Danio rerio, commonly known as zebrafish, is an established model organism for the developmental and cell biology studies. Although significant progress has been made in the analysis of the D. rerio genome, cytogenetic studies face challenges due to the unclear identification of chromosomes. Here, we present a novel approach to the study of the D. rerio karyotype, focusing on the analysis of lampbrush chromosomes isolated from growing oocytes. Lampbrush chromosomes, existing during diplotene, serve as a powerful tool for high-resolution mapping and transcription analysis due to their profound decondensation and remarkable lateral loops decorated by RNA polymerases and ribonucleoprotein (RNP) matrix. In D. rerio, lampbrush chromosomes are about 20 times longer than corresponding metaphase chromosomes. We found that the lampbrush chromosome stage karyotype of D. rerio is generally undifferentiated, except for several bivalents bearing distinct marker structures, including loops with complex RNP matrix and locus-associated nuclear bodies. Locus-associated nuclear bodies were enriched for coilin and snRNAs; the loci where they formed presumably correspond to the histone gene clusters. Further, we observed the accumulation of splicing factors in giant terminal RNP aggregates on one bivalent. DAPI staining of Danio rerio lampbrush chromosomes revealed large and small chromomeres non-uniformly distributed along the axis. For example, D. rerio lampbrush chromosome 4, comprising the sex-determining region, is divided into two halves-with small chromomeres bearing long lateral loops and with large dense chromomeres bearing no or very tiny lateral loops. As centromeres were not distinguishable, we identified centromeric regions in all bivalents by FISH mapping of pericentromeric RFAL1, RFAL2, and RFAM tandem repeats. Through a combination of morphological analysis, immunostaining of marker structures, and centromere mapping, we developed cytological maps of D. rerio lampbrush chromosomes. Finally, by RNA FISH we revealed transcripts of pericentromeric and telomeric tandem repeats at the lampbrush chromosome stage.

• Klíčová slova
• Centromere, FISH-mapping, Germinal vesicle, Histone locus bodies, Karyotype analysis, Lampbrush chromosomes, Non-coding RNA, Oocyte nucleus, Tandem repeats, Telomere, Zebrafish,
• MeSH
• chromozomy * genetika   ultrastruktura   MeSH
• dánio pruhované * genetika   MeSH
• hybridizace in situ fluorescenční MeSH
• karyotyp MeSH
• karyotypizace MeSH
• oocyty MeSH
• ribonukleoproteiny metabolismus   genetika   MeSH
• zvířata MeSH
• Check Tag
• ženské pohlaví MeSH
• zvířata MeSH
• Publikační typ
• časopisecké články MeSH
• práce podpořená grantem MeSH
• Názvy látek
• ribonukleoproteiny MeSH

Most eukaryotes maintain the stability of their cellular genome sizes to ensure genome transmission to offspring through sexual reproduction. However, some alter their genome size by selectively eliminating parts or increasing ploidy at specific developmental stages. This phenomenon of genome elimination or whole genome duplication occurs in animal hybrids reproducing asexually. Such genome alterations occur during gonocyte development ensuring successful reproduction of these hybrids. Although multiple examples of genome alterations are known, the underlying molecular and cellular processes involved in selective genome elimination and duplication remain largely unknown. Here, we uncovered the process of selective genome elimination and genome endoreplication in hemiclonal fish hybrids from the genus Hypseleotris. Specifically, we examined parental sexual species H. bucephala and hybrid H. bucephala × H. gymnocephala (HB × HX). We observed micronuclei in the cytoplasm of gonial cells in the gonads of hybrids, but not in the parental sexual species. We also observed misaligned chromosomes during mitosis which were unable to attach to the spindle. Moreover, we found that misaligned chromosomes lag during anaphase and subsequently enclose in the micronuclei. Using whole mount immunofluorescent staining, we showed that chromatid segregation has failed in lagging chromosomes. We also performed three-dimensional comparative genomic hybridization (3D-CGH) using species-specific probes to determine the role of micronuclei in selective genome elimination. We repeatedly observed that misaligned chromosomes of the H. bucephala genome were preferentially enclosed in micronuclei of hybrids. In addition, we detected mitotic cells without a mitotic spindle as a potential cause of genome duplication. We conclude that selective genome elimination in the gonads of hybrids occurs through gradual elimination of individual chromosomes of one parental genome. Such chromosomes, unable to attach to the spindle, lag and become enclosed in micronuclei.

