Genetic Sex Determination Research Articles

A Fish of Multiple Faces, Which Show Us Enigmatic and Incredible Phenomena in Nature: Biology and Cytogenetics of the Genus Carassius.

Sexual vs. asexual reproduction—unisexual vs. bisexual populations—diploid vs. polyploid biotypes—genetic vs. environmental sex determination: all these natural phenomena are associated with the genus of teleost fish, Carassius. This review places emphasis on two Carassius entities with completely different biological characteristics: one globally widespread and invasive Carassius gibelio, and the other C. carassius with a decreasing trend of natural occurrence. Comprehensive biological and cytogenetic knowledge of both entities, including the physical interactions between them, can help to balance the advantages of highly invasive and disadvantages of threatened species. For example, the benefits of a wide-ranged colonization can lead to the extinction of native species or be compensated by parasitic enemies and lead to equilibrium. This review emphasizes the comprehensive biology and cytogenetic knowledge and the importance of the Carassius genus as one of the most useful experimental vertebrate models for evolutionary biology and genetics. Secondly, the review points out that effective molecular cytogenetics should be used for the identification of various species, ploidy levels, and hybrids. The proposed investigation of these hallmark characteristics in Carassius may be applied in conservation efforts to sustain threatened populations in their native ranges. Furthermore, the review focuses on the consequences of the co-occurrence of native and non-native species and outlines future perspectives of Carassius research.

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.

Biotechnological manipulation of the transition from genetic to temperature-dependent sex determination to obtain high quality neomale in aquaculture

Sex control is an essential technology to produce mono-sex populations with high growth rates and product quality in aquaculture. To produce an all-female population in fish with the XX/XY sex determination system, the generation of XX neomales is the critical step and they have been successfully created by different approaches, such as androgens, aromatase inhibitors and high temperature treatment. However, there is no comprehensive comparison of testis structure, quality of sperm and offspring in XX neomales produced by different approaches. Yellow catfish are an excellent model to study sex determination and sex reversal since robust sex-specific molecular markers and genetic sex determination have been identified. Previously, we observed that 17α-methyltestosterone (MT) treatment could not induce functional testes in XX yellow catfish. In this study, we found that high-temperature treatment caused a delay in ovarian development instead of sex reversal in yellow catfish. However, high temperature combined with aromatase inhibitor (Letrozole, LZ) treatment could dramatically improve the efficiency of sex reversal compared with using LZ alone (the neomale was designated XXm-LZ), while they showed the same testis phenotype. Interestingly, the offspring of XXm-LZ transitioned from genetic to temperature-dependent sex determination, which could be easily sex reversed after treatment with high temperature (the neomale was designated XXm-HT-1). Then, we evaluated the reproductive performances and sperm parameters in XY male, XXm-LZ and XXm-HT-1 neomales. Strikingly, the fertilization rate, hatching rate and survival rate of XXm-HT-1 catfish were as good as those of XYm yellow catfish and better than those of XXm-LZ yellow catfish, while the malformation rate of fries produced by XXm-HT-1 catfish was significantly lower than that of XXm-LZ catfish. In addition, the expression of miR-141/−200a/−429b and dmrt1/pfpdz1 was negatively and positively correlated with sperm quality, respectively. However, the transition from genetic to temperature-dependent sex determination was transient and cannot be completely inherited by the next generation in yellow catfish. We concluded that precise application of the transition from genetic to temperature-dependent sex determination could produce the highest quality neomales for sex control in aquaculture after comparing different approaches.

Sex locus and sex markers identification using whole genome pool-sequencing approach in the largemouth bass (Micropterus Salmoides L.)

