How a Tiny Critter Has Seven (Yes, Seven) Sexes

If you thought the battle of the sexes was chaotic, you ain’t seen nothing yet. Meet Tetrahymena thermophila, a tiny, single-celled organism that has seven sexes.

Researchers have known since the 1950s that Tetrahymena—a freshwater ciliate covered in hair-like projections—has seven sexes, Types I through VII, and each type can mate with every sex but its own (such that Type I can mate with individuals that have Types II through VII, and so on). But exactly how Tetrahymena determines its sexes remained a mystery—until now. (Get a genetics overview.)

The freshwater critter Tetrahymena is covered in hair-like projections. Image by Aaron J. Bell, Science Source

Scientists have discovered that a random process of DNA splicing and rearrangement during development determines which sexes Tetrahymena will become.

Tetrahymena mostly reproduces asexually, dividing into two identical daughter cells. When Tetrahymena reproduces, one of its two nuclei—known as the germline nucleus—doesn’t do anything, and only the genes on its other nucleus—called the somatic nucleus—are activated. (See Synthetic DNA Created, Evolves on Its Own.”)

But when food is scarce, the organism can opt for sexual reproduction, which creates novel genetic combinations that may give daughter cells a better chance of surviving in a harsh environment.

Such tough conditions also drive the organisms to develop more sexes—and thus more mating options, said study leader Eduardo Orias, a biologist at the University of California, Santa Barbara.

“If you only have two sexes, there’s only a 50 percent chance that someone you meet will be of the right type,” Orias said. With seven different sexes, however, a ciliate’s chances of meeting Mr. or Mrs. Right increase to about 85 percent.

Seven Genes for Seven Sexes

But when scientists sequenced the Tetrahymena genome in 2006, they didn’t find any genes that jumped out at them as determining sex.

So in the lab, Orias and colleagues first deprived a strain of Tetrahymena of food to induce it to start mating sexually. To make things even more confusing, the strain used by the researchers had only six sexes, not seven.

Catching the ciliates in flagrante delicto, the researchers compared genes that were expressed during mating and those that were expressed during normal growth. The researchers identified one pair of genes that was active only during mating. When they inactivated this pair of genes, Tetrahymena no longer mated, making those genes good candidates for those that determine sex. (Also see Heat Triggers Sex Change in Lizards by ‘Turning Off’ Key Gene.”)

The researchers then scoured the genome sequence of Tetrahymena, and they found six copies of these mating-type genes in the germline nucleus, one for each sex.

These six sexes were clustered together on a small segment of DNA, one right after the other. This array is bookended by the standard start and finish of a protein-coding gene. The next question the scientists had to answer was how Tetrahymena got from six or seven pairs of mating-type genes down to one.

Mating Roulette

In experiments, the team showed that, during mating, Tetrahymena randomly sort, snip, and rearrange the six or seven pairs of the mating-type genes until the organism is left with just one pair of genes on its somatic nucleus.

“It’s as if they had a roulette wheel with numbers on it and just gave it a spin,” Orias said.

Tetrahymena retains the full array of mating-type gene pairs on its germline nucleus for the next time it finds itself hungry and hears the soft sounds of Barry White playing in the background, so to speak.

What’s more, this one gene pair is likely expressed on the membrane of the cell. This makes sense, Orias explained, because Tetrahymena frequently touch before mating. It could be that this touch lets Tetrahymena determine the sex of the individual, not unlike how parents frequently dress their babies in either pink or blue to indicate gender. (See “Behind the Scenes at a NatGeo Baby Cover Shoot.”)

Understanding Tetrahymena‘s strange sexual behaviors has a larger impact than just figuring out how one particular group of species finds a mate, the team added. The research also can help researchers better understand how DNA is spliced and recombined in other species, which is important in the development of immune function and even potentially cancer cells.

If Tetrahymena ever needed a dating service, however, these seven-sexed critters would likely pay a fortune.


Meet the Author
Carrie is a freelance science writer living in Virginia. When she's not writing about cool critters, she's spending time outside, drinking coffee, or knitting. You can visit her website at