Phylogeny, divergence time estimates, and phylogeography of the diploid species of the polypodium vulgare complex (Polypodiaceae)

© 2014 by the American Society of Plant Taxonomists. The Polypodium vulgare complex (Polypodiaceae) comprises a well-studied group of fern taxa whose members are cryptically differentiated morphologically and have generated a confusing and highly reticulate species cluster. Once considered a single species spanning much of northern Eurasia and North America, P. vulgare has been segregated into 17 diploid and polyploid taxa as a result of cytotaxonomic work, hybridization experiments, and isozyme studies conducted during the 20th century. Despite considerable effort, however, the evolutionary relationships among the diploid members of the P. vulgare complex remain poorly resolved. Here we infer a diploids-only phylogeny of the P. vulgare complex and related species to test previous hypotheses concerning relationships within Polypodium sensu stricto. Using sequence data from four plastid loci (atpA, rbcL, matK, and trnG-trnR), we recovered a monophyletic P. vulgare complex comprising four well-supported clades. The P. vulgare complex is resolved as sister to the Neotropical P. plesiosorum group and these, in turn, are sister to the Asian endemic Pleurosoriopsis makinoi. Using divergence time analyses incorporating previously derived age constraints and fossil data, we estimate an early Miocene origin for the P. vulgare complex and a late Miocene-Pliocene origin for the four major diploid lineages of the complex, with the majority of extant diploid species diversifying from the late Miocene through the Pleistocene. Finally, we use our node age estimates to reassess previous hypotheses, and to propose new hypotheses, about the historical events that shaped the diversity and current geographic distribution of the diploid species of the P. vulgare complex.