The mechanism of sex determination in teleost evolves in an amazing speed contrary to mammals and birds. Different populations of the same species may use different sex determination systems. Largemouth bass is an aquaculture importance and recreational fish, which belongs to the family of Centrarchidae, genus Micropterus. However, molecular basis of genetic sex determination currently remains unclear. We applied a whole genome pool-sequencing approach to better characterize sex determination in this fish. Our results confirmed that sex determination system in largemouth bass is male heterogametic (XX/XY). In this study, males and females genomic DNA pools were sequenced using a whole genomic sequencing approach. Using genome comparative analysis, we identified a large sex differentiated region (~ 1.7 Mb) at the beginning of chromosome 7 on largemouth bass. This non-recombination region contained enriched male-biased fragments and high-density of male-biased genetic polymorphisms. Meanwhile, sequence synteny analysis between/within sex chromosomes showed that many chromosomal rearrangement events have taken place in this region. Based on this genetic information, we successfully developed male specific genetic markers. Additionally, 96 genes were located on that sex differentiated region, whereas “usual suspects” sex determining gene didn't exist in this region. It is implied, in largemouth bass, a new gene has evolved as the master sex determining gene. Our results exhibited that largemouth bass has a large sex locus on Chromosome 7, which is likely the largemouth bass sex chromosome. The presence of numerous chromosomal rearrangements and the absence of “usual suspects” sex determining gene in this sex differentiated region indicate recombination is suppressed on that region and a new master sex determining gene may be co-opted in largemouth bass. Besides, we also developed sex-linked genetic markers. Our study will facilitate future researches on sex determination and aquaculture applications in largemouth bass.

Possible stochastic sex determination in Bursaphelenchus nematodes

Sex determination mechanisms evolve surprisingly rapidly, yet little is known in the large nematode phylum other than for Caenorhabditis elegans, which relies on chromosomal XX-XO sex determination and a dosage compensation mechanism. Here we analyze by sex-specific genome sequencing and genetic analysis sex determination in two fungal feeding/plant-parasitic Bursaphelenchus nematodes and find that their sex differentiation is more likely triggered by random, epigenetic regulation than by more well-known mechanisms of chromosomal or environmental sex determination. There is no detectable difference in male and female chromosomes, nor any linkage to sexual phenotype. Moreover, the protein sets of these nematodes lack genes involved in X chromosome dosage counting or compensation. By contrast, our genetic screen for sex differentiation mutants identifies a Bursaphelenchus ortholog of tra-1, the major output of the C. elegans sex determination cascade. Nematode sex determination pathways might have evolved by “bottom-up” accretion from the most downstream regulator, tra-1.

Induction of meiotic gynogenesis in Lanzhou catfish ( Silurus lanzhouensis ) with heterologous sperm and evidence for female homogamety

As a critical factor in the sustainability of fisheries, gynogenesis can be used to provide a shortcut for the genetic improvement of the endangered and degraded germplasm resources of the important Yellow River fish Silurus lanzhouensis. In this study, ultraviolet-inactivated yellow catfish Pelteobagrus fulvidraco sperm were used to induce meiotic gynogenesis in S. lanzhouensis through cold shock, and the optimal conditions for the genetic inactivation of the sperm and cold shock treatment were selected. Using microsatellite markers and gonadal histology, the genetic purity and sex determination of the gynogenetic offspring of S. lanzhouensis were analysed. The optimal induction conditions were as follows: treatment with P. fulvidraco sperm with 107- µW/cm2 ultraviolet irradiation for 30 min, fertilization of S. lanzhouensis eggs for 6 min and subsequent cold shock treatment at 4–6°C for 25 min. Flow cytometry and microsatellite markers confirmed the successful induction of meiogynogenesis. The homozygosity of the gynogenetic group (0.63) was higher than that of the normal control group (0.32). The sex ratio was determined through gonad histological observations, and 100% of the offspring in the gynogenetic groups were female, strongly indicating that the sex determination system of S. lanzhouensis is the XX/XY-type. In this study, inactivated heterologous sperm were used to successfully obtain meiogynogenetic individuals of S. lanzhouensis, and the study provides an important basis for the establishment of a pure line of these endangered fish and sex-controlled breeding of S. lanzhouensis.

Establishment of the Y-linked Dmrt1Y as the candidate sex determination gene in spotbanded scat (Selenotoca multifasciata)

