热带环太平洋间断分布格局的成因探讨-以樟科樟属为例

Tropical regions harbor almost two-thirds of the flowering plants, where amphi-Pacific disjunctions occur commonly within and among plant genera due to vicariance, boreotropical migration via the North Atlantic land bridges, and/or transoceanic long-distance dispersal. These are groups of plants that occur today primarily or exclusively in the tropical areas of Asia and the lands bordering the eastern Pacific Rim. The means by which these terrestrial organisms have come to occupy areas that are today separated by thousands of miles of ocean are not well understood.The tropical flowering plant genus Cinnamomum (Lauraceae) offers a good opportunity to explore the origin and evolution of amphi-Pacific tropical disjunct pattern in flowering plants due to its antiquity, widely distributed fossil record and ecological prominence. This research proposal aims to study the phylogenetic relationships of Cinnamomum using DNA sequence data to test competing hypotheses on the evolution of organisms with "amphi-Pacific tropical" distributions. One hypothesis explains this pattern by the process of intercontinental dispersion across the land bridges connecting North America and Eurasia around the Eocene-Oligocene boundary through the high-latitude regions of Beringia and the North Atlantic. A competing hypothesis maintains that these plants were pantropical in the Late Cretaceous, when the Atlantic Ocean was much narrower than today and dispersal over wide areas was easier.The investigators will expand the sampling scheme in Cinnamomum and conduct phylogenetic, molecular dating, and biogeographic analyses to reconstruct the evolutionary diversification history of Cinnamomum based on three nuclear markers (LEAFY, RPB2 and ITS). The aims of this project are to: (1) reconstruct the phylogeny of Cinnamomum; (2) infer the intercontinental splitting times of major subgroups and the ancestral areas for Cinnamomum using dated phylogenetic lineage divergences; and (3) test competing hypotheses on the evolution of tropical amphi-Pacific disjunctions. These data will in turn be interpreted in the context of the extensive fossil record of Cinnamomum to arrive at a synthetic view of the biogeographical history of this group. The project will integrate laboratory work with field collection of samples from several countries, in Southeastern Asia, Australasia and Central to South America. The broader scientific significance and societal ramifications of this project will benefit both environmental protection and science education. By facilitating inferences about the past movements of tropical vegetation, the knowledge derived from this project will contribute to the broader issues of the ecological effects of climate changes that are occurring presently. Further, it will help develop and refine conservation strategies in tropical countries, as many Cinnamomum species are rare and endangered.

全球约2/3有花植物集中分布于呈洲际间断分布的热带地区，分布于热带亚洲和热带美洲的很多生物类群呈现出热带环太平洋间断分布的格局。为了探讨热带环太平洋间断分布格局的形成原因，本项目以起源古老、化石记录丰富而广泛、经历了众多地质历史事件和环境变迁，在新旧世界热带地区森林群落中扮演重要角色，其现今的物种分布式样代表了典型热带环太平洋间断分布格局的樟科樟属植物为例，开展生物地理学研究。借助核基因分子序列完成樟属植物的系统发育重建，利用化石记录作为时间校正点，进行分化时间和祖先分布区域的分析，确定樟属的起源与分化时间，推测其祖先区域，结合化石记录以及古地质和古气候等学科证据，探讨热带环太平洋间断分布格局的成因，分析新旧热带森林群落的演化历史，探究新旧热带生物多样性空间格局形成的历史过程，在大的时间与空间尺度上揭示新旧热带森林群落对全球环境变化的响应，为制定有效的保护策略提供重要的理论依据。

全球约有三分之二的有花植物集中分布于洲际间断分布的热带地区，分布于热带美洲和热带亚洲的很多生物类群呈现出热带环太平洋间断分布的格局。目前这两大生物多样性富集区正面临着巨大危机已是全球性的共识，而开展热带环太平洋间断分布格局成因与机制的探讨，将有助于分析新、旧世界热带森林群落的演化历史，探究新、旧世界热带生物多样性空间格局形成的历史过程，在全球的空间尺度上揭示森林群落对全球环境变化的响应。.樟属群包含了樟属(Cinnamomum)在内的四个属植物，全球约有350余种。除分布于非洲的一种之外，其余物种均分布于洲际间断分布的太平洋两岸热带地区，是热带和亚热带常绿阔叶林的建群树种之一。本项目联合澳大利亚、美国和德国等多国学者，选取了来自75个樟属群物种的94个样本，涵盖了樟属群植物的整个分布区，对该类群植物的系统发育和生物地理学问题开展了深入研究，揭示发现：1)樟属群植物包含三个高度支持的单系分支，且基本对应于亚洲的樟组、肉桂组和美洲樟属类群；2)来自非洲的物种和澳洲樟属植物均聚在亚洲樟属肉桂组中，而来自热带美洲的Aiouea和Mocinnodaphne样本与美洲樟属聚为一支；3)樟属群起源于距今约55个百万年前的劳亚古陆，正值北热带植物群兴盛时期，此时樟属群植物可以通过白令海峡或北大西洋陆桥在欧亚古陆和北美大陆之间进行扩散传播；4)发生在距今约50至48个百万年前的第三纪第一次全球降温过程导致了北美大陆和欧亚大陆樟属群植物发生隔离，而发生在距今约42至38个百万年前的第三纪第二次全球降温过程，导致了非洲物种与亚洲樟属的隔离分化；5)在全球降温变冷的过程中，樟属和樟属群植物向低纬度迁移从高纬度消失，并在向低纬度南迁的过程中，发生了多次进入南美和一次进入澳洲的扩散事件，长距离扩散在其中可能发挥着关键作用，其中取食樟属群植物肉质果实的鸟类可能扮演着重要角色；6)很多被放入樟属的叶化石，尤其是晚白垩纪的樟属叶化石，仅仅依靠叶化石的叶脉特征而归入樟属并不可靠。项目研究证明，全球气候变迁，北热带植物群的兴盛、衰落和瓦解，以及生物类群的长距离扩散，最终塑造了樟属群的现代分布区;而樟属的化石记录显示其在早第三纪曾有一个广布北半球的古老分布区，其现代分布区只是其古老分布区的孑遗。