Natural hybridization in the orchid flora of South Africa: Comparisons among genera and floristic regions

Abstract

Natural hybridization is an important evolutionary process that can act as a bridge for gene flow between species, generate new variation, and even precipitate formation of new species. Orchids are an ideal plant group for assessing the incidence of natural hybridization as they are well-studied and form hybrids that are relatively easy to identify using floral traits. Here I provide an illustrated compendium of the putative natural orchid hybrids recorded in South Africa and ask whether natural hybridization is concentrated in particular lineages or floristic regions. There are 0.082 reported natural hybrids for every orchid species in South Africa, which is similar to the value of 0.09 reported for angiosperms in general. The largest number of putative natural hybrids is found in Disa (17 hybrid combinations involving 25 species) and Satyrium (11 hybrid combinations involving 13 species). The weighted average hybridization propensity (percentage of possible hybrid combinations realized) for South African orchids is estimated to be 0.92%, which is much lower than the value of 6% reported for orchids in seven other mainly temperate northern hemisphere floras. Natural hybridization appears to be more frequent in the Cape Floristic Region (documented for 15% of orchid species) than in the rest of South Africa (documented for 8% of orchid species). The factors that govern variation in the frequency of natural hybridization among orchid species are still largely unknown and require further investigation.