Given the chance, parasites evolve to cause less harm

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How do you make a parasite less harmful? It might be as simple as forcing them to stick around.

Amanda Gibson from Indiana University performed experimental evolution with the nematode worm Caenorhabditis elegans and a parasitic bacteria (Serratia marcescens), comparing different scenarios for how the parasite was passed on. Under most circumstances, parasites continued to cause their hosts harm throughout the 20-generation experiment. Only when each strain of parasite was allowed to infect the same host strain, generation after generation, did the parasite evolve to become less harmful.

The authors explain that in these conditions, the host and parasite can ‘co-evolve’ – adapting to either other’s characteristics – because their long-term success is linked. Parasites do better when their host produces more offspring for them to infect, so it pays for the parasites to play nice. This phenomenon produces a pattern of local adaptation, where hosts can tolerate local strains of parasite, but infection with an unfamiliar strain can be extremely harmful. Gibson and colleagues note that co-evolution may be an important factor in understanding how diseases evolve.

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