Abstract
1. The Ashida R. strain of the small race of the striated spinous loach, Cobitis taenia striata Ikeda, was crossed with the sympatric and allopatric strains of the middle race of the same subspecies. The two kinds of hybrids were compared in terms of male sterility and hybrid breakdown.
2. Both F1 males derived from the crosses between sympatric races and those from the crosses between allopatric ones are almost completely sterile. There was scarcely any remarkable difference in abnormality of the spermatogenesis between the two kinds of F1 males.
3. Both RF1 hybrids derived from the crosses between sympatric races and those from the crosses between allopatric ones partially showed inviability. Any significant difference in degree of inviability was scarcely found between the two kinds of RF1 hybrids.
4. As far as the present study is concerned, it may be concluded that there is scarcely any positive evidence for the existence of especially strengthened hybrid sterility and hybrid breakdown in the contact regions of the two races.