• Klíčová slova
• Asexual, Carp gudgeon, Gonocytes, Histone modification, Hybridogenesis, Micronucleus,
• MeSH
• chromozomy genetika   MeSH
• délka genomu MeSH
• genom * MeSH
• gonády metabolismus   MeSH
• hybridizace genetická MeSH
• mitóza genetika   MeSH
• ryby genetika   MeSH
• segregace chromozomů genetika   MeSH
• zvířata MeSH
• Check Tag
• mužské pohlaví MeSH
• ženské pohlaví MeSH
• zvířata MeSH
• Publikační typ
• časopisecké články MeSH

INTRODUCTION: Reproductive isolation and hybrid sterility are mechanisms that maintain the genetic integrity of species and prevent the introgression of heterospecific genes. However, crosses of closely related species can lead to complex evolution, such as the formation of all-female lineages that reproduce clonally. Bighead catfish (Clarias macrocephalus) and North African catfish (C. gariepinus) diverged 40 million years ago. They are cultivated and hybridized in Thailand for human consumption. Male hybrids are sterile due to genome-wide chromosome asynapsis during meiosis. Although female hybrids are sometimes fertile, their chromosome configuration during meiosis has not yet been studied. METHODS: We analyzed meiosis in the hybrid female catfish at pachytene (synaptonemal complexes) and diplotene (lampbrush chromosomes), using immunostaining to detect chromosome pairing and double-stranded break formation, and FISH with species-specific satellite DNAs to distinguish the parental chromosomes. RESULTS: More than 95% of oocytes exhibited chromosome asynapsis in female hybrid catfish; however, they were able to progress to the diplotene stage and form mature eggs. The remaining oocytes underwent premeiotic endoreplication, followed by synapsis and crossing over between sister chromosomes, similar to known clonal lineages in fish and reptiles. DISCUSSION: The occurrence of clonal reproduction in female hybrid catfish suggests a unique model for studying gametogenic alterations caused by hybridization and their potential for asexual reproduction. Our results further support the view that clonal reproduction in certain hybrid animals relies on intrinsic mechanisms of sexually reproducing parental species, given their multiple independent origins with the same mechanism.

• Klíčová slova
• clariid catfish, lampbrush chromosomes, satellite DNA, synaptonemal complex, telomeric sequence,
• Publikační typ
• časopisecké články MeSH

The cellular and molecular mechanisms governing sexual reproduction are conserved across eukaryotes. Nevertheless, hybridization can disrupt these mechanisms, leading to asexual reproduction, often accompanied by polyploidy. In this study, we investigate how ploidy level and ratio of parental genomes in hybrids affect their reproductive mode. We analyze the gametogenesis of sexual species and their diploid and triploid hybrids from the freshwater fish family Cobitidae, using newly developed cytogenetic markers. We find that diploid hybrid females possess oogonia and oocytes with original (diploid) and duplicated (tetraploid) ploidy. Diploid oocytes cannot progress beyond pachytene due to aberrant pairing. However, tetraploid oocytes, which emerge after premeiotic genome endoreplication, exhibit normal pairing and result in diploid gametes. Triploid hybrid females possess diploid, triploid, and haploid oogonia and oocytes. Triploid and haploid oocytes cannot progress beyond pachytene checkpoint due to aberrant chromosome pairing, while diploid oocytes have normal pairing in meiosis, resulting in haploid gametes. Diploid oocytes emerge after premeiotic elimination of a single-copied genome. Triploid hybrid males are sterile due to aberrant pairing and the failure of chromosomal segregation during meiotic divisions. Thus, changes in ploidy and genome dosage may lead to cyclical alteration of gametogenic pathways in hybrids.

• MeSH
• gametogeneze MeSH
• haploidie MeSH
• máloostní * genetika   MeSH
• tetraploidie MeSH
• triploidie * MeSH
• zvířata MeSH
• Check Tag
• mužské pohlaví MeSH
• ženské pohlaví MeSH
• zvířata MeSH
• Publikační typ
• časopisecké články MeSH
• práce podpořená grantem MeSH

Asexual reproduction can be triggered by interspecific hybridization, but its emergence is supposedly rare, relying on exceptional combinations of suitable genomes. To examine how genomic and karyotype divergence between parental lineages affect the incidence of asexual gametogenesis, we experimentally hybridized fishes (Cobitidae) across a broad phylogenetic spectrum, assessed by whole exome data. Gametogenic pathways generally followed a continuum from sexual reproduction in hybrids between closely related evolutionary lineages to sterile or inviable crosses between distant lineages. However, most crosses resulted in a combination of sterile males and asexually reproducing females. Their gametes usually experienced problems in chromosome pairing, but females also produced a certain proportion of oocytes with premeiotically duplicated genomes, enabling their development into clonal eggs. Interspecific hybridization may thus commonly affect cell cycles in a specific way, allowing the formation of unreduced oocytes. The emergence of asexual gametogenesis appears tightly linked to hybrid sterility and constitutes an inherent part of the extended speciation continuum.

