Twin oil sacs facilitate the evolution of a novel type of pollination unit (meranthium) in a South African orchid.

The unique floral morphology of the South African orchid H. pulchra, with its twin meranthia, is best explained as an adaptation to pollination by oil-collecting bees. Flowers consisting of meranthia (floral parts that function as single pollination units; commonly observed in garden Iris) are extremely rare among the angiosperms and their significance poorly understood. Unlike all other known examples of meranthia, the novel type described for H. pulchra is not bilabiate. All Huttonaea species are unique in having twin petal sacs with glandular verrucae that secrete oil and are pollinated by Rediviva (Melittidae) oil-collecting bees. But only Huttonaea pulchra has long and widely divergent petal claws that place the oil sacs well beyond the reach of a centrally positioned bee. The wide separation of these sacs forces the pollinator, R. colorata, to visit each side of the flower independently and effectively divides the flower into two meranthia. Molecular data indicate that the evolution of the Huttonaea-type meranthium was dependent on the prior evolution of the oil flower/oil bee relationship. Meranthium evolution was also facilitated by the presence of oil in two separate structures (petal sacs) that were not physically constrained to remain in close proximity.