex-limited expression contributes to resolving sexual conflict in cichlids. This article is part of the theme issue 'Challenging the paradigm in sex chromosome evolution: empirical and theoretical insights with a focus on vertebrates (Part II)'.

Dynamics of sex chromosome evolution in a rapid radiation of cichlid fishes

Sex is a fundamental trait determined by environmental and/or genetic factors, including sex chromosomes. Sex chromosomes are studied in species scattered across the tree of life, yet little is known about tempo and mode of sex chromosome evolution among closely related species. Here, we examine sex chromosome evolution in the adaptive radiation of cichlid fishes in Lake Tanganyika. Through the analysis of male and female genomes from 244 cichlid taxa (189 described species with 5 represented with two local variants/populations; 50 undescribed species) and of 396 multitissue transcriptomes from 66 taxa, we identify signatures of sex chromosomes in 79 taxa, involving 12 linkage groups. We find that Tanganyikan cichlids have the highest rates of sex chromosome turnover and heterogamety transitions known to date. We show that sex chromosome recruitment is not at random. Moreover convergently emerged sex chromosomes in cichlids support the “limited options” hypothesis of sex chromosome evolution.

Epigenetic Regulation and Environmental Sex Determination in Cichlid Fishes

Studying environmental sex determination (ESD) in cichlids provides a phylogenetic and comparative approach to understand the evolution of the underlying mechanisms, their impact on the evolution of the overlying systems, and the neuroethology of life history strategies. Natural selection normally favors parents who invest equally in the development of male and female offspring, but evolution may favor deviations from this 50:50 ratio when environmental conditions produce an advantage for doing so. Many species of cichlids demonstrate ESD in response to water chemistry (temperature, pH, and oxygen concentration). The relative strengths of and the exact interactions between these factors vary between congeners, demonstrating genetic variation in sensitivity. The presence of sizable proportions of the less common sex towards the environmental extremes in most species strongly suggests the presence of some genetic sex-determining loci acting in parallel with the ESD factors. Sex determination and differentiation in these species does not seem to result in the organization of a final and irreversible sexual fate, so much as a life-long ongoing battle between competing male- and female-determining genetic and hormonal networks governed by epigenetic factors. We discuss what is and is not known about the epigenetic mechanism behind the differentiation of both gonads and sex differences in the brain. Beyond the well-studied tilapia species, the 2 best-studied dwarf cichlid systems showing ESD are the South American genus Apistogramma and the West African genus Pelvicachromis. Both species demonstrate male morphs with alternative reproductive tactics. We discuss the further neuroethology opportunities such systems provide to the study of epigenetics of alternative life history strategies and other behavioral variation.

How did the guppy Y chromosome evolve?

The sex chromosome pairs of many species do not undergo genetic recombination, unlike the autosomes. It has been proposed that the suppressed recombination results from natural selection favouring close linkage between sex-determining genes and mutations on this chromosome with advantages in one sex, but disadvantages in the other (these are called sexually antagonistic mutations). No example of such selection leading to suppressed recombination has been described, but populations of the guppy display sexually antagonistic mutations (affecting male coloration), and would be expected to evolve suppressed recombination. In extant close relatives of the guppy, the Y chromosomes have suppressed recombination, and have lost all the genes present on the X (this is called genetic degeneration). However, the guppy Y occasionally recombines with its X, despite carrying sexually antagonistic mutations. We describe evidence that a new Y evolved recently in the guppy, from an X chromosome like that in these relatives, replacing the old, degenerated Y, and explaining why the guppy pair still recombine. The male coloration factors probably arose after the new Y evolved, and have already evolved expression that is confined to males, a different way to avoid the conflict between the sexes.

