The Evolution of Virulence in a Plant Virus

Abstract

The evolution of virulence is a rapidly growing field of research, but few reports deal with the evolution of virulence in natural populations of parasites. We present here an observational and experimental analysis of the evolution of virulence of the plant virus Cucumber mosaic virus (CMV) during an epidemic on tomato in eastern Spain. Three types of CMV isolates were found that caused in tomato plants either a systemic necrosis (N isolates), stunting and a severe reduction of leaf lamina (Y isolates), or stunting and leaf curl (A isolates). These phenotypes were due to the presence of satellite RNAs (satRNAs) necrogenic (in N isolates) or attenuative (in A isolates) of the symptoms caused by CMV without satRNA (Y isolates). For these three types of isolates, parameters of virulence and transmission were estimated experimentally. For virulence the ranking of isolates was N > Y > A, for transmissibility, Y > A > N. The predictions of theoretical models for the evolution of virulence were analyzed with these parameters and compared with observations from the field. A single-infection model predicted adequately the observed long-term evolution of the CMV population to intermediate levels of virulence. A coinfection model that considered competition between isolates with an effect on transmission explained the invasion of the CMV population by N isolates at the beginning of the epidemic, and its predictions also agreed with field data on the long-term evolution of the CMV population. An important conclusion from both models was that the density of the aphid vector's population is a major factor in the evolution of CMV virulence. This may be relevant for the design of control strategies for CMV-induced diseases.

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

“…For instance, it has been shown that when Barley stripe mosaic virus evolved by serial horizontal transfers, its virulence increased with no concomitant increase in the viral load (45). Despite having radically different levels of virulence, necrogenic and nonnecrogenic variants of Cucumber mosaic virus did not differ in their accumulation levels in tomato (20,21). Finally, in a recent study, a lack of correlation between fitness and virulence was also reported for foot-and-mouth disease virus (26).…”

Estudio de la interacción entre naranjo amargo y el virus de la tristeza de los cítricos

“…Aunque diversos modelos teóricos de evolución de la virulencia han asumido una correlación positiva entre el nivel de multiplicación del patógeno y sus efectos en el huésped (Sacristán et al, 2005) y dicha correlación ha sido observada en algunos sistemas virus-planta (Heijbroek et al, 1999;Fargette et al, 2002;Martín et al, 2005), en otros casos se ha encontrado que virulencia y carga viral no estaban relacionados o incluso se correlacionaban negativamente (Rodríguez-Cerezo et al, 1991;Shi et al, 2002;Escriu et al, 2003;Sacristán et al, 2005;Pagán et al, 2007). El presente estudio es el primero en identificar una ausencia de correlación entre virulencia y carga viral en un closterovirus y en un huésped leñoso.…”

Experimental evolution of plant RNA viruses

“…Therefore, all else being equal, in a natural situation it may be costly for the virus to harm its plant host. To test this prediction, Escriu et al (2003) performed field and laboratory studies seeking to understand the evolution of virulence (for example, reduction in lifespan biomass) and epidemiology of different CMV strains. The data gathered by these authors support the predicted negative correlation between virulence and the rate of transmission (Figure 3c: partial correlation coefficient controlling for strain type: r ¼ À0.96, d.f.…”