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Seven sexes!? Scientists figure out how these microbes juggle mates

The ASSET Program / Cornell

An image produced by a scanning electron microscope shows two Tetrahymena cells in the act of mating.

Biologists have known for decades that there are up to seven sexes of the single-celled organism known as Tetrahymena thermophila — but they didn't know exactly how those different sexes "did it." Until now.

When it's time for Tetrahymena to mate, two organisms of different mating types recognize each other and get together to swap DNA. The results of the hookup are totally random. One critter may be mating type No. 1, the other may be mating type No. 3, and the two resulting progeny may turn out to be, um, mating type No. 6. How do they do that?

In the journal PLOS Biology, researchers report that the hooked-up organisms almost literally roll the genetic dice to determine what the sex of the progeny will be.

The researchers say the key to Tetrahymena's sexual proclivities lies in its double genome: Every cell has a "somatic" genome that manages its everyday life, plus a "germline" genome that serves a function similar to that of the ovaries or testes in humans. The germline genome contains incomplete gene pairs for each of six or seven sexes, depending on the cell line. (In this case, the cells came in six sexual flavors.)

Random sex
When two microbes hook up, the progeny's newly created somatic genome latches onto one of those incomplete gene pairs, producing one complete sex-specific gene pair. The other sexy bits from the germline genome are wiped out. The random rearrangement leaves the resulting cells with exactly one complete sex-specific gene pair — and one mating type.

"It's completely random, as if they had a roulette wheel with six numbers, and wherever the marble ends up is what they get," senior researcher Eduardo Orias, a research professor emeritus at the University of California at Santa Barbara, explained in a news release. "By chance they may have the same mating type as the parents — but it's only by chance. It's a fascinating system."

Most of the time, Tetrahymena reproduces asexually, simply by having a parent cell divide into two progeny cells. But the organisms tend to pair up sexually when food is scarce, apparently as part of an evolutionary mechanism that takes advantage of genetic diversity. Sex-specific proteins on the surface of the cells serve as a signal that mating is likely to result in more diverse progeny. That's how two cells of the same mating type avoid pairing up with each other.

This type of mating process doesn't by itself increase the Tetrahymena population: Two cells hook up, and after recombining DNA, two cells separate again. "This is sex without reproduction," Orias said during a telephone interview. After mating, the recombined genetic information is passed down from parents to progeny through asexual reproduction — until it's time for the next hookup.

What it means for humans
Although the process sounds totally alien to us two-sex types, the lessons from Tetrahymena could have implications for human health.

"Tetrahymena has about as many genes as the human genome," Orias said in the news release. "For thousands of those genes, you can recognize the sequence similarity to corresponding genes in the human genome with the same biological function. That's what makes it a valuable organism to investigate important biological questions."

For example, Tetrahymena may reveal new tricks relating to the methods that cells use to recognize friend vs. foe. That could have implications for studying human immune response. Also, the way that the organisms rearrange their DNA may point to new strategies for fighting cancer, which often results from the faulty rearrangement of genetic material.

"The hope is that at some point, there may be useful applications for medicine," Orias told NBC News.

More about microbial marvels:

In addition to Orias, the authors of "Selecting One of Several Mating Types Through Gene Segment Joining and Deletion in Tetrahymena Thermophila" include Marcella D. Cervantes, Eileen P. Hamilton, Jie Xiong, Michael J. Lawson, Dongxia Yuan, Michalis Hadjitomas and Wei Miao.
