抄録
Two freshwater snail species of the genus Semisulcospira, S. libertina (Gould) and S. reiniana (Brot), inhabit various areas in the Japanese Islands. It has been pointed out that these two species are indistinguishable by their adult shell morphology. In the present study, genetic and morphological variations of the adult shell of the two species were examined, and the genetic relationship between them was inferred through starch gel electrophoretic analysis. The individuals examined of 19 populations mainly in the areas of the central to eastern Japan were separated into two almost-isolated groups without natural random mating between them. The Mpi locus is fixed for the allele Mpi^A in one group (MpiAA type) and for the allele Mpi^B in the other group (MpiBB type) except a few heterozygous individuals. The MpiAA type individuals live throughout the central to northeastern areas and Okinawa island in Japan, while the MpiBB ones are distributed in the area west of the Kanto district. As to the morphological features of adult shells, 85 percent of the MpiAA type individuals have smooth shells with no ribs, and pronounced ribs are recognized on the shell of 53 percent of the MpiBB type individuals. Except for the rib feature, there is no appreciable difference in the adult shell morphology between the two groups. Considering these morphological features and previous taxonomical research, the MpiAA type and MpiBB type individuals are assignable to S. libertina and S. reiniana respectively. According to molecular phylogenetic tree based on the gene frequency at 12 gene loci, MpiBB type individuals are inferred to be derived from a population of MpiAA type individuals.
