The origin of natural tetraploid loach Misgurnus anguillicaudatus (Teleostei: Cobitidae) inferred from meiotic chromosome configurations

In the loach, or Oriental weatherfish Misgurnus anguillicaudatus (Teleostei: Cobitidae), diploid (2n = 50) and tetraploid individuals (4n = 100) are often sympatric in central China. The evolutionary mechanism of this tetraploidization was analyzed with the observation of meiotic behavior of chromosomes in both the germinal vesicles of mature oocytes and the primary spermatocytes in diploid and tetraploid loaches. Whereas diploid specimens usually showed 25 bivalents in meiotic cells, tetraploid loaches exhibited 0-6 quadrivalents and 38-50 bivalents in both sexes, with the modal number of quadrivalents as three in females and four in males. In the diploid specimens, the two largest metacentric chromosomes bearing nucleolar organizing regions (NORs) identified by chromomycin A(3) staining and fluorescence in situ hybridization with a 5.8S + 28S rDNA probe formed one bivalent with terminal association. In the tetraploids, four NOR-bearing chromosomes never formed a quadrivalent, but were organized into two terminally-associated bivalents. These findings suggest an autotetraploid origin of the natural tetraploid loach and subsequent rediploidization of whole genome. The latter process, however, seems still in progress as inferred from the concurrence of up-to several quadrivalents and the majority of bivalents.