Biology subjects

Majtanova, Z.

Publications and source records attributed to Majtanova, Z..

4 recordsLinked to original sources

bioRxiv2026

Achiasmatic meiosis underlies propagation of clonal genome and B chromosomes in hexaploid Carassius gibelio females but not males

Sexual reproduction in eukaryotes relies on meiosis, recombination, and fertilization, yet hybridization can lead to transitions to asexuality. Asexual vertebrate hybrids require modified gametogenesis to produce unreduced gametes, but the underlying mechanisms remain poorly understood in various hybrid complexes. Here, we investigated the Carassius gibelio complex, which combines clonal genome propagation to the offspring along with the transmission of B chromosomes. We analyzed meiotic progression and gametogenesis in sexual tetraploid and asexual hexaploid lineages, focusing on sex-specific differences. Cytological analyses of synaptonemal complexes and diplotene chromosomes revealed that both sexes in hexaploid lineages undergo achiasmatic meiosis, characterized by the absence of homologous pairing, recombination, and chiasmata. Chromosomes persist as univalents throughout meiotic prophase. Despite this shared meiotic program, outcomes differ between sexes: females complete oogenesis and produce unreduced eggs, whereas males exhibit disrupted spermatogenesis and reduced fertility. Females bypass the reductional division, enabling clonal gamete formation, while males fail to segregate univalents properly. Furthermore, B chromosomes were detected in both mitosis and meiosis of hexaploid males and females, but not in sexual lineages. B chromosomes were consistently present and varied in number among individuals. B chromosomes varied in number and consistently formed univalents during meiosis, similar to other chromosomes. Significance statementMeiosis normally depends on chromosome pairing and recombination, yet asexual vertebrates can bypass these processes. We demonstrate that asexual hexaploid Carassius gibelio reproduces through achiasmatic meiosis, in which chromosomes fail to pair and recombine. While females successfully produce unreduced clonal eggs, males display reduced fertility, revealing striking sex-specific differences in the ability to overcome meiotic dysfunction. We further show that B chromosomes persist as meiotic univalents and are stably inherited.

genetics↗

bioRxiv2025

Sex-specific modifications of gametogenesis in natural and lab-bred Fundulus spp. hybrids

Successful interspecific hybridization can be limited by a variety of reproductive barriers, including the emergence of asexual lineages which, while relatively rare in animals, is an example of a strong post zygotic barrier. In vertebrates, asexual reproduction typically arises through gametogenic alterations such as premeiotic genome endoreplication or meiotic failure, yet the relative frequency of and conditions leading to these alterations remain poorly understood. Here, we investigate natural, all female clonal hybrid lineages and laboratory-bred hybrids of both sexes between two North American killifishes, the Banded Killifish (Fundulus diaphanus) and Common Killifish (F. heteroclitus), to determine the mechanisms underlying and maintaining the emergence of asexual reproduction. By combining cytogenetic, molecular, and gametogenetic analyses, we show that all natural and some lab-bred F1 hybrid females can produce diploid and tetraploid oocytes, with natural hybrids producing a higher proportion of tetraploid eggs. In diploid oocytes, we observed both bivalents and univalents. In contrast, tetraploid oocytes exclusively formed bivalents with normal crossover formation, consistent with premeiotic genome endoreplication restoring proper meiotic pairing and facilitating clonal reproduction. Wild and lab-bred hybrid males do not undergo premeiotic genome duplication and exhibit extensive chromosomal mispairing, leading to aberrant meiotic divisions, elevated apoptosis, and decreased fertility or sterility. Together, these findings highlight sex-specific differences in gametogenesis, paralleling patterns observed in other systems, and emphasize the relative ease with which asexual reproduction can arise in hybrid females, in contrast to the sterility commonly observed in hybrid males.

evolutionary biology↗

bioRxiv2022

Sex-linked markers in an Australian frog Platyplectrum ornatum with a small genome and homomorphic sex chromosomes