We report new findings concerning the long-studied the guppy XY pair, which has remained somewhat mystifying. We show that it can be understood as a case of a recent sex chromosome turnover event in which an older, highly degenerated Y chromosome was lost, and creation of a new sex chromosome from the ancestral X. This chromosome acquired a male-determining factor, possibly by a mutation in (or a duplication of) a previously X-linked gene, or (less likely) by movement of an ancestral Y-linked maleness factor onto the X. We relate the findings to theoretical models of such events, and argue that the proposed change was free from factors thought to impede such turnovers. The change resulted in the intriguing situation where the X chromosome is old and the Y is much younger, and we discuss some other species where a similar change seems likely to have occurred.

Integrative genomics reveals paths to sex dimorphism in Salix purpurea L

Sex dimorphism and gene expression were studied in developing catkins in 159 F2 individuals from the bioenergy crop Salix purpurea, and potential mechanisms and pathways for regulating sex development were explored. Differential expression, eQTL, bisulfite sequencing, and network analysis were used to characterize sex dimorphism, detect candidate master regulator genes, and identify pathways through which the sex determination region (SDR) may mediate sex dimorphism. Eleven genes are presented as candidates for master regulators of sex, supported by gene expression and network analyses. These include genes putatively involved in hormone signaling, epigenetic modification, and regulation of transcription. eQTL analysis revealed a suite of transcription factors and genes involved in secondary metabolism and floral development that were predicted to be under direct control of the sex determination region. Furthermore, data from bisulfite sequencing and small RNA sequencing revealed strong differences in expression between males and females that would implicate both of these processes in sex dimorphism pathways. These data indicate that the mechanism of sex determination in Salix purpurea is likely different from that observed in the related genus Populus. This further demonstrates the dynamic nature of SDRs in plants, which involves a multitude of mechanisms of sex determination and a high rate of turnover.

The timing of genetic degeneration of sex chromosomes

Genetic degeneration is an extraordinary feature of sex chromosomes, with the loss of functions of Y-linked genes in species with XY systems, and W-linked genes in ZW systems, eventually affecting almost all genes. Although degeneration is familiar to most biologists, important aspects are not yet well understood, including how quickly a Y or W chromosome can become completely degenerated. I review the current understanding of the time-course of degeneration. Degeneration starts after crossing over between the sex chromosome pair stops, and theoretical models predict an initially fast degeneration rate and a later much slower one. It has become possible to estimate the two quantities that the models suggest are the most important in determining degeneration rates-the size of the sex-linked region, and the time when recombination became suppressed (which can be estimated using Y-X or W-Z sequence divergence). However, quantifying degeneration is still difficult. I review evidence on gene losses (based on coverage analysis) or loss of function (by classifying coding sequences into functional alleles and pseudogenes). I also review evidence about whether small genome regions degenerate, or only large ones, whether selective constraints on the genes in a sex-linked region also strongly affect degeneration rates, and about how long it takes before all (or almost all) genes are lost. This article is part of the theme issue 'Challenging the paradigm in sex chromosome evolution: empirical and theoretical insights with a focus on vertebrates (Part I)'.

Are Geckos Special in Sex Determination? Independently Evolved Differentiated ZZ/ZW Sex Chromosomes in Carphodactylid Geckos

Amniotes possess astonishing variability in sex determination ranging from environmental sex determination (ESD) to genotypic sex determination (GSD) with highly differentiated sex chromosomes. Geckos are one of the few amniote groups with substantial variability in sex determination. What makes them special in this respect? We hypothesized that the extraordinary variability of sex determination in geckos can be explained by two alternatives: 1) unusual lability of sex determination, predicting that the current GSD systems were recently formed and are prone to turnovers; and 2) independent transitions from the ancestral ESD to later stable GSD, which assumes that geckos possessed ancestrally ESD, but once sex chromosomes emerged, they remain stable in the long term. Here, based on genomic data, we document that the differentiated ZZ/ZW sex chromosomes evolved within carphodactylid geckos independently from other gekkotan lineages and remained stable in the genera Nephrurus, Underwoodisaurus, and Saltuarius for at least 15 Myr and potentially up to 45 Myr. These results together with evidence for the stability of sex chromosomes in other gekkotan lineages support more our second hypothesis suggesting that geckos do not dramatically differ from the evolutionary transitions in sex determination observed in the majority of the amniote lineages.

