Phenotypic variation and leaf fluctuating asymmetry in isolated populations of an endangered dwarf birch Betula ovalifolia in Hokkaido, Japan

Betula ovalifolia is an endangered tetraploid that is restricted to two isolated sites, Betsukai and Sarabetsu, in Hokkaido, Japan. Among 50 ramets sampled along transect(s) in the each site, 45 genets in Betsukai and 49 in Sarabetsu were discriminated by simple sequence repeat (SSR) phenotypes. Multivariate patterns in seven measurements for leaf morphology and frequency distributions of fragment lengths in four SSRs significantly differed between Betsukai and Sarabetsu, but the ranges of the morphological measurements and fragment lengths were largely overlapped between the two sites. In spite of the smaller population size in Sarabetsu than in Betsukai, coefficients of variation in most leaf measurements and richness/diversity of fragment lengths in all the SSRs were not smaller in Sarabetsu than in Betsukai. This result suggests that both morphological and molecular variation has been maintained even in the smaller population. Leaf fluctuating asymmetry (FA) was higher in Sarabetsu than in Betsukai. Longer and narrower leaves had higher FA than shorter and wider leaves. However, SSR variation was not significantly associated with leaf FA. These results imply that environmental stress due to dryness of habitats and competition with other plants, which may be stronger in Sarabetsu than in Betsukai, causes both the difference in leaf shape and the increase in leaf FA.