Virulence and Local Adaptation of a Horizontally Transmitted Parasite

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Science  19 Aug 1994:
Vol. 265, Issue 5175, pp. 1084-1086
DOI: 10.1126/science.265.5175.1084


Parasites are thought to maximize the number of successfully transmitted offspring by trading off propagule production against host survival. In a horizontally transmitted microparasitic disease in Daphnia, a planktonic crustacean, increasing geographic distance between host and parasite origin was found to be correlated with a decrease in spore production and virulence. This finding indicates local adaptation of the parasite, but contradicts the hypothesis that long-standing coevolved parasites are less virulent than novel parasites. Virulence can be explained as the consequence of balancing the positive genetic correlation between host mortality and strain-specific spore production.