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

拖鞋大戟是一组大约 14 种墨西哥和加勒比植物物种，构成一品红科（大戟科）大戟属大戟属中的足花枝属分支。大戟的特征是花朵减少，并组织成一种特殊的花序结构，称为“cyathium”。在拖鞋大戟中，杯状花通常是色彩缤纷的，并饰有含花蜜的短刺，其形状和大小各不相同，似乎与授粉动物的身份有关。拖鞋大戟还因其生命形式的多样性而引人注目，从热带潮湿森林的常绿小树到甜点多肉植物。大多数拖鞋大戟物种的分布范围有限，其中 10 种是墨西哥当地特有的物种，另外两种从墨西哥南部延伸到中美洲北部。然而，大戟属 (Euphorbia tithymaloides) 的分布极其广泛：其八个公认的亚种分布在从佛罗里达州到南美洲北部的地区，种群遍及墨西哥、中美洲和加勒比海周围的弧形岛屿。据推测，加勒比盆地周围的 E. tithylamoides 的分化沿着两个前沿发生，形成一个种群圈，其两端在大安的列斯群岛相交。如果这两个大安的列亚亚种非常接近并且表现得像不同的物种，那么 E. tithymaloides 有可能成为环种。已知的环物种例子很少，其中大多数存在于动物中，它们因其揭示物种形成机制的潜力而引起人们极大的兴趣。拟议的研究将研究整个Pedilanthus分支的进化历史及其与地理的联系。这将为测试有关 Cyathium 进化、肉质起源以及群体中昆虫授粉的转变的进化假设提供一个框架。对 E. tithymaloides 亚种的核和质体基因 DNA 序列分析，结合形态变异研究，将用于检验 E. tithymaloides 构成环种的假设。新环物种的潜在发现将有助于阐明生态专业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海生物地理学的理解，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.