Evolutionary dynamics of chromosomal races in the Japanese allotetraploid spined loach, Cobitis sp. ‘yamato’ species complex

The Japanese spined loach, Cobitis sp. ‘yamato’ species complex (hereafter Yamato spined loach), is an allotetraploid group originating from two diploid Cobitis species hybridization . Although specific regional populations have ancestral 2n = 98, decreases in 2n have been reported, indicating ongoing rediploidization. However, distribution patterns and phylogenetic relationships of each chromosomal race remain unknown. Using conventional cytogenetics, we identified seven 2n values across 45 populations (2n = 80, 82, 86, 90, 92, 94, and 98), including two newly reported karyotypes (2n = 80 and 92). Five races clustered in specific geographical areas, whereas two races (2n = 80 and 90) formed discontinuous distribution ranges. Mitochondrial phylogenies ( cytb + ND2 ) revealed three major clusters with shallow differentiation and ancestral polymorphism retention across races, indicating rapid rediploidization. Races with few chromosomes belonged to derivative clusters, which reflect a gradual chromosome number reduction during Yamato spined loach evolution. Polyphyly within races indicates parallel and independent chromosomal reductions. Flow cytometry showed no positive correlation between chromosome number and relative DNA content, suggesting that karyotype rearrangements, rather than the loss of specific chromosomes, contributed to the decrease in chromosome or fundamental numbers.