The Dryas iulia Genome Supports Multiple Gains of a W Chromosome from a B Chromosome in Butterflies

In butterflies and moths, which exhibit highly variable sex determination mechanisms, the homogametic Z chromosome is deeply conserved and is featured in many genome assemblies. The evolution and origin of the female W sex chromosome, however, remains mostly unknown. Previous studies have proposed that a ZZ/Z0 sex determination system is ancestral to Lepidoptera, and that W chromosomes may originate from sex-linked B chromosomes. Here, we sequence and assemble the female Dryas iulia genome into 32 highly contiguous ordered and oriented chromosomes, including the Z and W sex chromosomes. We then use sex-specific Hi-C, ATAC-seq, PRO-seq, and whole-genome DNA sequence data sets to test if features of the D. iulia W chromosome are consistent with a hypothesized B chromosome origin. We show that the putative W chromosome displays female-associated DNA sequence, gene expression, and chromatin accessibility to confirm the sex-linked function of the W sequence. In contrast with expectations from studies of homologous sex chromosomes, highly repetitive DNA content on the W chromosome, the sole presence of domesticated repetitive elements in functional DNA, and lack of sequence homology with the Z chromosome or autosomes is most consistent with a B chromosome origin for the W, although it remains challenging to rule out extensive sequence divergence. Synteny analysis of the D. iulia W chromosome with other female lepidopteran genome assemblies shows no homology between W chromosomes and suggests multiple, independent origins of the W chromosome from a B chromosome likely occurred in butterflies.

Gene Variant of Barrier to Autointegration Factor 2 (Banf2w) Is Concordant with Female Determination in Cichlids

Oreochromis fishes exhibit variability of sex-determination (SD) genes whose characterization contributes to understanding of the sex differentiation network, and to effective tilapia farming, which requires all-male culture. However, O. niloticus (On) amh is the only master-key regulator (MKR) of SD that has been mapped (XY/XX SD-system on LG23). In O. aureus (Oa), LG3 controls a WZ/ZZ SD-system that has recently been delimited to 9.2 Mbp, with an embedded interval rich with female-specific variation, harboring two paics genes and banf2. Developing genetic markers within this interval and using a hybrid Oa stock that demonstrates no recombination repression in LG3, we mapped the critical SD region to 235 Kbp on the orthologous On physical map (p < 1.5 × 10-26). DNA-seq assembly and peak-proportion analysis of variation based on Sanger chromatograms allowed the characterization of copy-number variation (CNV) of banf2. Oa males had three exons capable of encoding 90-amino-acid polypeptides, yet in Oa females, we found an extra copy with an 89-amino-acid polypeptide and three non-conservative amino acid substitutions, designated as banf2w. CNV analysis suggested the existence of two to five copies of banf2 in diploidic Cichlidae. Disrupting the Hardy-Weinberg equilibrium (p < 4.2 × 10-3), banf2w was concordant with female determination in Oa and in three cichlids with LG3 WZ/ZZ SD-systems (O. tanganicae, O. hornorum and Pelmatolapia mariae). Furthermore, exclusive RNA-seq expression in Oa females strengthened the candidacy of banf2w as the long-sought LG3 SD MKR. As banf genes mediate nuclear assembly, chromatin organization, gene expression and gonad development, banf2w may play a fundamental role inducing female nucleus formation that is essential for WZ/ZZ SD.

