The switch to parasitism on an ecological and evolutionary scale

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Pathogenic organisms that are harmless under certain conditions can suddenly become extremely harmful under different circumstances. Indeed levels of parasitism vary continuously in nature, with some species shifting along a continuum from benign to pathogenic over ecological and evolutionary time. But even more fundamental to this issue is the process by which parasitism per se evolved. Understanding the ecological and evolutionary factors that facilitate the acquisition of parasitism is important because it allows epidemiologists to more accurately predict when and where infectious diseases will emerge, and develop effective control measures. I will investigate the life-history evolution of parasites that express variation in host exploitation strategies, and identify the selective pressures that lead to the transition to a parasitic lifestyle. Facultative parasites present a unique and interesting opportunity for addressing these questions because they regularly shift from free-living to parasitic lifestyles. The facultative ectoparasitic mite, Macrocheles subbadius feeds and reproduces on highly ephemeral habitats - rotting cacti. However, mites become parasitic under certain circumstances by attaching to and feeding on cactophilic drosophilid hosts. Therefore, it appears the mites are capable of context-dependent expression of infectivity (ability to parasitize a host). The flies and mites co-occur naturally in the wild and are highly amendable to laboratory microcosm experiments. I plan to manipulate environmental conditions to test the ecological consequences of environmental heterogeneity on mite behaviour and infection (aim 1). To determine the heritability of the infectivity trait, I will perform experimental evolution by artificially selecting for mites with divergent levels of infectivity; the selected lines will be used to measure the evolutionary trade-offs between the evolution of parasitism and other key parasite life history traits (aim 2). Additionally, I will identify the ecological factors that select for increased reliance on hosts over a long period of time, i.e., the evolutionary transition to parasitism (aim 3). ************