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Monday, 22 May, 2000, 20:20 GMT 21:20 UK
The breed without males
By Helen Sewell of BBC Science

Biologists have discovered that an all-female group of animals has been breeding successfully without males for 40 million years.

Normally animals need a mixture of genetic information from parents of both sexes. This prevents dangerous mutations from killing off whole populations.

The discovery that tiny water animals - called bdelloid rotifers - have overcome this risk could have implications for understanding the mutations which can cause genetic conditions in humans, such as Alzheimer's Disease.

The discovery was made by researchers from Harvard University in the United States.

Minuscule males

The scientists explain that not only have the animals been surviving, they have been very successful. They have diversified into more than 300 species, each one adapted to its own particular environment.

"These are all females," researcher Matthew Meselson said. "They lay eggs, out hatch more females, they lay eggs and out hatch even more females. Long, long ago there must have been one original bdelloid rotifer. That radiated to give 350 different species. All are female. No males are known."

In the past, scientists have suspected other species of being single sexed. But closer checks have revealed that there was some form of sexual reproduction going on - it just was not obvious.

For example, a species of scale-insects was thought to be only female, until minuscule males were found clinging to their legs. However, the DNA of the bdelloids shows a very high probability that they are all female and have been reproducing without sexual partners.

Benefits of sex

The next step for the Harvard team, which report their findings in the journal Science, is to study the genes to find out how the bdelloids - which range in size from 0.1 mm to 1 mm - have survived for this long without being killed off by dangerous mutations.

Benjamin Normark at Harvard University in Cambridge, Massachusetts, and Olivia Judson of Imperial College at Silwood Park in Ascot, UK, said it was possible the bdelloids had found a way to achieve the benefits of sex without the messy realities of having two genders.

"Perhaps the bdelloids have invented a kind of local recombination," they wrote in a commentary in Science.

"When we understand the full range of consequences of the loss of sex - the rapid extinction of the many and the enduring survival of the few - we will finally understand why most of us can't do without it."

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