Using GC Content to Compare Recombination Patterns on the Sex Chromosomes and Autosomes of the Guppy, Poecilia reticulata, and Its Close Outgroup Species

Genetic and physical mapping of the guppy (Poecilia reticulata) have shown that recombination patterns differ greatly between males and females. Crossover events occur evenly across the chromosomes in females, but in male meiosis they are restricted to the tip furthest from the centromere of each chromosome, creating very high recombination rates per megabase, as in pseudoautosomal regions of mammalian sex chromosomes. We used GC content to indirectly infer recombination patterns on guppy chromosomes, based on evidence that recombination is associated with GC-biased gene conversion, so that genome regions with high recombination rates should be detectable by high GC content. We used intron sequences and third positions of codons to make comparisons between sequences that are matched, as far as possible, and are all probably under weak selection. Almost all guppy chromosomes, including the sex chromosome (LG12), have very high GC values near their assembly ends, suggesting high recombination rates due to strong crossover localization in male meiosis. Our test does not suggest that the guppy XY pair has stronger crossover localization than the autosomes, or than the homologous chromosome in the close relative, the platyfish (Xiphophorus maculatus). We therefore conclude that the guppy XY pair has not recently undergone an evolutionary change to a different recombination pattern, or reduced its crossover rate, but that the guppy evolved Y-linkage due to acquiring a male-determining factor that also conferred the male crossover pattern. We also identify the centromere ends of guppy chromosomes, which were not determined in the genome assembly.

Origin of a Giant Sex Chromosome

Chromosome size and morphology vary within and among species, but little is known about the proximate or ultimate causes of these differences. Cichlid fish species in the tribe Oreochromini share an unusual giant chromosome that is ∼3 times longer than the other chromosomes. This giant chromosome functions as a sex chromosome in some of these species. We test two hypotheses of how this giant sex chromosome may have evolved. The first hypothesis proposes that it evolved by accumulating repetitive elements as recombination was reduced around a dominant sex determination locus, as suggested by canonical models of sex chromosome evolution. An alternative hypothesis is that the giant sex chromosome originated via the fusion of an autosome with a highly repetitive B chromosome, one of which carried a sex determination locus. We test these hypotheses using comparative analysis of chromosome-scale cichlid and teleost genomes. We find that the giant sex chromosome consists of three distinct regions based on patterns of recombination, gene and transposable element content, and synteny to the ancestral autosome. The WZ sex determination locus encompasses the last ∼105 Mb of the 134-Mb giant chromosome. The last 47 Mb of the giant chromosome shares no obvious homology to any ancestral chromosome. Comparisons across 69 teleost genomes reveal that the giant sex chromosome contains unparalleled amounts of endogenous retroviral elements, immunoglobulin genes, and long noncoding RNAs. The results favor the B chromosome fusion hypothesis for the origin of the giant chromosome.

Sex determination through X-Y heterogamety in Salix nigra

The development of non-recombining sex chromosomes has radical effects on the evolution of discrete sexes and sexual dimorphism. Although dioecy is rare in plants, sex chromosomes have evolved repeatedly throughout the diversification of angiosperms, and many of these sex chromosomes are relatively young compared to those found in vertebrates. In this study, we designed and used a sequence capture array to identify a novel sex-linked region (SLR) in Salix nigra, a basal species in the willow clade, and demonstrated that this species has XY heterogamety. We did not detect any genetic overlap with the previously characterized ZW SLRs in willows, which map to a different chromosome. The S. nigra SLR is characterized by strong recombination suppression across a 2 MB region and an excess of low-frequency alleles, resulting in a low Tajima's D compared to the remainder of the genome. We speculate that either a recent bottleneck in population size or factors related to positive or background selection generated this differential pattern of Tajima's D on the X and autosomes. This discovery provides insights into factors that may influence the evolution of sex chromosomes in plants and contributes to a large number of recent observations that underscore their dynamic nature.

When and how do sex-linked regions become sex chromosomes?

