First author

“It would be so nice if something made sense for a change,” said the title character in Alice in Wonderland. This sentiment held true for Ellen Decaestecker of the Catholic University of Leuven in Belgium. She and her colleagues dug mud cores from a pond in Heverlee, and isolated different generations of the water flea Daphnia magna and its microbial parasite, Pasteuria ramosa, from different depths. The team grew these in the lab and monitored how flea–parasite pairs from different generations interacted. They found that parasitic virulence increased over time, but that the water flea's evolution kept pace, so infection rates remained steady (see page 870). Decaestecker discusses this 'Red Queen' effect — named after another Lewis Carroll character who said, “It takes all the running you can do, to keep in the same place.”

What is the Red Queen effect?

The Red Queen hypothesis says that because the host and parasite are fighting each other all the time, no one wins. Red Queen dynamics are rare, and those that occur result from the maintenance of sexual reproduction. As the parasite adapts to the host, the host changes, so the parasite has to adapt all the time. If the host can create a lot of variation through sexual reproduction, it can fight off the parasites.

How did you find this particularly parasite-prone pond?

By coincidence. I started my PhD work on the parasites of Daphnia, and the pond is near the university. When infected, Daphnia change colour depending on the parasite. This pond is a parasite treasure trove, because it is quite shallow and nutrient-rich.

Was it hard to recreate the pond conditions in the lab?

The only tricky thing was getting enough spores from the parasites and growing them — we lost a lot of lines. We also had to choose among hundreds of possible flea-parasite combinations. We wanted to do every one, but we had to make choices of samples from several depths and strains representing the past, present and future.

What do your findings tell us about the arms race?

In terms of infectivity, there's a stalemate. But if you look at spore loads, then you do see a strong increase in parasite numbers. In terms of spore production, you could say the host loses the battle, because it doesn't seem to find a quick response. However, the effect of increased spores may not be strong enough to warrant counterbalancing by the host.