The genus Lasthenia consists of 17 species, mostly annuals, distributed primarily in California. One species occurs in central Chile. They occupy a variety of habitats and are particularly common in vernal pools. In early spring, the most common species, L. californica (Section Baeria), can cover thousands of acres of grasslands and open woodlands with a rich characteristic orange-yellow color thus giving the genus its common name, “Goldfields.” Although the genus comprises five sections now, some sections were previously considered as separate genera. Chromosome numbers n=4,5,6,7,8,12,16,24 indicate that aneuploidy and polyploidy exist. This study explores some of the questions posed by earlier morphological studies of Lasthenia and has resulted in a plausible phylogeny for the genus. The nuclear ribosomal ITS region was sequenced in two outgroup species and in at least three replicate samples of each taxon representing geographically separated populations. Interspecific sequence divergence was found to be as high as 13 percent. All but one taxa were found to possess low intraspecific variation. Most interestingly, an approximately 15 base pair sequence deletion was found in some L. californica populations but not in others. This variation did not correspond to whether the L. californica population was diploid or tetraploid. This deletion was also found in L. leptalea (Section Burrielia) which is morphologically very similar to L. californica. Most clades that correspond to current recognized sections are well supported. However, a few ITS lineages conflict with sectional circumscription. Preliminary sequence data from the trnK intron region of chloroplast DNA suggest a phylogeny that is similar to the one obtained from the ITS region. This information will serve as a basis for testing the phylogenetic utility of other DNA regions in Lasthenia.

Key words: Asteraceae, ITS, Lasthenia, phylogeny, trnK intron