Low frequency of mutations with strongly deleterious but nonlethal fitness effects.

Most experimentally detectable effects of mutations in cellular organisms are either lethal or mildly deleterious. A possible explanation for the paucity of strongly detrimental but nonlethal mutations is that the processes constituting cellular metabolism are either essential or largely redundant. Alternatively, the partition between lethal and inconspicuous mutations exists within important biological processes. To test this, we measured maximum growth rates of yeast strains each carrying the deletion of a single gene in one of 38 protein complexes. We also used relevant data from previous high-throughput phenotypic studies of the yeast gene-deletion collection. The complexes typified well-defined sets of genes engaged in a common process. Within virtually all essential complexes there were two clear modes of phenotypic effects, that is the cessation of growth or slowdown of growth by a few percent. This uniformity is striking given that complexes differ extensively in function, size, and proportion of essential proteins. The pattern of bimodality is observed both under optimal and suboptimal environmental conditions. The generic paucity of strong effects and abundance of small ones relates to the feasibility of analyses of quantitative traits and epidemiological surveys, irrespective of the particular element of metabolism under study.