被子植物基部类群花粉形态多样性和适应性进化研究

From the time of Charles Darwin, no topic in plant evolution has been more controversial or generated a large literature than the origin of the angiosperm and its process. The basal angiosperms are the earliest-diverging flowering plants from the ancestral angiosperms, comprised of a few separate evolutionary lineages with a few hundred species,i.e., Amborellanae, Nymphaeanae, Austrobaileyanae, Magnolianae and Ceratophyllanae. Nevertheless, the phylogenetic relationships still remain incompletely resolved. As a key trait of plant, pollen plays a very substantial role on revealing some important phylogenetic relationships of angiosperms at order level in the APG III (Angiosperm Phylogeny Group, 2009) classification system. In this study, based on recent molecular evidence, previous pollen information of the basal angiosperms and related taxa incorporates new data obtained presently to investigate pollen diversity and evolution.Firstly, we trace evolution of selected pollen characters onto several alternative molecular-based phylogenetic estimates from most recent studies, to reveal pollen plesiomorphic and synapomorphic (or likely synapomorphic) states and corresponding evolutionary pattern for inferring the first pollen type of angiosperms, and evaluating pollen systematic value for attempting to provide evidence for some uncertain relationships among these early-diverging lineages from discrepant tree topologies. Secondly, we address evolution of selected pollen characters onto maximum likelihood tree generated from the recent molecular data by three methods, i.e., Fitch parsimony, maximum likelihood, and hierarchical Bayesian inference, to test difference of pollen evolutionary pattern and systematic significance among these methods. We discuss the possibilities of the first pollen type and palynological evolution of early flowering plants based on integrated data. Ultimately, we perform a character correlated evolution analysis between pollen traits and habitat，distribution and pollination etc., to provide more insights on the adaptive evolutionary mechanism of the basal angiosperms.

自达尔文时代起被子植物起源和演化一直存在许多悬而未绝的问题。分子系统学研究提出了从被子植物中最早分化出的被子植物基部类群（basal angiosperms）的概念，基部类群内部系统发育关系成为争论的焦点之一。近年来，花粉形态特征在植物系统发育研究中的重要性被进一步突显，并为定义以"目"这一分类等级为重点的被子植物APG系统关键分支提供了重要的广义形态学证据。本研究依据APGIII系统，以目下科属为基本单元，选择近年来代表不同拓扑结构并具影响力的分子系统树，运用最大简约法、最大似然法和贝叶斯法分析基部类群花粉性状演化样式，增进对基部类群花粉形态演化历程一些重要脉络的认识；发现共近裔性状，筛选用于揭示相关类群有价值的花粉性状，研究系统关系复杂或不确定类群，为深度研究被子植物花粉形态演化规律提供有价值的基本框架；对花粉性状与生境、地理分布和传粉方式等进行综合分析，探讨基部类群的适应性进化规律。

本项目基于APG III（2009）系统框架，大尺度收集被子植物基部类群及相关类群花粉形态数据进行综合和民主编码，以分子系统发育框架为背景，开展基部类群和相关类群花粉形态多样性和适应性进化研究，了解该类群在被子植物花粉形态演化中的地位和意义，取得以下成果: 1）分析基部类群9目28科57属的21个花粉性状，揭示了8个明确祖征，花粉主要从球形，远极单沟，中型或大型，穿孔状、皱波状至网状的覆盖层，覆盖层下蜂窝状结构向其他类型的状态演化。为36个科及以上阶元找到了共衍征或可能共衍征，支持了Qiu et al. (2006) 三个线粒体基因树的拓扑结构（睡莲目和无油樟目互为姐妹群，金鱼藻目和金粟兰目互为姐妹群）。传粉方式与花粉性状显著相关，在早期被子植物花粉形态演化中扮演重要角色，为白垩纪中后期被子植物多样化和特化传粉方式的存在提供了重要线索。2）分析单子叶分支12目71科120属的19个花粉性状，揭示了12个明确祖征。为具争议的鸭跖草分支五个目的关系、禾本目的基部类群等提供了花粉证据支持。花粉形态在阿尔布阶到土伦阶出现了频繁的性状转变，可能与该时期气候波动有关。生境与传粉方式，特别是前者在单子叶分支花粉形态演化中可能起到重要作用。3）分析了基部真双子叶类群4目13科187属的20个花粉性状，揭示了16个明确祖征，伴随着萌发孔数目增多、外萌发孔形状由沟状向孔状演化、萌发孔位置由沿赤道分布向周面分布演化，覆盖层纹饰由简单向复杂演化的趋势。为58个族及以上阶元找到了共衍征或可能共衍征。复合萌发孔首次出现，可能与水分调节以及花粉管萌发和早期生长有关。该类群花粉受传粉方式影响较小，但形状等级与植物生活型有一定的相关性。4）比较研究揭示了基部类群和单子叶分支的花粉较基部真双子叶类群和桔梗分支性状变异性和多样性更高。传粉方式、生境湿度、生活型分别对不同类群花粉形态演化产生不同影响。扩大取样，结合多学科手段和新方法，可深入剖析适应性进化机制。