Evolution of Caulanthus amplexicaulis var. barbarae (Brassicaceae), a rareserpentine endemic plant: A molecular phylogenetic perspective

Intra- and interspecific phylogenetic relationships of the rare serpentine endemic taxon Caulanthus amplexicaulus var. barbarae and related taxa in th e "Streptanthoid Complex" of genera (Streplanthus, Caulanthus, Guillenia) w ere examined using nuclear ribosomal internal transcribed spacer (ITS) and chloroplast trnL intron sequences. Phylogenetic hypotheses generated from 8 1 variable ITS nucleotide sites and six variable trnL nucleotide sites indi cate that Streptanthus and Caulanthus are nonmonophyletic groups. Caulanthu s amplexicaulis var. barbarae and its more widespread nonserpentine sister taxon Caulanthus amplexicaulis var. amplexicaulis formed a distinct monophy letic group. Among the taxa in our study, C. amplexicaulis was most closely related to Streptanthus tortuosus. The ITS sequences supported monophyly o f subgenus Euclesia, which includes the bulk of the serpentine endemics in the Streptanthoid Complex. The serpentine taxa were nonmonophyletic, occurr ing in at least three distinct clades, suggesting that tolerance to serpent ine may be gained or lost through relatively few genetic changes. Intraspec ific ITS1 and ITS2 sequence divergence within C. amplexicaulis (1.3-1.8%) w as higher than in comparable species (0.0-0.3%) implications of this geneti c differentiation for the conservation status of C. amplexicaulis var. barb arae are discussed. Evidence is presented that supports a "biotype depletio n" model for the origin of this rare endemic taxon.