Unraveling the phylogeny of polygrammoid ferns (Polypodiaceae and Grammitidaceae): exploring aspects of the diversification of epiphytic plants

We explore the phylogeny of the polygrammoid ferns using nucleotide sequences derived from three plastid loci for each of 98 selected species. Our analyses recovered four major monophyletic lineages: the loxogrammoids, two clades consisting of taxa restricted to the Old World, and a largely neotropical clade that also includes the pantropical Grammitidaceae. The loxogrammoid lineage diverges first and is sister to a large clade comprising the three remaining species-rich lineages. One paleotropical clade includes the drynarioid and selligueoid ferns, whereas the second paleotropical clade includes the platycerioids, lepisoroids, microsoroids, and their relatives. The grammitids nest within the neotropical clade, although the sister taxon of this circum-tropic, epiphytic group remains ambiguous. Microsorum and Polypodium, as traditionally defined, were recovered as polyphyletic. The relatively short branch lengths of the deepest clades contrast with the long branch lengths leading to the terminal groups. This suggests that the polygrammoid ferns arose through an old, rapid radiation. Our analysis also reveals that the rate of substitution in the grammitids is remarkably higher relative to other polygrammoids. Disparities in substitution rate may be correlated with one or more features characterizing grammitids, including species richness, chlorophyllous spores, and an extended gametophytic phase.