Classification of Edible, Indigenous Dioscorea Species in Japan by Leaf Morphology and Cross-compatibility

To speciate the edible, indigenous Dioscorea in Japan, 425 plants collected from 24 regions were classified as Dioscorea japonica Thunb., although some plants have leaf morphology characteristic of Dioscorea opposita Thunb. Typical D. japonica specimen were collected from 21 regions, whereas those with characteristics of D. opposita were collected from 17 regions. The former were collected exclusively from seven regions with mild climate, while the latter were collected from three regions with cold climate, indicating that the two species were adapted to different climatic zones. When D. japonica and cultivated D. opposita were hand-pollinated to observe the hybrid seed formation, few normal seeds were produced, indicating cross-incompatibility. Similarly, D. japonica and plants with characteristics of D. opposita were also incompatible, producing no normal seeds. Many crosses within D. japonica and those between D. japonica with characteristics of D. opposita and locally cultivated D. opposita produced viable seeds. These findings indicate that cross-incompatible plants with characteristics of D. opposita should not be classified as D. japonica but rather as D. opposita or its close relative.