• Klíčová slova
• Cobitis, asexual reproduction, evolutionary biology, gametogenesis, hybrid sterility, hybridization, speciation, spined loaches,
• MeSH
• fylogeneze MeSH
• hybridizace genetická MeSH
• infertilita * MeSH
• karyotyp MeSH
• lidé MeSH
• nepohlavní rozmnožování * genetika   MeSH
• Check Tag
• lidé MeSH
• mužské pohlaví MeSH
• ženské pohlaví MeSH
• Publikační typ
• časopisecké články MeSH
• práce podpořená grantem MeSH

Intrinsic postzygotic isolation typically appears as reduced viability or fertility of interspecific hybrids caused by genetic incompatibilities between diverged parental genomes. Dobzhansky-Muller interactions among individual genes, and chromosomal rearrangements causing problems with chromosome synapsis and recombination in meiosis, have both long been considered as major mechanisms behind intrinsic postzygotic isolation. Recent research has, however, suggested that the genetic basis of intrinsic postzygotic isolation can be more complex and involves, for example, overall divergence of the DNA sequence or epigenetic changes. Here, we review the mechanisms of intrinsic postzygotic isolation from genic, chromosomal, genomic, and epigenetic perspectives across diverse taxa. We provide empirical evidence for these mechanisms, discuss their importance in the speciation process, and highlight questions that remain unanswered.

• MeSH
• chromozomy * MeSH
• genom MeSH
• genomika * MeSH
• hybridizace genetická MeSH
• reprodukční izolace MeSH
• vznik druhů (genetika) MeSH
• Publikační typ
• časopisecké články MeSH
• práce podpořená grantem MeSH
• přehledy MeSH

Sexual reproduction is the primary mode of reproduction in eukaryotes, but some organisms have evolved deviations from classical sex and switched to asexuality. These asexual lineages have sometimes been viewed as evolutionary dead ends, but recent research has revealed their importance in many areas of general biology. Our review explores the understudied, yet important mechanisms by which sperm-dependent asexuals that produce non-recombined gametes but rely on their fertilization, can have a significant impact on the evolution of coexisting sexual species and ecosystems. These impacts are concentrated around three major fields. Firstly, sperm-dependent asexuals can potentially impact the gene pool of coexisting sexual species by either restricting their population sizes or by providing bridges for interspecific gene flow whose type and consequences substantially differ from gene flow mechanisms expected under sexual reproduction. Secondly, they may impact on sexuals' diversification rates either directly, by serving as stepping-stones in speciation, or indirectly, by promoting the formation of pre- and postzygotic reproduction barriers among nascent species. Thirdly, they can potentially impact on spatial distribution of species, via direct or indirect (apparent) types of competition and Allee effects. For each such mechanism, we provide empirical examples of how natural sperm-dependent asexuals impact the evolution of their sexual counterparts. In particular, we highlight that these broad effects may last beyond the tenure of the individual asexual lineages causing them, which challenges the traditional perception that asexual lineages are short-lived evolutionary dead ends and minor sideshows. Our review also proposes new research directions to incorporate the aforementioned impacts of sperm-dependent asexuals. These research directions will ultimately enhance our understanding of the evolution of genomes and biological interactions in general.

• Klíčová slova
• apparent competition, hybridization, meiosis, population dynamics, speciation,
• Publikační typ
• časopisecké články MeSH
• přehledy MeSH

Unisexual reproduction, which generates clonal offspring, is an alternative strategy to sexual breeding and occurs even in vertebrates. A wide range of non-sexual reproductive modes have been described, and one of the least understood questions is how such pathways emerged and how they mechanistically proceed. The Amazon molly, Poecilia formosa, needs sperm from males of related species to trigger the parthenogenetic development of diploid eggs. However, the mechanism, of how the unreduced female gametes are produced, remains unclear. Cytological analyses revealed that the chromosomes of primary oocytes initiate pachytene but do not proceed to bivalent formation and meiotic crossovers. Comparing ovary transcriptomes of P. formosa and its sexual parental species revealed expression levels of meiosis-specific genes deviating from P. mexicana but not from P. latipinna. Furthermore, several meiosis genes show biased expression towards one of the two alleles from the parental genomes. We infer from our data that in the Amazon molly diploid oocytes are generated by apomixis due to a failure in the synapsis of homologous chromosomes. The fact that this failure is not reflected in the differential expression of known meiosis genes suggests the underlying molecular mechanism may be dysregulation on the protein level or misexpression of a so far unknown meiosis gene, and/or hybrid dysgenesis because of compromised interaction of proteins from diverged genomes.

