Simulation of models involving mixed selfing and random mating : II. Effects of selection and linkage in finite populations.
The joint effects of linkage, inbreeding, and drift due to finite population size were investigated in terms of population changes under selection involving gene interaction. Six-locus models with the same amount of recombination between adjacent pairs of loci, mixed selfing and random mating, and selection of basically three forms (heterotic, optimizing and mixed optimum-heterotic) were used for Monte Carlo simulation. The results were primarily described in terms of certain measures of gene dispersion, genetic variability, gametic unbalance (linkage disequilibrium) and the approach to stable gene frequency equilibria. Under both cumulative and diminutive heterosis models, a steady state with polymorphisms could be attained with random gene dispersion being small and different replicate populations evolved high degrees of gametic unbalance in the direction of excess of either coupling or repulsion phase linkages depending on the random drift in gene frequencies. Under optimum models, on the other hand, all populations approached steady decay toward fixation at all loci although gene dispersion was governed by rather complex interactions between the parameters of selfing, linkage and selection intensity. Gene dispersion was not necessarily proportionately greater with the higher levels of inbreeding. An excess of repulsion linkages with mean population fitness approaching unity was noted in all runs with the optimum models, more so with tight linkage and heavy inbreeding. Any asymmetry in the sense of selection favoring one or the other allele tends to reinforce gene fixation particularly under inbreeding. Heterozygote advantage, on the other hand, seemed to play a relatively greater role under inbreeding in terms of retaining heterozygosity. Mixed optimum-heterotic models provide a favorable compromise between these conflicting attributes of multilocus systems in terms of the maintenance of polymorphisms and the maximization of fitness in relation to certain optimal linked gene complexes. In general, for moderate to large population size these results are, as expected, in line with those reported previously for two-locus deterministic models.
Department of Agronomy, University of California, Davis.
S K
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Jain