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.

• Keywords
• Asexual, Carp gudgeon, Gonocytes, Histone modification, Hybridogenesis, Micronucleus,
• MeSH
• Chromosomes genetics   MeSH
• Genome Size MeSH
• Genome * MeSH
• Gonads metabolism   MeSH
• Hybridization, Genetic MeSH
• Mitosis genetics   MeSH
• Fishes genetics   MeSH
• Chromosome Segregation genetics   MeSH
• Animals MeSH
• Check Tag
• Male MeSH
• Female MeSH
• Animals MeSH
• Publication type
• Journal Article MeSH

Gametogenesis produces gametes as a piece of genetic information transmitted to the offspring. While during sexual reproduction, progeny inherits a mix of genetic material from both parents, asexually reproducing organisms transfer a copy of maternal or paternal DNA to the progeny clonally. Parthenogenetic, gynogenetic and hybridogenetic animals have developed various mechanisms of gametogenesis, however, their inheritance is not fully understood. Here, we focused on the inheritance of asexual gametogenesis in hybrid Pelophylax esculentus (RL), emerging after crosses of P. lessonae (LL) and P. ridibundus (RR). To understand the mechanisms of gametogenesis in hybrids, we performed three-generation experiments of sexual P. ridibundus females and hybrids from all-male hybrid populations. Using fluorescent in situ hybridization, micronuclei analysis, flow cytometry and genotyping, we found that most adult hybrid males simultaneously produced two types of clonal sperm. Also, most male tadpole progeny in two successive backcrossed generations simultaneously eliminated L and R parental genomes, while some progeny produced only one type of sperm. We hypothesize that the reproductive variability of males producing two kinds of sperm is an adaptive mechanism to reproduce in mixed populations with P. ridibundus and may explain the extensive distribution of the all-male lineage across the European River Basin.

• Keywords
• Amphispermy, Crossing experiments, FISH., Male hybridogenesis, Micronuclei, Monospermic, Spermatogenesis,
• MeSH
• Genotype MeSH
• Hybridization, Genetic * MeSH
• In Situ Hybridization, Fluorescence MeSH
• Reproduction, Asexual * genetics   MeSH
• Rana esculenta genetics   MeSH
• Ranidae genetics   physiology   MeSH
• Spermatozoa physiology   MeSH
• Inheritance Patterns genetics   MeSH
• Animals MeSH
• Check Tag
• Male MeSH
• Female MeSH
• Animals MeSH
• Publication type
• Journal Article 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.

• Keywords
• clariid catfish, lampbrush chromosomes, satellite DNA, synaptonemal complex, telomeric sequence,
• Publication type
• Journal Article 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
• Gametogenesis MeSH
• Haploidy MeSH
• Cypriniformes * genetics   MeSH
• Tetraploidy MeSH
• Triploidy * MeSH
• Animals MeSH
• Check Tag
• Male MeSH
• Female MeSH
• Animals MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH

Hybrid taxa from the genus Pelophylax can propagate themselves in a modified way of sexual reproduction called hybridogenesis ensuring the formation of clonal gametes containing the genome of only one parental (host) species. Pelophylax grafi from South-Western Europe is a hybrid composed of P. ridibundus and P. perezi genomes and it lives with a host species P. perezi (P-G system). Yet it is unknown, whether non-Mendelian inheritance is fully maintained in such populations. In this study, we characterize P. perezi and P. grafi somatic karyotypes by using comparative genomic hybridization, genomic in situ hybridization, fluorescent in situ hybridization, and actinomycin D-DAPI. Here, we show the homeology of P. perezi and P. grafi somatic karyotypes to other Pelophylax taxa with 2n = 26 and equal contribution of ridibundus and perezi chromosomes in P. grafi which supports F1 hybrid genome constitution as well as a hemiclonal genome inheritance. We show that ridibundus chromosomes have larger regions of interstitial (TTAGGG)n repeats flanking the nucleolus organizing region on chromosome no. 10 and a high quantity of AT pairs in the centromeric regions. In P. perezi, we found species-specific sequences in metaphase chromosomes and marker structures in lampbrush chromosomes. Pericentromeric RrS1 repeat sequence was present in perezi and ridibundus chromosomes, but the blocks were stronger in ridibundus. Various cytogenetic techniques applied to the P-G system provide genome discrimination between ridibundus and perezi chromosomal sets. They could be used in studies of germ-line cells to explain patterns of clonal gametogenesis in P. grafi and broaden the knowledge about reproductive strategies in hybrid animals.

• Keywords
• Pelophylax grafi, Pelophylax perezi, comparative genomic hybridization, fluorescent in situ hybridization, hybridogenesis, karyotype,
• MeSH
• Centromere * genetics   MeSH
• In Situ Hybridization, Fluorescence MeSH
• Karyotyping MeSH
• Ranidae * genetics   MeSH
• Comparative Genomic Hybridization MeSH
• Animals MeSH
• Check Tag
• Animals MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH

Paternal genome elimination-a type of reproduction in which males inherit but fail to pass on their father's genome-evolved independently in 6-8 arthropod clades. Thousands of species, including several important for agriculture, reproduce via this mode of reproduction. While paternal genome elimination is well established in some of the clades, the evidence in globular springtails (Symphypleona) remains elusive, even though they represent the oldest and most species-rich clade putatively reproducing via paternal genome elimination. We sequenced genomic DNA from whole bodies of Allacma fusca males with high fractions (>27.5%) of sperm to conclusively confirm that all the sperm carry 1 parental haplotype only. Although it is suggestive that the single haplotype present in sperm is maternally inherited, definitive genetic proof of the parent of origin is still needed. The genomic approach we developed allows for the detection of genotypic differences between germline and soma in all species with sufficiently high fraction of germline in their bodies. This opens new opportunities for scans of reproductive modes in small organisms.

• Keywords
• evolutionary biology, genome, paternal genome elimination, reproduction, springtails,
• MeSH
• Arthropods * genetics   MeSH
• Genome MeSH
• Genomics MeSH
• Semen * MeSH
• Animals MeSH
• Check Tag
• Male MeSH
• Animals MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't 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.

• Keywords
• Pelophylax esculentus, aneuploidy, endoreplication, genome elimination, hybridogenesis, in situ hybridization, polyploidy, spermatogenesis,
• Publication type
• Journal Article MeSH