R-059. The Distribution of Beneficial Mutations and their Pleiotropic Effects

S. G. Comeau, V. S. Cooper;
Univ. of New Hampshire, Durham, NH.

Despite its central importance to countless biological processes, the nature of pleiotropy, defined as the ability of single mutations to affect multiple traits, remains poorly understood. We captured the first adaptive steps in two sets of evolving bacterial populations to quantify the relationship between adaptation to a common environment and correlated effects in alternative environments. In doing so we tested two hypotheses: 1) the distribution of adaptive steps will follow a common form because of the inherent rarity of beneficial mutations, and 2) the abundance of antagonistic pleiotropic effects will increase with greater specialization of the ancestral genotype. We used two different strains, a natural isolate of Burkholderia cenocepacia and a laboratory strain of Escherichia coli (specialist) to test these hypotheses. Each ancestor was marked with two neutral markers, a lactose-utilizing marker (blue-white) for B. cenocepacia and an arabinose-utilizing marker (red-white) for E. coli. Beneficial mutations were collected as follows: a 50:50 mixture of the two marked ancestors was combined into ten replicate cultures and then serially passaged each day in a galactose minimal media until the marker ratio skewed significantly from 50:50. When a skew in marker ratio was seen, both populations were then competed against the wild-type ancestor to determine the conferred fitness advantage in the evolved isolates. Beneficial mutations were then assessed in 12 other environments to quantify the pleiotropic effects on general stress, outer membrane function, carbon source transport, and iron limiting environments to evaluate how adaptation to a single carbon source changes growth in alternative environments. Our results support our first hypothesis of a common underlying distribution of beneficial mutations despite different ancestral genotypes. Our evidence for the second hypothesis is more preliminary, but suggests that pleiotropic effects of early adaptation are generally positive for unspecialized B. cenocepacia and more mixed (both positive and antagonistic) for the more specialized E. coli strain.