Spotbanded scat (Selenotoca multifasciata) has become a valuable marine culturing fish species in the southeast coastal areas of China. Herein, sex-linked markers were developed in spotbanded scat to elucidate its genetic foundation of sex determination. Eighty-four (84) cultured spotbanded scat were randomly chosen and used to screen and test the sex-linked markers. The average weight of the female (249 g, n = 32) is significantly higher than that of the male (212 g, n = 52). The whole genome of a male spotbanded scat was sequenced using next-generation sequencing technology. Phylogenetic analysis showed that spotbanded scat diverged from its closely related species, spotted scat (Scatophagus argus) 13.1 million years ago.Three sex-linked markers (Marker-1, 2 and 3) were obtained in spotbanded scat. Marker-1 and 2 could only amplify 562 bp and 881 bp Dmrt1 specific bands in male fish, while there were no amplified bands in female fish. Marker-3 amplified a single band of 373 bp in all female, and double bands of 373 bp and specific 294 bp in male. Sequence analysis showed that the 294 and 373 bp are Dmrt1 homologous sequences. These results indicate that the spotbanded scat possesses XY sex-determination system (XX females, XY males). Two types of Dmrt1 cDNA sequences were cloned and named Dmrt1X and Dmrt1Y according to their locations on the X and Y chromosomes, respectively. The amino acid (aa) sequences coded by Dmrt1X (309 aa) and Dmrt1Y (305 aa) are highly similar (85.8%). Dmrt1Y mRNA is specifically expressed in testis, with no expression in the ovary. Hence, Dmrt1Y is male-specific and the candidate SD gene. Knowledge of genetic sex determination and the development of sex-linked markers in spotbanded scat, will promote artificial reproduction and breeding in aquaculture.

Exploration of an XX/XY Sex Determination System and Development of PCR-Based Sex-specific Markers in Procambarus clarkii Based on Next-Generation Sequencing Data.

Many economic crustacean species have sex dimorphisms during their growth. Exploring the sex determination system and developing sex-specific molecular marker(s) are very helpful for carrying out sex control breeding, and next-generation sequencing has been used as an efficient way to explore them in recent years. In this study, first, the genetic sex determination system of P. clarkii was explored as an XX/XY system by analyzing the 2b-RAD sequencing data. Furthermore, DNA samples of male and female individuals from a P. clarkii family were pooled separately for whole-genome resequencing. Based on the data of whole-genome resequencing, the 9,163 male- and female-specific bias sites with higher feasibility were obtained based on the assumption of the XX/XY sex determination system, and four sites were selected to design the sex-specific marker primers. One efficient sex-specific marker was identified with a sex discrimination rate of 99.49% (195/196) when applied to five different geographical groups with 196 individuals. The results of this study would provide a foundation for the realization of P. clarkii sex control and could provide some reference for investigating the sex determination system and sex molecular marker(s) of other crustacean species based on next-generation sequencing data.

Heterochiasmy and the establishment of gsdf as a novel sex determining gene in Atlantic halibut.

Atlantic Halibut (Hippoglossus hippoglossus) has a X/Y genetic sex determination system, but the sex determining factor is not known. We produced a high-quality genome assembly from a male and identified parts of chromosome 13 as the Y chromosome due to sequence divergence between sexes and segregation of sex genotypes in pedigrees. Linkage analysis revealed that all chromosomes exhibit heterochiasmy, i.e. male-only and female-only meiotic recombination regions (MRR/FRR). We show that FRR/MRR intervals differ in nucleotide diversity and repeat class content and that this is true also for other Pleuronectidae species. We further show that remnants of a Gypsy-like transposable element insertion on chr13 promotes early male specific expression of gonadal somatic cell derived factor (gsdf). Less than 4.5 MYA, this male-determining element evolved on an autosomal FRR segment featuring pre-existing male meiotic recombination barriers, thereby creating a Y chromosome. Our findings indicate that heterochiasmy may facilitate the evolution of genetic sex determination systems relying on linkage of sexually antagonistic loci to a sex-determining factor.

Intensive masculinization caused by chronic heat stress in juvenile Cynoglossus semilaevis: Growth performance, gonadal histology and gene responses

The sea temperature has been observed to chronically increase during the past decades, leaving unpredictable influences to the marine biological resources. Thus, it is of vital significance to study the biological responses of ocean inhabited organisms with the artificially stimulated heat stress environment. Cynoglossus semilaevis provides us with an ideal model to study the influence of chronic heat stress on the sexual differentiation in marine teleosts for its genetic sex determination (GSD) + environmental effected (EE) sex determination system. In this study, the comparative experiment was conducted employing heated seawater (HT group) and ambient seawater (CT group) to cultivate juvenile C. semilaevis respectively. Significant differences were exhibited in growth performance and a delayed germ cell development effect was found in pseudomales formed under chronic heat stress. Using transcriptome analysis, the transcription profile of 55 days post fertilization (dpf) and 100 dpf juveniles’ gonads were studied. A total of 47 libraries were constructed with an average mapping rate of 94.63% after assembling. GO and KEGG enrichment were proceeded using DEGs screened out between (1) pseudomale gonads at 55 dpf and 100 dpf in HT and CT group (2) pseudomale and female gonads at 55 dpf and 100 dpf in HT and CT group. Terms and pathways involved in steroid stimulation, reproduction ability, germ cell proliferation et al. were shed light on. The expression pattern of 29 DEGs including amh, hsp90b1, pgr et al. were also provided to supplement the results of functional enrichment. Weighted gene co-expression networks analysis (WGCNA) was constructed and hspb8-like, histone H2A.V were exhibited to play vital roles in the heat-induced masculinization. Our findings facilitate the understanding for transcriptional variations in intensive masculinization cause by chronic heat stress of C. semilaevis and provide referable study of the influences on the teleosts in elevated sea temperature.

