Communicating Editor: Aaron Liston

Mosaically sympatric populations of Blepharizonia plumosa subsp. plumosa and B. plumosa subsp. viscida occur in the eastern San Francisco Bay region, California. Results from pollen and chromosomal studies of artificial hybrids yielded evidence of low interfertility between individuals of different taxa. Low fertility of hybrids between taxa was associated with meiotic irregularities (e.g., failure of meiosis I). Phylogenetic analyses of 18S–26S nuclear ribosomal DNA sequences of the internal transcribed spacer (ITS) region from representatives of multiple populations of both groups yielded evidence for two well-supported evolutionary lineages corresponding to B. plumosa subsp. plumosa and B. plumosa subsp. viscida. Pairwise distances between ITS sequences of the two taxa approximate or exceed pairwise ITS sequence distances between species of tarweed genera that are closely related to Blepharizonia. Rate constancy of ITS sequence evolution allowed us to estimate a five-fold more ancient divergence between lineages corresponding to the two taxa in Blepharizonia than between lineages corresponding to the two outgroup taxa in Hemizonia. We conclude that the two taxa currently recognized in Blepharizonia are ancient, highly divergent groups worthy of continued taxonomic recognition and best treated as species, B. plumosa and B. laxa (= B. plumosa subsp. viscida). Although natural hybridization appears to occur between B. laxa and B. plumosa, we found no convincing evidence of introgression that could lead to loss of genetic integrity of the rare and endangered B. plumosa.