A dried up celibate kleptomaniac

If there’s one thing that biologists agree on, it’s that sex is good. Really good*.

Huge amounts of time and energy are invested in it. Well, alas, not so much in the act per se, as much as in finding a way to have sex; to tempt a mate or dispatch a rival. But is it really worth the ego busting knock-backs, the frustration, the time and energy?

One quirky animal certainly doesn’t think so; it’s had a dry patch on a geological timescale. For the last 80 million years or so a small pond and fresh water dwelling animal called a bdelloid rotifer has done away with sex, and males, entirely. How has it managed for so long without sex?

Simple. It steals.

Sex is not something a species can easily dismiss; it has huge long term benefits. Sex shuffles and rearranges genes, creating diversity. This diversity, when environmental conditions change, gives an evolutionary advantage. Some individuals will be better suited to the change than others, they’re more likely to pass on their genes, and the species survives. Over a long enough time frame, a more adapted species evolves.  Ordinarily a lack of diversity is an evolutionary dead-end where bad mutations irreversibly build up, each one taking a species closer to extinction.

An electron micrograph of bdelliod rotifer.

Yet, despite its ancient vow of celibacy, almost 300 different species of bdelloid rotifer have somehow evolved around the world; an “evolutionary scandal” as John Maynard Smith put it. It’s an asexual animal, living in a world of clones, of mothers and their genetically identical daughters, its offspring developing from unfertilised eggs in a process called ‘parthenogenesis’. So, just how the bdelloid rotifer has lasted so long, and even evolved into different species, is something of a mystery.

In an effort to get a handle on this achievement their DNA has been poked, probed, sequenced and examined. The results were…surprising.

What are fungal and bacterial genes doing in an animal? For that matter, what the heck are algae and plant genes doing there?

Somehow bdelloid rotifers have been snaffeling up the genes of other species. Something called horizontal gene transfer (HGT). Normally genes pass vertically, from parent to child, but sometimes—and it may be more common that we once imagined—something happens that allows genes to jump sideways, from one unrelated species to another.

When the human genome was completed, it was revealed that something like 6-8% of our DNA came from viruses. And it works both ways. Human herpes virus 8, at some point in its history, entirely by chance, copied a number of human genes into it’s own genome. Given a long enough time frame, it’s possible to imagine how HGT could happen in animals, with a gene borrowed from one species accidentally dropped off in another, but they seem to be rare events. The exception rather than the rule.

But as with sex, bdelloid rotifers seem to be the exception, HGT seems to be their driver of genetic innovation, opening up that evolutionary dead-end. They have a huge number of genes that we wouldn’t have expected to see in an animal, genes far more closely related to other species.

Until recently—while we assumed at least some of these stolen genes had a function—it wasn’t clear what these stolen genes might do. Not all DNA codes for proteins, some has other functions, but now a new study from a group at the University of Cambridge has taken a look at only the genes that might result in actual proteins, the ‘transcriptome’, from a species of rotifer called Adineta ricciae.

The protein making machinery of cells can’t ‘read’ DNA, so it is first transcribed into a format that it can called mRNA. The Cambridge team extracted all of the mRNA, and then reverse transcribed it back into DNA. By doing this the team could sequence and compare it to libraries of genes from other species.

The results showed that roughly 10% of the transcriptome is of foreign origin, of this about 80% code for enzymes. The foreign DNA would give rotifers unique biochemical functions, such as in metabolism or toxin breakdown, not seen in other animals and could help explain another curious adaptation of these weird little animals: reanimation.

Bdelloid rotifers can be dessicated, completely dried out, with no sign of life, yet when conditions are more favourable again—i.e. wet—they come back to life and carry on as normal, something called anhydrobiosis. It’s an adaptation that helps them survive the hard times, like when the small pond or puddle they currently call home dries up. Some bdelloid rotifers can stay in this state for up to 9 years and still come back to life.

It could even be the ability to dry out and reanimate that lets them steal so many genes. Being dried out is tough on a cell, membranes, and DNA in particular, become damaged. It seems the bdelloid rotifer has a pretty good repair kit, able to reconstruct and repairing damaged DNA. This may be the scene of the crime; stray bits of DNA from the environment, be it plant, fungal, bacterial or whatever find their way into the cell and get added to its genome. If this happens in an egg cell, then it can be passed onto the next generation.

It’s just a hypothesis, yet to be confirmed, but it seems a plausible way for these weird sisters to diversify, and drive straight through evolutions dead end.

Reference: Boschetti, C. et al., 2012. Biochemical Diversification through Foreign Gene Expression in Bdelloid Rotifers J. Zhang, ed. PLoS Genetics, 8(11), p.e1003035. Available at: http://dx.plos.org/10.1371/journal.pgen.1003035 [Accessed November 16, 2012].

Image credit

Top image used with permission by Wim Van Egmond. For more stunning rotifer images and information go here – http://www.microscopy-uk.org.uk/micropolitan/index.html


* feel free to interpret that how you will…