Variation in the strength of inbreeding depression across environments: Effects of stress and density dependence

Abstract

In what types of environments should we expect to find strong inbreeding depression? Previous studies indicate that inbreeding depression, δ, is positively correlated with the stressfulness of the environment in which it is measured. However, it remains unclear why stress, per se, should increase δ. To our knowledge, only "competitive stress" has a logical connection to δ. Through competition for resources, better quality (outbred) individuals make the environment worse for lower quality (inbred) individuals, accentuating the differences between them. For this reason, we expect inbreeding depression to be stronger in environments where the fitness of individuals is more sensitive to the presence of conspecifics (i.e., where fitness is more density dependent). Indeed, some studies suggest a role for competition within environments, but this idea has not been tested in the context of understanding variation in δ across environments. Using Drosophila melanogaster, we estimated δ for viability in 22 different environments. These environments were simultaneously characterized for (1) stressfulness and (2) density dependence. Although stress and density dependence are moderately correlated with each other, inbreeding depression is much more strongly correlated with density dependence. These results suggest that mean selection across the genome is stronger in environments where competition is intense, rather than in environments that are stressful for other reasons. K E Y W O R D S :Competition, deleterious mutations, density dependence, inbreeding depression, selection, stress.

Reduction in the cumulative effect of stress-induced inbreeding depression due to intragenerational purging in Drosophila melanogaster

“…Thus, our results do not support the findings of Yun and Agrawal (2014), who found that ID was strongest when competition was intense. We found that, although competition resulted in a slightly higher ID than the other stressors, its effect was very similar to that of heat, while the other two stressors (ethanol and bacteria) induced very similar levels of ID (see Figure 2a), pairings that suggest no particular pattern.…”

“…Predicting the outcome of inbreeding-stress interactions centers on the concept that stress alters the strength of selection against deleterious alleles that cause ID (Agrawal and Whitlock, 2010;Yun and Agrawal, 2014). The finding that the intensity of stress scales positively with ID in a variety of taxa (Fox and Reed, 2011;Enders and Nunney, 2012;Schou et al, 2015) supports the general prediction that selection is greater in stressful environments.…”

“…Although ID is a common phenomenon in nature, understanding what factors contribute to the high degree of variability in magnitude observed across environments remains a challenge (Armbruster and Reed, 2005;Fox and Reed, 2011;Yun and Agrawal, 2014). Predicting the outcome of inbreeding-stress interactions centers on the concept that stress alters the strength of selection against deleterious alleles that cause ID (Agrawal and Whitlock, 2010;Yun and Agrawal, 2014).…”

“…However, different stress types have been shown to vary in their ability to increase selection against mutations (Agrawal and Whitlock, 2010). Recent work in Drosophila also found the degree of density-dependent intraspecific competition associated with a stressful environment was a better predictor of ID than the level of stress alone (Yun and Agrawal, 2014), further suggesting that stressor-specific factors influence the expression of ID.…”

Heterozygosity–fitness correlations in blue tit nestlings (Cyanistis caeruleus) under contrasting rearing conditions

“…In most cases, the negative effects of inbreeding (or the positive effects of outbreeding) have been found more often among females than males. The differences in the effects of heterozygosity or inbreeding between males and females may be explained by sex-specific gene expression (Yun and Agrawal 2014), maternal investments (Charpentier et al 2006), food acquisition (Charpentier et al 2006), growth strategies (Coulson et al 1998), sexual selection pressure and life history (Ebel and Phillips 2016).…”