Incipient speciation in the Japanese brown frog, Rana japonica

本研究试图通过杂交实验、同工酶和线粒体DNA分析来全面阐明日本褐蛙（Rana japonica）的早期物种形成过程。对本州和九州20个种群161只青蛙进行了101个组合的种群间杂交试验。虽然在这些种群中既不存在配子隔离，也不存在杂种不育，但在东、西、西北种群群体中，雄性在互惠杂交中占有显著优势。这三个群体通过雄性杂交不育相互分离。东西部群体杂交的雄性不育程度最大，西北与东部群体杂交的雄性不育程度最小。对25个居群505只青蛙的15种酶和3种血蛋白的25个基因座进行了淀粉-凝胶电泳分析，结果表明东西部居群差异明显。在西北种群中，如秋田种群和久津种群的几个位点上发现了东部等位基因的渐渗。采用RFLP分析了本州16个种群78只蛙的线粒体DNA。东西部人群的平均核苷酸差异为4.83%。秋田西北种群与东部6个种群的平均差异为1.70%，与西部9个种群的平均差异为6.42%。这表明秋田种群的mtDNA来源于东部种群。根据杂交实验、等位酶和mtDNA分析结果，将西部种群和东部种群分别命名为广岛种群和一野关种群，将西北地区称为接触区也不是不合理的。

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.