Abstract
Life history traits of pathogens are the products of a long coevolution with their host(s). They determine the pathogen ability to colonize, disperse and survive, and thus reflect the potential of invasiveness and persistence within natural and agricultural systems. Natural selection retains reproductive pattern exhibiting higher fitness, this latter being modulated by several factors such as host resistance or multiple infections. Understanding these host-pathogen and pathogen-pathogen interactions is a key issue for dealing with adaptation of pathogen populations to host resistance. The aim of this PhD thesis is to study impact of these two scales of interaction on reproductive strategies of Phytophthora infestans, the pathogen responsible for late blight of potato (Solanum tuberosum). More precisely, this work aims (i) to determine which omponents of partial resistance impact sexual reproduction of the pathogen; (ii) to describe reproductive strategies adopted in response to multiple infections for sexually incompatible isolates; and (iii) to compare reproductive strategies between compatible and incompatible isolates during multiple infections. Partial resistance impacts similarly sexual and asexual reproductions of P. infestans. Furthermore, varieties which increase the latent period seem to limit oospore formation, which are survival structures resulting from sexual reproduction. Therefore, sex can be seen as an adaptation to less resistant varieties, which rapidly succumb to the disease. Our results also highlight the diversity of reproductive strategies observed during multiple infections, and raise the question of the cost of sex for the pathogen. Two contrasted strategies were identified: some genotypes, called asexual competitive, are able to improve their reproductive rate during incompatible infections, but less performing when sex is possible; the others, named sexual competitive, respond in the opposite way. The importance of the competing genotype is highlighted. The plasticity of reproductive strategies explains why P. infestans adaptation can be observed at short time scales, such as a cultural season, during which the frequencies of multiple infections are expected to increase. This work underlines the importance to take reproductive strategies and host effect into account when measuring the fitness of pathogens. These assessments, which are highly dependent on the system complexity, should allow to test the adaptation to host resistance at the population and landscape levels, using models of evolutionary ecology. This knowledge could eventually contribute to develop plans for the durable management of host resistance.