Big-Benefit Mutations in a Bacteriophage Inhibited with Heat

Abstract

High temperature inhibits the growth of the wild-type bacteriophage phiX174. Three different point mutations were identified that each recovered growth at high temperature. Two affected the major capsid protein (residues F188 and F242), and one affected the internal scaffolding protein (B114). One of the major capsid mutations (F242) is located in a beta strand that contacts B114 in the procapsid during viral maturation, whereas the other capsid mutation (F188) is involved in subunit interactions at the threefold axis of symmetry. Selective coefficients of these mutations ranged from 13.9 to 3.8 in the inhibitory, hot environment, but all mutations reduced fitness at normal temperature. The selective effect of one of the mutations (F242) was evaluated at high temperature in four different genetic backgrounds and exhibited epistasis of diminishing returns: as log fitness of the background genotype increased from -0.1 to 4.1, the fitness boost provided by the F242 mutation decreased from 3.9 to 0. 8. These results support a model in which viral fitness is bounded by an upper limit and the benefit of a mutation is scaled according to the remaining opportunity for fitness improvement in the genome.

Specificity of genome evolution in experimental populations of Escherichia coli evolved at different temperatures

“…The convergence scenario requires that antagonistic pleiotropy not be so pervasive that most mutations that are adaptive in one environment are deleterious in other conditions (42,43). However, as more mutations accumulate in any population, whether by selection, hitchhiking, or drift, there are more chances that a mutation that is deleterious in some other conditions will fix.…”

Genome Properties and the Limits of Adaptation in Bacteriophages

“…These phages were used from the collection of IJM, except that Q␤ was obtained as an infectious clone from D. Mills (Health Science Center, State University of New York, Brooklyn, NY), RB69 as well as T6 were obtained from S. Abedon (Ohio State University, Mansfield, OH), and G4 was ultimately obtained from B. Fane (University of Arizona, Tucson, AZ). The X174 used to start the selection was the isolate Af from Bull et al (2000; adapted for rapid growth), and the G4 used to start the selection was an equal mix of three isolates from different stages of adaptation in the study of Holder and Bull (2001). In the latter case, it was thought that the pool of variation would facilitate the adaptation, as 37ЊC is near the upper end of temperatures tolerated by wild-type G4, and Holder and Bull (2001) had specifically adapted G4 to higher temperatures.…”

“…Each phage was separately adapted to IJ1862 in a manner slightly modified from that used in previous studies (Bull et al 2000;Rokyta et al 2002). Our goal was to select phages with maximal growth rates by growing them in an environment in which hosts were physiologically healthy and more abundant than the phages.…”

The genetic basis of parallel and divergent phenotypic responses in evolving populations ofEscherichia coli

“…Thus, different mutations may be substituted in separate lineages because they provide similar improvements to a selected trait, yet they may cause diverse suites of pleiotropic effects that become important only after an environmental change or some further change in the gene pool. Differences in phenotype that arise from heterogeneous pleiotropic effects can thereby influence the likelihood of subsequent substitutions, leading to divergent evolutionary trajectories over the long term (Mani & Clarke 1990;Travisano et al 1995a,b;Hodgkin 1998;Bull et al 2000;MacLean & Bell 2003;MacLean et al 2004). …”