THE FITNESS CONSEQUENCES OF MULTIPLE-LOCUS HETEROZYGOSITY UNDER THE MULTIPLICATIVE OVERDOMINANCE AND INBREEDING DEPRESSION MODELS

There is a growing body of literature suggesting that the fitness of an individual increases with the observed number of heterozygous loci. Broad theoretical considerations indicate that under various sorts of balancing selection, this is what one should generally expect in a population of multiple-locus genotypes. To date, however, it has not been possible to distinguish between two potential explanations of the phenomenon. The first explanation is that the loci examined are themselves responsible for the fitness differences observed (or, equivalents, are very closely linked to those that do). The genetic variation in question is thought to be maintained in polymorphic equilibrium by some form of balancing selection. The second explanation assumes that the observed loci are themselves selectively irrelevant but that their heterozygosity reflects that of the total genome. Genomic heterozygosity is thought to be predictive of fitness, being an obverse measure of generalized inbreeding depression. We provide a formal derivation of an explicit relationship between fitness and multiple-locus genotype for a simple form of the first explanation, the multiplicative overdominance model. The inbreeding depression model is a degenerate special case of this more general formulation. A formal estimation and testing framework is constructed that should facilitate evaluation of the two models with empiric data on heterozygosity and fitness.