How to survive without sex: Rotifer genome reveals its strategies

Jul 22, 2013
How to survive without sex: Rotifer genome reveals its strategies
This shows the bdelloid rotifer Adineta vaga, birefringence image, polarized light microscope. Credit: Credit: M. Shribak and I. Arkhipova, MBL

How a group of animals can abandon sex, yet produce more than 460 species over evolutionary time, became a little less mysterious this week with the publication of the complete genome of a bdelloid rotifer (Adineta vaga) in the journal Nature.

Rather than the standard way of using to weed out harmful mutations to its DNA, this tiny aquatic animal appears to have adopted other strategies to maintain over millennia that aren't burdened by or killed off altogether, says David Mark Welch of the Marine Biological Laboratory (MBL) in Woods Hole.

Mark Welch and his MBL colleague, Irina Arkhipova, are the U.S. leads on the international project to sequence the rotifer genome and analyze what it reveals.

Neither males nor meiosis (cell division to produce sperm or eggs) have ever been observed in a bdelloid rotifer. Instead, the unfertilized eggs just divide to produce offspring. This , which for most animals would be an evolutionary dead end, is borne out by the rotifer's genome, the structure of which "is completely consistent with what you would expect to see with a long-term absence of meiosis," Mark Welch says.

"It's hard to prove a negative, and we can never say there is no chance the rotifer is ever having sex. But it would have to be some kind of crazy meiosis," Mark Welch says.

In most , alternative forms of the same gene (alleles) are found in the same spot on two different —one from the mother, one from the father—which pair during meiosis, and segregate into new sperm and . In the bdelloid genome, gene copies either don't match up positionally along or are located on the same chromosome. This means the would not be able to pair up normally during meiosis and segregate evenly into new sperm and egg cells.

If bdelloids are not having sex, how can they avoid the accumulation of deleterious mutations or generate new diversity? The bdelloid genome shows evidence for other ways of maintaining healthy genes and viable lineages. One is gene conversion, in which one allele replaces another through DNA repair mechanisms or other strategies. The other is horizontal gene transfer (HGT), the transfer of DNA from one organism to another, which is common among microbes yet rarely seen in animals. At least 8 percent of the rotifer's genes, more than in any other animal, are likely to have been acquired by HGT.

"In general, animal germ lines are fairly well protected from acquiring DNA from external sources," Arkhipova says. Bdelloids are unusual, though, in that they can completely dry up (desiccate) for weeks or months at a time, and then spring back to life when water becomes available. During their desiccation phases, their DNA breaks up into many pieces. "When they rehydrate, this might be an opportunity for foreign DNA fragments from ingested bacteria, fungi, or microalgae to transfer into the rotifer genome," Arkhipova says.

More significantly, this may also be a chance for the rotifer to incorporate genes from other rotifers. This would be quite useful if it needed to pick up genes to repair damaged ones through gene conversion, the authors suggest. "In this way, the processes of mutation and DNA repair mimic some aspects of sex," Mark Welch says.

Another striking finding in the bdelloid rotifer genome was the extremely low number of transposons, "pieces of DNA sometimes called 'genetic parasites' that are capable of moving around the genome and causing harmful mutations," Arkhipova says. While about 50 percent of the mammalian genome is transposons, they constitute only about 3 percent of the bdelloid genome, "and their proliferative capacity appears to be severely limited," Arkhipova says. This affords the rotifer a layer of protection from mutations that most animals don't enjoy. "We are interested in dissecting this multi-layered defense system in our future studies," she says.

Explore further: Mycobacteria get all the advantages of sex with none of the downsides

More information: Nature DOI: 10.1038/nature12326

Related Stories

Mining the botulinum genome

May 14, 2013

(Norwich BioScience Institutes) Scientists at the Institute of Food Research have been mining the genome of C. botulinum to uncover new information about the toxin genes that produce the potent toxin behind ...

Clues to chromosome crossovers

Feb 13, 2013

Neil Hunter's laboratory in the UC Davis College of Biological Sciences has placed another piece in the puzzle of how sexual reproduction shuffles genes while making sure sperm and eggs get the right number ...

Rotifers avoid sex for millions of years by blowing away

Jan 28, 2010

(PhysOrg.com) -- They haven't had sex in some 30 million years, but some very small invertebrates named bdelloid rotifers are still shocking biologists - they should have gone extinct long ago. Cornell researchers ...

Recommended for you

New alfalfa variety resists ravenous local pest

23 hours ago

(Phys.org) —Cornell plant breeders have released a new alfalfa variety with some resistance against the alfalfa snout beetle, which has ravaged alfalfa fields in nine northern New York counties and across ...

New patenting guidelines are needed for biotechnology

Apr 22, 2014

Biotechnology scientists must be aware of the broad patent landscape and push for new patent and licensing guidelines, according to a new paper from Rice University's Baker Institute for Public Policy.

Rainbow trout genome sequenced

Apr 22, 2014

Using fish bred at Washington State University, an international team of researchers has mapped the genetic profile of the rainbow trout, a versatile salmonid whose relatively recent genetic history opens ...

User comments : 0

More news stories

When things get glassy, molecules go fractal

Colorful church windows, beads on a necklace and many of our favorite plastics share something in common—they all belong to a state of matter known as glasses. School children learn the difference between ...

FCC to propose pay-for-priority Internet standards

The Federal Communications Commission is set to propose new open Internet rules that would allow content companies to pay for faster delivery over the so-called "last mile" connection to people's homes.

SK Hynix posts Q1 surge in net profit

South Korea's SK Hynix Inc said Thursday its first-quarter net profit surged nearly 350 percent from the previous year on a spike in sales of PC memory chips.