High Population Genetic Diversity and Regional Differentiation in the Endemic Vincetoxicum katoi-V. yamanakae Complex (Apocynaceae; Asclepiadoideae) in Japan.
We examined morphological differentiation and allozyme diversity in nine populations of Vincetoxicum katoi and its close relative V. yamanakae. Although, V. yamanakae has been distinguished from V. katoi in having a larger corolla and gynostegium, the principal component analysis based on 12 floral and three leaf characters did not support the distinction of the two taxa. The six allozyme loci examined revealed that population of V. katoi and V. yamanakae maintained high genetic diversity (P=72.1, A=2.40, h=0.313). The nine populations were clustered into three regions (Kanto-Tokai; middle Kiniki; and Kii-Shikoku) based on genetic differences, contradicting the current taxonomic treatment. Considering the evidence, it is appropriate to treat V. yamanakae as a synonym of V. katoi. The high genetic differentiation among regions suggested that disjunct distribution of the V. katoi-V. yamanakae complex might reflect the persistence of refugia since the last glacial period. In particular, the middle Kinki (Hyogo) population is located outside of known evergreen forest refugia, suggesting that it might have survived during the glacial period within cooler vegetative zones, such as in temperate forests, whereas the populations on the Pacific-side retreated to warm-temperature coastal refugia.