专性异交植物地果性系统格局的转换及其对花粉限制的响应

It is adaptively important for plants to adjust their sex expression in response to pollen limitation. Stronger pollen limitation usually imposes stronger natural selections on traits important for reproduction. However, the idea that pollen limitation might be important in the plant sex system evolution was not took seriously until recent years. It is demonstrated the plants relying on specific pollinators are more prone to pollen limitation. Ficus species and their pollinating fig wasps are among the most specific mutualisms. The sex expressions of Ficus species underwent strong selection by the pollination behavior and pollination efficiency of their species-specific pollinating fig wasps. Divergence in the anther-to-ovule ratios (A/O ratios) and distribution patterns of male flowers were demonstrated significant between actively- and passively-pollinated Ficus species. The Chinese endemic Ficus tikoua has an apparently-active pollinator, but atypical high variation of A/O ratios were detected among its natural populations in Southwest China. The distribution patterns of male flower showed continuous transitions along geographic clines. The high A/O ratios and scattered distribution pattern of male flowers deviated from the expectation of the active pollination mode. We plan to further investigate variations in A/O ratios among different natural populations of F. tikoua in order to map the distribution of pollination modes in relation to environmental variables; to estimate the level of pollen limitation of focal populations through a comparison of seed production under open pollination and artificial pollination; to assess the genetic divergence and population sizes of the pollinating fig wasps using both ecological and molecular methods and to compare behavior of the wasps in different areas. We thereby aim to explore evolutionary and ecological adaptations to pollen limitation in F. tikoua. This will aid interpretation of the responses of obligate-outcrossing species to pollinator population declines and changes in pollinator behavior, and contribute to a better understanding of the evolution of the plant breeding systems.

花粉限制诱导的性表达变化是植物适应环境的重要方式，花粉限制强度越大植物相关繁育性状受到的自然选择就越强烈，但花粉限制对植物性系统进化的作用近年来才被真正重视。依赖专一性传粉者的植物被证实往往遭受更强烈的花粉限制，榕属物种与其传粉小蜂是目前已知的专一性最强的传粉共生体系之一，传粉小蜂的传粉行为对宿主榕的性表达有极强的选择作用，主动与被动传粉榕进化出不同的传粉综合征。地果为主动传粉榕，却呈现出不同于其他主动传粉榕的性系统特征，且在不同地域连续变化。我们拟比较不同类型地果的典型和交错带种群，评价其性表达的时空格局；野外调查和人工补充授粉相结合评价地果自然种群的花粉限制强度、传粉小蜂的传粉效率；野外调查和遗传分析相结合探究传粉小蜂的分化及种群统计动态；以厘清地果性表达格局转换的进化机理，解读强制异交物种对花粉限制强度、传粉者种群大小和行为波动的响应，深化对被子植物繁育系统进化的认识。

繁殖是生物适合度的最终表现，物种能否适应环境变化在很大程度上取决于有性生殖对环境变化的响应速度。被子植物的性系统影响着雌、雄配子在种群中的频率、交配机会和交配方式；性系统的表达与分配受遗传和环境因素的共同作用，其中花粉限制对植物繁育性状产生强烈的自然选择，导致分化。地果广泛分布于我国西南，自然种群的性表达在不同地域有显著分化，呈现出异域分布的两种生态型（OS型、SS型），其中OS型对雄性功能的投入显著低于SS型。为厘清地果性表达格局的进化机理，项目组对地果不同生态型典型种群的种子繁殖策略、遗传分异、专性传粉小蜂种群动态、花粉供应格局的环境驱动因子进行了分析，探究地果性表达的遗传控制，及其对环境变化和传粉者种群动态的响应。结果显示，不同地域的地果种群有明确的遗传分化，地果与地果传粉小蜂的遗传格局在大尺度上相互匹配，青藏高原、西南山地的地质历史及天空之岛的隔离驱动了两者的分化。地果的性表达可能受核基因的控制，花粉流在地果生态型地理分布格局的形成与维持中起到重要作用。地果不同生态型的适生区呈现出环境条件的显著差异，影响了传粉小蜂的种群动态、寿命和行为，进而选择了宿主的花粉供应格局和繁殖策略。OS种群所处生境，季节分明，冬季低温导致传粉小蜂种群波动较大，不稳定的传粉者供应选择了宿主较低的花粉供应水平和r繁殖策略。项目组发现不同生态型响应环境条件的差异，形成了不同的叶片结构和生理特征，因而增加了同质园种植条件下不同生态型植株的叶片结构与光合特征比较实验，探讨营养供应特征对地果性表达的影响。同时增加了基因组和转录组的研究，探讨地果性表达的分子机制。项目组的研究将为揭示榕属物种传粉模式的进化机理提供参考，深化对被子植物繁育系统进化的理解。