Hybrid origin of Bauhinia blakeana Dunn (Leguminosae: Caesalpinioidaea) inferred using morphological, reproductive and molecular data

Abstract

Bauhinia blakeana Dunn (Leguminosae subfam. Caesalpinioideae tribe Cercideae), or the HongKong Orchid Tree, is of great horticultural value. It is completely sterile, and is shown here to be the result of hybridization between the largely sympatric species, B. purpurea and B. variegata. The analysis of patterns of morphological variation in the flowers of B. blakeana and its putative parents reveal only a few examples of phenotypic intermediacy; these are largely restricted to length/width ratios, indicating intermediacy in shape rather than size. Study of Intersimple Sequence Repeat (ISSR) markers, however, enables unequivocal identification of the parental species due to the presence of additive inheritance of alleles, and the absence of any bands that are unique to B. blakeana. Investigation of aspects of the reproductive biology of the taxa(based on field observations and controlled pollination experiments) furthermore reveal that the parental species are largely xenogemous, have flowering periods that overlap seasonally and temporally, and share common pollinators(mainly Apis and Xylocope bees, and Papilio butterflies). Evidence is provided to show that B. blakeana is not naturally stabilized, and is only maintained horticulturally by artificial propagation. It is therefore recommended that the hybrid be regarded as a horticultural cultivar rather than a naturally occurring species.