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

拖鞋spurges，一组14左右的墨西哥和加勒比植物物种，包括大戟属大戟枝，在一品红科（大戟科）。大戟的特点是有退化的花，这些花被组织在一个特殊的花序结构中，称为cyathium。在拖鞋spurges中，cyathium通常是彩色的，并装饰着含有花蜜的马刺，其形状和大小各不相同，似乎与授粉动物的身份有关。从热带潮湿森林的常绿树苗到甜点多肉植物，拖鞋藻的生命形式的多样性也很引人注目。大多数拖鞋的分布受到限制，10种是墨西哥当地特有的，两种从墨西哥南部延伸到中美洲北部。然而，有一种叫做大戟的植物分布极为广泛：它的8个已知亚种分布范围从佛罗里达到南美洲北部，种群遍布墨西哥、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的分化沿着两条前沿发生，形成了一个种群圈，其两端在大安的列斯群岛相遇。如果这两个大安的列斯亚种发生得很近，表现得像不同的物种，那么E. tithymaloides有可能是一个环种。已知的环状物种的例子很少，它们大多存在于动物中，它们非常有趣，因为它们有可能揭示物种形成的机制。这项拟议的研究将研究整个系枝的进化史，以及它与地理的关系。这将提供一个框架来测试进化假设，关于cyathium进化，多肉的起源，以及在群体中转向昆虫授粉。对该亚种的核基因和质体基因的DNA序列分析，结合形态变异的研究，将用于验证该亚种为环种的假设。新的环状物种的发现将有助于阐明生态特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理解，并将阐明一些受威胁物种的系统分类学，可能有助于确定保护重点。

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.