Monarch butterflies give clue to parasitic dilemma
Scientists studying monarch butterflies have uncovered evidence of how the insects' parasites strike a balance between harming their hosts and ensuring they stay alive long enough to infect other butterflies. The study, which was partly funded by the EU, represents the first empirical evidence from a natural system of the so-called 'trade-off hypothesis'. When parasites reproduce within their hosts, they consume host resources, damage the host's body and provoke an immune response. The parasite must therefore find a trade-off between increasing its reproduction and transmission levels and minimising damage to the host, so that it does not die before the parasite has been transmitted to other hosts. In this latest piece of research, scientists studied female monarch butterflies which had been infected with different levels of spores from a parasite called Ophryocystis elektroscirrha, which is often found in wild monarch populations. The results are published online by the journal the Proceedings of the National Academy of Sciences (PNAS). The researchers found that butterflies which were heavily infected with the parasite often died before mating or, if they survived, failed to mate. Meanwhile butterflies with a low parasite load lived for a long time and had many offspring, but only a few of these offspring were infected with the parasite. The best result, from the parasites' point of view, came from the butterflies bearing an intermediate parasite load; these lived for a long time, and laid lots of eggs. 'Parasites have to harm their host to replicate and be transmitted,' said Jacobus de Roode of Emory University in the US, the lead author of the paper and a recipient of a Marie Curie Outgoing International Fellowship from the EU. 'But what this study found is that if they harm their host too much, they'll suffer too. On the other hand, this study also shows that it does not pay to be maximally benign, because those parasites don't replicate enough to be effectively transmitted.' The scientists also wanted to find out what was happening in wild populations. The eastern monarch population famously migrates between its wintering sites in Mexico and its breeding grounds in Canada. The round trip is some 5,000 km long. In contrast, the western population travels just a third of that distance along the Californian coast. 'We thought that if the parasites are going to be more benign in any population, it's going to be in the eastern monarchs, because those butterflies fly the furthest distances, and parasites that kill their hosts during this long journey won't produce any offspring,' said Sonia Altizer of the University of Georgia, US. The researchers exposed butterflies from both populations to strains of parasite taken from their own population and the other population. Parasites taken from the western population caused the butterflies to die sooner, confirming the scientists' prediction that the eastern parasites would be less harmful to their hosts. The researchers underline the fact that their findings have implications for human health, as many researchers are interested in how the trade-off hypothesis could influence the effectiveness of attempts to block the transmission of human pathogens.
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