In recent years there has been growing interest in applying frameworks from evolutionary ecology to understand infectious disease. It is becoming increasingly apparent that the interactions between parasites within the host environment can shape parasite phenotypes underlying infection dynamics and transmission. However, the spread of the disease will crucially depend on both within-host and between-host dynamics. Bridging these scales is challenging and for vector borne parasites, such as malaria and trypanosomes, will involve gaining a much better understanding of infection dynamics both within the host and vector. I apply evolutionary ecology frameworks including social evolution, life history theory, and phenotypic plasticity to investigate how parasite phenotypes are shaped by within-host and within-vector environments and examine the implications for inhost survival and between-host transmission. Specifically, I demonstrate that; 1. Within the host; i. In accordance with theory malaria parasites detect and respond to the presence of competitors by altering reproductive strategies to maximise in-host survival. Furthermore, these strategies are fine tuned in response to variation in the within-host environment, including the availability of resources. ii. The reproductive investment strategies of malaria parasites can be applied to explain the transmission strategies of African trypanosomes. This shows how general evolutionary frameworks can be applied to a novel parasite species and demonstrates the explanatory power of an evolutionary approach. iii. The complexity of the within-host environment poses specific statistical challenges for examining the temporal dynamics of parasite life history traits that are often not adequately dealt with, potentially leading to type 1 errors. Methods to evaluate levels of autocorrelation and how to deal with it are applied to datasets of within-infection dynamics. 2. Within the vector; i. Malaria parasites undergo programmed, apoptotic cell death. The occurrence of, and putative explanation for, apoptosis in protozoan parasites is controversial. I demonstrate the importance of quantitative methods and parasite ecology in testing the evolutionary explanations for parasite apoptosis. ii. The links between within-host dynamics and within-vector dynamics are complex and can lead to counter-intuitive implications for the success of between-host transmission. Density-dependent processes result in diverse fitness costs to parasites of crowding. More broadly, these processes could explain why parasites undergo apoptosis. In general my results demonstrate, across vertebrate hosts and insect vectors, how the interactions between parasites and with their environment shapes traits important for the transmission of infectious disease.

Les interactions intimes et répétées entre hôtes et parasites peuvent engendrer la spécialisation d’un parasite à son hôte, grâce à des adaptations comportementales, morphologiques et/ou génétiques, combinées avec un flux de gènes limité. C’est un processus clef car il participe à l’évolution de la biodiversité parasitaire et peut ainsi permettre de mieux comprendre l’émergence d’organismes pathogènes. Encore peu étudié, une spécialisation d’hôtes a néanmoins été démontré lors de précédentes études chez deux espèces de tiques nidicoles : chez Ixodes uriae une tique dure, parasite des oiseaux marins coloniaux en zone arctique, et dans un complexe de tiques molles Ornithodoros capensis sensu lato, parasitant aussi de nombreuses espèces d’oiseaux marins, mais cette fois-ci en zones tempérées et tropicales. Ces deux espèces sont vectrices d’une grande diversité d’agents pathogènes incluant des virus, des bactéries et des protozoaires. Cependant, les facteurs impliqués dans le phénomène de spécialisation d’hôte restent inconnus. Dans ce cadre, le but de ma thèse était donc de déterminer 1) si l’évolution des divergences en fonction des hôtes est toujours accompagnée par les mêmes changements phénotypiques et 2) si ces changements pourraient permettre d’identifier les facteurs de sélection sous-jacents. Dans ce contexte, des campagnes d’échantillonnage de tiques ont été menées durant la période de reproduction des hôtes oiseaux dans les différentes zones de leur répartition et nous avons réalisé des analyses morphométriques, basées sur l’utilisation de landmarks et de contours sur chaque individu tique et des analyses phylogénétiques et génétiques des populations sur les mêmes individus. L’ensemble de ces résultats suggère la présence de convergences morphologiques au sein de ces systèmes et souligne un rôle de la sélection dans ce processus de divergence. En effet, les caractéristiques écologiques des hôtes mais aussi le micro-habitat exercent des pressions sélectives importantes dans ces deux systèmes pouvant être à l’origine de la divergence observée entre les populations. De plus, les caractéristiques biologiques de chaque espèce de tiques, telle que la capacité de dispersion, entrent également en jeu et peuvent fortement modifier l’épidémiologie des agents infectieux dont elles sont vectrices.Mots clés : Argasidae, écologie de la transmission, évolution convergente, interactions hôte-parasite, Ixodidae, oiseaux marins. / Intimate and repeated interactions between hosts and parasites can lead to parasite specialization to a given host via behavioral, morphological and/or genetic adaptations that act in combination with restricted gene flow. Specialization is a key process leading to the generation of parasite biodiversity and can help us understand the emergence of pathogenic organisms. Although little studied, host specialization has already been demonstrated to occur in previous studies of two nidicolous tick species: Ixodes uriae a hard tick parasitizing colonial seabirds in polar regions, and soft ticks of the complex Ornithodoros capensis sensu lato, that also exploit colonial seabirds, but this time in temperate and tropical zones. Both of these species act as vector to a wide variety of pathogenic organisms, including viruses, bacteria and protozoa. However, the factors involved in host specialization remain unknown. In this context, the aim of my thesis was to determine 1) whether the evolution of host specialization is always accompanied by the same phenotypic changes and 2) whether these changes could help to identify the selective factors that influence this phenomenon. In this context, tick collections were conducted during the breeding period of the host birds in different areas of their distribution and morphometric analyses, based on landmark and contour methods, were performed on each individual tick. Phylogenetic and population genetic analyses were also carried out using the same individuals. Overall, the results demonstrate that morphological convergence occurs within these systems, highlighting the role of selection in the divergence process. Indeed, the ecological characteristics of the hosts, but also their micro-habitat, may exert significant selective pressures on ticks and may cause the observed divergence among populations. Likewise, the biological characteristics of each tick species, particularly in relation to dispersal capacity, may also come into play and will greatly modify the epidemiology of associated infectious agents.Keywords: Argasidae, convergent evolution, host-parasite interactions, Ixodidae, transmission ecology, seabirds.