苦苣苔科镜像花的多样性及演化

泛热带分布的苦苣苔科(Gesneriaceae)在我国南方具有极高的物种丰富度与特有率,花部特征变化丰富,是研究物种形成与适应演化的代表类群。镜像花(mirror-image flowers)是极为特化的传粉系统,在苦苣苔科中出现了较多的不同类型,可能与苦苣苔科物种多样性形成与维持有关。该研究总结与分析了苦苣苔科镜像花的类型多样性以及系统分布与适应演化等,讨论了镜像花对苦苣苔科物种形成与维持的积极意义。结果表明:镜像花仅分布在亚洲和非洲的苦苣苔亚科(Didymocarpoideae)的7个属,在历史上就至少发生了5次独立起源。长冠苣苔属(Rhabdothamnopsis)、南洋苣苔属(Henckelia)及长蒴苣苔属(Didymocarpus)镜像花的花柱与可育雄蕊分别向左、右两侧偏转,形成互补镜像花;蛛毛苣苔属(Paraboea)、喜鹊苣苔属(Ornithoboea)、非洲堇属(Saintpaulia)镜像花缺乏与花柱对应侧偏的可育雄蕊(非互补镜像花);而海角苣苔属(Streptocarpus)直立堇兰亚属(subg.Streptocarpella Engler)则同时出现了互补、非互补镜像花。不同于其他被子植物(离瓣花、缺乏花冠筒),苦苣苔科中的镜像花大多伴随着明显的花冠筒、内藏的雄蕊、合生的花药,以非互补镜像花为主;传粉者以小型的无垫蜂(Amegilla spp.)和熊蜂(Bombus spp.)为主。这些特殊的花部综合征与特化的传粉机制,提高了传粉精确性,可能促进了传粉隔离与物种适应辐射。今后的一个研究重点应通过分子系统发育方法,进一步揭示苦苣苔亚科互补与非互补镜像花的进化顺序及其在物种分化与长距离扩散过程中的可能作用。

Diversity and evolution of mirror-image flowers in Gesneriaceae

Gesneriaceae is a pantropical family with diverse floral traits and forms a diversification and endemism center in Southwest China. Mirror-image flowers, a highly specialized pollination system, were discussed in Gesneriaceae with an emphasis on its diversity, systematic distribution, and evolution. Mirror-image flowers was found in seven genera in the subfamily Didymocarpoideae, among which three genera have reciprocal mirror-image flowers(Rhabdothamnopsis, Henckelia, Didymocarpus)and other three genera possess nonreciprocal mirror-image flowers(Paraboea, Ornithoboea, Saintpaulia). Subg. Streptocarpella Engler of Streptocarpus contains both reciprocal and nonreciprocal mirror-image flowers. Gesneriaceae was dominated by nonreciprocal mirror-image flowers with united anthers and hidden sexual organs in floral tubes, distinctive from the typical mirror-image flowers in other families. The main pollinators to mirror-image flowers in these species are small-bodied bees such as Amegilla spp. These unusual floral traits suggest the mirror-image flowers in Gesneriaceae probably had experienced distinctive evolutionary history. Phylogenetic relationships indicate that mirror-image flowers are restricted to the subfamily Didymocarpoideae, which is endemic to the Old World. According to molecular phylogenetic studies, at least five independent origins of mirror-image flowers were found. The highly-specialized pollination mechanism associated with mirror-image flowers might facilitate pollination accuracy and consequently accelerate speciation rate of Didymocarpoideae. It is suggested that molecular phylogenetic methods can be used to explore the evolutionary relationship of reciprocal and nonreciprocal mirror-image flowers in Gesneriaceae to determine the possible role of evolutionary transitions of these two types of mirror-image flowers in the family''s speciation histories and long-distance dispersal.