Re-terrestrialization in the phylogeny of epiphytic plant lineages: Microsoroid ferns as a case study

The fern family Polypodiaceae, with over 1600 species, is not only one of the most species-rich families of ferns, but also a major contributor to the vascular epiphytic diversity throughout the tropics. Although the vast majority of species belonging to this family prefer to grow as epiphytes, several species colonize successfully rheophytic, lithophytic, and even terrestrial habitats. Here, we explore the hypothesis that non-epiphytic habitat preferences, including terrestrial growth, evolved secondarily with epiphytes being the plesiomorphic habitat preference. The results of phylogenetic analyses, based on dense taxon sampling and four chloroplast DNA regions, were integrated with divergence time estimates and ancestral character state reconstructions to test these predictions. Both fossils and secondary calibration data were incorporated to obtain divergence time estimations. The results support the prediction of multiple transitions from epiphytic/lithophytic to terrestrial/rheophytic habitats occurring mainly in the Microsoreae lineage. The change in niche preferences coincides with niche colonization opportunities created by climatic fluctuations and geographical changes during the Oligocene and Miocene periods.