Incipient Intraspecific Isolating Mechanisms in the Japanese Brown Frog Rana japonica

Abstract

In order to elucidate the extent to which the reproductive isolating mechanism is established during the intraspecific differentiation of Rana japonica, hybridization experiments were carried out in 101 combinations using 161 frogs collected from 20 populations throughout the Honshu and Kyushu Islands of Japan. 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 groups. In crosses within each group, 52.7-55.7% of frogs were males, whereas 93.0 and 88.0% of frogs were males in the hybrid crosses between the eastern and western groups and between the northwestern and western groups, respectively. Spermatogenesis in male progeny from crosses within each of the three groups was almost always normal, whereas most of the male reciprocal hybrids among the three groups were more or less abnormal in spermatogenesis. The abnormality in spermatogenesis was largest in the hybrids between the western and eastern groups and smallest in the hybrids between the northwestern and eastern groups. These results suggest that the eastern and western groups had come into contact in the northwestern region before they were reproductively isolated from each other through hybrid sterility. It is thus not unreasonable to label the western and eastern groups as the Hiroshima and Ichinoseki races, respectively, and the northwestern region as the hybrid zone. The Japanese brown frog Rana japonica is widely distributed through Japan, from Honshu (except for the northern end), to Kyushu, and into China (except for the northeastern regions) (Okada, 1931, 1966; Maeda and Matsui, 1989). In Japan, the Ichinoseki population (eastern Honshu) of Rana japonica is reproductively isolated from the Hiroshima population (western Honshu) by the sterility of the male hybrids (Sumida, 1981). Although the two populations are not morphologically distinct, they are distinguishable both karyologically and biochemically. The karyotypes of two populations differ slightly in the centromere positions of chromosomes Nos. 6 and 9, and these two populations are further distinguished by diagnostic alleles at the Pep-A and Hb-II loci. In reciprocal crossings between these two populations, the sex ratio is skewed in favor of males in the reciprocal hybrids. These male hybrids show e Japan se brown frog Rana japonica is idely distributed through Japan, fr m Hon(except for the north rn end), to Kyushu, into China (except for the nor heastern ions) (Ok da, 193 , 1966; M e a and Matsui, 89). In Japan, the Ichinoseki population (eastHonshu) of Rana japonica is reproductively lated from the Hiroshima population (westHonshu) by the sterility of th male hybrids ida, 1981). Althoug he two populations e not morphologically dis inct, th y are disguishable both karyologic lly and biochema ly. The karyotypes of two populations differ i htly in the centromere p sitions of chrosomes Nos. 6 and 9, and these two populans are further distinguished by diagnostic leles at the Pep-A and Hb-II loci. In reciprocal o sings between these two populations, the varying degrees of testicular abnormality, and in the first spermatogenetic meiosis, homologo s chromosomes fail to pair, resulting in formation of univalents and varying degrees of degeneration of the spermatids (Sumida, 1994). Starch gel electrophoresis of enzymes and blood proteins also distinguishes the eastern and western groups of Rana japonica (Sumida and Nishioka, 1994a). The locus linked with the sexdetermining gene differs with the local popul tions (Sumida and Nishioka, 1994b). It is assumed that the eastern and western groups of R. japonica are at the stage of incipient speciation. It would be very interesting to measure the extent to which the reproductive isolating mechanisms are established during the intraspecific differentiation of R. japonica. The present study reports extensive intraspecific crossing experiments among many populations from Honshu to Kyushu to elucidate the arying degrees of testicular abno mality, and the first permatogenetic meiosis, homoloous c romosomes fail to pair, resulting in fortion of univalents and v rying degrees of egeneration of the spermatids (Sumida, 1994). tarch gel e ectrophoresis of enzymes and blood roteins also distinguishes the eastern and est rn groups of Rana japonic (Sumida and i hioka, 1994a). The locus linked with the sexe ermin g gene differs with the local popu333 This content downloaded from 207.46.13.76 on Fri, 09 Sep 2016 04:37:35 UTC All use subject to http://about.jstor.org/terms