Relationship between heterozygosity and asymmetry: a test across the distribution range.

The genetic basis of developmental stability, as measured by bilateral asymmetry, has been debated for over 50 years among developmental and evolutionary biologists. One of the central theories dealing with this relationship suggests that higher levels of genetic diversity, as reflected in heterozygosity, result in increased stability during development and thus in lower asymmetry. In this study, we aimed to test the relationship between asymmetry and heterozygosity at two levels: (1) the population level, where mean heterozygosity within a population is predicted to be negatively correlated with mean population asymmetry and (2) the individual level, where the proportion of heterozygous loci of an individual and its bilateral asymmetry estimates are predicted to be negatively correlated. While previous studies often focused on local populations, work across species ranges can answer the following questions. Are levels of heterozygosity correlated with levels of developmental instability, as estimated by bilateral asymmetry? Are patterns consistent across the distribution range, from the periphery towards the core? Does the relationship between genetic stress and bilateral asymmetry depend on the degree of environmental stress? We tested heterozygosity levels in 26 loci and asymmetry in third toe length in 11 populations of the chukar partridge (Alectoris chukar) across a sharp climatic gradient in Israel from the arid periphery, through the Mediterranean-desert ecotone towards the Mediterranean areas located further away from the range boundaries. Genetic diversity, as estimated using both observed and expected heterozygosity, was not associated with asymmetry at either the population or at the individual level. Whereas heterozygosity showed a hump-shaped pattern, peaking at the ecotone, asymmetry monotonically increased towards the range periphery. We argue that whereas asymmetry may serve as a useful tool for estimating changes in environmental stress, it may not be widely applicable for estimating genetic stress.