Abstract
Spontaneous mutations are the source of new genetic variation and are thus central to the evolutionary process. In molecular evolution and quantitative genetics, the nature of genetic variation depends critically on the distribution of effects of mutations on fitness and other quantitative traits. Spontaneous mutation accumulation (MA) experiments have been the principal approach for investigating the overall rate of occurrence and cumulative effect of mutations but have not allowed the phenotypic effects of individual mutations to be studied directly. Here, we crossed MA lines of the green alga Chlamydomonas reinhardtii with its unmutated ancestral strain to create haploid recombinant lines, each carrying an average of 50% of the accumulated mutations in a large number of combinations. With the aid of the genome sequences of the MA lines, we inferred the genotypes of the mutations, assayed their growth rate as a measure of fitness, and inferred the distribution of fitness effects (DFE) using a Bayesian mixture model. We infer that the DFE is highly leptokurtic (L-shaped). Of mutations with absolute fitness effects exceeding 1%, about one-sixth increase fitness in the laboratory environment. The inferred distribution of effects for deleterious mutations is consistent with a strong role for nearly neutral evolution. Specifically, such a distribution predicts that nucleotide variation and genetic variation for quantitative traits will be insensitive to change in the effective population size.
Highlights
Understanding evolution requires an understanding of the origin of new genetic variation from mutation, including the rates of mutation at individual loci and the magnitudes of their effects on fitness and other traits
We developed a Markov chain Monte Carlo (MCMC) approach with a random-effects model in which mutation effects are assumed to be sampled from some distribution or a mixture of distributions
To directly infer the distribution of fitness effects (DFE), we crossed six C. reinhardtii mutation accumulation (MA) lines derived from the CC-2931 strain to an ancestral strain of the same genetic background and the opposite mating type to produce a total of 1,526 recombinant line (RL)
Summary
Understanding evolution requires an understanding of the origin of new genetic variation from mutation, including the rates of mutation at individual loci and the magnitudes of their effects on fitness and other traits. We analysed a model in which positive- and negative-effect mutations can have either the same or different absolute means, but their distributions have the same shape parameter.
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