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Bdelloids: No sex for over 40 million years.



Talk about a dry spell. Microscopic bdelloid rotifers have seemingly evolved without sex for millions of years and probably don't exist in male form, say Harvard University Harvard University, mainly at Cambridge, Mass., including Harvard College, the oldest American college. Harvard College


Harvard College, originally for men, was founded in 1636 with a grant from the General Court of the Massachusetts Bay Colony.
 biologists.

The bdelloid genome shows an odd pattern of differences between versions of the same genes, report David Mark David Mark is the President of the Senate of Nigeria. He is a member of the People's Democratic Party (PDP)

    [
 Welch and Matthew Meselson Matthew Stanley Meselson (b. May 24, 1930) is an American geneticist and molecular biologist whose research was important in showing how DNA replicates, recombines and is repaired in cells. . This pattern most likely arose during eons without sex, they argue in the May 19 SCIENCE.

If further tests prove them right, they will have confirmed the first example of ancient asexuals, organisms much sought after in biology.

Of the planet's 2 million named species, only about 2,000 appear totally asexual asexual /asex·u·al/ (a-sek´shoo-al) having no sex; not sexual; not pertaining to sex.

a·sex·u·al
adj.
1. Having no evident sex or sex organs; sexless.

2.
, Meselson notes. Hardly any of these line-ages seem old, and fossil evidence has suggested that asexuality a·sex·u·al  
adj.
1. Having no evident sex or sex organs; sexless.

2. Relating to, produced by, or involving reproduction that occurs without the union of male and female gametes, as in binary fission or budding.

3.
 is a dead end. "It's not because asexual species don't appear," Meselson says. "It's because they don't last."

"Although evolutionary biologists agree that sex is essential, they cannot agree on why," say Olivia P. Judson of Imperial College at Silwood Park in England and Benjamin B. Normark of Harvard in the same issue of SCIENCE. They note some 20 explanations that they find "ranging from the sublime to the ridiculous."

Meselson raises hopes that creatures that evolved for a long time without sex could reveal what's so special about it. "This is the beginning of a molecular attack on the problem," he says.

One of the strongest candidates for ancient asexuals, bdelloid rotifers date back at least 40 million years. That's the age of the oldest bdelloid recovered from amber. Despite bdelloids' asexuality, they've diversified into 360 species.

Extrapolating from sexual rotifers, bdelloid specialist Bill Birky at the University (If Arizona in Tucson speculates that males, if they existed, would be "small swimming hypodermic syringes full of sperm." He pictures them zooming up to females to inject sperm right through the body wall. Instead, females just seem to produce eggs that hatch into more bdelloids without fertilization. Among biologists, bdelloids have been called an evolutionary scandal.

Several past claims for asexual species collapsed, recalls sex evolutionist ev·o·lu·tion·ism  
n.
1. A theory of biological evolution, especially that formulated by Charles Darwin.

2. Advocacy of or belief in biological evolution.
 Laurence D. Hurst at the University of Bath in England. For example, a scale insect scale insect, common name for members of a highly modified group of insects belonging to several families of the superfamily Coccoidea. Scales possess antennae and are characterized by reduced legs. Only the males have wings; females are always wingless.  turned out to have males, albeit "wee things that stick to the females' legs," he says. "And how often do you see humans having sex? If you were a Martian looking around, you'd be pretty sure we were asexual."

For a molecular test, Meselson focussed on four bdelloid genes. Sexual organisms inherit a copy of a gene from each parent. If two-parent reproduction vanishes, the organism just keeps copying its own genes. Like a fax of a fax of a fax, the genome gets glitches. If they aren't harmful, they build up. One copy of a gene in an ancient asexual can develop very different mistakes from the same organism's other copy. Without sex to spread them around, copies of the same gene within an organism can look as different from each other as if they began diverging di·verge  
v. di·verged, di·verg·ing, di·verg·es

v.intr.
1. To go or extend in different directions from a common point; branch out.

2. To differ, as in opinion or manner.

3.
 when sex stopped.

That's what Welch and Meselson found when they checked genes in four bdelloid species. A few more scenarios, all complicated, could create this pattern and must be ruled out before the asexuality claim is bulletproof Refers to extremely stable hardware and/or software that cannot be brought down no matter what unusual conditions arise. See industrial strength.

bulletproof - Used of an algorithm or implementation considered extremely robust; lossage-resistant; capable of correctly
, Meselson says.

Still, Judson and Normark welcome the new report as "robust evidence," and Birky calls it "very solid." Hurst comments, "They haven't fully nailed it, but they've tilted the balance of probability very, very firmly." The remaining possibilities, he says, "mean that bdelloids are doing something seriously weird Seriously Weird was a show that played on YTV in Canada. In the UK, it was shown on ITV. Introduction
The show revolves around the misadventures of 3 teenagers in Draper High in upstate New York.
 with their genome."

This study alone won't topple, or prove, any of the major theories of the importance of sex, Meselson predicts. Most can be tweaked See tweak.  to allow long-term asexuality under certain extraordinary circumstances.

Such theories cluster into two groups, Meselson says. One asserts that sex speeds the process of dumping bad mutations, and the other focuses on spreading benefits. The Red Queen hypothesis, for instance, finds the main benefit of sex in shuffling the genome quickly, making it difficult for parasites to lock onto weaknesses.

Meselson ranks "what goes wrong without sex" as "one of the deepest questions in biology," bearing on who perishes and who prevails. "Extinction and splitting into other species--we're not exempt from that," he points out. "We're a species, too."
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Title Annotation:bdelloid rotifers
Author:Milius, S.
Publication:Science News
Article Type:Brief Article
Date:May 20, 2000
Words:699
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