Sex-specific splicing of Z- and W-borne nr5a1 alleles suggests sex determination is controlled by chromosome conformation

Pogona vitticeps has female heterogamety (ZZ/ZW), but the master sex-determining gene is unknown, as it is for all reptiles. We show that nr5a1 (Nuclear Receptor Subfamily 5 Group A Member 1), a gene that is essential in mammalian sex determination, has alleles on the Z and W chromosomes (Z-nr5a1 and W-nr5a1), which are both expressed and can recombine. Three transcript isoforms of Z-nr5a1 were detected in gonads of adult ZZ males, two of which encode a functional protein. However, ZW females produced 16 isoforms, most of which contained premature stop codons. The array of transcripts produced by the W-borne allele (W-nr5a1) is likely to produce truncated polypeptides that contain a structurally normal DNA-binding domain and could act as a competitive inhibitor to the full-length intact protein. We hypothesize that an altered configuration of the W chromosome affects the conformation of the primary transcript generating inhibitory W-borne isoforms that suppress testis determination. Under this hypothesis, the genetic sex determination (GSD) system of P. vitticeps is a W-borne dominant female-determining gene that may be controlled epigenetically.

Comparative phylotranscriptomics reveals putative sex differentiating genes across eight diverse bivalve species

Mollusks, especially bivalves, exhibit a great diversity of sex determining mechanisms, including both genetic and environmental sex determination. Some bivalve species can be gonochoristic (separate sexes), while others are hermaphroditic (sequential or simultaneous). Several models have been proposed for specific bivalve species, utilizing information gained from gene expression data, as well as limited RAD-seq data (e.g., from Crassostrea gigas). However, these mechanisms are not as well studied as those in model organisms (e.g., Mus musculus, Drosophila melanogaster, Caenorhabditis elegans) and many genes involved in sex differentiation are not well characterized. We used phylotranscriptomics to better understand which possible sex differentiating genes are in bivalves and how these genes relate to similar genes in diverse phyla. We collected RNAseq data from eight phylogenetically diverse bivalve species: Argopecten irradians, Ensis directus, Geukensia demissa, Macoma tenta, Mercenaria mercenaria, Mya arenaria, Mytilus edulis, and Solemya velum. Using these data, we assembled representative transcriptomes for each species. We then searched for candidate sex differentiating genes using BLAST and confirmed the identity of nine genes using phylogenetics analyses from nine phyla. To increase the confidence of identification, we included ten bivalve genomes in our analyses. From the analysis of doublesex and mab-3 related transcription factor (DMRT) genes, we confirmed the identify of a Mollusk-specific sex determining DMRT gene: DMRT1L. Based on gene expression data from M. edulis and previous research, DMRT1L and FoxL2 are key genes for male and female development, respectively.

Prediction of the Sex-Associated Genomic Region in Tunas (Thunnus Fishes).

Fish species have a variety of sex determination systems. Tunas (genus Thunnus) have an XY genetic sex determination system. However, the Y chromosome or responsible locus has not yet been identified in males. In a previous study, a female genome of Pacific bluefin tuna (T. orientalis) was sequenced, and candidates for sex-associated DNA polymorphisms were identified by a genome-wide association study using resequencing data. In the present study, we sequenced a male genome of Pacific bluefin tuna by long-read and linked-read sequencing technologies and explored male-specific loci through a comparison with the female genome. As a result, we found a unique region carrying the male-specific haplotype, where a homolog of estrogen sulfotransferase gene was predicted to be encoded. The genome-wide mapping of previously resequenced data indicated that, among the functionally annotated genes, only this gene, named sult1st6y, was paternally inherited in the males of Pacific bluefin tuna. We reviewed the RNA-seq data of southern bluefin tuna (T. maccoyii) in the public database and found that sult1st6y of southern bluefin tuna was expressed in all male testes, but absent or suppressed in the female ovary. Since estrogen sulfotransferase is responsible for the inactivation of estrogens, it is reasonable to assume that the expression of sult1st6y in gonad cells may inhibit female development, thereby inducing the individuals to become males. Thus, our results raise a promising hypothesis that sult1st6y is the sex determination gene in Thunnus fishes or at least functions at a crucial point in the sex-differentiation cascade.

