Sunday, October 14, 2007

1 organism : No sex for 80 mn yrs

The mystery of how an animal has survived for 80 million years without sex has been solved by UK scientists. A Cambridge team says the creature owes its existence to a genetic quirk that offers some recompense for its prolonged celibacy. Many asexual organisms have died out because they cannot adapt to changes in the natural world.

But an evolutionary trick allows this pond-dweller to survive when conditions change, researchers report in Science . Some have billed it as the longest “headache” ever.

The animal is a tiny invertebrate known as a bdelloid rotifer. It lives in freshwater pools. If deprived of water, it survives in a desiccated state until water becomes available again. They then revive, having suffered no ill effect, once water becomes available again. And without even a scintilla of sex the egg laying microscopic creatures - widely distributed in mosses, creeks and ponds - have managed to diverge into nearly 400 species.

The secret to this novel survival mechanism lies in a twist of asexual reproduction, whereby the animal is able to make two separate proteins from two different copies of a key gene.

Dr Alan Tunnacliffe, from the Institute of Biotechnology at the University of Cambridge, who led the research, said his team had been able to show for the first time that gene copies in asexual animals can have different functions.

“It’s particularly exciting that we’ve found different, but complementary, functions in genes which help bdelloid rotifers survive desiccation,” he explained.

“Evolution of gene function in this way can’t happen in sexual organisms, which means there could be some benefit to millions of years without sex after all.”

The researchers discovered that two copies of a particular gene, known as LEA, in the asexual pond-dweller are different - giving rise to proteins with separate functions that protect the animal during dehydration. One copy stops other essential protein molecules from clumping together as the animal dries out, while the second copy helps to maintain the fragile membranes that surround the creature’s cells.

Humans and most other types of organism reproduce sexually. The union of sperm and eggs results in two copies (or a pair) of genetic instructions within a cell, one copy inherited from each parent.

This produces two nearly identical copies of each gene in each cell, and therefore two nearly identical proteins.

The “re-shuffling” of genetic material over many generations allows sexual animals to adapt to changes in their natural environment. In contrast, many asexual organisms have died out because their rigid genetic make-up means they are unable to adapt in this way. The latest discovery explains why the bdelloids have likely escaped this fate with their mechanism for generating genetic diversity in the absence of sexual reproduction.

This is the first time that this evolutionary trick - given the technical name of the Meselson effect - has been shown in any asexual animal, and shows that lack of sex can be positively life enhancing.

The study reported in Science magazine was conducted on a species of bdelloid rotifer known as Adineta ricciae.

Sex is crucial because it scrambles genes much more quickly than random DNA mutations and helps new species to evolve and emerge as the environment changes. Even bacteria have a primitive form of sex, where they exchange genes to add variety to their entire genetic complement, or genome.

But this pond life has been the one ugly fact undermining the beauty of sex: the translucent organisms abandoned sex such a long time ago that they were once denounced as an ‘evolutionary scandal’ by the late biologist John Maynard Smith.