The Rare, Serpentine Endemic Streptanthus morrisonii (Brassicaceae) Species Complex, Revisited Using Isozyme Analysis

The Streptanthus morrisonii (Brassicaceae) complex is a group of six narrowly-distributed obligate serpentine endemic taxa whose habitat is threatened by geothermal development. Isozyme analysis of this little-studied complex supports the delineation of two species, S. morrisonii and S. brachiatus, but is at odds with the treatment of two subspecies based on morphology. These results may be influenced by small sample sizes but genetic studies of other Streptanthus taxa have shown patterns of relatedness that often transgress subspecies boundaries based on morphology. The present study further shows that members of the S. morrisonii complex share high genetic identity values (mean = 0.87) and are not genetically depauperate (mean value for percent of loci polymorphic = 37%, average number of alleles per locus = 1.48, and average heterozygosity per locus = 0.137). Preservation of their serpentine outcrop habitat is essential to the survival of these plants. The Streptanthus morrisonii F. W. Hoffman complex is a group of very narrowly distributed obligate serpentine endemics. These plants have been little studied or collected. Their habitat is remote, several taxa are known from only single populations and the plants are difficult o distinguish based on morphological traits. They have, however, received recent attention because their habitat is threatened by geothermal energy procurement and mining activities. This study was undertaken to determine whether biochemical genetic data from isozyme analysis support the classification based on morphology. The Streptanthus morrisonii complex comprises two species: S. morrisonii, with three described subspecies, and S. brachiatus F. W. Hoffman, with two subspecies. The complex was originally placed in the subgen. Euclisia Nutt. ex Torrey & A. Gray, based on the co-occurrence of zygomorphic flowers, nonbracteate inflorescences, and one or two pairs of stamens with connate or partially connate filaments (Hoffman 1952). Hoffman also noted a biennial (i.e, monocarpic perennial) life history for these plants, and the presence of glabrous, glaucous cabbage-like rosette leaves not found in other taxa in the subgenus. This distinction led Kruckeberg and Morrison (1983) to place the complex in a new section, Biennes Kruckeberg & Morrison. Related taxa in subgen. Euclisia include the more widely distributed annual serpentine endemic S. glandulosus Hook., S. breweri Gray, S. tortuosus Kell., S. polygaloides Gray, S. niger Greene, and S. insignis Jepson species complexes (Morrison 1941). Hoffman (1952) thought the S. morrisonii complex most closely related to S. breweri. Streptanthus morrisonii has four recognized subspecies. Streptanthus morrisonii subsp. morrisonii grows on serpentine outcrops and adjacent serpentine chaparral in north central Sonoma County. This subspecies, the most widely distributed taxon in the complex, grows in the large serpentine area known as "The Cedars" (Fig. 1; Tables 1, 2). Streptanthus morrisonii subsp. hirtiflorus F. W. Hoffman, a highly restricted taxon known from a single 100 m2 area of outcrop and totaling only several hundred plants, is found within the range of S. morrisonii subsp. morrisonii. Disjunct to the east of The Cedars is S. morrisonii subsp. elatus F. W. Hoffman, on the Lake-Napa county line (Fig. 1). This taxon is restricted to several closely spaced outcrops. A fourth subspecies, S. morrisonii subsp. kruckebergii Dolan & LaPre occurs in extreme eastern LakeNapa County line near Knoxville and has recently been reported from Mendocino County (L. Vorobik, pers. comm.). Streptanthus brachiatus subsp. brachiatus is a morphologically uniform taxon that has been collected only from the immediate vicinity of Socrates Mine in the Mayacmas Mountains of northern Sonoma County (Fig. 1; Tables 1, 2). The habitat has been severely altered by past mining. Plants occur primarily on open outcrop surfaces but are also found scattered in adjacent