Life History and the Transitions to Eusociality in the Hymenoptera

Although indirect selection through relatives (kin selection) can explain the evolution of effectively sterile offspring that act as helpers at the nest (eusociality) in the ants, bees, and stinging wasps (aculeate Hymenoptera), the genetic, ecological, and life history conditions that favor transitions to eusociality are poorly understood. In this study, ancestral state reconstruction on recently published phylogenies was used to identify the independent transitions to eusociality in each of the taxonomic families that exhibit eusociality. Semisociality, in which a single nest co-foundress monopolizes reproduction, often precedes eusociality outside the vespid wasps. Such a route to eusociality, which is consistent with groups consisting of a mother and her daughters (subsocial) at some stage and ancestral monogamy, is favored by the haplodiploid genetic sex determination of the Hymenoptera (diploid females and haploid males) and thus may explain why eusociality is common in the Hymenoptera. Ancestral states were also reconstructed for life history characters that have been implicated in the origins of eusociality. A loss of larval diapause during unfavorable seasons or conditions precedes, or coincides with, all but one transition to eusociality. This pattern is confirmed using phylogenetic tests of associations between state transition rates for sweat bees and apid bees. A loss of larval diapause may simply reflect the subsocial route to eusociality since subsociality is defined as females interacting with their adult daughters. A loss of larval diapause and a gain of subsociality may be associated with an extended breeding season that permits the production of at least two broods, which is necessary for helpers to evolve. Adult diapause may also lower the selective barrier to a first-brood daughter becoming a helper. Obligate eusociality meets the definition of a major evolutionary transition, and such transitions have occurred five times in the Hymenoptera.

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.

Molecular and morphological changes in Nile tilapia ( Oreochromis niloticus ) gonads during high‐temperature‐induced masculinization

Genetic sex of Nile tilapia (Oreochromis niloticus) is determined by genotype (genetic sex determination). However, these fish are temperature-sensitive, and high-temperature-induced sex reversal is a common practice in research and tilapia farming. In this study, we characterized the molecular and morphological sex differentiation of Nile tilapia during the process of sex reversal induced by high temperatures. The offspring of parental pairs of XX and XY Nile tilapia underwent high-temperature treatment for 20 d beginning at 5 dpf (days post fertilization). Gonads were isolated from Nile tilapia at seven time points between 10–180 dpf. The histology results showed that gonads of high-temperature-induced XX pseudomales exhibited morphological differences from XX females during 25–35 dpf. qRT-PCR analysis revealed that the expression levels of cyp19a1a and foxl2 were temperature-sensitive, and were lowly expressed in the gonads of pseudomales from 35 dpf onward. Such findings were also consistent with the histological observations of gonadal morphological differentiation. The expression of the TGFβ superfamily member, gsdf, was increased in the gonads of pseudomales from 12 dpf onward. The transcription of kdm6bb was elevated by high-temperature treatment at 10–25 dpf and displayed sexual dimorphism from 35 dpf onward. Dmrt1 was specifically expressed in phenotypic males from 12 dpf onward. Collectively, this study identified the roles of specific genes involved in the process of high-temperature-induced masculinization in Nile tilapia and established robust molecular markers of phenotypic sex in Nile tilapia that will be useful for ongoing research related to sex control.

Perinatal burials at pre‐Hispanic noncemetery sites in Gran Canaria: Tophet, infanticide, or natural mortality?

