青藏高原植物银露梅(蔷薇科)花大小性二态的进化选择研究

Sexual dimorphism in flower size was considered to be one of the important characteristics between male and female plants in dioecious flowering plants inhabiting temperate regions. Two nonexclusive theories, detailed as sexual selection and natural selection respectively, were suggested to be responsible for the evolution of sexual dimorphism in flower size. However, the contributions of sexual selection and natural selection still remain unclear and need more experimental supports. We found that Potentilla glabra, one of the dominant shrubs on the Qinghai-Tibet Plateau, consisted of male, female and hermaphrodite plants in each of the four populations. Flower size of male flowers were significantly larger than those of female and hermaphroditic flowers, suggesting sexual dimorphism in flower size in this species. In this research proposal, we would perform field observations and experiments to find the effects of sexual dimorphism in flower size on floral traits, pollinator preference and reproductive fitness of male, female and hermaphroditic flowers, with the aims to explain the contributions of sexual selection and natural selection on sexual dimorphism in flower size in P. glabra. Our results would also be of great help in understanding the relationships between polyploidization and evolution of dioecy. This proposal will provide a good example of researches on the sexual dimorphism in flower size of plant species on the Qinghai-Tibet Plateau.

花大小性二态现象是温带雌雄异株植物的重要特征之一，对这一现象的解释主要来自于性选择和自然选择，但这两种选择压力在花大小性二态进化中的作用和贡献仍然需要进一步阐明。野外调查发现青藏高原分布的银露梅的四个种群均由雄性、雌性和两性植株组成，而且雄花的花瓣长度显著高于雌花和两性花，表现出典型的花大小性二态现象。本项目以四个银露梅种群为研究对象，通过野外观察和室内实验揭示花大小性二态对花综合特征、传粉者偏好访问和繁殖适合度的影响，阐明性选择和自然选择在银露梅花大小性二态进化过程中的作用，并探讨染色体倍性变化与雌雄异株进化的关系。本项目将为青藏高原植物花大小性二态的进化研究提供一个研究范例。

自然选择和性选择是维持雌雄异株植物的重要原因，前期调查研究表明有些植物具有两性植株和雌雄异株植物共存的现象，并且存在花大小性二态现象，而自然选择还是性选择维持了花大小的性二态现象还需要更多的证据。本项目以青藏高原地区广泛分布的蔷薇科具有雌株、雄株和两性株共存的银露梅（Dasiphora glabra）和金露梅（D. fruticosa）植物为研究对象，围绕多倍化、性别分化以及花大小性二态的维持机制等科学问题开展研究。研究发现：1）单性植株比两性植株具有更高的倍性，表明两个物种的性别分化都与多倍化相关，并且物种内单性植株和两性植株之间存在繁殖隔离，但银露梅和金露梅的倍性变化存在差异；2）两个物种都具有明显的性二态特征，如单性植株产生更多的花粉和胚珠，银露梅的雄花显著大于其他性别的花；3）花粉流的实验表明种群内有效的花粉转移很低，并且存在很强的传粉限制；4）银露梅种群主要的传粉者为双翅目的蝇类，传粉者偏好更大的雄花，可能更大的雄花具有更大的雄性适合度。本项目研究结果表明，多倍化以及由此引起的性别分化和性二态维持了三种性别植株的共存，而雄花的大小差异表明存在一定程度的性选择，但昆虫介导的自然选择弱化了性选择的强度。此外，两性植株和单性植株间存在繁殖隔离，一方面表明多倍化在青藏高原的植物多样化过程中具重要的贡献，而另一方面因多倍化植株与二倍体植株的形态差异较小而导致低估了青藏高原的植物多样性。