Multiple infections and virulence – MULTIVIR

Theory predicts that virulence should be higher when multiple strains of parasites regularly compete within hosts. This is because, under conditions of multiple infection, parasites that exhaust limiting host resources more rapidly will leave more descendents than those that follow a more prudent strategy. Furthermore, plasticity of virulence expression, as well higher mean virulence, may evolve, allowing parasites to respond optimally to their local infection environment (i.e., multiple versus simple infection, relatedness of co-infecting strains…). Additional theoretical developments, however, suggest that the evolutionary trajectory of virulence as well as the value of optimal virulence under conditions of multiple infection will depend on the nature of within-host competition as well as on the degree of relatedness between interacting, competing parasites. For example, virulence is expected to decrease rather than increase under multiple infections in cases of interference (competitive inhibition) between different co-infecting parasites, particularly if parasite strains are unrelated. To date very few data are available with which to confront these theoretical predictions. In particular there are very few experiments that vary degree of genetic relatedness between strains of interacting or competing parasites. Here we propose a set of experiments in the field and the greenhouse using three different plant pathogen-host associations: Hyaloperonospora, an oomycete parasite of Arabidopsis thaliana, Microbotryum, a basidiomycete anther smut of the Caryophyllaceae, and Cryphonectria parasitica, the chestnut blight fungus, that itself is parasitized by a double stranded hypovirulence virus. All three are model systems for which a battery of genetic tools are rapidly becoming available, but more importantly, all present multiple infections in natural populations and all are well suited to this kind of experimental manipulation. We will perform experimental inoculations of multiple parasite strains and measure the response in terms of virulence. We will also follow the course of multiple infections to determine whether parasite strains can coexist, can do so in a perennial fashion or whether one strain can exclude the other, and whether this depend on relatedness between them. We will measure the phenotypic effect of single versus multiple infections, and in particular of multiple infections that vary for the degree of genetic relatedness between co-infecting parasite strains to determine whether parasites exhibit plasticity for the negative effects on their hosts and are sensitive to the kin status of their competitors. This is timely because the role of kin selection in the evolution of social behaviour in general and costly parasite traits that can cause damage to hosts is currently the object of some controversy. We intend to contribute to this debate in an informed manner, by providing data on how kin versus non-kin parasites interact when they compete for host resources. Despite the breadth of the biological details of our systems, most of our questions are general to all three. This work should elucidate the effects of within-host competition between more or less related parasite strains on how damaging parasites are to their hosts. Our results will aid in designing control programmes that take into consideration the long-term effects of modifying the competitive environment of parasites and the identity of likely competitors. Our project proposes basic research that nonetheless has important applications for better design of informed parasite control programmes that are of extreme importance for human health, food security and management of forests.