Perspectives for Virulence Management: Relating Theory to Experiment

This paper reviews our current knowledge about the evolution of virulence in pathogen-host systems, with an emphasis on the interface between the theoretical and experimental literature. After giving a methodically oriented overview of the field, stressing restrictions and caveats, the paper attempts to summarize the main results on virulence evolution gleaned from the literature. From that perspective the authors identify what they see as gaps in our current knowledge that need to be filled to transform the study of virulence evolution and management into a mature science

Evolution Restricts the Coexistence of Specialists and Generalists - The Role of Trade-Off Structure

Environmental variability and adaptive foraging behavior have been shown to favor coexistence of specialists and generalists on an ecological time scale. This leaves unaddressed the question whether such coexistence can also be expected on an evolutionary time scale. In this article we study the attainability, through gradual evolution, of specialist-generalist coexistence, as well as the evolutionary stability of such communities when allowing for immigration. Our analysis shows that the potential for specialist-generalist coexistence is much more restricted than originally thought, and strongly depends on the trade-off structure assumed. We establish that ecological coexistence is less likely for species facing a trade-off between per capita reproduction in different habitats than when the trade-off acts on carrying capacities alone. We also demonstrate that coexistence is evolutionarily stable whenever it is ecologically stable, but that in most cases such coexistence cannot be reached through gradual evolution. We conclude that an evolutionarily stable community of specialists and generalists may only be created through immigration from elsewhere or through mutations of large effect. Our results highlight that trade-offs in fitness-determining traits can have counterintuitive effects on the evolution of specialization

Evolution of parasite virulence to vectors

Vectorborne parasites are commonly predicted to be less virulent to the vector than to the definitive host as the parasite gains little by harming its main route of transmission. Here we assess the empirical evidence from systems where insects vector vertebrate parasites. The body of evidence supports lower (but non-zero) parasite virulence to vectors than to plant or invertebrate hosts but not to vertebrate hosts. We consider why this might be by assessing evolutionarily stable strategies for an insect parasite that can infect both vector and definitive host and can have distinct virulences in these two potential hosts. In a homogeneous environment the parasite is predicted to be equally virulent to vector and host. However, in a patchy environment it is predicted to become benign towards the more mobile of the two potential hosts, provided competitive displacement among strains in a patch is weak. This prediction may not meet reality for two different reasons. First, relative mobility of vector to host depends on the spatial scale under consideration: malaria mosquitoes are the more mobile hosts from house to house within a human settlement, but human hosts may be more mobile from one settlement to the other. Second, as in malaria, the main host and therefore probably also the vector may be multiply infected and this is likely to increase virulence and to level off differences between vector and definitive hos