Evolution and ecology of phenotypic plasticity in parasite life history strategies

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The environments experienced by parasites inside hosts and vectors are highly changeable. For example, resources become scarce as hosts get sick, immune responses directed at parasites develop, and parasites often have to compete with other strains and species sharing their host. Given that parasites live in the bodies of others, with which they are engaged in a life-and-death struggle, knowledge about how parasites cope with the challenges of their lifestyle is remarkably shallow. Understanding the strategies that parasites have evolved to cope with the challenging environments they face inside hosts and vectors is the aim of this proposal. Parasite strategies are important because they are determinants of how sick hosts get (virulence) and how infectious parasites are (transmission). In evolutionary ecology, plasticity in life-history traits is a well-known adaptation to life in variable environments. Adaptive plasticity enables organisms to change their phenotype in response to changes in their circumstances in ways that maintain fitness across environments. That the environment is complex and shapes phenotypes has been known for over a century but because parasites have been viewed as creatures with inflexible strategies, plasticity in their traits - and whether it is beneficial - has been overlooked. Mounting evidence is revealing considerable plasticity in parasite traits that underpin the ability to survive during infections and to transmit to new hosts. Our recent research shows that parasites adjust their traits in response to the conditions (resource availability, competition with other strains, and anti-parasite drugs) they experience in the host. We know want to investigate why parasites do this by testing theory that predicts they adjust their traits in ways that maximise fitness. We will identify how much variation in parasite traits is explained by parasite strategies and complex interactions between hosts and parasites, whether plasticity benefits parasites, and whether plasticity has evolved to enable to parasites to cope with variable environmental conditions. Therefore, our project will reveal how sophisticated parasite strategies for survival and transmission really are, and reveal weaknesses that could be exploited for disease control.Malaria (Plasmodium) parasites offer an excellent system to explain the ecological and evolutionary drivers of variation in traits that shape the virulence and transmission phenotypes of parasites. A key fitness-determining trait for malaria parasites is the trade-off between the allocation of resources to asexually replicating stages (required for within-host survival) and sexually reproducing stages (required for transmission), which is called "reproductive effort" in evolutionary biology. Our project will determine how host and parasite factors contribute to plasticity in reproductive effort, quantify the patterns generated across environmental gradients and parasite genetic backgrounds, test whether parasite strategies are adjusted in response to the environment in ways that benefit survival and transmission, and investigate whether plasticity is costly by testing if it is lost under selection in constant environmental conditions. Explaining variation in the virulence and infectiousness of parasites is a major aim in evolutionary biology and predicting how parasites adapt to their environment is becoming increasingly important. For example, changes in parasite ecology (e.g. composition of host-parasite-vector communities, and habitats) are implicated in the emergence of new infectious diseases of wildlife, crops, livestock, and humans. For applied science, a better understanding of parasites is necessary because evolution continually erodes efforts to control diseases responsible for considerable mortality and morbidity.