Plastid atpA data provide improved support for deep relationships among ferns

DNA sequence data and phylogenetic approaches have contributed greatly to our understanding of fern relationships. Nonetheless, the datasets analyzed to date have not been sufficient to definitively resolve all parts of the global fern phylogeny; additional data and more extensive sampling are necessary. Here, we explore the phylogenetic utility of the plastid atpA gene. Using newly designed primers, we obtained atpA sequences for 52 fern and 6 outgroup taxa, and then evaluated the capabilities of atpA relative to four other molecular markers, as well as the contributions of atpA in combined analyses. The five single-gene datasets differed markedly in the number of variable characters they possessed; and although the relationships resolved in analyses of these datasets were largely congruent, the robustness of the hypotheses varied considerably. The atpA dataset had more variable characters and resulted in a more robustly supported phylogeny than any of the other single gene datasets examined, suggesting that atpA will be exceptionally useful in more extensive studies of fern phylogeny and perhaps also in studies of other plant lineages. When the atpA data were analyzed in combination with the other four markers, an especially robust hypothesis of fern relationships emerged. With the addition of the atpA data, support increased substantially at several nodes; three nodes, which were not well-supported previously, received both good posterior probability and good bootstrap support in the combined 5-gene (> 6 kb) analyses.