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 biologists.
The bdelloid genome shows an odd pattern of differences between versions of the same genes, report David Mark Welch and Matthew Meselson. 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, Meselson notes. Hardly any of these lineages seem old, and fossil evidence has suggested that asexuality 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 of 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 Laurence D. Hurst at the University of Bath in England. For example, a scale insect 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 focused 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 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, 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 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 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.”