7 Choices?! Strange Creature Picks Its Sex at Random

Single-cell Tetrahymena mate
Two Tetrahymena mating. (Image credit: The ASSET (Advancing Secondary Science Education with Tetrahymena) Program at Cornell University)

With seven different sexes to choose from, the single-celled organism Tetrahymena thermophila determines its biological mating type in a game of molecular chance, new research finds.

Tetrahymena are oval-shaped protozoa that live in freshwater. These microscopic organisms come in seven different "sexes," or mating types. Any Tetrahymena sex can mate with any other mating type except its own.

Even more intriguing to biologists is that it doesn't matter what mating types two Tetrahymena parents are. In fact, their offspring can be any one of the seven. That observation had some strange genetic implications, as parents typically would pass on their own mating-type genes to their progeny.

But even though scientists have known about Tetrahymena's seven mating varieties for 60 years, they have only now discovered how the individuals select their type.

"Finally, we had the resources to get at the molecular basis of it ― to actually discover the mating-type genes, what their sequence is, and how it is that the cells have the potential for many mating types and only end up expressing one ― a random one," study researcher Eduardo Orias, a biologist at the University of California, Santa Barbara, told LiveScience.

By peering into the Tetrahymena genome, the researchers found the equivalent of a cellular roulette wheel. The organisms have two nuclei apiece. One, the somatic nucleus, contains the DNA that does the daily work of the cell. The other, the germline nucleus, acts like the cells in the ovaries or testes of humans. The DNA in the germline passes along traits to offspring. [Sex Quiz: How Animals Really Do It]

When two Tetrahymena fuse in their version of single-cell sex, they produce a gamete nucleus, which is the protozoan equivalent of a fertilized egg in humans. This fertilization nucleus starts making copies of itself, some of which are destined to become germline nuclei and some of which are somatic.

It is during this step that the mating type is chosen, the researchers found. Each germline nucleus holds an array of incomplete gene pairs ― one for each of the organism's seven sexes. The cell joins and completes one of these gene pairs randomly, thus setting the cell's mating type. The rest of the incomplete gene pairs are thrown out, said the report, released by Orias and his colleagues today (March 26) in the journal PLOS Biology.

"We had no idea what a beautifully organized system this turned out to be," Orias said. "It's very modularly organized and very symmetrical in some ways and very ― to us ― aesthetically exciting."

Having seven mating types, instead of only two, may make it more likely for Tetrahymena to run into a cell they can reproduce with when they meet and greet in a pond, Orias said.

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Stephanie Pappas
Live Science Contributor

Stephanie Pappas is a contributing writer for Live Science, covering topics ranging from geoscience to archaeology to the human brain and behavior. She was previously a senior writer for Live Science but is now a freelancer based in Denver, Colorado, and regularly contributes to Scientific American and The Monitor, the monthly magazine of the American Psychological Association. Stephanie received a bachelor's degree in psychology from the University of South Carolina and a graduate certificate in science communication from the University of California, Santa Cruz.