Genetic population structure and relatedness of Colorado mule deer (Odocoileus hemionus) and incidence of chronic wasting disease

Watry, Mary Kay; Wunder, Bruce

Understanding genetic variation among individuals is important to understanding disease transmission. In particular, genotypes are believed to influence susceptibility of mule deer (Odocoileus hemionus) to chronic wasting disease. We examined the genetic structure, PrP genotypes and genetic relationships of mule deer in north-central Colorado in relation to occurrence of chronic wasting disease (CWD). Genotypes from 8 microsatellite loci in 250 mule deer captured between January 2003 and April 2005 in Estes Valley, Colorado, Middle Park, Colorado and on the west side of the continental divide in Rocky Mountain National Park, Colorado were analyzed based on sampling location, sex, and CWD status. The fixation index, FST, and differences in allele frequency distributions provided evidence of significant difference between sampled groups (FST=0.0092, S.E.=0.0003, P=0.0499; allele frequency P=0.015). These results were primarily attributed to differences observed between the Middle Park and Estes Valley sampled groups (FST=0.0118, S.E.=0.0052, P=0.015; allele frequency P=0.00134). Analyses conducted on each sex separately revealed that only females had significant multilocus differences between Middle Park and Estes Valley (FST=0.0174, S.E.=0.0085, P=0.043; allele frequency P=0.0011). When deer were analyzed based on CWD status we found little evidence of genetic structure by CWD status. We also found that there were fewer CWD-positive deer than expected in the heterozygous serine/phenylalanine genotype at codon 225 of the PrP gene and more CWD-positive deer than expected in the homozygous serine genotype, however, these differences were only significant in males (P=0.020). There was no evidence that CWD-positive deer as a group or CWD-positive males were more closely related to each other than would be expected based on the reference population (all deer, P=0.20; males P=0.98), however, we found that CWD-positive females were more closely related to each other than expected (P=0.027). Our results indicate that 1) there is genetic differentiation between deer wintering in Estes Valley and Middle Park but these differences appear attributable to females indicating that mixing of populations is likely dominated by males, 2) genotypes and genetic structure based on the eight microsatellite loci in our study did not reveal any genotypes that were specifically associated with CWD, 3) there were more CWD-positive males observed with the homozygous serine genotype and less observed with the heterozygous serine/phenylalanine genotype than expected, and 4) CWD-positive female deer are more closely related than expected supporting the hypothesis that once a member of a female matrilineal group becomes infected with CWD it is passed to other closely related females within the group.