AbstractIn this paper, we perform the first multidisciplinary study of perinatal burials at a pre‐Hispanic domestic site in Gran Canaria. For that, we carried out bioanthropological analyses, the genetic determination of the molecular sex of the individuals, and the contextualization of their chronological and cultural affiliations. To date, these remains had been associated to a local re‐interpretation of a Phoenician‐Punic tophet or as evidence of female infanticide within the mechanisms of demographic control of the ancient Canarians. However, these proposals did not take into account the recurrence of this type of human remains outside the normative cemeteries of the indigenous population or considered the demographic profile of coetaneous necropolises. In the light of the age of death and the molecular sex of the individuals, and the results obtained from the taphonomic and chronological analyses, we propose an alternative historical scenario, directly linked to high perinatal mortality rates and the development of specific burial practices for those individuals that died around the time of birth.

Lizards as Model Organisms of Sex Chromosome Evolution: What We Really Know from a Systematic Distribution of Available Data?

Lizards represent unique model organisms in the study of sex determination and sex chromosome evolution. Among tetrapods, they are characterized by an unparalleled diversity of sex determination systems, including temperature-dependent sex determination (TSD) and genetic sex determination (GSD) under either male or female heterogamety. Sex chromosome systems are also extremely variable in lizards. They include simple (XY and ZW) and multiple (X1X2Y and Z1Z2W) sex chromosome systems and encompass all the different hypothesized stages of diversification of heterogametic chromosomes, from homomorphic to heteromorphic and completely heterochromatic sex chromosomes. The co-occurrence of TSD, GSD and different sex chromosome systems also characterizes different lizard taxa, which represent ideal models to study the emergence and the evolutionary drivers of sex reversal and sex chromosome turnover. In this review, we present a synthesis of general genome and karyotype features of non-snakes squamates and discuss the main theories and evidences on the evolution and diversification of their different sex determination and sex chromosome systems. We here provide a systematic assessment of the available data on lizard sex chromosome systems and an overview of the main cytogenetic and molecular methods used for their identification, using a qualitative and quantitative approach.

Sex chromosomes in meiotic, hemiclonal, clonal and polyploid hybrid vertebrates: along the 'extended speciation continuum'.

We review knowledge about the roles of sex chromosomes in vertebrate hybridization and speciation, exploring a gradient of divergences with increasing reproductive isolation (speciation continuum). Under early divergence, well-differentiated sex chromosomes in meiotic hybrids may cause Haldane-effects and introgress less easily than autosomes. Undifferentiated sex chromosomes are more susceptible to introgression and form multiple (or new) sex chromosome systems with hardly predictable dominance hierarchies. Under increased divergence, most vertebrates reach complete intrinsic reproductive isolation. Slightly earlier, some hybrids (linked in ‘the extended speciation continuum') exhibit aberrant gametogenesis, leading towards female clonality. This facilitates the evolution of various allodiploid and allopolyploid clonal (‘asexual’) hybrid vertebrates, where ‘asexuality' might be a form of intrinsic reproductive isolation. A comprehensive list of ‘asexual' hybrid vertebrates shows that they all evolved from parents with divergences that were greater than at the intraspecific level (K2P-distances of greater than 5–22% based on mtDNA). These ‘asexual' taxa inherited genetic sex determination by mostly undifferentiated sex chromosomes. Among the few known sex-determining systems in hybrid ‘asexuals', female heterogamety (ZW) occurred about twice as often as male heterogamety (XY). We hypothesize that pre-/meiotic aberrations in all-female ZW-hybrids present Haldane-effects promoting their evolution. Understanding the preconditions to produce various clonal or meiotic allopolyploids appears crucial for insights into the evolution of sex, ‘asexuality' and polyploidy.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)’.

Sperm quality of spontaneously occurring gynogenetic males in the yellowtail tetra Astyanax altiparanae

Gynogenesis is a chromosomal manipulation technique where an offspring is generated exclusively from maternal inheritance. In the yellowtail tetra (Astyanax altiparanae) almost all gynogenetic fish were females, suggesting a genetic sex determination system of XX/XY. However, due to environmental (e.g. temperature) or unknown genetic factors, the rise of males are described. Therefore, the aim of this study was to analyze the reproductive parameters of spontaneously gynogenetic males. The parameters analyzed were spermatozoon viability, motility and ploidy. Additionally, the gonads were processed for histology and the genetic inheritance confirmed by genotyping. Sperm parameters of gynogenetic males did not show differences when compared with semen of normal males. These data supports that gynogenetic males presents fertilization capacity and can be used for future studies of chromosome manipulation or possible production of monosex populations in A. altiparanae.