Pollination of Plectranthus L'Her. (Lamiaceae) along the Eastern seaboard of southern Africa.
Potgieter, Christina Johanna.
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Pollination data is provided for a third of the Plectranthus species in southern Africa. In the largest genus of Lamiaceae in the region (53 species), 18 were studied, plus two species of allied genera (Pycnostachys urticifolia and Aeollanthus parvifolius). Study of these 20 species aimed to describe the groups of pollinators that have driven pollinator specialisation. Case histories are provided upon which future studies of Lamiaceae pollination, breeding systems and speciation may be based. Bees (Apidae) and flies (Nemestrinidae, Acroceridae and Tabanidae) are the main pollinating insect groups. Seven straight-tubed Plectranthus species show a match between corolla tube- and proboscis length of nectar-feeding pollinators. Long-proboscid nemestrinid flies are specialised on long-tubed Plectranthus species (P. ambiguus, P. hilliardiae, P. reflexus and P. saccatus), while shorter-proboscid flies of all three families are important pollinators of straight-tubed species with medium- and short corolla tubes. Seven species with sigmoid corolla tubes are bee-pollinated, with fly-pollination prevalent in some. Bent corolla tubes, coupled with length, act as barriers to illegitimate nectarfeeders and ensures alignment of pollinators for effective pollen placement and carryover. It is suggested that straight-tubed species may have evolved from sigmoid-tubed species. Long-tubed species with straight corollas in other Lamiaceae may show convergent pollination by long-proboscid flies, with the guild being dependent on habitat and distribution of plants and flies. Formal establishment of the Stenobasipteron wiedemanni pollination guild extends the study from Plectranthus to selected Acanthaceae, Orchidaceae, Balsaminaceae, Gesneriaceae and Iridaceae, occurring in forested habitat along the Eastern seaboard. Micro-distillation of essential oils confirmed parentage of a putative natural hybrid; once established, hybrid data allows studies of the importance of natural hybridisation events in explaining pollinator fidelity. Nectar sugar studies in Plectranthus mostly showed sucrose dominance; cases of hexose dominance are noted and discussed. Nectar volume and concentration proved variable and do not fit any trends. Pollination by medium-proboscid acrocerid flies has importance for ‘medium-tubed’ plants, since six of the Plectranthus species are solely or partially reliant on Acroceridae for pollination. An appendix with consolidated data describes the 20 study species i.t.o. morphology, habitat, study sites, field work, pollinator observations and insect vouchers.