Résumé
Parasites induce changes in their hosts that can be viewed either as pathological effects of infection (i.e., symptoms), adaptive defense by the host, or adaptive manipulation on the part of the parasite. The pollinator-borne anther-smut fungus Microbotryum violaceum is a prevalent parasite of the common roadside weed Silene latifolia (Caryophyllaceae). The fungal disease alters the phenotype of the host plant in several important ways. In a greenhouse experiment, we show that the disease accelerated flowering and that diseased plants invested relatively less in root biomass than did healthy plants. In a second experiment, diseased plants also produced smaller, more irregularly shaped flowers with different nectar-sugar production than did healthy plants. The magnitude of some of these effects of the disease differed for different fungal crosses. Differential alteration of host phenotype may result in pollinator-mediated selection on this pathogen. Whereas some of these changes in host phenotype may represent parasite adaptations, others may represent side effects of disease. It also appears that although the parasite modifies host phenotypes, trade-offs become evident, such as those between flower size and flower number and investment to vegetative compared with storage biomass. This indicates that this fungus cannot produce a plant phenotype optimal for pollinator attraction and disease transmission.