Comparing the population history of Neotropical annual species: The role of climate change and hybridization between Tithonia tubaeformis and T. rotundifolia (Asteraceae).

Historical climate changes and interspecific gene flow have played an important role in shaping the distribution and genetic diversity of the biota in the Neotropics. In this study, we explored the role of both the Pleistocene climate changes and hybridization on the contemporary geographic structure of two Neotropical weedy species: Tithonia tubaeformis and T. rotundifolia. Distribution shifts under past and current climate conditions were explored through ecological niche modelling (ENM). We then tested these hypotheses using chloroplast microsatellite (cpSSR) data in T. tubaeformis and compared the patterns deduced with those previously reported for T. rotundifolia using the same cpSSR loci. Lastly, we searched for shared haplotypes between species. Both species exhibited significant downwards altitudinal shifts during the last interglacial (LIG) and the Last Glacial Maximum (LGM). For instance, T. rotundifolia showed large suitable habitat areas since the LIG at xeric conditions in western Mesoamerica. Tithonia tubaeformis showed a ~five-fold range contraction during the LIG compared to current climate conditions, followed by a range expansion in the LGM. Despite the large shared refugial areas predicted through ENM, we found a low number of shared haplotypes, suggesting a minor role of hybridization in shaping the geographic structure of these species. Our results provide additional patterns of the population history of the northern Neotropics during the Quaternary, and we suggest that weedy widespread species are a well-suited group for the study of the effects of historic climatic changes on the biota of this region.