In Lake Alexandrina, New Zealand, a population of snails is under threat from a parasitic flatworm, a fluke aptly known as Microphallus. The fluke chemically castrates its snail host and uses its body as a living incubator for its larvae. But the snails have a weapon against these body-snatching foes – sex.
The New Zealand mud snail Potamopyrgus antipodarum is found throughout island’s freshwater habitats. They breed either sexually or asexually through cloning, and the two strategies vary in prevalence throughout the lake. In the shallower waters round its margins, sex is the name of the game, but in the deeper waters towards the lake’s centre, snails are more likely to opt for cloning.
Kayla King from Indiana University has shown that it’s the concentration of the local parasites that drives this gradient of sex. The flukes spend their adult lives in ducks and they rely on the birds inadvertently scooping up their larvae while feeding. In Lake Alexandrina, ducks only feed in the shallow waters around the lake’s margins so these areas are hotspots for parasites, and for co-evolutionary wars between them and their snail hosts. Sex provides the snails with the genetic ammunition they need to stay in the game.
The snails and their parasites beautifully support and illustrate the principles of the Red Queen hypothesis, which suggests that one of the chief benefits of sex lies in providing the genetic innovation necessary to outfox parasites in evolutionary arms races.
This is the fifth of eight posts on evolutionary research to celebrate Darwin’s bicentennial.
Life can sometimes be a futile contest. Throughout the natural world, pairs of species are locked in an evolutionary arms race where both competitors must continuously evolve new adaptations just to avoid ceding ground. Any advantage is temporary as every adaptive move from a predator or parasite is quickly neutralised by a counter-move from its prey or host. Coerced onward by the indifferent force of natural selection, neither side can withdraw from the stalemate.
These patterns of evolution are known as Red Queen dynamics, after the character in Lewis Carroll’s Through the Looking Glass who said to Alice, “It takes all the running you can do, to keep in the same place.” These arms races are predicted by evolutionary theory, not least as an explanation for sex. By shuffling genes from a mother and father, sex acts as a crucible for genetic diversity, providing a species with the raw material for adapting to its parasites and keep up with the arms race.
We can see the results of Red Queen dynamics in the bodies, genes and behaviours of the species around us but actually watching them at work is another matter altogether. You’d need to study interacting species over several generations and most biologists have neither the patience nor lifespan to do so. But sometimes, players from generations past leave behind records of the moves they made. Ellen Decaestecker and colleagues from Leuven University found just such an archive in the mud of a Belgian lake.
The lake is home to a small crustacean called a water flea (Daphnia magra) and a parasitic bacteria Pasteuria ramosa that lives inside it. Both species can undergo dormant states, and Decaestecker found that the lake’s sediment preserves members of this sleeping fauna from up to 39 years ago. Every layer of sediment acts as a time capsule, preserving members from previous generations