The Evolution of Virulence under Conditions of Frequent Multiple Infections

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A single host individual often harbors more than one parasite species. The interactions between multiple parasite species can affect the evolutionary trajectory of virulence (the parasite-induced loss in host fitness), with important ecological and epidemiological consequences. Our current understanding of virulence evolution is seriously constrained by the lack of long-term studies, particularly those involving different parasite species with diverse modes of transmission and exploitation strategies. There is a gap between short-term laboratory studies, which mostly exclude between-host selection, and predictions from evolutionary models, which are based on a combination of within- and between-host selection. The goal of the proposed work is to investigate the long-term implications of multiple infections by different parasite species on the evolution of virulence, using Daphnia and their microparasites as a model system. In contrast to previous work, our experimental approaches will include between-host selection. Moreover, we will study virulence evolution under different scenarios: when competing parasites exploit different/the same host tissues; have different/the same transmission mode; and have different/the same onset of transmission. Importantly, we will relate the results of laboratory experiments to findings from field studies of natural populations, as understanding complex interactions among parasite species depends critically on the scale under consideration. The proposed research, a result of close collaboration between Israeli and German scientific teams, will further our understanding of the processes that shape the evolution of virulence. This project will also stimulate new and refined predictions regarding the consequences of the spread of parasites into new regions. Such invasion scenarios can result in co-infection between parasites that have never previously co-occurred. The match between the two applying researchers and their complementary scientific expertise promises a successful collaboration whose results will be a significant contribution to the field of evolutionary ecology.