Genomic constitution and atypical reproduction in polyploid and unisexual lineages of the Misgurnus loach, a teleost fish.

The loach (Misgurnus anguillicaudatus) is an excellent animal model to elucidate biological origin and evolutionary significance of genome duplication and unisexual reproduction because artificially induced and naturally occurring polyploids and parthenogenetic (gynogenetic, androgenetic) animals can be compared. First, we summarize the chromosome manipulation techniques to induce triploids and tetraploids by inhibiting meiotic or mitotic divisions of inseminated eggs, respectively, as well as parthenogenetic animals, obtained after fertilization with genetically inactivated gametes. Then, we review the knowledge on natural polyploid and unisexual lineages found in Misgurnus loaches. A natural diploid-tetraploid complex occurs in wild populations in central China, and these diploid and tetraploid loaches reproduce bisexually. Chinese tetraploids are considered autotetraploid, which may have arisen by doubling of the entire genome of an ancestral diploid, based on cytogenetic results from FISH (fluorescence in situ hybridization) karyotypes and meiotic configurations. In contrast, gynogenetically reproducing clonal diploid lineages have been discovered in a few wild populations in Japan, although most wild-type individuals are bisexually reproducing diploids. Such clonal diploid loaches sometimes produce triploid progeny by accidental incorporation of a sperm nucleus into an unreduced diploid egg, and the resulting triploid generates haploid eggs by meiotic hybridogenesis. Unreduced diploid gametes of clonal loaches are generated by a cytological mechanism, premeiotic endomitosis, which likely occurs in the early (gonium stage) germ cells. Initiation of gynogenetic development is related to a failure of decondensation of the male (sperm) pronucleus in unreduced diploid eggs of a clonal loach. Clonal lineages may have arisen from a past hybrid event between genetically divergent groups, but their exact origins are unknown at present. See also the sister article focusing on plants by Hegarty et al. in this themed issue.