Allozyme diversity and geographical differentiation of Parasenecio adenostyloides (Asteraceae), an endemic forest herb in Japan

In this study, allozyme variation at 15 putative loci was analyzed in 11 populations of Parasenecio adenostyloides (Asteraceae), a perennial herb endemic to Shikoku and Honshu, Japan. This species occurs mainly in the understory of subalpine coniferous forests and shows an islandlike pattern of geographical distribution. Genetic variation at the species level (P = 73.3, mean number of alleles per locus (A) = 4.00, expected heterozygosity (H-E) = 0.237) and at the population level (percentage of polymorphic loci [P] = 42.4, A = 2.05, H-E = 0.173) was considerably higher than the means for other short-lived perennial herbs and endemic species. A moderate but significant genetic differentiation was observed among the populations (G(ST) = 0.179). The distribution of genetic variation showed an evident geographical pattern. In addition to the pattern of isolation by distance, a genetic distance phenogram and a principal components analysis based on allele frequencies roughly supported three clusters of the populations in central Honshu, northern Honshu, and Shikoku. In particular, two isolated populations in Shikoku were also characterized by remarkably lower genetic variation than those in Honshu, possibly resulting from strong genetic drift associated with their isolated distribution. This geographical structure of genetic variation was not in accord with the predictions based on migration history of subalpine coniferous forest in Japan, and it exemplified the importance of geographical isolation in establishing the genetic structure of P. adenostyloides.