Distribution of Fitness and Virulence Effects Caused by Single-Nucleotide Substitutions in Tobacco Etch Virus

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.

Experimental evolution of plant RNA viruses

“…Combining the estimate of U d s obtained by de la Iglesia and and the estimate of s obtained by Carrasco et al (2007b), it is possible to infer an estimate for TEV deleterious mutation rate as U d ¼ 0.1, a value that is consistent with the one reported for TMV (Malpica et al, 2002).…”

Efficient escape from local optima in a highly rugged fitness landscape by evolving RNA virus populations

“…Solid green lines represent likely adaptive walks (always connecting beneficial genotypes) and dashed black lines pathways that involve genotypes not significantly better than the ancestral (neutral pathways). The landscape is highly epistatic and contains two disconnected adaptive peaks corresponding to genotypes 01001 and 00110 (see , where R 0 and R t are the ratios of accumulations estimated for the mutant and reference viruses, respectively, at inoculation and after t days of growth [29].…”

The acquisition of molecular determinants involved in potato virus Y necrosis capacity leads to fitness reduction in tobacco plants

“…The proportions of each genotype in infected tissues (Fig. 3) were used to calculate, according to Carrasco et al (2007b), the fitness of the genotypes, using PVY KRED or PVY N as the reference genotypes ( …”