Amphibians have highly diverse sex-determining modes leading to a notable interest in vertebrate sex determination and sex chromosome evolution. The identification of sex-determining systems in amphibians, however, is often difficult as a vast majority consist of homomorphic sex chromosomes making them hard to distinguish. In this study, we used Diversity Array Technology sequencing (DArTseq) to identify the sex-determining system in the ornate burrowing frog from Australia, Platyplectrum ornatum. We applied DArTseq to 44 individuals, 19 males and 25 females, collected from two locations to develop sex-linked markers. Unexpectedly, these 44 individuals were classified into two distinct population clusters based on our SNP analyses, 36 individuals in cluster-1, and 8 individuals in cluster-2. We then performed sex-linkage analyses separately in each cluster. We identified 35 sex-linked markers from cluster-1, which were all associated with maleness. Therefore, P. ornatum cluster-1 is utilising a male heterogametic (XX/XY) sex-determining system. On the other hand, we identified 210 sex-linked markers from cluster-2, of which 89 were male specific, i.e., identifying XX/XY sex determining system and 111 were female specific, i.e., identifying ZZ/ZW sex determining system, suggesting existence of either male or female heterogametic sex determining system in cluster-2. We also performed cytogenetic analyses in 1 male and 1 female from cluster-1; however, we did not detect any visible differentiation between the X and Y sex chromosomes. We also mapped sex-linked markers from the two clusters against the P. ornatum genome and our comparative analysis indicated that the sex chromosomes in both clusters shared homologies to chromosome 10 (autosome) of Rana temporaria and ZWY sex chromosome of Xenopus tropicalis. It is plausible that the cluster-2 has a potential to be either male or female heterogamety in sex determination, requiring further investigation.

genomics↗

bioRxiv2019

Parthenogenesis as a solution to hybrid sterility: the mechanistic basis of meiotic distortions in clonal and sterile hybrids

Formation of species generally occurs in a continuum from potentially intermixing populations to independent entities isolated from other species by pre- and postzygotic barriers. Especially the establishment of hybrid sterility (HS) is a hallmark of speciation, which usually emerges at different rates between hybrid sexes. However, although HS is frequently observed, the underlying molecular mechanisms remain poorly understood. Here we report that speciation proceeds through a previously unnoticed stage at which gene flow is completely interrupted on side of both hybrids sexes, although only male hybrids are sterile, while female fertility is rescued due to a particular gametogenetic deviation leading to the formation of clonal gametes. Specifically, analysis of gametogenetic pathways in hybrids between fish species Cobitis elongatoides and C. taenia revealed that male HS resulted from extensive asynapses and crossover reduction among elongatoides-taenia chromosomal pairs followed by apoptosis. By contrast, hybrid females exhibited premeiotic genome endoreplication which ensured proper formation of bivalents between identical chromosomal copies. This deviation ultimately restored fertility in females but since it simultaneously leads to the production of unreduced clonal gametes, it restricts interspecific gene flow thereby directly contributing to speciation. In conclusion, our data demonstrate that the emergence of asexuality may remedy HS in a sex-specific manner and is intermingled with the speciation process. Although gametogenetic mechanisms employed by asexual animals and plants have rarely been scrutinized, available evidence suggests that premeiotic endoreplication is relatively widespread. This suggests that observed link between HS and clonality may have general validity in taxa able of asexual reproduction.\n\nAuthors summarySpecies are fundamental evolutionary units that presumably evolve in a continuum from potentially intermixing populations to independent entities isolated from other species by pre- and postzygotic barriers. Especially the establishment of hybrid sterility (HS) is a hallmark of speciation, which usually emerges at different rates between hybrid sexes. However, although HS is frequently observed, the underlying molecular mechanisms remain poorly understood. Here we report the existence of a previously unnoticed stage of speciation at which gene flow is completely interrupted, although only male hybrids are sterile, while female fertility is rescued due to a particular gametogenetic deviation leading to formation of clonal gametes. Specifically, HS resulted from extensive asynapses in male gonads, but in females the hybridization provoked premeiotic endoreplication which rescued chromosome pairing and fertility. Simultaneously, this meiotic deviation caused clonal transmission of maternal genome, thereby effectively restricting the interspeficic gene flow. Our results emphasize that emergence of clonality is a type of hybrid incompatibility that is intermingled with the formation of biological species and may remedy hybrid sterility in a sex-specific manner.

evolutionary biology↗