Academic journal article Genetics

Comparing Mutational and Standing Genetic Variability for Fitness and Size in Caenorhabditis Briggsae and C. Elegans

Academic journal article Genetics

Comparing Mutational and Standing Genetic Variability for Fitness and Size in Caenorhabditis Briggsae and C. Elegans

Article excerpt

ABSTRACT

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^sub G^) is predicted to be proportional to the mutational variance for the trait (V^sub M^); V^sub M^ is proportional to the mutation rate for the trait. The ratio V^sub M^/V^sub G^ predicts the average strength of selection (S) against a new mutation. Here we compare V^sub M^ and V^sub G^ for lifetime reproductive success ([asymptotically =] 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^sub G^ and V^sub 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^sub M^/V^sub 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.

(ProQuest: ... denotes formulae omitted.)

THE genetic variation present in a species is a composite function of mutation, population size, and natural selection. The relationship between the standing genetic variance (V^sub G^) and the per-generation input of genetic variance by mutation (the mutational variance, V^sub M^) has a straightforward interpretation under two evolutionary scenarios. Under a deterministic mutation- (purifying) selection balance (MSB) model, V^sub G^ [asymptotically =] ..., where S is the average selection coefficient against a new mutation (Barton 1990; Crow 1993; Houle et al. 1996). The ratio ...(i.e., ...) can be interpreted as the "persistence time" (tP) of a new mutation, i.e., the expected number of generations a mutant allele remains in the (infinite) population before it is eliminated by selection (Crow 1993; Houle et al. 1996). The more deleterious the allele, the faster it is removed from the population. At the opposite extreme, under a strict neutral model of mutation-drift equilibrium (MDE), for self-fertilizing taxa, V^sub G^ [asymptotically =] 4NeV^sub M^, where Ne is the effective population size (Lynch and Hill 1986). For a quantitative trait, V^sub M^ = UE(a2), where U is the genomic mutation rate and a is the additive phenotypic effect of a new mutation (Lynch and Walsh 1998, p. 329).

The unifying factor in these different scenarios is V^sub M^. Under both the MSB and MDE scenarios, we expect V^sub G^ to be proportional to V^sub M^ and thus the persistence time tP = ... to be constant if selection is uniform. Changes in the relationship between V^sub G^ and V^sub M^ among groups must be due to differences in natural selection. Thus, if tP differs between groups, the difference must be due to historical differences in the strength or efficiency of natural selection. This principle has been demonstrated byHoule et al. (1996), who found that the average tP for life history traits was about half that for morphological traits in a variety of taxa, consistent with the expected stronger correlation of life history traits with fitness. Average persistence times differed significantly between species, but the traits differed between species, the species were phylogenetically disparate, and included taxa not likely to be at equilibrium and/or to have experienced recent strong artificial selection. To our knowledge, there has been no comparison of mutational variance and standing genetic variance for the same traits in natural populations of related taxa. …

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