Unravelling the genetic diversity and population dynamics of three Tacinga species (Cactaceae: Opuntioideae) in the Caatinga

Abstract The Caatinga biogeographical region, located in the semi-arid region of northeastern Brazil, is characterized by high levels of cactus diversity and endemism. In this study, we investigated the genetic diversity, differentiation, and phylogeography of three Tacinga species (Tacinga inamoena, Tacinga subcylindrica, and Tacinga palmadora), which are endemic and widely distributed in the Caatinga. We collected 222 samples from 108 locations and used plastid (3ʹrps16-5ʹtrnK, PsbE-PetL) and nuclear (PhyC) molecular markers for a multilocus approach. Our results suggest that diversification of the three Tacinga species occurred during the middle Pleistocene, with the Southern Sertaneja Depression ecoregion of the Caatinga being identified as the ancestral area. Interestingly, analyses of genetic structure did not show any evidence that the major river in the region (the Sao Francisco River) and the Caatinga ecoregions acted as biogeographical barriers for gene flow in these species. This suggests that the three Tacinga species exhibited different responses to widely accepted geographical and geomorphological barriers known to affect other taxonomic groups in the Caatinga. Overall, our findings provide new insights into the biogeographical history of the Caatinga and the factors that have shaped diversification and genetic structure of species in this region. Our study underlines the importance of considering the unique natural history and biological characteristics of individual taxonomic groups when investigating the biogeography of the Caatinga.