Tuesday, June 10, 2008

Bdelloid rotifers

The bdelloid rotifers are certainly in the news a lot recently.

They may not be into sex very much, but it turns out they are world-class gene snarfers.

No sex, but plenty of gene transfer
Where do you get your genes? If you are an animal, you inherit them from your parents at the moment of conception, and that's about it. No later incorporation of environmental DNA for you, unless you become host to a parasite or an endosymbiont that somehow transfers bits of its genome into yours (which is a rarely documented event). Unless you are a bdelloid rotifer, that is.

This odd, microscopic, freshwater animal is making news once again, this time for the startling discovery of numerous chunks of foreign DNA in its genome. In a paper published this week in Science, evidence for massive horizontal gene transfer—from bacteria, fungi, even from plants—into the bdelloid rotifer genome is presented by Irina Arkhipova and Matthew Meselson. ...

While horizontal gene transfer is common in bacterial species, it was unheard of in the animal kingdom on such a massive scale – until this study.

"It is quite amazing that bdelloids are able to recruit foreign genes, which were acquired from remarkably diverse sources, to function in the new host," says Arkhipova. "Bdelloids may have the capacity for tapping into the entire environmental gene pool, which may be of (evolutionarily) adaptive significance during expansion into new ecological niches, and may even contribute to bdelloid speciation," she says.

So the first question one has is, how have all these genes from totally different species gotten into the bdelloid genome? The most likely answer is: as a direct consequence of how bdelloids are able to survive such traumatic events as complete dessication or levels of radiation deadly to almost all other species.

Apparently bdelloids are able to do this because of a nearly unique ability to reconstitute and repair heavily damaged DNA. Evidently, in the process, they sometimes incorporate foreign DNA from other creatures they have eaten, or which may simply be lying around. And since some of this reconstituted bdelloid DNA is included in bdelloid egg cells, which eventually become separate individual animals, it is passed on to subsequent generations.

(Something similar to this process, but on a much smaller scale, occurs in "higher" animals too, such as humans. Our genome is full of remnants of the DNA ("exogenous DNA") of viruses that infected our ancestors unknown millions of years ago.)
How bdelloids have been able to gobble up such a variety of genes from their environment and incorporate it into their genome is a good question. Typically in animals, the germ line–the heritable egg and sperm cells–are protected from environmental assaults, such as intrusion of foreign DNA, by the rest of the body cells, which are not heritable and serve to "sequester" the germ line. Ideas on why the bdelloids' germ line is so exposed to environmental exchange, Arkhipova says, "are all speculative. But we talk about this a lot!"

One clue is the unusual ability of bdelloids to survive total desiccation (drying out), which is fatal for most organisms. When water disappears from their environment, bdelloids enter a kind of suspended, dehydrated state, and can stay there for months or even years. But once water returns, they spring back to action, move around, eat things, and start reproducing again.

During the desiccation phase, Arkhipova says, "you would imagine there is potential for membrane damage and DNA damage in the rotifer. And not only the rotifer desiccates, but also everything it just consumed." If the DNA of both the rotifer and its food are broken up during desiccation, "this would provide an opportunity for the (foreign) DNA to enter the rotifer's germ line. During rehydration, the DNA breakage is somehow repaired, and the foreign DNA may get incorporated," she says.

From the details mentioned so far, there may be some other things you could be wondering about.

For instance, do any of these foreign genes actually work like normal genes once they've become part of the bdelloid genome? Generally, they do not. But there is evidence that a few foreign genes are sufficiently intact that they are transcribed into messenger RNA. And some of this RNA may even be used to make proteins, such as enzymes to catalyze basic metabolic processes.

Interestingly enough, some bacterial genes that have entered the bdelloid genome now have embedded introns, which is normal for eukaryotic genes, but unheard of in prokaryotic (bacterial) genes. (The introns are like punctuation marks that separate distinct segments of a gene, called exons. This makes it possible to construct a number of different proteins from a single gene, by the process of "alternative splicing".)

Another more general question that might occur to you is: why do asexual animals such as bdelloids have egg cells, like more normal sexual creatures? And how, exactly, do they reproduce, anyhow? The answer is probably that at some time back in the evolutionary history of bdelloids (estimated at 50 to 100 million years ago), bdelloids diverged from a rotifer ancestor species that was sexual, as are many species of rotifers today.

So bdelloids still have egg cells, with paired chromosomes that carry two separate copies of the genome, just as "normal" sexual animals do. However, bdelloid eggs do not undergo the process of meiosis, in which egg cells divide, and the paired chromosomes are separated, to be later combined with chromosomes from another individual during fertilization.

Instead, bdelloid eggs are eventually expelled from the mother's body, and go on to develop into a new individual by parthenogenesis, which also occurs occasionally in a few other species capable of asexual reproduction, such as some insects, fish, snakes, and lizards. (Or even sharks and komodo dragons.)

In case you enjoy thinking about sex, you probably have many other questions about what's up with bdelloids. Answers to a lot of these questions can be found in the following.

Further reading:

Common Aquatic Animal's Genome Can Capture Foreign DNA – another 5/29/08 press release about the research

Water creatures caught stealing DNA – 5/30/08 ABC/Reuters news story on the research

Who needs sex when you can steal DNA? – 5/29/08 Reuters news story

Scientists find promiscuous genes in an asexual animal – informative 5/29/08 article from Science News

Massive Horizontal Gene Transfer in Bdelloid Rotifers – 5/30/08 research article in Science (sub. rqd.)

The Weird Sisters – very informative 6/3/08 New York Times blog article, by Olivia Judson

An Evolutionary Scandal – 11/2000 Harvard Magazine article on bdelloid rotifers

Who Needs Sex (or Males) Anyway? – 3/20/07 PLOS Biology article on bdelloids

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