Diversity of flies visiting and pollinating flowers of the Southern African genus Riocreuxia (Apocynaceae: Ceropegieae, Anisotominae)

The subtribe Stapeliinae (Apocynaceae: Asclepiadoideae) contains ca. 720 species and, with fly pollination being ubiquitous in the group, appears to have diversified without shifts between major groups of pollinators. In contrast, the sister subtribe Anisotominae, which only has ca. 30 species, appears to include some shifts between different pollinator groups, but there is a paucity of data on pollination in this group. To address this gap, we collected flower visitors of five Riocreuxia (Anisotominae) species, identified pollinators, assessed pollen transfer efficiency (PTE) and recorded fruit set, where possible. We found that the flowers of Riocreuxia species are functionally specialized for pollination by flies. The flies collected belong to 14 families, whereby each Riocreuxia species was visited by representatives of a minimum of 3 and a maximum of 10 fly families. The degree of pollinator specialization per Riocreuxia species ranged between 1 and 4 fly families and the number of pollinating species within these families ranged between 1 and 21. There was zero overlap in the pollinating fly species between the Riocreuxia species. PTE was high and averaged 15.5% across species. Fruit set was recorded only for R. alexandrina and was very low (3% of flowers developed into fruits). This study indicates that Riocreuxia species are ecologically specialized for pollination by specific fly groups, predominantly microdiptera.