Theory predicts that threatened species living in small populations will
experience high levels of inbreeding that will increase their negative
genetic load but recent work suggests that the impact of load may be
minimized by purging resulting from long term population bottlenecks.
Empirical studies that examine this idea using genome-wide estimates of
inbreeding and genetic load in threatened species are limited. Here we
use genome resequencing data to compare levels of inbreeding, levels of
genetic load and population history in threatened Eastern massasauga
rattlesnakes (Sistrurus catenatus) which exist in small isolated
populations and closely-related yet outbred Western massasauga
rattlesnakes (S. tergeminus). In terms of inbreeding, S. catenatus
genomes had a greater number of ROHs of varying sizes indicating
sustained inbreeding through repeated bottlenecks when compared to S.
tergeminus. At the species level, outbred S. tergeminus had higher
genome-wide levels of genetic load in the form of greater numbers of
derived deleterious mutations compared to S. catenatus presumably due to
long-term purging of deleterious mutations in S. catenatus. In contrast,
mutations that escaped the “drift sieve” and were polymorphic within
S. catenatus populations were more abundant and more often found in
homozygote genotypes than in S. tergeminus suggesting a reduced
efficiency of purifying selection in smaller S. catenatus populations.
Our results support an emerging idea that the historical demography of a
threatened species has a significant impact on the type of genetic load
present which impacts implementation of conservation actions such as