Conservation genetics of Sclerocactus brevihamatus subsp. tobuschii and an assessment of its genetic relationship with Sclerocactus brevihamatus subsp. brevihamatus

Background/Question/Methods

Sclerocactus brevihamatus subsp. tobuschii (Tobusch fishhook cactus) is a federally endangered species in the United States. This taxon appears morphologically similar to the more commonly occurring Sclerocactus brevihamatus subsp. brevihamatus and the two subspecies have a slight overlap in their geographic distribution. Although information on genetic diversity is necessary for conservation of rare and endangered taxa, such data are not available for Sclerocactus brevihamatus subsp. tobuschii. Moreover, the genetic distance between the two subspecies has not been assessed. The objectives of this study were to document genetic diversity within and among nine populations of Sclerocactus brevihamatus subsp. tobuschii and within one population of Sclerocactus brevihamatus subsp. brevihamatus, and to investigate the genetic distance between the two subspecies. Self-developed, species-specific nuclear microsatellite primers were tested and seven were selected for genotyping. Across both taxa, 255 individuals were analyzed at seven microsatellite loci and fragment length data were statistically analyzed.

Results/Conclusions

Sclerocactus brevihamatus subsp. tobuschii exhibited lower genetic diversity (mean N_a= 4.9, H_o= 0.38, H_e= 0.59) compared to the published estimates in a related and threatened species Sclerocactus glaucus. However, each population of subsp. tobuschii had higher heterozygosity compared to heterozygosity (H_o= 0.26, H_e= 0.52) in one population of subsp. brevihamatus. Indirect estimates of inbreeding (F_is) were high, ranging from 0.15 to 0.63 for subsp. tobuschii and 0.47 for subsp. brevihamatus. The nine populations of subsp. tobuschii showed low genetic differentiation (mean F_st = 0.058) with pairwise F_st ranging from 0.015 to 0.10, each of which was statistically significant. Results from AMOVA (Ф_PT = 0.07, p = 0.00) supported the differentiation estimates. A Mantel test indicated no correlation between isolation and distance (r² = 0.05, p = 0.14) for subsp. tobuschii, and the IAM model detected significant bottleneck (P < 0.01) in three of its populations. Clustering results were consistent across STRUCTURE, principal coordinate analysis, and the neighbor joining dendrogram, yielding three distinct clusters. Individuals of subsp. brevihamatus clustered separately. Based on the polymorphism across seven nuclear microsatellites, populations of subsp. tobuschii exhibit low genetic differentiation, however the taxon appears sufficiently differentiated from subsp. brevihamatus to retain its current taxonomic status.