The role of pollinators in plant species diversification in the Greater Cape Floristic Region, South Africa

Supervised by Tonya Lander, Department of Plant Sciences

The question of why there are so many species is one of the most fundamental in biology. Species richness has been correlated with paleoclimatic stability, energy and water availability, geological diversity, and fire regime (niche diversity), but the study of the actual mechanisms that have created the diversity is in its infancy. A feedback loop of increasing specialization in plant-pollinator mutualisms has been proposed as a potential mechanism driving plant diversification, but examples of species that are actually in the process of divergent evolution linked directly to plant-pollinator specialization are rare.

South Africa is one of the hottest biodiversity hotspots in the world.  The country covers 2% of the global terrestrial surface, but is host to 5.5% of global invertebrate diversity and 10% of global plant diversity, 32% of which is endemic. Our research focuses on Rediviva oil-bees (Melittidae), and their Diascia oil-plants (Scrophulariaceae) from the Greater Cape Floristic Region of South Africa. Diascia plants produce oil in a pair of floral spurs which the Rediviva bees collect with their forelegs, and it is thought that the oil is used to waterproof the bee’s underground nests and perhaps provision their brood. Interestingly, there is co-variation between Rediviva foreleg length and Diascia spur length. This is observed between species, such that long-fore-legged Rediviva species appear to specialize on long-spurred Diascia species, but there is also co-variation within species. Our collaborator Prof Michael Kuhlmann from the Zoological Museum of Kiel University has found that foreleg length of a single Rediviva species and spur length of a single Diascia species may be significantly shorter at one site, and within a few hundred meters, be significantly longer. These differences in spur and fore-leg length within species suggest that the populations are genetically distinct, but Diascia plants exist in patches separated by distances shorter than the Rediviva bees’ estimated flight capacity of 1-2.2km.
Our project uses population genetic and related molecular techniques to understand whether and why Rediviva bees might be travelling between some patches of Diascia plants and not others, with the goal of understanding (1) to what extent is there pollen movement between spatially distinct patches of Diascia, and (2) is it was possible to identify geological or ecological features which serve to limit or encourage Rediviva movement between particular Diascia patches. The answers to these questions will help us understand the role of this highly specialized pollinator in the apparent morphological divergence of Diascia plants.