Inbreeding, body condition, and heterozygosity-fitness correlations in isolated populations of the endangered eastern massasauga rattlesnake (Sistrurus c. catenatus)

Abstract

Endangered species often occur in small populations that may have a greater risk of short-term extinction due to the negative effects of inbreeding depression. The eastern massasauga (Sistrurus c. catenatus) is an endangered rattlesnake that is found in isolated populations of varying size throughout its range. Here, we investigate whether variation in an indirect measure of individual fitness (relative body condition) can be explained by genome-wide levels of genetic variation (based on 19 microsatellite loci) and other factors. To do this, we use genetic and phenotypic data from individual snakes sampled from 14 populations throughout the species’ range. We tested for levels of inbreeding by comparing observed mean multi-locus heterozygosity (MLH) for each population (an estimate of average inbreeding) with the expected distribution under random mating. We then looked for evidence of heterozygosity-fitness correlations (HFCs) using a measures of individual MLH and relative body condition. In all but one population, observed MLH values are indistinguishable from those generated under a model of random mating implying low levels of inbreeding in most populations. There was significant variation in both mean MLH and mean body condition within and among populations but evidence for inbreeding depression was equivocal: in support, there were some high (but largely non-significant) HFCs effect sizes within a number populations, including one that showed significant evidence for both inbreeding and a HFC. Overall, however, there was no significant correlation between MLH and body condition across all populations after controlling for non-genetic factors such as sex, season of capture and year of capture. Our results suggest that among-population and individual differences in fitness (measured as body condition) in these snakes are better explained by short-term ecological factors rather than genetic mechanisms, but leave open the possibility that limited undetected effects of inbreeding depression are present.