The attention given to heteromorphism and genetic degeneration of "classical sex chromosomes" (Y chromosomes in XY systems, and the W in ZW systems that were studied first and are best described) has perhaps created the impression that the absence of recombination between sex chromosomes is inevitable. I here argue that continued recombination is often to be expected, that absence of recombination is surprising and demands further study, and that the involvement of selection in reduced recombination is not yet well understood. Despite a long history of investigations of sex chromosome pairs, there is a need for more quantitative approaches to studying sex-linked regions. I describe a scheme to help understand the relationships between different properties of sex-linked regions. Specifically, I focus on their sizes (differentiating between small regions and extensive fully sex-linked ones), the times when they evolved, and their differentiation, and review studies using DNA sequencing in nonmodel organisms that are providing information about the processes causing these properties.

An Investigation of ZZ/ZW and XX/XY Sex Determination Systems in North African Catfish (Clarias gariepinus, )

An investigation of sex-specific loci may provide important insights into fish sex determination strategies. This may be useful for biotechnological purposes, for example, to produce all-male or all-female fish for commercial breeding. The North African catfish species, Clarias gariepinus, has been widely adopted for aquaculture because its superior growth and disease resistance render the species suitable for hybridization with other catfish to improve the productivity and quality of fish meat. This species has either a ZZ/ZW or XX/XY sex determination system. Here, we investigate and characterize these systems using high-throughput genome complexity reduction sequencing as Diversity Arrays Technology. This approach was effective in identifying moderately sex-linked loci with both single-nucleotide polymorphisms (SNPs) and restriction fragment presence/absence (PA) markers in 30 perfectly sexed individuals of C. gariepinus. However, SNPs based markers were not found in this study. In total, 41 loci met the criteria for being moderately male-linked (with male vs. female ratios 80:20 and 70:30), while 25 loci were found to be moderately linked to female sex. No strictly male- or female-linked loci were detected. Seven moderately male-linked loci were partially homologous to some classes of transposable elements and three moderately male-linked loci were partially homologous to functional genes. Our data showed that the male heterogametic XX/XY sex determination system should co-exist with the ZZ/ZW system in C. gariepinus. Our finding of the co-existence of XX/XY and ZZ/ZW systems can be applied to benefit commercial breeding of this species in Thailand. This approach using moderately sex-linked loci provides a solid baseline for revealing sex determination mechanisms and identify potential sex determination regions in catfish, allowing further investigation of genetic improvements in breeding programs.

Identification of a novel sex determining chromosome in cichlid fishes that acts as XY or ZW in different lineages

Sex determination systems are highly conserved among most vertebrates with genetic sex determination, but can be variable and evolve rapidly in some. Here, we study sex determination in a clade with exceptionally high sex chromosome turnover rates. We identify the sex determining chromosomes in three interspecific crosses of haplochromine cichlid fishes from Lakes Victoria and Malawi. We find evidence for different sex determiners in each cross. In the Malawi cross and one Victoria cross the same chromosome is sex-linked but while females are the heterogametic sex in the Malawi species, males are the heterogametic sex in the Victoria species. This chromosome has not previously been reported to be sex determining in cichlids, increasing the number of different chromosomes shown to be sex determining in cichlids to 12. All Lake Victoria species of our crosses are less than 15,000 years divergent, and we identified different sex determiners among them. Our study provides further evidence for the diversity and evolutionary flexibility of sex determination in cichlids, factors which might contribute to their rapid adaptive radiations.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s10750-021-04560-7.

Network architecture and sex chromosome turnovers: Do epistatic interactions shape patterns of sex chromosome replacement?

Recent studies have revealed an astonishing diversity of sex chromosomes in many vertebrate lineages, prompting questions about the mechanisms of sex chromosome turnover. While there is considerable population genetic theory about the evolutionary forces promoting sex chromosome replacement, this theory has not yet been integrated with our understanding of the molecular and developmental genetics of sex determination. Here, we review recent data to examine four questions about how the structure of gene networks influences the evolution of sex determination. We argue that patterns of epistasis, arising from the structure of genetic networks, may play an important role in regulating the rates and patterns of sex chromosome replacement.