• Klíčová slova
• Achiasmatic, Crossing-over, Meiosis, Oogenesis, Parthenogenesis, Recombination, Synaptonemal complex, Transcriptome,
• MeSH
• meióza MeSH
• sperma MeSH
• transkriptom MeSH
• živorodka * genetika   MeSH
• zvířata MeSH
• Check Tag
• mužské pohlaví MeSH
• ženské pohlaví MeSH
• zvířata MeSH
• Publikační typ
• časopisecké články MeSH
• práce podpořená grantem MeSH
• Geografické názvy
• Taiwan MeSH

Hybridogenesis is a hemiclonal reproductive strategy in diploid and triploid hybrids. Our study model is a frog P. esculentus (diploid RL and triploids RLL and RRL), a natural hybrid between P. lessonae (LL) and P. ridibundus (RR). Hybridogenesis relies on elimination of one genome (L or R) from gonocytes (G) in tadpole gonads during prespermatogenesis, but not from spermatogonial stem cells (SSCs) in adults. Here we provide the first comprehensive study of testis morphology combined with chromosome composition in the full spectrum of spermatogenic cells. Using genomic in situ hybridization (GISH) and FISH we determined genomes in metaphase plates and interphase nuclei in Gs and SSCs. We traced genomic composition of SSCs, spermatocytes and spermatozoa in individual adult males that were crossed with females of the parental species and gave progeny. Degenerating gonocytes (24%-39%) and SSCs (18%-20%) led to partial sterility of juvenile and adult gonads. We conclude that elimination and endoreplication not properly completed during prespermatogenesis may be halted when gonocytes become dormant in juveniles. After resumption of mitotic divisions by SSCs in adults, these 20% of cells with successful genome elimination and endoreplication continue spermatogenesis, while in about 80% spermatogenesis is deficient. Majority of abnormal cells are eliminated by cell death, however some of them give rise to aneuploid spermatocytes and spermatozoa which shows that hybridogenesis is a wasteful process.

• Klíčová slova
• Pelophylax esculentus, aneuploidy, endoreplication, genome elimination, hybridogenesis, in situ hybridization, polyploidy, spermatogenesis,
• Publikační typ
• časopisecké články MeSH

Hybridization and genome duplication have played crucial roles in the evolution of many animal and plant taxa. The subgenomes of parental species undergo considerable changes in hybrids and polyploids, which often selectively eliminate segments of one subgenome. However, the mechanisms underlying these changes are not well understood, particularly when the hybridization is linked with asexual reproduction that opens up unexpected evolutionary pathways. To elucidate this problem, we compared published cytogenetic and RNAseq data with exome sequences of asexual diploid and polyploid hybrids between three fish species; Cobitis elongatoides, C. taenia, and C. tanaitica. Clonal genomes remained generally static at chromosome-scale levels but their heterozygosity gradually deteriorated at the level of individual genes owing to allelic deletions and conversions. Interestingly, the impact of both processes varies among animals and genomic regions depending on ploidy level and the properties of affected genes. Namely, polyploids were more tolerant to deletions than diploid asexuals where conversions prevailed, and genomic restructuring events accumulated preferentially in genes characterized by high transcription levels and GC-content, strong purifying selection and specific functions like interacting with intracellular membranes. Although hybrids were phenotypically more similar to C. taenia, we found that they preferentially retained C. elongatoides alleles. This demonstrates that favored subgenome is not necessarily the transcriptionally dominant one. This study demonstrated that subgenomes in asexual hybrids and polyploids evolve under a complex interplay of selection and several molecular mechanisms whose efficiency depends on the organism's ploidy level, as well as functional properties and parental ancestry of the genomic region.

• Klíčová slova
• asexual reproduction, gene conversions, hemizygous deletions, hybridization, loss of heterozygosity, polyploidy,
• MeSH
• diploidie MeSH
• genom rostlinný MeSH
• hybridizace genetická MeSH
• máloostní * genetika   MeSH
• molekulární evoluce MeSH
• polyploidie * MeSH
• ztráta heterozygozity MeSH
• zvířata MeSH
• Check Tag
• zvířata MeSH
• Publikační typ
• časopisecké články MeSH
• práce podpořená grantem MeSH