Variable pleiotropic effects from mutations at the same locus hamper prediction of fitness from a fitness component

The relationship of genotype, fitness components, and fitness can be complicated by genetic effects such as pleiotropy and epistasis and by heterogeneous environments. However, because it is Often difficult to measure genotype and Pitiless directly, fitness components are commonly used to estimate fitness Without regard to genetic architecture. The small bacteriophage phi X174 enables direct evaluation of genetic and environmental effects on fitness components and fitness. We used 15 mutants to Study Mutation effects oil attachment rate and fitness in six hosts. The mutants differed from our lab strain of phi X174 by only one or two amino acids in the major capsid protein (gpF, sites 101 and 102). The sites are variable in natural and experimentally evolved phi X174 populations and affect phage attachment rate. Within the limits Of detection Of Our assays, all Mutations were neutral or deleterious relative to the wild type; I I mutants had decreased host range. While fitness was predictable from attachment rate in most cases, 3 mutants, had rapid attachment but IOW fitness Oil Most hosts. Thus, some initiations had a pleiotropic effect oil a fitness component. other than attachment rate. In addition, on one host most mutants had high attachment rate but decreased fitness, suggesting that pleiotropic effects also depended oil host. The data highlight that even in this simple, well-characterized system, prediction of fitness from a fitness component depends oil genetic architecture and environment.