Background and aims – The populations of Epimedium in the western part of Chugoku district on the Japanese island of Honshu are morphologically diverse: most consist of individuals showing wide variation in morphological characteristics, and the character composition varies with population. We explored the morphological variations between Epimedium populations in Chugoku district to test the hypothesis that these variations are the result of natural hybridization between E. diphyllum and E. sempervirens var. rugosum, and to elucidate to what extent natural hybridization between these two species has occurred in this region. Methods – We measured the length of three flower parts using a digital caliper and counted the number of teeth per leaflet. The leaf architecture was also categorized. In addition, we quantified flower colour and leaflet apex shape using a spectrophotometer and principal component analyses of elliptic Fourier descriptions, respectively. From these measurements, we calculated Anderson's hybrid index for each individual. Key results – The populations in Chugoku district showed wide variation in the morphological characters examined. In particular, the populations located near the center of the hybrid zone tended to show greater variation than those located at the periphery. The variation observed across the populations showed similar patterns among the morphological characters examined. No correlations were found among the examined characters. Conclusions – The wide variation in morphological characters likely resulted from natural hybridization between E. diphyllum and E. sempervirens var. rugosum. Hybridization between the two species occurred intensively where the distributions of the species overlap, and bi-directional introgressions of these species are advancing throughout the region. To clarify the factors that produced the hybrid zone, it will be necessary to compare fitness between hybrids and their parental species throughout the parental species' distribution areas and the hybrid zone.

BERBERIDACEAE; EPIMEDIUM; HYBRID ZONE; HYBRIDIZATION; MORPHOLOGICAL VARIATIONS