“金虎尾路线”植物的花进化与传粉转变

Floral evolution and pollination shifts of the “Malpighiaceae route” taxa, a classical model for biogeographical study

以金虎尾科植物地理分布格局及迁移历史总结出来的"金虎尾路线",是解释热带植物洲际间断分布与长距离扩散格局的重要模式。金虎尾路线阐明了金虎尾科植物历史时期7次独立的从起源中心(南美洲)向旧世界(非洲和亚洲)的洲际长距离扩散事件。本文总结了金虎尾路线植物起源地与扩散地主要类群的花部特征与传粉系统,以探讨这些类群及类似植物长距离扩散后的花进化与传粉转变等适应规律。金虎尾科的南美洲类群都有分泌油脂的萼片腺体,花的形态结构非常保守,是与美洲当地特有的条蜂科集油蜂长期协同进化的结果。金虎尾科的非洲类群花保守性消失,花白色、辐射对称且无萼片腺体,繁育系统为雄花–两性花异株(功能性的雌雄异株),传粉者是采集花粉的蜜蜂科昆虫。亚洲的一些属发生了类似非洲类群的泛化适应转变,但风筝果属(Hiptage)出现了镜像花、异型雄蕊和极度反折的花瓣,且传粉者是亚洲特有的大蜜蜂(Apis dorsata),显示出了非常特化的适应性转变。风筝果属所在支系的现存类群涵盖了南美洲、中美洲、非洲和亚洲等地的地方特有属,体现了金虎尾路线整个迁移历史过程,是认识金虎尾路线及其进化适应规律的关键类群,值得在今后的研究中加以重视。

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 spe- cialized oil-collecting Anthophoridae bees. These bees, however, are absent from the Old World. In the Afri- can 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 pollina- tion system, in the Asia-endemic genus Hiptage. Mirror-image flowers in Hiptage show a sexual polymor- phism 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 honey- bees. 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 experi- mental 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.