Morphological evidence for introgressive hybridization in the genus Psoralea L. (Psoraleeae, Fabaceae)

Introgression is interesting for plant evolutionary studies because it yields significant numbers of new genotypes, thus increasing genetic diversity, but also blurs species boundaries. Here we report a case of introgressive hybridization from a hybrid zone on the Swartberg Pass in the species rich Core Cape Subregion of South Africa. We analysed morphological data from 52 individuals between the hybridizing species Psoralea sp. 15 (P. “forbesiae” sp. nov. ined) and P. sordida C.H. Stirt and Muasya. The data were examined using the Andersonian hybrid index method and multivariate analyses. The results showed that P. “forbesiae” and P. sordida were distinct species linked through a range of intermediates and introgressants to both species and that the sampled populations comprised an introgressive hybrid swarm. Habitat disturbance via road-building has played an important role in the formation and maintenance of the swarm. While introgressive hybridization can increase genetic diversity in the short term in Psoralea it is unclear as to the future of the genetic integrity of the parent species and hybrid populations given the current differences in habitat specialisation (i.e. natural and disturbed habitats, respectively).