Lack of prezygotic reproductive isolation between Rhododendron eriocarpum and R. indicum; overlapping flowering phenology and cross compatibility

Flowering phenology in Rhododendron eriocarpum and R. indicum in Yakushima Island and its surrounding islands and cross compatibility between the species were investigated. We found that their flowering periods partially overlapped from May to early June. The flowering peaks of the riverside populations were variable and longer than those of the seaside populations where the flowering patterns synchronized. In artificial intra- and inter-specific outcross experiment, >60% crosses were successful. Weak self-incompatibility was observed in the both species. Their cross compatibility has been maintained in their presumptive hybrids. These results indicate that there is little prezygotic reproductive isolation between the two species and the lack of such reproductive isolation is likely to have contributed the diversification of 'Satsuki' cultivars.