Examining the genetic integrity of a rare endemic Colorado cactus (Sclerocactus glaucus) in the face of hybridization threats from a close and widespread congener (Sclerocactus parviflorus)

Abstract

Natural interspecific hybridization within plants is relatively common and plays an important evolutionary role. Hybridization between rare and common taxa is of concern due to questions of protection status related to hybrids. However, field based identification of hybrid plants can be challenging. Sclerocactus glaucus (Cactaceae), the Colorado hookless cactus, is protected under the Endangered Species Act and has a small range around Grand Junction, Colorado, where populations are threatened by anthropogenic activities. Field biologists frequently observe S. glaucus populations containing individuals with hooked spines that are thought to be either hybrids or individuals of a closely related, parapatrically distributed species, Sclerocactus parviflorus. We used 13 nuclear microsatellites and two chloroplast DNA sequence regions to examine genetic diversity and structure within S. glaucus and potential hybridization with S. parviflorus. The two species were found to be genetically distinct based on microsatellites (average FST = 0.14 among species) and chloroplast sequence data (no shared haplotypes among species). Evidence of minimal hybridization was documented (5.7 % of sampled S. glaucus individuals with >10 % S. parviflorus signal). Hybrid individuals did not show a clear geographic or morphological pattern. Sclerocactus glaucus populations contained significant genetic structure falling into north and south groups associated with two separate river drainages. Overall, genetic integrity of sampled S. glaucus populations is intact, with relatively high diversity (Ho = 0.47, He = 0.66) and low levels of inbreeding (FIS = 0.28), indicating management should focus on protecting existing populations and preventing movement of material among genetically distinct regions.