Genetic Introgression from Horticultural Varieties to Wild Species in Japanese Evergreen Azalea

In Japan, cultivation of azalea has been very popular since the Edo era (1603-1867). Cultivars selected from wild plants and chance seedlings have been commonly distributed and planted throughout the country. Rhododendron x transiens and R. x hannoense are endemic to the Kanto region which surrounds Tokyo (formerly named Edo) and rarely inhabit in suburban forests. These putative hybrid species are found growing with R. kaemferi populations which are widely distributed in the low lands of Japan. Morphological analysis suggested that R. x transiens and R. x hannoense have hybrid features between R. kaemferi and R. ripense or R. indicum, respectively. However the native habitats of R. ripense and R. indicum are different from these putative hybrid species. We employed five SSR markers to analyze the genetic relationships of R. x transiens, R. x hannoense and their morphologically-related species and cultivars. In Structure analysis, R. kaempferi was assigned to all clusters, R. x transiens wild plants were assigned to a R. ripense / R. mucronatum cluster, and R. x hannoense wild plants are assigned to a R. indicum cluster. R. mucronatum is presumed to be a hybrid between R. ripense and R. macrosepalum. Both R. mucronatum and R. indicum have many cultivars and have been used as garden shrubs and traditional pot plants (Bonsai) since the Edo era. These results suggest that R. x transiens and R. x hannoense originated from hybrids between native R. kaempferi individuals and cultivars of R. mucronatum or R. indicum which had been planted in rural areas nearby the suburban forests.