Genetic variation and differentiation in populations of a threatened tree, Magnolia stellata: factors influencing the level of within-population genetic variation

Genetic variation and differentiation in Magnolia stellata were studied in 20 populations distributed across most of the species' range using 10 microsatellite markers, and the factors influencing their levels of within-population genetic variation were examined. Generally, populations distributed intermittently from southern Gifu to central Aichi Prefectures showed substantially higher levels of genetic variation (exceptions included populations located at unusually high altitude sites or western and southern edges of the range) than more isolated populations on the Atsumi Peninsula of southern Aichi Prefecture and in northern Mie Prefecture. Significant isolation-by-distance patterns were detected in genetic differentiation among the studied populations, and a neighbor-joining tree based on D(A) distances among the populations reflected well the geographical positions of the populations. The level of within-population genetic variation was significantly influenced not only by the size of the populations (represented by the number of reproductive individuals) but also by their degree of isolation (represented by the number of populations within a radius of 0.5 km around them). Therefore, areas within radii of 0.5 km may encompass M. stellata metapopulations, in which gene flow may usually occur. We suggest that this area may be a suitable standard for constructing conservation units for the species.