Flora of North America species comparison

Stephanomeria pauciflora generally grows as an intricately branched, often rounded bush. Occasional plants, usually from Arizona, New Mexico, Texas, and southern Utah, have long, flexuous stems and branches, an architecture that resembles one of the typical forms of S. tenuifolia. Some plants of S. pauciflora have white pappi, also typical of S. tenuifolia. It is not known if these plants represent uncommon and unusual individuals or if they are from populations in which all plants have those traits. It is also not known whether such plants of S. pauciflora grow near populations of S. tenuifolia; if so, they may result from interspecific hybridization. That is a possibility; experimental hybrid plants produced by crossing individuals from the two species were about 20% fertile. Such fertility suggests the species are sufficiently compatible that fully fertile segregants with variously intermediate morphologies could be expected where they hybridize in nature. The experimental crosses were made reciprocally between S. pauciflora from Riverside County, California (L. D. Gottlieb 6653), and S. tenuifolia from Wheeler County, Oregon (L. D. Gottlieb 6692); specimens of the six F1 hybrid plants that were produced are deposited at DAV. Plants of S. pauciflora that are densely tomentose throughout are occasionally found, particularly in the deserts of California and Nevada, and have been named S. cinerea or S. pauciflora var. parishii.

(Discussion copyrighted by Flora of North America; reprinted with permission.)

Species 16 (14 in the flora).

Because all the species of Stephanomeria have not previously been examined at one time, the present treatment provides the first unified picture of their variability, ecologic specializations, and geographic distributions. The genus includes six annual species (all in the flora) and ten perennial species (eight in the flora, one in the mountains of northern Baja California, and one known only from Guadalupe Island, Mexico).

Taxonomic distinctions among annual species of Stephanomeria did not become evident until their morphology and geographic distributions were correlated with their chromosome numbers and reproductive compatibilities (L. D. Gottlieb 1971, 1972). The same studies also provided an hypothesis that satisfactorily accounted for their variability. Studies showed that S. exigua and S. virgata differed for a relatively large number of characters and that other annual species originated from genetic segregates that were formed by hybridization, at both diploid and tetraploid levels, as well as directly from S. exigua.

Stephanomeria exigua has five subspecies; S. virgata has two. Within each species, the subspecies share numerous morphologic features as well as chromosomal karyotype. They are recognized as polytypic because reproductive compatibility between any pair of subspecies of S. exigua or between subspecies of S. virgata is substantially higher than is the compatibility between the two species. The two species appear to represent a fundamental phylogenetic divergence within annuals; nevertheless their different features are combined in different ways in S. elata and S. diegensis.

Stephanomeria paniculata and S. malheurensis probably evolved more or less directly from S. exigua subsp. coronaria. The speciation process that gave rise to S. malheurensis (L. D. Gottlieb 1978) has been examined in a series of studies (Gottlieb 1973b, 1977, 1979; S. Brauner and Gottlieb 1987, 1989). The origin of the highly self-pollinating S. paniculata may have been similar but much less evidence is available. Stephanomeria malheurensis has served as a model for reintroduction of a species back into its original habitat after local extinction, in its case by competition from invasive cheatgrass (Bromus tectorum).

Information about evolution and speciation is not so available for the perennials as for the annuals. Treatment of perennials is based almost entirely on examination of herbarium specimens plus published information describing their chromosome numbers. Although little is known about phylogenetic relationships among perennial species of Stephanomeria, a recent DNA sequencing study of nuclear rDNA (J. Lee et al. 2002) showed that the genus does not include either Munzothamnus blairii (previously S. blairii) or Pleiacanthus spinosus (previously S. spinosa). Without them, Stephanomeria is a well-supported, monophyletic group of species.

The DNA analysis suggested that Stephanomeria tenuifolia, S. runcinata, S. fluminea, and S. thurberi comprise a subclade. Those four species are perennial and all have fully plumose, white pappus bristles. They differ markedly in their ecologic specializations, as indicated in their treatments below. The DNA studies also showed a very close relationship between S. malheurensis and S. exigua subsp. coronaria consistent with results of previous studies (cited above). It is to be hoped that taxonomic information presented below will make species of Stephanomeria more easily accessible to continuing studies.

(Discussion copyrighted by Flora of North America; reprinted with permission.)