When our lives are in danger, some humans go on the run, seeking refuge in other countries far away from the threats of home. Animals too migrate to escape danger but one group – the pond-living bdelloid rotifers – have taken this game of hide-and-seek to an extreme.
If they are threatened by parasitic fungi, they completely remove any trace of water in their bodies, drying themselves out to a degree that their parasites can’t stand. In this desiccated state, they ride the wind to safety, seeking fresh pastures where they can establish new populations free of any parasites.
This incredible strategy may be partially responsible for another equally remarkable one – the complete abandonment of sex. For over 80 million years, the bdelloids (pronounced with a silent ‘b’) have lived an asexual existence. Daughters are identical clones of their mothers
, budded off from her body. No males have ever been discovered. For this reason, Olivia Judson once described bdelloid rotifers as an “evolutionary scandal”. Their sexless lifestyles simply shouldn’t work in the long run.
Ditching sex allows an animal to efficiently pass all of its genes to the next generation without having to seek out a mate. This should give asexual animals a big advantage but not so. Sex provides fuel for evolution. Every time two individuals meet in flagrante, their chromosomes are joined, shuffled and re-dealt to the next generation. In this way, sex begets diversity, remixing genes into exciting new combinations.
This diversity is a vital weapon in the never-ending war against parasites. Parasites, with their large populations and short generations, are quick to evolve new ways of exploiting their hosts. They could have their run of a genetically uniform population and soon bring it to its knees. A sexually active species is a harder target. With genes that shuffle every generation, new anti-parasite adaptations are always just one bout of mating away. And so it goes, again and again, with hosts constantly having to outrun their parasites and sex acting as the getaway vehicle.
So asexual reproduction, for all its immediate gains, should be a poor long-term strategy compared to the dynamic nature of sex. Bdelloids have clearly addressed this problem and thanks to the last few years of research, we know how. They have evolved ways of achieving every single one of the many benefits of sex, without actually doing the deed. Escape parasites? They’ve got that covered. Shuffle their genes? They do that too. Generate genetic diversity? Check.
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
Male insects have a tough time of it. Aside from the usual threats of predators, competitors and the odd hungry female, many are plagued by discriminatory parasites intent on killing them, while leaving their female peers unharmed. These “male-killers” are incredibly successful and infect a wide range of insects, who are themselves a very successful group. One of these killers, a bacteria known as Wolbachia, may well be the world’s most successful parasite.
The male-killers are paragons of selfishness. Their success hinges on successfully infecting females, for whithin egg cells, they find easy passage into the next generation. But they can’t infect sperm and as a result, males are useless to them; a reproductive cul-de-sac that they can afford to do away with. At worst, eliminating males from a host population does the parasites no harm and at best, it provides the surviving female siblings with fewer competitors and even a potential source of food.