Thomas R. O'Donnell

Those wild and crazy snails are back, with lessons about sex

In University research on April 27, 2016 at 11:50 am
 In a study involving multiple generations of a freshwater snail in New Zealand, researchers at the University of Iowa found that polyploidy doesn't appear to be an asset—nor is it a drawback—for females bearing offspring without the help of a male. Instead, it's the snails' sexuality that creates the advantage: Asexual females, the study found, grew twice as fast during the late juvenile phase and reached reproductive maturity 30 percent faster than female snails that mated with males. Photo by Justin Torner.

In their cups: University of Iowa researchers grew multiple generations of tiny freshwater snails in the lab to study whether having multiple genomes provides advantages. Photo by Justin Torner from the U of I news website.

The snails are back. Or more precisely, researchers using snails as a model to understand the biological benefits of sexual reproduction are back with results.

I wrote about the research about two years ago, when conservative news outlets began ridiculing an $876,000 National Science Foundation grant to study “snail sex.” Two University of Iowa researchers, Maurine Neiman and John Logsdon, were among those receiving the grant.

Although multiple conservative outlets had reported and commented on the grant, none had asked the researchers to explain its significance. I was the first writer to contact them for any more than a cursory question. To me it was an example of a gap in science reporting in Iowa and conservative bias against government spending.

The bottom line: The tiny New Zealand snails are good models to study the evolutionary benefits of sexual reproduction, the true purpose of the study. The snails, Potamopyrgus antipodarum, have two genetic lines, one that reproduces sexually and another asexually, allowing the scientists to compare their genes for signs of advantages or disadvantages to sex.

Now results are coming out of this and related snail research, and the results are surprising. Sex and its biology, it turns out, aren’t as simple as scientists thought.

In January, Neiman, an associate professor of biology, published a paper (PDF) that examined the question of polyploidy: when an organism has more than two genomes – complete sets of genes. (Most, including humans, have just two.) The asexually producing female snails have three or four sets of genes. The sexually reproducing female snails have the standard two.

Neiman and her colleagues theorized that the extra genes would be detrimental to the asexual snails, since all that extra DNA and RNA takes energy to maintain. Instead, they found polyploidy made no difference. In fact, asexual females had an advantage: They grew faster and bore children sooner than the snails that mated with males, as a U of I release notes.

If asexual reproduction provides that kind of advantage, then why are there sexually reproducing snails at all? “This is making the role of sex even harder to explain,” Neiman said.

The release also tells how it took three years to complete the project, as Neiman and students Katelyn Larkin and Claire Tucci raised thousands of snails in individual cups. “We learned that these snails grow at a snail’s pace,” Larkin said, somewhat obviously.

Maurine Neiman, collecting freshwater snails at Lake Grasmere, on the South Island of New Zealand. Neiman and colleagues found sexual reproduction yields new gene combinations that help the snails fend off threats. Photo courtesy of Maurine Neiman, from the U of I news website.

Maurine Neiman, collecting freshwater snails at Lake Grasmere, on the South Island of New Zealand. She and her colleagues found sexual reproduction yields new gene combinations that help the snails fend off threats. Photo courtesy of Neiman, from the U of I news website.

The researchers offered one possible explanation for the existence of a sexually reproducing genetic line: A parasitic worm that infests the snails. The offspring of asexual females, with their identical genomes, are easier for the worms to target. The genetic diversity gained through sexual reproduction could afford some protection.

A second paper, published earlier this month, seems to point in that direction. Neiman and colleagues from Carleton College and Indiana University-Bloomington studied the concentration of sexual and asexual females in a New Zealand lake. There were lots of male snails in areas where the worm was prevalent, indicating there were also sexually reproducing females. (Boys, of course, usually go to where the girls are.)

The results support the idea that the genetic diversity sexual reproduction provides helps protect offspring from evolving external threats.

“Snails born with rare gene combinations would be harder to infect because the parasites have rarely, if ever, encountered those shuffled genetic combinations,” Neiman said in a U of I release.

While snails are the subject, the research has broader implications. “You could argue that our genome is the most important thing we have, yet we don’t know why humans have two copies when a lot of organisms do fine with one, or three, or more,” Neiman said. “This research speaks to that question.”

Even if it sometimes proceeds at a crawl.

Addendum:  After I wrote this I found that Neiman and Logsdon, with several other researchers, posted a paper April 23 at the BioRxiv online preprint site that reports on their NSF-supported research. I haven’t digested it completely yet, but the abstract indicates their findings support the notion that sexual reproduction in the snails is more effective at removing radical gene mutations (ones that have a greater effect on the organism’s physiology) than asexual reproduction. That means harmful mutations may linger in successive generations of asexually reproducing snails than in sexually reproducing snails. It’s another possible example of an advantage sex offers.

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