Phylogenetic relationships among New World Scrophularia L. (Scrophulariaceae): new insights inferred from DNA sequence data

The genus Scrophularia L. (Scrophulariaceae) comprises 200-300 species, of which approximately 19 are distributed in North America and the Greater Antilles. To investigate phylogenetic and biogeographic relationships of the New World species, two intergenic spacers (trnQ-rps16 and psbA-trnH) of chloroplast DNA and nuclear ribosomal ITS were sequenced. Phylogenetic analyses revealed three distinct New World clades that correspond to their geographical distribution and are corroborated by morphological characters. Phylogenetic inference indicates the eastern American S. marilandica L. as sister to all Antillean species; for colonization of the Caribbean archipelago, a late Miocene dispersal event from the North American mainland is assumed. There is evidence for a hybrid origin of the most widespread North American species, S. lanceolata Pursh. The results further suggest that S. nodosa L. is sister to all New World and three Japanese species of Scrophularia. The latter form an Eastern Asian-Eastern North American (EA-ENA) disjunction with six New World species. We propose an eastern Asian origin for the New World taxa of Scrophularia. Divergence times estimated using a relaxed molecular clock model suggest one or more Miocene migration events from eastern Asia to the New World via the Bering Land Bridge followed by diversification in North America.