Virulence evolution of a sterilizing plant virus: Tuning multiplication and resource exploitation

Virus Evolution, 2017, 3(2): vex033 doi: 10.1093/ve/vex033 Research article Virulence evolution of a sterilizing plant virus: Tuning multiplication and resource exploitation Viji Vijayan,† Silvia López-González,† Flora Sánchez, Fernando Ponz, and Israel Pagán*,‡ Centro de Biotecnologı́a y Genómica de Plantas (UPM-INIA), Autopista M-40, km 38, Campus Montegancedo, Pozuelo de Alarcón, 28223 Madrid, Spain *Corresponding author: E-mail: † These authors contributed equally to this work. ‡ http://orcid.org/0000-0001-8876-1194 Abstract Virulence evolution may have far-reaching consequences for virus epidemiology and emergence, and virologists have devoted increasing effort to understand the modulators of this process. However, still little is known on the mechanisms and determinants of virulence evolution in sterilizing viruses that, as they prevent host reproduction, may have devastating effects on host populations. Theory predicts that sterilizing parasites, including viruses, would evolve towards lower virulence and absolute host sterilization to optimize the exploitation of host resources and maximize fitness. However, this hypothesis has seldom been analyzed experimentally. We investigated the evolution of virulence of the sterilizing plant virus Turnip mosaic virus (TuMV) in its natural host Arabidopsis thaliana by serial passage experiments. After passaging, we quantified virus accumulation and infectivity, the effect of infection on plant growth and development, and virulence of the ancestral and passaged viral genotypes in A. thaliana. Results indicated that serial passaging increased the proportion of infected plants showing absolute sterility, reduced TuMV virulence, and increased virus multiplication and infectivity. Genomic comparison of the ancestral and passaged TuMV genotypes identified significant mutation clustering in the P1, P3, and 6K2 proteins, suggesting a role of these viral proteins in the observed phenotypic changes. Our results support theoretical predictions on the evolution of virulence of sterilizing parasites and contribute to better understand the phenotypic and genetic changes associated with this process. Key words: virulence evolution; Turnip mosaic virus; Arabidopsis thaliana; host resource exploitation. 1. Introduction Virulence is an intrinsic property of parasites, defined as the deleterious effects of infection on the host fitness (Read 1994). Changes in virulence have been associated with modifications of parasite epidemiology and host–parasite co-evolutionary dynamics, with biodiversity loss and local extinction of host populations, and disease emergence and re-emergence (Bull 1994; Malmstrom et al. 2006; Little et al. 2010; Berngruber et al. 2013). Thus, explaining virulence evolution is fundamental to understand the life history of parasites and may be of socioeconomic relevance given the significant impact of parasites on human, animal, and plant welfare (Dieckman et al. 2002). Most theoretical elaborations on the evolution of virulence derive from the trade-off hypothesis. According to this hypothesis, maximal parasite fitness would result from optimizing the within-host multiplication and the between-host transmission C The Author 2017. Published by Oxford University Press. V This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/ licenses/by-nc/4.0/), which permits non-commercial re-use, distribution, and reproduction in any medium, provided the original work is properly cited. For commercial re-use, please contact 1 2 | Virus Evolution, 2017, Vol. 3, No. 2 components of parasite fitness (Anderson and May 1982; Ebert and Bull 2003; Cressler et al. 2016). Two key assumptions underlie the trade-off hypothesis: greater parasite load in an infected host increases the probability of transmission to a susceptible host; and parasite load is positively correlated with virulence measured as increased host mortality. A trade-off occurs because higher virulence, by reducing host longevity, reduces the infectious period and the probability of transmission. Thus, the trade-off hypothesis predicts that parasites would evolve to intermediate levels of virulence (Anderson and May 1982; Ebert and Bull 2003; Cressler et al. 2016). However, not every parasite may necessarily play with these rules. This is the case of sterilizing parasites, defined as those whose infection results in draining of host reproductive resources, eventually leading to castration (Laffertty and Kuris 2009). Reproduction draws energy away from survival, so by lessening host reproduction parasites can keep their host alive longer. This reduces parasite fitness costs associated with increased host mortality (Obrebski 1975) and allows the storage of sterilization-liberated resources into host growth until the parasite can exploit them (Ebert et al. 2004). Thus, modifications of the trade-off hypothesis to accommodate sterilizing parasites predict that they would maximize fitness at absolute sterilization (maximum amount of host resources dedicated to host survival), and at lower virulence (reduced mortality), provided that they are not vertically transmitted (Jaenike 1996; O’Keefe and Antonovics 2002; Ebert et al. 2004; Hall et al. 2007). Sterilizing parasites have been described in a wide range of animal (Laffertty and Kuris 2009) and plant (Clay 1991; Clay and Kover 1996) hosts. As they prevent host reproduction, the effect of this type of parasites in host populations can be enormous. For instance, at high prevalence they may impose strong selection pressures leading to selection for resistant host genotypes (Laffertty and Kuris 2009). In addition, various mathematical models predict that, in the absence of resistance, higher prevalence of sterilizing parasites would reduce host population density (Antonovics 2002; Negovetich and Esch 2008; Laffertty and Kuris 2009). Moreover, if infection affects the recruitment capacity of the host population, preventing migration of host individuals from other populations with lower or no parasite burden, sterilizing parasites may even cause the local extinction of the host populations. These predictions are supported by several empirical analyses (reviewed by Laffertty and Kuris 2009). Despite the great potential of sterilizing parasites to affect the population dynamics of their hosts, experimental analyses on the evolution of virulence of such parasites are scarce. Evidence supporting predictions of the trade-off hypothesis (reduced mortality) comes from a handful of studies in parasite–invertebrate interactions (Jaenike 1996; Sorensen and Minchella 2001; Ebert et al. 2004; Jensen et al. 2006). Equivalent studies in plants are even scarcer and limited to fungal parasites (Burdon 1991; Kover 2000; Sloan et al. 2008). Remarkably, the predictions of the tradeoff hypothesis for sterilizing parasites have not been tested for (...truncated)