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.
