Weird Science thinks seven sexes is enough for anybody

Ars Technica » Scientific Method 2013-03-30

Dance of the seven veils sexes. Tetrahymena is a single-celled organism that looks a bit like a microscopic ball of fuzz, since it's covered with cilia. Despite its unshaven appearance, the cells can mate, although they'll also simply divide asexually if there's nobody around. (A Weird Science Fact: Tetrahymena has a set of chromosomes they carry around that are reserved for having sex. The rest of the time, a completely different set help run the cell.) Of course, that creates a problem: as soon as they divide, there is somebody around, but mating with a cell that's genetically identical is a bit of a waste of time. So, the organism uses something akin to sexes to avoid this. If it divides, the two cells that result will be the same sex, and can't mate.

All very sensible, until you get to one tiny detail: for no obvious reason, Tetrahymena have decided they need seven sexes. The new paper figures out how that actually works. In the chromosomes used for mating, they have a set of seven inactive half-genes, located next to a different half gene that will ensure that the protein made from it ends up on the cell's surface. When mating occurs, one of the set of seven is selected at random, linked up to the other half, and converted into a functional gene. When the gene product is on the cell's surface, that cell will not be able to mate with any other cell that has the same arrangement.

Now we'll need an extra set of lasers. If you were to happen across this two-headed shark foetus, what would your first thought be? If you were a biologist, it would apparently be "I wonder if this is a case of the fusion of twin shark embryos, or if this is really a single embryo with two heads?" Fortunately, said biologists now have an answer: one shark, two heads.

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