Phylogenetic relationships within the Western United States species of Lepidium l.

Abstract

The genus Lepidium L. is one of two global genera in the Brassicaceae. The genus has been arranged by species (geographic regions) worldwide, but no formal levels below the genus are recognized. Recent efforts to evaluate phylogenetic relationships have been performed at the global scale for about 20 percent of the species in the genus. The genus is recognized as having subtle and variable morphological characteristics to define species limits. Several nuclear and chloroplast DNA methods have been used to construct phylogenetic relationships within the genus. Incongruences between various phylogenetic trees indicate likely hybridization and/or hybrid origin of multiple species and a genus blurred with a reticulate evolutionary past. Internal Transcribed Spacer (ITS) ribosomal DNA (rDNA) sequences were developed here and combined with other ITS sequences on Genbank for other North American species of Lepidium. Two phylogenetic trees were developed, one comparing North American and another dominated by Intermountain West species. Results of a limited Intermountain Lepidium phylogenetic tree were compared to a cladistic tree developed from 123 morphological traits for select species of Lepidium from the western United States. A comprehensive ITS tree was developed to evaluate species relationships in the genus throughout this region. Ploidy levels of 22 taxa of Intermountain species of Lepidium were evaluated to assess whether ploidy levels were associated with any geographic or morphologic patterns within the group. The results show closely related species and varieties with several ploidy levels, but are lacking any relationships to morphological features. Neither ITS nor ploidy levels provided a clear understanding into the current taxonomic treatment of the many faint morphologically different taxa in the group. But Intermountain Lepidium, as a geographic group and clade, is distinct from other west coast members in the genus. The species most associated with all the radiant speciation, and the least understood, is L. montana.