Gaura neomexicana ssp. coloradensis, commonly known as the Colorado butterfly plant, is a regional endemic restricted to approximately 1700 acres of habitat in Laramie County, Wyoming, western Kimball County Nebraska, and Weld County, Colorado. The taxon was recently listed as Threatened under the Endangered Species Act (U.S. Fish and Wildlife Service, October 2000). The three largest populations of G. neomexicana ssp. coloradensis are confined to F.E. Warren Air Force Base, Cheyenne, Wyoming, and are known in the literature as the Crow Creek, Diamond Creek, and "unnamed drainage" populations. For each population, DNA samples were obtained from 50 different randomly selected individuals. An initial survey of ISSR locus variation, using three primers on the entire sample of 150, revealed very low levels of variation, both within and between populations. In an effort to reveal additional ISSR locus variation, subsets of eight individuals were randomly chosen from each population sample of 50. The reduction of sample size from 150 to 24 permitted examination with more ISSR primers. A total of 12 ISSR primers were applied to the sample of 24, revealing 88 loci. A sample-by-locus matrix was constructed, and loci were scored as present or absent (or as missing data if PCR failed). The matrix contained 3.4% missing data, with most coming from one individual. Results from cluster analysis revealed a high level of genetic similarity (0.88) across the entire sample, and no similarity clustering by population. The apparent ISSR locus homogeneity across these three populations was not unexpected. Prior to the establishment (1867), and subsequent expansion of the military base, these three populations were very likely lobes of a larger, contiguous population.

Key words: Gaura neomexicana ssp. coloradensis, ISSR, Onagraceae, rare species