探究植物丢失花蜜的演化及适应性意义

花蜜是植物分泌的含糖液汁，是提供给传粉者的报酬。花蜜是不稳定的演化特征，许多植物类群在演化中丢失了花蜜。前人的演化分析常以蜜腺等花部特征推断植物是否分泌花蜜，且较少调查相应传粉者的觅食行为。前人对植物丢失花蜜的意义提出多种假说，其中节约资源假说认为花蜜过于耗能；促进异交假说认为花蜜增加同株异花授粉；侵扰豁免假说认为花蜜增加花受破坏的风险。这些假说缺少充足实证支持，且缺乏考虑对传粉者行为和植物雄性适合度的影响。同时，我们结合前期成果提出行为转变假说，认为丢失花蜜可使传粉者转变出更利于植物适合度的行为模式。为解决以上问题，本项目拟以熊蜂传粉的马先蒿属植物为研究系统，在野外调查采样中获取花蜜特性与传粉者行为，进行系统发生分析；在定点研究中从雌、雄性适合度的角度验证花蜜丢失的三个已有假说和行为转变假说。项目期望获知植物丢失花蜜的演化模式和适应性意义，为植物—传粉者互作下花部特征的演化提供新的视角。

Nectar is the liquid containing sugar secreted by plants, offering to pollinators as the reward of transferring pollen. Nectar is a labile evolutionary character, many plant taxa have lost nectar during their evolution processes. Previous evolutionary analyses usually used indirect evidences to infer whether nectar was secreted in a given plant species, and usually neglected the investigation of foraging behavior of corresponding pollinators. Previous studies proposed several hypotheses to explain the adaptive significance of nectar loss, within which the resource-saving hypothesis considered that producing nectar would cost too much resource; cross-pollination-promoting hypothesis considered that nectar would increase the risk of geitonogamy; attack-exemption hypothesis considered that nectar would increase the risk of flowers being attacked by herbivores or nectar robbers. These hypotheses lack the support of adequate evidences, and lack the consideration on the effect of pollinator behavior on plant male fitness. Meanwhile, we propose an extra hypothesis according to our previous study, that considers the loss of nectar would make the pollinators shift their behavior to a pattern that better benefit plants. To solve the problems listed above, this program would use bumblebee-pollinated Pedicularis species as the study system, investigating nectar features and pollinator behaviors in field surveys, and conduct phylogenetic analysis; testing the three previously proposed and one newly proposed hypothesis in field site experiments. This program is about to acquire the evolutionary pattern and adaptive significance of nectar loss in plants, which may provide new insight to the evolution of floral traits under the circumstance of plant-pollinator interactions.