Movement of transposable elements contributes to cichlid diversity

African cichlid fishes are a prime model for studying speciation mechanisms. Despite the development of extensive genomic resources, it has been difficult to determine which sources of genetic variation are responsible for cichlid phenotypic variation. One of their most variable phenotypes is visual sensitivity, with some of the largest spectral shifts among vertebrates. These shifts arise primarily from differential expression of seven cone opsin genes. By mapping expression quantitative trait loci (eQTL) in intergeneric crosses of Lake Malawi cichlids, we previously identified four causative genetic variants that correspond to indels in the promoters of either key transcription factors or an opsin gene. In this comprehensive study, we show that these indels are the result of the movement of transposable elements (TEs) that correlate with opsin expression variation across the Malawi flock. In tracking the evolutionary history of these particular indels, we found they are endemic to Lake Malawi, suggesting that these TEs are recently active and are segregating within the Malawi cichlid lineage. However, an independent indel has arisen at a similar genomic location in one locus outside of the Malawi flock. The convergence in TE movement suggests these loci are primed for TE insertion and subsequent deletions. Increased TE mobility may be associated with interspecific hybridization, which disrupts mechanisms of TE suppression. This might provide a link between cichlid hybridization and accelerated regulatory variation. Overall, our study suggests that TEs may be an important driver of key regulatory changes, facilitating rapid phenotypic change and possibly speciation in African cichlids.

High-quality chromosome-level genomes of two tilapia species reveal their evolution of repeat sequences and sex chromosomes

Tilapias are one of the most farmed fishes that are coined as "aquatic chicken" by the food industry. Nile tilapia and blue tilapia exhibit very recent transition of sex chromosome systems since their divergence approximately five million years ago, making them a great model for elucidating the molecular and evolutionary mechanisms of sex chromosome turnovers. Studies of their sex-determining pathways are also critical for developing genetic sex control in aquaculture. We report here the newly produced genomes of Nile tilapia and blue tilapia that integrate long-read sequencing and chromatin conformation data. The two nearly complete genomes have anchored over 97% of the sequences into linkage groups (LGs), and assembled majorities of complex repetitive regions including telomeres, centromeres and rDNA clusters. In particular, we inferred two episodes of repeat expansion at LG3 respectively in the ancestor of cichlids and that of tilapias. The consequential large heterochromatic region concentrated at one end of LG3 comprises tandem arrays of mRNA and small RNA genes, among which we have identified a candidate female determining gene Paics in blue tilapia. Paics shows female-specific patterns of single-nucleotide variants, copy numbers and expression patterns in gonads during early gonadogenesis. Our work provides a very important genomic resource for functional studies of cichlids, and suggested that unequal distribution of repeat content that impacts the local recombination rate might make some chromosomes more likely to become sex chromosomes.

Multiple Sex Chromosomes and Evolutionary Relationships in Amazonian Catfishes: The Outstanding Model of the Genus Harttia (Siluriformes: Loricariidae)

The armored Harttia catfishes present great species diversity and remarkable cytogenetic variation, including different sex chromosome systems. Here we analyzed three new species, H. duriventris, H. villasboas and H. rondoni, using both conventional and molecular cytogenetic techniques (Giemsa-staining and C-banding), including the mapping of repetitive DNAs using fluorescence in situ hybridization (FISH) and comparative genomic hybridization (CGH) experiments. Both H. duriventris and H. villasboas have 2n = ♀56/♂55 chromosomes, and an X₁X₁X₂X₂ /X₁X₂Y sex chromosome system, while a proto or neo-XY system is proposed for H. rondoni (2n = 54♀♂). Single motifs of 5S and 18S rDNA occur in all three species, with the latter being also mapped in the sex chromosomes. The results confirm the general evolutionary trend that has been noticed for the genus: an extensive variation on their chromosome number, single sites of rDNA sequences and the occurrence of multiple sex chromosomes. Comparative genomic analyses with another congeneric species, H. punctata, reveal that the X₁X₂Y sex chromosomes of these species share the genomic contents, indicating a probable common origin. The remarkable karyotypic variation, including sex chromosomes systems, makes Harttia a suitable model for evolutionary studies focusing on karyotype differentiation and sex chromosome evolution among lower vertebrates.

