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. 2007 Dec;81(23):12979-84.
doi: 10.1128/JVI.00524-07. Epub 2007 Sep 26.

Distribution of fitness and virulence effects caused by single-nucleotide substitutions in Tobacco Etch virus

Purificación Carrasco  1 , Francisca de la Iglesia, Santiago F Elena
Affiliations

Distribution of fitness and virulence effects caused by single-nucleotide substitutions in Tobacco Etch virus

Purificación Carrasco et al. J Virol. 2007 Dec.

Abstract

Little is known about the fitness and virulence consequences of single-nucleotide substitutions in RNA viral genomes, and most information comes from the analysis of nonrandom sets of mutations with strong phenotypic effect or which have been assessed in vitro, with their relevance in vivo being unclear. Here we used site-directed mutagenesis to create a collection of 66 clones of Tobacco etch potyvirus, each carrying a different, randomly chosen, single-nucleotide substitution. Competition experiments between each mutant and the ancestral nonmutated clone were performed in planta to quantitatively assess the relative fitness of each mutant genotype. Among all mutations, 40.9% were lethal, and among the viable ones, 36.4% were significantly deleterious and 22.7% neutral. Not a single case of beneficial effects was observed within the level of resolution of our measures. On average, the fitness of a genotype carrying a deleterious but viable mutation was 49% smaller than that for its unmutated progenitor. Deleterious mutational effects conformed to a beta probability distribution. The virulence of a subset of viable mutants was assessed as the reduction in the number of viable seeds produced by infected plants. Mutational effects on virulence ranged between 17% reductions and 24.4% increases. Interestingly, the only mutations showing a significant effect on virulence were hypervirulent. Competitive fitness and virulence were uncorrelated traits.

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Figures

FIG. 1.
FIG. 1.
Fit of the beta distribution to the observed cdf for mutational fitness effects.
FIG. 2.
FIG. 2.
Distribution of mutational effects on virulence. Genotypes are ordered according to the magnitude of their virulence. A dashed line has been drawn to indicate the location of the ancestral TEV-7DA in the distribution. Error bars represent ± standard errors of means. Asterisks indicate the cases that were significantly different from TEV-7DA.
FIG. 3.
FIG. 3.
Relationship between relative fitness and relative virulence. Dashed axes are drawn as a reference to the values of the ancestral TEV-7DA.
See this image and copyright information in PMC

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