Abstract
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 inS. 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 genetic
rescue.
Keywords : ROHs; inbreeding; demographic bottlenecks; genetic
load; Sistrurus catenatus