Mutation, Selection, and the Maintenance of Life-History Variation in a Natural Population

Abstract

In an effort to provide insight into the role of mutation in the maintenance of genetic variance for life-history traits, we accumulated spontaneous mutations in 10 sets of clonal replicates of Daphnia pulex for approximately 30 generations and compared the variance generated by mutation with the standing level of variation in the wild population. Mutations for quantitative traits appear to arise at a fairly high rate in this species, on the order of at least 0.6 per character per generation, but have relatively small heterozygous effects, changing the phenotype by less than 2.5% of the mean. The mean persistence time of a new mutation affecting life-history/body-size traits is approximately 40 generations in the natural population, which requires an average selection coefficient against new mutations of approximately 3% in the heterozygous state. These data are consistent with the idea that the vast majority of standing genetic variance for life-history characters may be largely a consequence of the recurrent introduction of transient cohorts of mutations that are at least conditionally deleterious and raise issues about the meaning of conventional measures of standing levels of variation for fitness-related traits.

Genomic Background and Generation Time Influence Deleterious Mutation Rates in Daphnia

“…The within-line variance represents the environmental variance (V E ) and the among-line variance represents the genetic variance (V G ). Estimates of the rate of change in the genetic variance were obtained by weighted least-squares regression of the V G estimates on generation number, with the slope of the regression representing an estimate of the rate of change in the variance due to mutational input, DV (Lynch and Walsh 1998). Data points in the regression were weighted by the inverse of the sampling variance of V G .…”

“…The final estimates of DV for Lake Marie SGT , Klamath SGT , and Klamath LGT lines were obtained by averaging the linespecific estimates. The standard error of this estimate was obtained by treating the line-specific estimates as independent as described in Lynch et al (1998). Estimates of the rate of change in the mean due to mutation accumulation, DM, were obtained by weighted least-squares regression of the assay-specific phenotypic means on generation number.…”

“…Regression analyses of mean control phenotype and among-line variance were then conducted such that estimates of DM and DV for MA lines could be corrected for change due to environmental variation among assays. Lowerbound estimates of the genomic mutation rate, U min = DM 2 / DV, and upper-bound estimates of the heterozygous mutational effect as a fraction of the initial mean phenotype (ž 0 ), a9 max = DV/DM/ž 0 , were obtained according to the method of Bateman (1959) and Mukai et al (1972) and modified for clonal lines (Lynch and Walsh 1998). Bateman-Mukai estimates were corrected for bias caused by sampling error in estimating DM and DV (Lynch 1994).…”

“…Bateman-Mukai estimates were corrected for bias caused by sampling error in estimating DM and DV (Lynch 1994). Standard errors for U min and a9 max were obtained using the formula for the standard error of a ratio, taking the sampling variances of the numerator and denominator into account (Lynch and Walsh 1998).…”

“…The magnitude of our trait-specific estimates of the rates of change in mean phenotype and the variance in D. pulicaria are generally lower than those obtained from MA experiments conducted under similar environmental conditions Table 2 Estimates of the initial mean phenotype (ž 0 ), per-generation rate of change in the mean phenotype (DM), per-generation rate of input of mutational variance (DV), genomic mutation rate (U min ), and average mutational effect as a fraction of the initial mean phenotype (a9 max ) for Lake Marie, Klamath SGT involving the pond-dwelling sister species, Daphnia pulex (Lynch et al 1998). These differences in rates of change in the mean and variance translate into lower estimates of the mutation rate and higher estimates of the average mutational effect in D. pulicaria relative to D. pulex.…”