Differential Tolerance to Direct and Indirect Density- Dependent Costs of Viral Infection in Arabidopsis thaliana Israel Paga ´n 1 , Carlos Alonso-Blanco 2 , Fernando Garcı´a-Arenal 1 * 1 Centro de Biotecnologı ´a y Geno ´ mica de Plantas (UPM-INIA) and E.T.S.I. Agro ´ nomos, Campus Montegancedo, Universidad Polite ´cnica de Madrid, Pozuelo de Alarco ´n (Madrid), Spain, 2 Departamento de Gene ´tica Molecular de Plantas, Centro Nacional de Biotecnologı ´a, Consejo Superior de Investigaciones Cientı ´ficas (CNB-CSIC), Darwin 3, Campus Universidad Auto ´ noma, Cantoblanco, Madrid, Spain Abstract Population density and costs of parasite infection may condition the capacity of organisms to grow, survive and reproduce, i.e. their competitive ability. In host–parasite systems there are different competitive interactions: among uninfected hosts, among infected hosts, and between uninfected and infected hosts. Consequently, parasite infection results in a direct cost, due to parasitism itself, and in an indirect cost, due to modification of the competitive ability of the infected host. Theory predicts that host fitness reduction will be higher under the combined effects of costs of parasitism and competition than under each factor separately. However, experimental support for this prediction is scarce, and derives mostly from animal– parasite systems. We have analysed the interaction between parasite infection and plant density using the plant-parasite system of Arabidopsis thaliana and the generalist virus Cucumber mosaic virus (CMV). Plants of three wild genotypes grown at different densities were infected by CMV at various prevalences, and the effects of infection on plant growth and reproduction were quantified. Results demonstrate that the combined effects of host density and parasite infection may result either in a reduction or in an increase of the competitive ability of the host. The two genotypes investing a higher proportion of resources to reproduction showed tolerance to the direct cost of infection, while the genotype investing a higher proportion of resources to growth showed tolerance to the indirect cost of infection. Our findings show that the outcome of the interaction between host density and parasitism depends on the host genotype, which determines the plasticity of life-history traits and consequently, the host capacity to develop different tolerance mechanisms to the direct or indirect costs of parasitism. These results indicate the high relevance of host density and parasitism in determining the competitive ability of a plant, and stress the need to simultaneously consider both factors to understand the selective pressures that drive host–parasite co-evolution. Citation: Paga ´n I, Alonso-Blanco C, Garcı ´a-Arenal F (2009) Differential Tolerance to Direct and Indirect Density-Dependent Costs of Viral Infection in Arabidopsis thaliana. PLoS Pathog 5(7): e1000531. doi:10.1371/journal.ppat.1000531 Editor: Dieter Ebert, University of Basel, Switzerland Received January 13, 2009; Accepted July 6, 2009; Published July 31, 2009 Copyright: ß 2009 Paga ´n et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: This work was partly supported by grants AGL2005-01122, and AGL2008-02458 from Plan Nacional de I+D, Spain, to FGA. IP was supported by a FPI fellowship from Ministerio de Ciencia y Tecnologı ´a, Spain. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * E-mail: fernando.garciaarenal@upm.es Introduction Competition is one of the major selection factors in nature, acting at all phases of development [1]. The key role of competition in shaping evolution is one of the bases of the Darwinian Theory, underlying the colonization success, expansion and suppression of genotypes and species. The relevance of competition for ecology and evolutionary biology has lead to a large body of work based on life-history theory, which states that competitive ability depends on trade-offs between the capacity of an organism to grow, survive and reproduce [2,3]. The optimal amount of resources allocated to each of these components may be modified depending on environmental conditions in order to maximize the organism’s fitness [4]. Experimental analyses have shown that competition due to increased population density may induce severe alterations in life history traits that are components of competitive ability such as mortality rate [5], time span to maturity [6], adult size [7] or fecundity [8]. The impact of population density on the effects of predation and herbivory have been widely investigated [9–11]. In contrast, density dependent effects on host-parasite systems, have received much less attention [12]. In host-parasite systems there are different compet- itive interactions: intra-class competition among uninfected hosts or among infected hosts, and inter-class competition between uninfect- ed and infected hosts. Each interaction may have different effects on host life-history traits, resulting in a direct cost of infection, due to parasitism itself, and in an indirect cost, due to modification of the competitive ability of the infected host, both being modulated by host population density [13]. Theory predicts that fitness reduction will be higher under the combined effects of host population density and parasitism than under each factor separately [14–17]. The outcome of this interaction may also depend on the prevalence of infection, the effect on the host competitive ability being less severe as prevalence increases [13]. Experimental analyses of these predictions derive mostly from animal-parasite systems [18–22], and few similar ones have been carried out with plants [23–26]. PLoS Pathogens | www.plospathogens.org 1 July 2009 | Volume 5 | Issue 7 | e1000531