Evidence is accumulating that individuals in poor physiologic condition may accumulate mutational damage faster than individuals in good condition. If poor condition results from pre-existing deleterious mutations, the result is "fitness-dependent mutation rate," which has interesting theoretical implications. Here we report a study in which 10 mutation accumulation (MA) lines of the nematode Caenorhabditis elegans that had previously accumulated mutations for 250 generations under relaxed selection were expanded into sets of "second-order" MA lines and allowed to accumulate mutations for an additional 150 generations. The 10 lines were chosen on the basis of the relative change in fitness over the first 250 generations of MA, five high-fitness lines and five low-fitness lines. On average, the mutational properties (per-generation change in mean relative fitness, mutational variance, and Bateman-Mukai estimates of genomic mutation rate and average mutational effect) of the high-fitness and low-fitness did not differ significantly, and averaged over all lines, the point estimates were extremely close to those of the first-order MA experiment after 200 generations of MA. However, several nonsignificant trends indicate that low-fitness lines may in fact be more likely to suffer mutational damage than high-fitness lines.
Keywords: Bateman-Mukai method; fitness-dependent mutation; genetic load; mutational variance.