Abstract
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.