Molecular evidence for hybridization and introgression in the neotropical coastal desert-endemic Palaua (Malveae, Malvaceae)

Recent years have seen an increasing awareness of the importance of hybridization and introgression in plant evolution. Both processes were also invoked to have had a strong impact in the evolution of Palaua, based on morphological intermediacy. Here, we used nuclear ITS and cellulose synthase DNA sequences to assess previous phylogenetic hypotheses and to uncover previously undescribed reticulate evolution in Palaua. Both nuclear regions recovered largely the same reticulate pattern, showing that interspecific crosses occurred between distantly related species of the Dissecta and Integrifolia clade. Hybridization between species occurred only in a narrow contact zone. Moreover, many sequences of Palaua dissecta were placed in the strongly divergent Integrifolia clade, a pattern that is best explained by introgression. These results support our view that the strong interannual fluctuations of humidity and habitat expansion provide opportunities for the isolation of taxa, establishment of hybrid species, and for introgression between incompletely reproductively isolated taxa following recurrent contact.