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.
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.
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 ââ¬â¢ 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.
