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Virulence-Transmission Trade-Offs and Population Divergence in Virulence in a Naturally Occurring Butterfly Parasite

Jacobus C. de Roode, Andrew J. Yates and Sonia Altizer
Proceedings of the National Academy of Sciences of the United States of America
Vol. 105, No. 21 (May 27, 2008), pp. 7489-7494
Stable URL: http://www.jstor.org/stable/25462625
Page Count: 6
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
Virulence-Transmission Trade-Offs and Population Divergence in Virulence in a Naturally Occurring Butterfly Parasite
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Abstract

Why do parasites harm their hosts? Conventional wisdom holds that because parasites depend on their hosts for survival and transmission, they should evolve to become benign, yet many parasites cause harm. Theory predicts that parasites could evolve virulence (i.e., parasite-induced reductions in host fitness) by balancing the transmission benefits of parasite replication with the costs of host death. This idea has led researchers to predict how human interventions-such as vaccines-may alter virulence evolution, yet empirical support is critically lacking. We studied a protozoan parasite of monarch butterflies and found that higher levels of within-host replication resulted in both higher virulence and greater transmission, thus lending support to the idea that selection for parasite transmission can favor parasite genotypes that cause substantial harm. Parasite fitness was maximized at an intermediate level of parasite replication, beyond which the cost of increased host mortality outweighed the benefit of increased transmission. A separate experiment confirmed genetic relationships between parasite replication and virulence, and showed that parasite genotypes from two monarch populations caused different virulence. These results show that selection on parasite transmission can explain why parasites harm their hosts, and suggest that constraints imposed by host ecology can lead to population divergence in parasite virulence.

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