Dojo loach, Misgurnus anguillicaudatus (2n = 50), is a freshwater fish distributed in East Asia. There are two genetically-diverse groups A and B within a single species and they reproduce bisexually. However, unisexual lineages with clonal genotypes reproduce by gynogenesis in certain localities of Japan. They are considered to originate from past hybridization event between the two groups, but mechanisms responsible for clonal gametogenesis have not been disclosed yet. Here, we succeeded to distinguish chromosomes derived from each group by fluorescence in situ hybridization (FISH) with group-B specific probe and 5.8S + 28S rDNA probe. Then, hybrid origin of the clone was clearly demonstrated both in somatic as well as meiotic cells by FISH. In addition, we detected chromosomes originated from the two different groups in diploid sperm of sex-reversed clonal males. Thus, we proved that clonal gametes should be produced by a mechanism of premeiotic endomitosis, pairing between sister chromosomes formed by doubling of ancestral chromosome from each group before entering into meiosis. Sister chromosome pairing assures production of unreduced isogenic clonal gametes because sister chromosomes behave as homologous chromosomes and recombination occurs between identical elements of sister chromosomes. Our results give insights into the mechanisms underlying in alterations to meiosis for atypical reproduction, as well as hybrid origin of clonal vertebrate.
Dojo loach, Misgurnus anguillicaudatus (2n = 50), is a freshwater fish distributed in East Asia. There are two genetically-diverse groups A and B within a single species and they reproduce bisexually. However, unisexual lineages with clonal genotypes reproduce by gynogenesis in certain localities of Japan. They are considered to originate from past hybridization event between the two groups, but mechanisms responsible for clonal gametogenesis have not been disclosed yet. Here, we succeeded to distinguish chromosomes derived from each group by fluorescence in situ hybridization (FISH) with group-B specific probe and 5.8S + 28S rDNA probe. Then, hybrid origin of the clone was clearly demonstrated both in somatic as well as meiotic cells by FISH. In addition, we detected chromosomes originated from the two different groups in diploid sperm of sex-reversed clonal males. Thus, we proved that clonal gametes should be produced by a mechanism of premeiotic endomitosis, pairing between sister chromosomes formed by doubling of ancestral chromosome from each group before entering into meiosis. Sister chromosome pairing assures production of unreduced isogenic clonal gametes because sister chromosomes behave as homologous chromosomes and recombination occurs between identical elements of sister chromosomes. Our results give insights into the mechanisms underlying in alterations to meiosis for atypical reproduction, as well as hybrid origin of clonal vertebrate.
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