Monarch butterflies help explain why parasites ...( Athens Ga. Its a paradox that has ...)

Athens, Ga. Its a paradox that has confounded evolutionary biologists since Charles Darwin published On the Origin of Species in 1859: Since parasites depend on their hosts for survival, why do they harm them?

A new University of Georgia and Emory University study of monarch butterflies and the microscopic parasites that hitch a ride on them finds that the parasites strike a middle ground between the benefits gained by reproducing rapidly and the costs to their hosts. The study, published in the early online edition of the journal Proceedings of the National Academy of Sciences, provides the first empirical evidence in a natural system of whats called the trade-off hypothesis.

Parasites have to harm their host to replicate and be transmitted, said lead author Jacobus de Roode, a former post-doctoral researcher at UGA and now an assistant professor at Emory University. But what this study found is that if they harm their host too much, theyll suffer too. On the other hand, this study also shows that it does not benefit the parasite to be maximally benign, because those parasites dont replicate enough to be effectively transmitted.

In a painstaking, three-year study conducted in the laboratory of Sonia Altizer, assistant professor in the UGA Odum School of Ecology, researchers infected monarch caterpillars with varying levels of spores from a protozoan parasite commonly found in wild populations. After the adult butterflies emerged, females were mated and placed in outdoor mesh cages. The butterflies spread the parasites when they deposit spores onto eggs or leaves of the milkweed plants that caterpillars feed on. These spores are then consumed by caterpillars as they feed. Each butterfly had one stalk of milkweed in its cage, and every day for up to 30 days the researchers gave the butterflies a new stalk while taking the previous stalk back to the lab for analysis. The spores on the eggs and on the milkweed were counted, which i
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