Reconstruction of the birth of a male sex chromosome present in Atlantic herring

Understanding the evolution of sex determination mechanisms and sex chromosomes is of fundamental importance in biology. Here we have reconstructed the evolution of the sex-determining region in the Atlantic herring. The region is small and contains only three intact genes. The candidate sex-determining factor BMPR1BBY is an evolutionary innovation in the herring lineage. It encodes a truncated form of a BMP type I receptor, which originated by gene duplication and underwent rapid protein evolution. The receptor has maintained its kinase activity and has the potential to induce development of testis. The other two genes in the sex-determining region, CATSPERG and CATSPER3Y, are male beneficial genes because they encode proteins predicted to be essential for sperm to fertilize the egg.

The mechanisms underlying sex determination are astonishingly plastic. Particularly the triggers for the molecular machinery, which recalls either the male or female developmental program, are highly variable and have evolved independently and repeatedly. Fish show a huge variety of sex determination systems, including both genetic and environmental triggers. The advent of sex chromosomes is assumed to stabilize genetic sex determination. However, because sex chromosomes are notoriously cluttered with repetitive DNA and pseudogenes, the study of their evolution is hampered. Here we reconstruct the birth of a Y chromosome present in the Atlantic herring. The region is tiny (230 kb) and contains only three intact genes. The candidate male-determining gene BMPR1BBY encodes a truncated form of a BMP1B receptor, which originated by gene duplication and translocation and underwent rapid protein evolution. BMPR1BBY phosphorylates SMADs in the absence of ligand and thus has the potential to induce testis formation. The Y region also contains two genes encoding subunits of the sperm-specific Ca²⁺ channel CatSper required for male fertility. The herring Y chromosome conforms with a characteristic feature of many sex chromosomes, namely, suppressed recombination between a sex-determining factor and genes that are beneficial for the given sex. However, the herring Y differs from other sex chromosomes in that suppression of recombination is restricted to an ∼500-kb region harboring the male-specific and sex-associated regions. As a consequence, any degeneration on the herring Y chromosome is restricted to those genes located in the small region affected by suppressed recombination.

Structure and Sequence of the Sex Determining Locus in Two Wild Populations of Nile Tilapia

In domesticated strains of the Nile tilapia, phenotypic sex has been linked to genetic variants on linkage groups 1, 20 and 23. This diversity of sex-loci might reflect a naturally polymorphic sex determination system in Nile tilapia, or it might be an artefact arising from the process of domestication. Here, we searched for sex-determiners in wild populations from Kpandu, Lake Volta (Ghana-West Africa), and from Lake Koka (Ethiopia-East Africa) that have not been subjected to any genetic manipulation. We analysed lab-reared families using double-digest Restriction Associated DNA sequencing (ddRAD) and analysed wild-caught males and females with pooled whole-genome sequencing (WGS). Strong sex-linked signals were found on LG23 in both populations, and sex-linked signals with LG3 were observed in Kpandu samples. WGS uncovered blocks of high sequence coverage, suggesting the presence of B chromosomes. We confirmed the existence of a tandem amh duplication in LG23 in both populations and determined its breakpoints between the oaz1 and dot1l genes. We found two common deletions of ~5 kb in males and confirmed the presence of both amhY and amh∆Y genes. Males from Lake Koka lack both the previously reported 234 bp deletion and the 5 bp frameshift-insertion that creates a premature stop codon in amh∆Y.