Dissertation Research: Systematics and Biogeography of the Pedilanthus Clade (Euphorbia-Euphorbiaceae)

拖鞋植物是由大约14种墨西哥和加勒比海地区的植物组成的分支，属于一品红属(大戟科)的一大属。一品红的特征是花被组织成一种特殊的花序结构，称为聚伞花序。在拖鞋中，海鞘通常是五颜六色的，并装饰着含有花蜜的刺突，其形状和大小各不相同，似乎与授粉动物的身份有关。拖鞋类也因其生命形式的巨大多样性而引人注目，从热带潮湿森林的常绿小树到甜点多肉植物。大多数拖鞋类物种的分布有限，其中10种为墨西哥当地特有种，两种从墨西哥南部延伸到中美洲北部。然而，有一种名为大戟的物种分布非常广泛：它的八个公认亚种从佛罗里达州到南美洲北部，种群分布在墨西哥、中美洲和加勒比海周围的弧形岛屿上。据推测，加勒比海盆地周围的石斑藻的分化是沿着两条战线进行的，形成了两端在大安的列斯群岛相遇的种群圈。如果这两个大安的列斯亚种出现在非常接近的地方，并且表现为不同的物种，那么E.tihyaloids有可能是一个环状物种。已知的环状物种例子很少，大多数在动物身上，它们因其揭示物种形成机制的潜力而引起人们的极大兴趣。这项拟议的研究将研究整个马蹄莲分支的进化史，以及它与地理的关联。这将提供一个框架，以检验关于海鞘进化的进化假说，多肉的起源，并切换到昆虫在群体中授粉。从核基因和叶绿体基因中分析DNA序列，结合形态变异的研究，将被用来检验蒂胸腺棘豆亚种构成环种的假设。一个新环种的潜在发现将有助于阐明生态专化和生殖隔离在植物物种形成过程中的作用。这项研究还将有助于我们对加勒比生物地理学的了解，并将阐明一些受威胁物种的系统学，可能有助于确定保护优先事项。

The slipper spurges, a group of 14 or so Mexican and Caribbean plant species, comprise the Pedilanthus clade within the large genus Euphorbia, in the poinsettia family (Euphorbiaceae). Euphorbia is characterized by having reduced flowers that are organized in a specialized inflorescence structure, called a cyathium. In the slipper spurges, the cyathium is often colorful and adorned with a nectar-containing spur, whose shape and size varies and appears to relate to the identity of pollinating animals. The slipper spurges are also remarkable for their great diversity in life form, from evergreen treelets of tropical moist forests to dessert succulents. Most slipper-spurge species have restricted distributions, 10 being local endemics in Mexico, and two extending from southern Mexico into northern Central America. However, one species, Euphorbia tithymaloides, has an extremely broad distribution: its eight recognized subspecies range from Florida to northern South America with populations throughout Mexico, Central America, and an arc of islands around the Caribbean. It has been hypothesized that the differentiation of E. tithylamoides around the Caribbean basin occurred along two fronts, forming a circle of populations of which the two ends meet in the Greater Antilles. If the two Greater Antillean subspecies occur in close proximity and behave as distinct species, then E. tithymaloides has the potential to be a ring-species. Very few examples of ring-species are known, most of them in animals, and they are of great interest for their potential to shed light on mechanisms of speciation. The proposed research will study the evolutionary history of the entire Pedilanthus clade, and its association with geography. This will provide a framework to test evolutionary hypotheses regarding cyathium evolution, the origin of succulence, and switches to insect pollination in the group. Analysis of DNA sequences from nuclear and plastid genes, combined with studies of morphological variation, for the subspecies of E. tithymaloides will be used to test the hypothesis that E. tithymaloides constitutes a ring-species. The potential discovery of a new ring- species would help elucidate the roles of ecological specialization and reproductive isolation in the process of plant speciation. This study will also contribute to our understanding of Caribbean biogeography and will shed light on the systematics of a number of threatened species, potentially helping to establish conservation priorities.