Pollination of Plectranthus spp. (Lamiaceae) with sigmoid flowers in southern Africa

Abstract

Within the South African Plectranthus species two specialized lines of corolla adaptations have evolved. Long-proboscid flies (Nemestrinidae) appear to have driven the development of Plectranthus species with long corolla tubes that are limited to the sub-continent. Plectranthus s.l. (including Coleus) species with sigmoid corollas are far more widespread and evidence presented here supports the hypothesis that this floral type has evolved as a response to melittophily. Thirty percent of southern African Plectranthus species have corolla tubes that are bent to some degree. Pollination of the following four labiate species with sigmoid corollas was studied in detail: Plectranthus petiolaris, P. laxiflorus, P. calycinus and Pycnostachys urticifolia. The pollination of three other species was investigated to a lesser degree: Plectranthus spicatus, P. rehmannii and Aeollanthus parvifolius. Bee pollination is confirmed for P. laxiflorus and Py. urticifolia and is recorded here for the first time in P. petiolaris and P. calycinus. A new group of floral visitors comprising nemestrinid flies of the genus Prosoeca with moderately long proboscids is recorded for P. laxiflorus and P. calycinus, where the corolla tube shape allows visits by medium-proboscid floral visitors. The sigmoid corolla shape limits the type and size of insects that can access nectar and act as pollinators. Explanations for the existence and function of the sigmoid corolla shape are suggested.