July 25, 2011 at 2:02 PM ET
For the scientists who study sex and its mysterious origins, animal species that skip sex when they spawn were at first puzzling, and then exciting. Now it turns out that many of these supposedly sexless species can swing both ways.
The latest peccadillo involves a type of ant that scientists thought had survived sans sexual reproduction for millions of years — until they discovered that the seemingly abstemious arthropods were covertly copulating.
From an evolutionary point of view, sex is a costly business. Nevertheless, most species mate to multiply. For researchers grappling with the question of why sex exists, asexual species provide a clue, one small nudge in the direction of an answer. "We study how normal things work by studying mutant version of those things," John Logsdon, a biologist at the University of Iowa, told me. "In this case, how they basically get around the rule, because the rule seems to be sex."
That's what makes the ants interesting. When scientists started scooping up Amazonian fungus-growing ants in Mexico, Argentina and other parts of South America, they believed that the all-female colonies of ant clones stayed strictly sex-free. But in a fresh set of samplings in new locations, the same ant species was found propagating sexually with the usual mingling of genomes of both genders.
What seems to have happened, the researchers who found the ants surmise, is that isolated ant colonies lost the ability to reproduce sexually, due to a genetic switch that was turned off over time. Once this change occurred in a colony, there was no going back, Christian Rabeling and his colleagues propose in a paper published last week in the Proceedings of the National Academy of Sciences.
The ants are the latest addition to a small list of species that have a sometimes sexy, sometimes sexless life. These invertebrates' genomes are a rich source of insights for scientists who are still puzzling over one of nature's most basic mysteries: why sex exists.
Worms do it, snails do it ... or don't
In July, scientists studying the sex lives of the nematode C. elegans offered up one reason as to why sex exists. The worm propagates in two ways: Sometimes it mates with another worm, and sometimes it fertilizes itself. Scientists compared the offspring of two worms with the self-fertilized spawn of a single parent, and found that genetically diverse offspring were less likely to be infected by parasites than offspring from a single parent. This aligns well with a big idea called the Red Queen hypothesis, which claims that sex, as a behavior that allowed a mix-and-match of genomes, stuck around to help species win out over co-evolving parasites, Indiana University's Curtis Lively and his colleagues write in the July 8 issue of Science.
Dodging parasites is probably just one chapter of the story. "My suspicion is that we're not going to come up with a universal solution to sex," Maurine Neiman, a biologist at the University of Iowa, told me. She expects the answer is going to be messy and complicated ... just like sex itself.
Neiman treks down to lakes in New Zealand every other summer to harvest a species of freshwater snail, which, like C. elegans, is sometimes asexual and sometimes sexual. For as yet unknown reasons, the snail's fate — to start a sexual or asexual lineage — is decided before the snails are born. Yet some lakes have both kinds of snails living in them. "They set the stage very nicely for comparing sexual and asexual genomes," Neiman told me. "You can compare populations that have lots and lots of sex with those that don't have any at all."
Like the nematodes, the snails have a natural parasite, and Neiman is looking into how the snails' sexual behavior relates to their ability (or lack of ability) to survive being infected. There's another odd secret that is hidden in the snail genome: Those that are built to stay single sometimes have many, many copies of their DNA packed into the same space where most species just have two copies.
Some species stay sexless
A few species appear to have stuck with sexlessness, what scientists call "ancient asexuality." Tanya Schwander, a geneticist at the University of Groningen, recently showed that two species of stick insects have stayed asexual for more than 1 million generations.
Schwander and her colleagues wrote about the stick insects in the June 12 issue of Current Biology, and they're continuing to investigate how they managed to do this without going extinct.
While 1 million generations may seem like a long time, the stars of sexlessness are still the bdelloid rotifers — single-celled singletons who appear to have kept sex-free for 40 million years. Their unusual genomes also come riddled with questions, but researchers suspect they're getting closer to the answers.
Whether by coincidence or causation, other extreme survival skills are coded into the rotifer genome — the superbugs can survive being blasted with radiation, and even bounce back to life after being dried out. Scientists such as the University of Iowa's Logsdon reason that the rotifers' exceptional talent for fixing errors in their DNA caused by radiation could explain how they fix unwanted changes that crop up in genomes that don't mix it up every so often.
"It's not a smoking gun, but we smell a connection," Logsdon says. "What the connection is, is still an open question."
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