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.
At this dose, their egg-laying capacity took a large hit and fell to 10% of previous levels, but even so, they weren't sterilised completely. Their daughters (who are all identical clones of their parents) also showed a similar lack of ill effects due to the radiation. These figures make the bdelloids the most radiation-resistant of all animals so far tested. Even other rotifer groups show similar levels of sterilisation at a fifth of the doses tolerated by bdelloids.
Their closest animal rivals are the tardigrades or 'water bears', impossibly cute aquatic animals that are quite possibly the hardest animals alive. Like the bdelloids, they can also enter a dormant, dried-out state where they can withstand extreme heat, temperatures close to absolute zero, poisonous gases and extreme radiation. As I've blogged about before, they may soon be revealed as the first animals to have survived the open vacuum of space. But even these hard-cases have been sterilised by the 500-1,000 Gray doses tolerated by the bdelloids.
Genomes in pieces
This resistance is all the more amazing because radiation affects the DNA of rotifers in the same way as other animals - it shreds it. Gladyshev and Meselson measured the size of the remaining pieces in one species, Adineta vaga, immediately after being exposed to radiation. They found that a 560 Gray burst broke the animal's genome in over 500 different places, and the 1,000 Gray doses that they contended with so well created over 1,000 double-stranded breaks.
The fact that the bdelloids survive and their offspring are fertile is a clear sign that they have an extraordinary ability to repair these breaks, or to protect the proteins that do so. But most places on earth, including the habitats frequented by rotifers, have very low levels of background radiation and without intense sources, there is no impetus for an animal to evolve extreme resistance. How then could it have evolved?
Other species provide a clue. Only bacteria can give the bdelloids a run for their money in the resistance stakes and one in particular, Deinococcus radiodurans, has a name that literally means "terrifying berry that withstands radiation". Like the bdelloids, it can reassemble its genome after it has been torn asunder into tiny fragments. In general, bacteria that are resistant to radiation also tend to be resistant to prolonged bouts of dehydration, a connection that the tardigrades also share.
It turns out that both drought and radiation pose similar challenges including the production of damaging reactive oxygen molecules and frequent DNA breaks. So Gladyshev and Meselson believe that the ability to shrug off killer doses of radiation is a happy side-effect of adaptations to dry-living.
Coping without sex
The resistance to drought may have given the bdelloids a competitive edge over parasites, predators and other rotifers that aren't so hardy. It may also have ensured their success when they first started to adopt an asexual way of life, by mitigating some of the more harmful side effects of this strategy.
Without the genetic shuffling that accompanies sex, asexual reproduction is often viewed as a poor long-term strategy that leaves a species unable to adapt quickly to new challenges. But some groups have argued that the process of shattering and reconstructing their genome may provide the bdelloids with genetic benefits that compensate for this drawback.
The bdelloids repair their broken DNA by using a duplicate piece as a template for copying the lost information at the site of the break. If this template strand contains a gene with a new beneficial mutation, the animal would suddenly have two copies, and a positive change that might otherwise have been genetically overlooked could more easily spread within a population.
The genomes of asexual animals are prone to invasion by genetic parasites called transposons, selfish bits of DNA that can jump around a genome. Our genomes are rife with these parasites and their spread is predicted to go unchecked in asexual lineages to the point where they do so much damage that the species goes extinct. Frequently repairing their DNA may give bdelloids a route for cutting out unwanted genetic parasites and indeed, these animals have remarkable streamlined and transposon-free genomes.
Images by Diego Fontaneto and David Mark Welch.
Reference: Gladyshev, E., Meselson, M. (2008). Extreme resistance of bdelloid rotifers to ionizing radiation. Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.0800966105