Genetic variation and gene flow within and between local populations of the timber rattlesnake, Crotalus horridus

Primers designed to amplify four microsatellite loci were used to screen 32 timber rattlesnakes (Crotalus horridus) representing five geographically distinct hibernacula within a 6000-ha area. Average heterozygosity at these loci approached 50% for the total sample of 32 individuals; therefore, these markers detected considerable genetic variation. Individuals using the same hibernaculum were more closely related to each other than they were to randomly selected individuals from the study site. In general, as geographic distance between hibernacula increased, genetic distance increased, and gene flow decreased; however, in some cases genetic distance did not correlate with geographic distance. Analysis of the movements of radiotracked snakes confirmed that patterns of gene flow depended upon specific structural habitat features and were not simply related to the geographic distance between hibernacula. The location of suitable basking habitat appeared to influence the direction of movements and the distribution of alleles. These results suggest that isolated hibernacula or a small number of hibernacula are the demographic units of C. horridus within this study site. We found evidence for the possible existence of nonamplifying ("null") alleles for one of these loci (7-144), suggesting the importance of screening for null alleles before carrying out microsatellite analysis of a population.