Evaluating the Extent of Hybridization between Mule and White-tailed Deer in Western Canada

Abstract

Mule deer (Odocoileus hemionus) are endemic to a wide variety of habitats in western North America, many of which are shared in sympatry with their closely related sister-species white-tailed deer (O. virginianus). Hybridization of mule and white-tailed deer has historically been observed on a semi-regular basis throughout this range. Close monitoring of this hybrid system continues to be a priority among contemporary evolutionary biologists because (a) hybridization is a volatile evolutionary phenomenon, with potential outcomes ranging from adaptive introgression to local extinction, (b) deer are ecologically, economically, and culturally significant species, and (c) chronic wasting disease (CWD) currently poses a threat to farmed and wild cervid populations in western Canada and northern United States. CWD is a communicable, fatal, neurodegenerative disease similar to scrapie in sheep and bovine spongiform encephalopathy in cattle. Molecular ecological studies have confirmed the presence of hybrids in various regions of species overlap, including Alberta, Montana, and Texas, but a lack of detection power has limited their identification abilities to the earliest, most admixed generations. Furthermore, female F1 hybrids can sometimes retain fertility, giving rise to the possibility of advanced-generation backcrosses, which require considerable diagnostic power to be reliably identified. In this thesis, I interrogated the Alberta region of the hybrid zone with the goals of (a) better characterizing its current state and evolutionary trajectory and (b) determining whether hybridization events provide an opportunity for CWD transmission to bridge the species gap. To answer these questions, I first developed a SNP assay using 40 species-discriminating loci to improve upon the previously available molecular methods of hybrid identification. After assessing its utility and efficacy through analysis of simulated hybrid populations, I used it to conduct a comprehensive survey of wild deer populations in western Canada. To investigate whether hybridization facilitates interspecific CWD spread, I designed sampling strategies to compare hybridization rates between infected and uninfected deer. We observed overall hybridization rates of 0.3-1.1%, slightly lower than reported elsewhere in the greater hybrid zone. The admixed individuals are independent of geographical factors and no significant association between hybridization and CWD infection status was found. Although the two species often exist in sympatry and share similar diets and life history traits, interspecific gene flow appears to be prevented by a number of pre and postzygotic barriers to reproduction, including fine-scale habitat segregation and severe underdominance of hybrid offspring. Here, I build upon past studies of introgressive hybridization of deer in Alberta by increasing hybrid detection power, expanding sample sizes, and providing a new molecular resource applicable to future research. This comprehensive, robust, and current assessment of the circumstances of hybrids in wild populations will inform stakeholders of patterns of contemporary hybridization so they may shape future research and policies accordingly.