Comparing Mutational and Standing Genetic Variability for Fitness and Size in Caenorhabditis briggsae and C. elegans

摘要

The genetic variation present in a species depends on the interplay between mutation, population size, and natural selection. At mutation-(purifying) selection balance (MSB) in a large population, the standing genetic variance for a trait ( V G) is predicted to be proportional to the mutational variance for the trait ( V M); V M is proportional to the mutation rate for the trait. The ratio V M /V G predicts the average strength of selection ( S ) against a new mutation. Here we compare V M and V G for lifetime reproductive success (≈ fitness) and body volume in two species of self-fertilizing rhabditid nematodes, Caenorhabditis briggsae and C. elegans , which the evidence suggests have different mutation rates. Averaged over traits, species, and populations within species, the relationship between V G and V M is quite stable, consistent with the hypothesis that differences among groups in standing variance can be explained by differences in mutational input. The average (homozygous) selection coefficient inferred from V M /V G is a few percent, smaller than typical direct estimates from mutation accumulation (MA) experiments. With one exception, the variance present in a worldwide sample of these species is similar to the variance present within a sample from a single locale. These results are consistent with specieswide MSB and uniform purifying selection, but genetic draft (hitchhiking) is a plausible alternative possibility.