Floral biology of Romulea (Iridaceae: Crocoideae): a progression from a generalist to a specialist pollination system

Field observations, floral dissections, and pollen load analyses of insects captured on 32 species of Romulea, including all the main flower types in the genus, show that flowers of this African and Eurasian genus of c. 90 species centered in the winter-rainfall zone of southern Africa are cross pollinated by a relatively narrow range of insects. Observations indicate that there are four modes of floral presentation in the southern African members of the genus. The Romulea flava group is typically pollinated largely by female bees representing four families of native Apoidea. In contrast, the Romulea monadelpha group is pollinated exclusively by hopliine beetles (Scarabaeidae). The Romulea eximia group combines morphological and pigmentation characters found in the other two groups and is pollinated by bees or hopliine beetles alone or in combination. Species in the Romulea hantamensis group have elongated floral tubes and are pollinated by long-proboscid flies (Nemestrinidae). Pollination systems within the genus are comparatively fewer than in other irid genera of similar size (e.g., Ixia, Lapeirousia). The relatively low level of adaptive radiation in Romulea appears to be a consequence of both a conservative floral phenology and floral architecture. Outgroup comparison strongly suggests that long-tongued fly pollination and exclusive beetle pollination represent relatively recent syndromes derived from pollination primarily by bees or a more generalist condition of combined bees and hopliines.