Molecular evidence for the hybrid origin of Bauhinia blakeana (Caesalpinioideae)

Bauhinia blakeana Dunn is the Hong Kong Special Administrative Region emblem and a popular horticultural species in many Asian countries. It was first described as a new species from Hong Kong almost a century ago. This plant is sterile and has long been considered a hybrid, possibly from two related species, B. purpurea and B. variegata. However, not much evidence based on molecular methods was available to support this hypothesis. In this study, sequences of internal transcribed spacer 1 (ITS1), rbcL and atpB-rbcL intergenic spacer for five Bauhinia species and two varieties of one of the species were determined and compared. There were two types of ITS1 sequences in B. blakeana, one indistinguishable from that of B. purpurea and the other one identical to that of B. variegata. This confirmed that B. blakeana was a hybrid of these two species. Chloroplast atpB-rbcL intergenic spacer sequence of B. blakeana was identical to that of B. purpurea, indicating that B. purpurea was the female parent. The hybridization event seemed to occur only recently and was a rare incident. Its occurrence was likely facilitated by interspecific pollen competition. It appeared that human efforts played a crucial role in the preservation and ubiquity of B. blakeana.