Floral biology, breeding systems and population genetic structure of three climbing bauhinia species (Leguminosae: Caesalpinioideae) in Hong Kong, China

The pollination ecology, breeding system and population genetic structure of three climbing Bauhinia species B. championii (4 populations, 23 individuals), B. corymbosa (2 populations, 25 individuals) and B. glauca (8 populations, 76 individuals) were studied in Hong Kong, southern China. We hypothesize that the climbing Bauhinia species will attract targeted pollinators to achieve out-cross success and high levels of self-incompatibility will be expected to maintain diversity, with local population expansion relying on vegetative propagation. All three species have inflorescences consisting of numerous small, pale, fragrant flowers, which show diurnal anthesis. Field observations revealed that all three species are predominantly pollinated by bees (particularly Apis mellifera) and butterflies (Graphium and Papilio species), although B. championii is also pollinated by wasps and flies. Bauhinia corymbosa and B. glauca have sucrose-dominant nectar, whereas B. championii has hexose-dominant nectar. In controlled-pollination experiments fruit and seed set were generally highest following artificial out-crossing. The index of self-incompatibility of B. championii is 1.07, indicating self-compatibility; B. corymbosa and B. glauca were obligately self-incompatible. The population genetic structure and variation of the Bauhinia species was investigated using ISSR markers. Generally the three species have moderate within-population (mean HS = 0.206) and high among-population genetic variation (mean GST = 0.284). No correlation exists between the geographical and genetic distance, possibly due to the small local population size. All three species showed high levels of heterozygosity as expected for predominantly out-crossing long-lived K-selected species.