Phylogeographic analysis reveals early Pleistocene divergence and strong genetic structure in Euphorbia phosphorea (Euphorbiaceae), an endemic species to the semi-arid Caatinga Biome

Abstract The largest continuous patch of Seasonally Dry Tropical Forests (SDTF) is the Caatinga Biome in north-eastern Brazil, which has the greatest biodiversity and highest degree of endemism compared to any other SDTF patch of South America. Our aim was to conduct a phylogeographic study with Euphorbia phosphorea to understand the evolutionary history of this species endemic to the Caatinga Biome. We studied genetic diversity and structure, demographic dynamics, as well as intraspecific phylogenetic relationships, based on plastidial and nuclear DNA. In addition, we conducted species distribution modelling (SDM) to infer connectivity since the Last Interglacial. Our results recovered a stronger genetic structure for plastidial than for nuclear genome, which probably result from distinct species capability to seed and pollen dispersal, and low levels of genetic diversity within populations. Although SDM results suggest higher connectivity during Last Maximum Glacial, demographic analyses detected no signs of demographic expansion in this period. In agreement with previous studies with rock specialists, our data reveal remarkable among-population differentiation and low within-population genetic diversity, which make populations more prone to drift effects. Furthermore, our results shed light on the importance of early Pleistocene events in the diversification of lineages within the Caatinga Biome, although further comparative work is required.