Phylogeny and biogeography of exindusiate Andean Polystichum (Dryopteridaceae)

Abstract

Uplift of the tropical Andes had a significant impact on the diversification of South American flora and fauna. Recent biogeographic inquiries have established patterns of Andean divergence, but investigations on ferns are scant. The fern genus Polystichum Roth (Dryopteridaceae) combines widespread geographic and elevational distribution with a large number of species to form an ideal system for investigation of the origin and diversification patterns of a fern lineage in the tropical Andes. The relationships among 42 Polystichum species, including taxa from all major biogeographic regions, were analyzed with 2591 aligned nucleotides from four plastid markers using maximum parsimony and Bayesian inference. The resulting phylogeny was then used to estimate divergence times and reconstruct both ancestral areas and ancestral elevations. Tropical Andean South American polystichums that lack an indusium (sori exindusiate) were confirmed to form a monophyletic group. This exindusiate Andean Polystichum clade diverged from a middle-elevation forest lineage now rich in species endemic to Mexico during the middle Miocene (13.12 million years ago). The majority of diversification that followed took place in the montane regions of the central Andes with radiations to the northern Andes, southeastern Brazil, and alpine regions. The monophyletic exindusiate Andean Polystichum lineage diverged from a Mexican lineage in the middle Miocene and diversified in the central Andes before dispersing northward. This south-to-north dispersal pattern, documented for many other Andean lineages, corresponds with episodes of uplift in the tropical Andes.