Co-evolution between mosquitoes and microsporidian transmission strategies

Parasite and host impose strong selection on each other. The first causes damages and mortality to the host, while the second responds by reducing the detrimental effects and the intensity and/or success of infection. The resulting co-evolutionary dynamics are profoundly affected by the ecological conditions, for these may influence many aspects of host-parasite interactions including life history evolution, virulence and transmission. It is therefore essential to study and incorporate environmental variation in the field of parasitology to gain an exhaustive understanding of how host and parasite evolve. In this thesis, a single generation and an evolutionary experimental approach were used to investigate the impact of the ecological and epidemiological conditions on several aspects of host-parasite interactions, with the main focus on parasite transmission strategies. Firstly, it was examined the effect of the availability of resources for the host, timing of infection, and co-infection on the virulence and transmission success of two parasites with conflicting transmission strategy. Next, it was tested how the environment influenced the trade-off between vertical and horizontal transmission in a parasite with a mixed mode of transmission and it was assessed the genetic contribution of the host to its transmission mode. Whether the vertical and horizontal component of this parasite and the associated virulence responded to restriction opportunities, represented by different availability of resources over several generations, was investigated with an evolutionary experiment. Finally, the presence of a plastically parasite-induced response on the recombination rate of the host as a potential cross-generational defence mechanism was explored. The experiments cover many key aspects of host-parasite interactions and emphasize the role of the ecological conditions on shaping these relationships. The results and their implications are discussed in detail throughout the thesis. Overall, this work highlights the dependence of crucial aspects of host-parasite interactions from the epidemiological and ecological conditions. Disentangling the various forces surrounding these interactions may help us to acquire a better knowledge of how a changing environment may drive the evolution of both host and parasite.