Antagonistic Pleiotropy Is Unexpectedly Rare In New Mutations

ABSTRACT Pleiotropic effects of mutations may underlie diverse biological phenomena such as ageing and specialization. In particular, antagonistic pleiotropy (“AP”: when a mutation has opposite fitness effects in different environments) generates tradeoffs, which may constrain adaptation. Models of adaptation typically assume that AP is common – especially among large-effect mutations – and that pleiotropic effect sizes are positively correlated. The rare empirical tests of these assumptions have largely focused on beneficial mutations observed under strong selection, whereas most mutations are actually deleterious or neutral, and are removed by selection. We quantified the incidence, nature and effect size of pleiotropy for carbon utilization across 80 single mutations in Escherichia coli that arose under mutation accumulation (i.e. weak selection). Although ~46% of the mutations were pleiotropic, only 11% showed AP, which is lower than expected given the distributions of fitness effects for each resource. In some environments, AP was more common in large-effect mutations (but not synergistic pleiotropy, SP); whereas pleiotropic effect sizes were positively correlated for SP (but not AP). Thus, AP is generally rare, is not consistently enriched in large-effect mutations, and often involves weakly deleterious antagonistic effects. Our unbiased quantification of mutational effects therefore suggests that antagonistic pleiotropy is unlikely to cause maladaptive tradeoffs.