Abstract:

The “Malpighiaceae route” is proposed based on the distribution pattern of the family Malpighiaceae to explain plant inter-continent disjunctions and long-distance dispersal during historical periods. The route involves seven inter-continent dispersals from New World (America) to Old World (Africa and Asia) in the Eocene (~ 65 Ma). Malpighiaceae has about 1,300 species, most of which are endemic to the New World and are characterized by “floral conservatism”. Floral conservatism in this family refers to its stereotyped yellow or pink flowers with paired oil-glands on each sepal, a result from co-evolution with specialized oil-collecting Anthophoridae bees. These bees, however, are absent from the Old World. In the African Malpighiaceae, floral conservatism disappeared as a result of adaptations to local pollen-collecting bees. Their flowers became white and radially symmetric, without sepal gland. Furthermore, the floral sex changed to be morphological androdioecy but functional dioecy. These results indicated that African Malpighiaceae had shifted from floral conservatism to generalized floral syndromes. In Asian Malpighiaceae, some studies reported in Aspidopterys and Ryssopterys generalized evolutionary adaptations in floral traits and pollination systems similar to African taxa. But a recent study found mirror-image flowers, a highly-specialized pollination system, in the Asia-endemic genus Hiptage. Mirror-image flowers in Hiptage show a sexual polymorphism in which the style deflects either to the left or the right side of the floral axis, which is a highly specialized mechanism promoting cross-pollination between left- and right-styled flowers via pollinators touching two sexual organs respectively with their left and right side of abdomens. Hiptage is also notable for its heteromorphic stamens, bilaterally-symmetric corolla, and extremely-reflected petals. Its main pollinators are the pollen-collecting honeybees such as Apis dorsata. These results indicate that the Asian Malpighiaceae could shift from floral conservatism to specialized pollination systems adapted to pollen-gathering honeybees. Hiptage is also distinctive for its position in a clade with complete migration history of Malpighiaceae route, having endemic genera respectively in South and Central America, Africa, and Asia. Further experimental studies on this genus and other Asia-endemic genera are needed to fully understand the Malpighiaceae route and its associated evolutionary adaptations, which will be helpful for studies on plant long-distance dispersal and inter-continent disjunctions.

Key words: adaptive evolution, breeding systems, long-distance dispersal, disjunction, Malpighiaceae