Phylogenetic and population genetic studies in the genus Streptocarpus Lindl. (Gesneriaceae DC.)
Thesis (PhD (Botany and Zoology))—-Stellenbosch University, 2008.
Streptocarpus Lindl. (Gesneriaceae DC.) is a genus of herbaceous plants containing approximately 160 species, of which the majority occur in Africa and Madagascar. They are largely restricted to shaded and moist habitats such as primary forest and rock outcrops. The genus contains considerable morphological variation, with subgenus Streptocarpella containing caulescent species and subgenus Streptocarpus mostly consisting of acaulescent growth forms, mainly the unifoliates, plurifoliates and rosulates. Preliminary molecular analyses conducted using nuclear ITS sequence data suggested that subgenus Streptocarpus evolved in tropical central Africa, before radiating in several independent waves into southern Africa. Streptocarpus has therefore only recently spread into South Africa. Amongst the South African species, 11 morphologically similar species were identified as being closely related, together forming the Cape primrose clade, based on the analysis of nuclear ITS sequence data. However, these analyses only contained a few South African species, and the ITS data did not provide enough resolution of relationships within this clade. In this study nuclear and chloroplast sequence data as well as nuclear microsatellite data were therefore employed to unravel the complex relationships amongst the South African Streptocarpus species. The analyses indicate that 16 rosulate, palynologically similar species (S. primulifolius, S. rexii, S. johannis, S. baudertii, S. modestus, S. formosus, S. gardenii, S. lilliputana, the S. cyaneus complex [S. cyaneus, S. parviflorus, S. fenestra-dei, S. kunhardtii and S. roseo-albus], S. floribundus, S. aylae and S. kentaniensis), the core Cape primrose species, are closely related, while five unifoliate/plurifoliate, palynologically more variable species (S. denticulatus, S. dunnii, S. pusillus, S. rimicola and S. bolusii) consistently emerged as more distantly related to the core Cape primrose species. However, the positions of a further ten species (S. meyeri, S. montigena, S. fanniniae, S. caeruleus, S. longiflorus, S. polyanthus, S. saundersii, S. porphyrostachys, S. grandis and S. vandeleurii) were more complex in the analyses, indicating that hybridization has played a role in their evolution. Five of these species (S. meyeri, S. montigena, S. fanniniae, S. caeruleus and S. longiflorus) are, however, palynologically homogenous and rosulates, and therefore are probably more closely related to the core Cape primrose species, while the other five (S. polyanthus, S. saundersii, S. porphyrostachys, S. grandis and S. vandeleurii) are unifoliates/plurifoliates that are palynologically more heterogenous, and are probably more distantly related to the core Cape primrose species. Amongst the core Cape primrose species, S. primulifolius emerged as being ancestral or having hybridized with many of the other species, while the S. cyaneus complex forms a geographically and genetically more isolated group. However, evolutionary relationships amongst these species were to a certain extent obscured by incomplete lineage sorting caused by limited interpopulation gene flow, frequent hybridization and rapid speciation. The analyses confirmed that the Pondoland Centre forests constitute important Pleistocene refugia, and revealed some of the historical migration routes along which the species had radiated.