Environment-dependence of the expression of mutational load and species' range limits

Abstract

Theoretical and empirical research on the causes of species range limits suggests the contribution of several intrinsic and extrinsic factors, with potentially complex interactions among them. An intrinsic factor proposed by recent theory is mutational load increasing towards range edges because of genetic drift. Furthermore, environmental quality may decline toward range edges and enhance the expression of load. Here we tested whether the expression of mutational load associated with range limits in the North American plant Arabidopsis lyrata is enhanced under stressful environmental conditions by comparing the performance of within- versus between-population crosses at common garden sites across the species distribution and beyond. Heterosis, reflecting the expression of load, increased with heightened estimates of genomic load and with environmental stress caused by warming, but the interaction was not significant. We conclude that range-edge populations suffer from a twofold genetic Allee effect caused by increased mutational load and stress-dependent load linked to general heterozygote deficiency, but no synergistic effect between them.