Most writers wouldn’t be pleased to see their name in a national newspaper next to the headline “I haven’t had sex for 40 million years. Should I worry?” But what are science writers, if not a little strange…
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 article is reposted from the old WordPress incarnation of Not Exactly Rocket Science. The blog is on holiday until the start of October, when I’ll return with fresh material.
Sex is, on the whole, a good thing. I know it, you know it, and natural selection knows it. But try telling it to bdelloid rotifers. These small invertebrates have survived without sex for some 80 million years.
While many animals, from aphids to Komodo dragons, can reproduce asexually from time to time, it’s incredibly rare to find a group that have abandoned sex altogether. The bdelloid rotifers (pronounced with a silent b) are an exception.
They live in an all-female world and since their discovery, not a single male has ever been found. Genetic studies have confirmed that they are permanently asexual, and females reproduce by spawning clone daughters that are genetically identical to them.
The bdelloids pose a problem for evolutionary biologists, who have struggled to explain how they could make do without a strategy that serves the rest of the animal kingdom very well. Now, Natalia Pouchkina-Stantcheva, Alan Tunnacliffe and colleagues from the University of Cambridge have found out how they do it.
Sexual animals have two copies of each gene that have only minimal differences between them. But the asexual bdelloid lifestyle has uncoupled the fates of each copy in a gene pair, allowing them to evolve in new directions. They get two genes for the price of one.
One of the vaunted benefits of sex is that it acts as a crucible for genetic diversity. Animals receive one pair of every gene from their mother and one from their father. As the pairs are united in the embryo, they are often shuffled into new combinations.
You inherited your genes from your parents, half from your father and half from your mother. Almost all other animals contend with the same hand-me-down processes, but not the bdelloid rotifers. This intriguing group of small freshwater creatures are not content with their genetic hand-me-downs; they import genes too. A new study shows that their genomes are rife with legions of foreign DNA, transferred from bacteria, fungi and even plants.
The swapping of genetic material is all part of a day’s activity for bacteria but it’s incredibly rare in animals. But bdelloids are bringing in external genes to an extent that’s completely unheard of in complex organisms. Each rotifer is a genetic mosaic, whose DNA spans almost all the major kingdoms of life.
Bdelloid rotifers are no strangers to attention-grabbing science. They are one of the only groups of animals to have completely abandoned sex. No male bdelloid has ever been found and for 80 million years, the females have been reproducing solely through cloning themselves. They are great survivors; at any point during their lives, they can be completely dried out and return from dormancy after being rehydrated. And as I’ve explained before, they are most radiation-resistant animals on the planet, and can tolerate doses a hundred times more than what would kill a human.
Bdelloid rotifers are one of the strangest of all animals. Uniquely, these small, freshwater invertebrates reproduce entirely asexually and have avoided sex for some 80 million years. At any point of their life cycle, they can be completely dried out and live happily in a dormant state before being rehydrated again.
This last ability has allowed them to colonise a number of treacherous habitats such as freshwater pools and the surfaces of mosses and lichens, where water is plentiful but can easily evaporate away. The bdelloids (pronounced with a silent ‘b’) have evolved a suite of adaptations for surviving dry spells and some of these have had an unexpected side effect – they’ve made the bdelloids the most radiation-resistant animals on the planet.
Ionising (high-energy) radiation is bad news for living cells. Far from granting superpowers, it damages DNA, often completely breaking both strands of the all-important molecule. If you think of DNA as a recipe book for the various parts of a living thing, the double-stranded DNA breaks that are caused by ionising radiation are like tearing the book up into small chunks.
Absorbed doses of radiation are measured in Grays and ten of these are more than enough to kill a human. In comparison, bdelloids are a hundred times harder. Eugene Gladyshev and Matthew Meselson from Harvard University found that two species shrugged off as much as 1,000 Grays and were still active two weeks after exposure.