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

拖鞋大戟，一组约14种墨西哥和加勒比植物物种，包括在一品红科（大戟科）的大属大戟属内的Pedilanthus分支。大戟属的特点是具有退化的花，这些花被组织成一种特殊的花序结构，称为杯状花序。在拖鞋大戟中，杯状花序通常是五颜六色的，并装饰着含有花蜜的刺，其形状和大小各不相同，似乎与授粉动物的身份有关。拖鞋大戟也是显着的生活形式的巨大多样性，从万年青乔木的热带潮湿的森林，沙漠多肉植物。大多数滑子大戟属物种的分布范围有限，其中10种为墨西哥的地方特有种，另外2种从墨西哥南部延伸到中美洲北方。然而，一个物种，Euphorbia tithymaloides，有一个非常广泛的分布：它的八个公认的亚种范围从佛罗里达到北方南美洲与整个墨西哥，中美洲和加勒比海周围的弧形岛屿的人口。有人推测E.加勒比海盆地周围的tithylamoides发生沿着两条战线，形成了一个人口圈，两端在大安的列斯群岛。如果这两个大安的列斯亚种出现在非常接近，并表现为不同的物种，那么E。tithymaloides具有成为环种的潜力。环物种的例子很少，其中大多数是在动物中，他们是非常感兴趣的，因为他们有可能揭示物种形成的机制。这项拟议中的研究将研究整个Pedilanthus分支的进化历史及其与地理的关系。这将提供一个框架，以测试进化假说有关杯状花序的进化，肉质的起源，并切换到昆虫授粉的组。本文对E. tithymaloides将被用来测试的假设，E. tithymaloides构成环种。新环种的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了解加勒比海的物种地理学，并将阐明一些受威胁物种的系统学，可能有助于建立保护优先事项。

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.