Phylogenetic placement and biogeography of the North American species of Valerianella (Valerianaceae: Dipsacales) based on chloroplast and nuclear DNA

Abstract

Although Valerianaceae is a relatively small group of angiosperms (ca. 350 species), sample sizes of previous phylogenetic studies have been limited and taxonomic sampling has been usually geographically biased to species from Europe and/or South America. One group that has never been included in any analyses to date is the North American representatives of Valerianella. In this study I sampled 17 additional accessions from Valerianaceae, including six of the 12 described North American Valerianella species and four additional samples of European Valerianella species. Phylogenies based on parsimony and Bayesian methods show strong support for placing the North American Valerianella nested within the European species. These analyses also found Fedia to be nested within Valerianella, making Valerianella paraphyletic, a result consistent with several previously published molecular phylogenies. Divergence times estimated from the molecular data using recently proposed Bayesian ‘relaxed’ clock methods, suggest Valerianella was in North America by the middle Miocene (ca. 16–14 million years ago). These dates would suggest the North American species of Valerianella arrived in the New World just prior to the North Atlantic land bridge (NALB) no longer being a viable conduit for migration. Given the large amount of uncertainty in the divergence time estimates, dispersal across the North Atlantic may not be the only viable alternative to explain the current distribution of Valerianella. However, biogeographic analyses using an explicit model based approach, strongly favor the NALB as the route of dispersal for Valerianella.