Incipient speciation in the Japanese brown frog, Rana japonica

点击翻译按钮获取中文摘要

This study attenpts to clarify the whole aspects of incipient speciation in the Japanese brown frog Rana japonica by hybridization experiments and allozyme and mitochondrial DNA analyzes. Interpopulational hybridization experiments were carried out in 101 combinations using 161 frogs of 20 populations from Honshu and Kyushu. Although neither gametic isolation nor hybrid inviability existed among these populations, a remarkable preponderance of males occurred in the reciprocal hybrids among the eastern, western, and northwestern population groups. These three groups were reproductively isolated from one another by male hybrid sterility. The degree of male hybrid sterility was largest in the hybrids between the western and eastern population groups, and smallest in the hybrids between the northwestern and eastern population groups. Starch-gel electrophoretic analyzes of 25 loci encoding 15 enzymes and three blood proteins of 505 frogs belonging to 25 populations showed distinct differentiation between the eastern and western population groups. The introgression of eastern alleles was noted at several loci in the northwestern population group such as the Akita and joetsu populations. Mitochondrial DNA was analyzed by RFLP using 78 frogs of 16 populations from Honshu. The mean nucleotide devergence between the eastern and wastern populations groups was 4.83%. The mean divergence between the Akita northwestern population and the six eastern populations was 1.70%, whereas that between the former and the nine western populations was 6.42%. This suggests that the mtDNA of the Akita population was derived from that of the eastern population group. Based on the results obtained from hybridization experiments and allozyme and mtDNA analyzes, it is not unreasonable that the western and eastern population groups might be named the Hiroshima and Ichinoseki races, respectively, and that the northwestern region can be called the contact zone.