Abstract
BackgroundEvolution of parasite traits is inextricably linked to their hosts. For instance one common definition of parasite virulence is the reduction in host fitness due to infection. Thus, traits of infection must be viewed in both protagonists and may be under shared genetic and physiological control. We investigated these questions on the oomycete Hyaloperonospora arabidopsis (= parasitica), a natural pathogen of the Brassicaceae Arabidopsis thaliana.ResultsWe performed a controlled cross inoculation experiment confronting six lines of the host plant with seven strains of the parasite in order to evaluate genetic variation for phenotypic traits of infection among hosts, parasites, and distinct combinations. Parasite infection intensity and transmission were highly variable among parasite strains and host lines but depended also on the interaction between particular genotypes of the protagonists, and genetic variation for the infection phenotype of parasites from natural populations was found even at a small spatial scale within population. Furthermore, increased parasite fitness led to a significant decrease in host fitness only on a single host line (Gb), although a trade-off between these two traits was expected because host and parasite share the same resource pool for their respective reproduction. We propose that different levels of compatibility dependent on genotype by genotype interactions might lead to different amounts of resources available for host and parasite reproduction. This variation in compatibility could thus mask the expected negative relationship between host and parasite fitness, as the total resource pool would not be constant.ConclusionThese results highlight the importance of host variation in the determination of parasite fitness traits. This kind of interaction may in turn decouple the relationship between parasite transmission and its negative effect on host fitness, altering theoretical predictions of parasite evolution.
Highlights
Evolution of parasite traits is inextricably linked to their hosts
Because host resources that are diverted by parasites are no longer available to the host, this must lead to a reduction in host fitness, termed virulence
Transmission and Infection intensity Transmission, estimated as the asymptote of the sigmoid curve fitted to the cumulated daily transmission data of the eight transmission events, differed among the different origins of the parasite strains, with the Orsay strains being the least transmitted, and among the six host lines
Summary
Evolution of parasite traits is inextricably linked to their hosts. For instance one common definition of parasite virulence is the reduction in host fitness due to infection. Darwinian medicine aims at controlling the evolution of human pathogens in order to drive them toward "milder" forms or, ideally, to extinction [1]. Such control requires knowledge of which traits of infections are adaptive for the host or the parasite (page number not for citation purposes). BMC Evolutionary Biology 2007, 7:189 http://www.biomedcentral.com/1471-2148/7/189 as well as the associated trade-offs between traits and constraints on their evolution. Parasites harm their hosts in a number of ways that may be adaptive or not [2]. I.e. how host fitness varies with parasite transmission success, has certain advantages because the different measures of virulence among studies are not always comparable nor do they necessarily imply the same thing for host or parasite fitness (e.g., host mortality versus weight loss)
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