Evolutionary and proximate mechanisms shaping host-parasite interactions : the case of "Daphnia magna" and its natural bacterial parasite "Pasteuria ramosa"

Host-parasite interactions are composed of a sequence of steps, all necessary for successful infection: parasites need to encounter their hosts, to enter into their bodies, and to proliferate within them. Selection will act on the mechanisms used in each of the steps; the parasite being selected to increase their efficiency, and the host selected to reduce it. I have proposed, and shown, that explicitly analyzing the factors that influence each of the steps and their impact on host and parasite fitness is of crucial importance for a complete understanding of host-parasite interactions. In my Ph.D. research work, I identified markers of different steps of the interaction between the host crustacean Daphnia magna and its natural bacterial parasite Pasteuria ramosa, and investigated factors influencing different steps, as well as the contribution of each of them to shaping the interaction between the two species.
I established that the infection of Daphnia magna by Pasteuria ramosa could be decomposed in at least five sequential steps (Chapter 1): 1) the encounter between the host and the parasite, 2) the activation of the parasite transmissible, resting stage, which happens once it contacts the host, 3) the attachment of the parasite to the host cuticula, 4) the penetration of the parasite into the host body cavity, and 5) the proliferation of the parasite within the host. The factors affecting the likelihood of encounter between host and parasite had been investigated before, in a study that revealed that there is a host genetic component, and polymorphism for the ability of the host to avoid encountering the parasite. Resolving the interaction into its different steps and focusing on steps affect the encounter allowed me to see that: i) different steps are under the influence of different factors (Chapter 1), ii) the traits underlying some steps, but not all, do not seem to be polymorphic (Chapter 1), iii) the parasite genotype specificity of the success of the attachment step can explain the genotype specificity of the host susceptibility (Chapter 1), iv) the speed with which the parasite penetrates the host body after attachment is crucial for the parasite success (Chapter 2), v) the molting, usually seen as a cost against parasite, can be beneficial to reduce the likelihood of infection, vi) once in the host body, the parasite will adapt to the environment that is characteristic of the most common host sex, here female characteristic (Chapters 3 and 4), vii) the success of proliferation of P. ramosa inside D. magna hosts is not influenced by previous host exposure to that same parasite (Chapter 5). All in all, I show that considering each of the steps explicitly provides new light into the mechanisms and selective pressures on hosts and their parasites. Each of the two interacting parties will, indeed, be under more or less strong selection to maximize their success at each of the steps. Below I will elaborate on this idea in relation to my specific findings and the research perspectives they open.