Patterns and processes of diversification in the Pedilanthus clade of Euphorbia.

摘要

In this work I focus on evolutionary diversification at two different scales, above and at the species level, in a small yet rather diverse group, the Pedilanthus clade of Euphorbia.;I estimated the phylogeny of the Pedilanthus clade using one plastid (matK) and three nuclear regions (ITS and two G3pdh loci), and used the results and a Yule model of diversification to test the hypothesis that the group's cyathial nectar spur is a key innovation. Slight changes in spur morphology could confer some degree of pre-mating isolation, explaining why clades with spurs tend to include more species than their spurless close relatives. The data are consistent with a key innovation hypothesis, but I cannot statistically reject the null model, perhaps because of the young age (hence small size) of the group. Additionally, my results confirm previous species designations and suggest a possible Central American origin of the group.;At a subspecific scale, I test whether the species Euphrobia tithymaloides is a ring species in the Caribbean. A ring species arises when a parental population encircles a patch of unsuitable habitat in such a way that when the two terminal ends of the chain of populations meet they behave as distinct species. Ring species offer great possibilities for the study of the forces causing species divergence but are extremely rare in nature, especially in plants. Here I present a molecular phylogenetic analysis of two nuclear markers (one G3pdh locus and a homolog of the Solanum Gene Network Unigene 342009) for the E. tithymaloides species complex in the Caribbean. The data are consistent with a two-fronted invasion of the Caribbean Basin from Mexico/Guatemala and show that, at least on the island of St. Croix, the two terminal forms coexist and lack evidence of ongoing gene flow. Thus, my results suggest that Euphorbia tithymaloides is indeed a ring species, making it the first well corroborated reing species discovered in plants.