Molecular phylogenies have been generated to investigate relationships among species of Streptocarpus Lindl., the largest genus of Gesneriaceae in Africa. Of the around 146 species described, 77 have been included in a parsimony analysis using the internal transcribed spacer (ITS) of ribosomal DNA. The studies show that Streptocarpus is paraphyletic and at least three other genera are nested within the genus. Incongruence occurs in relation to the subgeneric division. Streptocarpus is currently divided into two subgenera, Streptocarpella and Streptocarpus, mainly on morphological grounds; the former including basically caulescent species with a functional shoot apical meristem, while the latter includes mainly unifoliates and rosulates with an abnormal shoot apical meristem. On the African continent the geographic distribution of caulescent species of subgenus Streptocarpella ranges across tropical Africa from Sierra Leone to the Indian Ocean, while species of subgenus Streptocarpus occur in central and eastern parts of Africa. The molecular data suggest a southwards migration and “progressive latitudinal speciation” of subgenus Streptocarpus in Africa. Some evidence suggests an origin of the genus on Madagascar but this is equivocal. The subgeneric split probably occurred before interchange between mainland Africa and Madagascar ceased, after the geological separation of Madagascar, and is consistent with the hypothesis of a land bridge connection between 45-26Mya.
Phylogenetic Studies in Streptocarpus (Gesneriaceae): Reconstruction of Biogeographic History and Distribution Patterns
Publication: Systematics and Geography of Plants