This Species Has Seven Sexes

The single-celled organism Tetrahymena thermophila has seven sexes, not just two, which you would think would improve your odds tremendously.

(Young healthy VII WLTM I, II, III, IV, V, VI for conjugation
Image: Volker Steger/E. Cole/SPL)

Tetrahymena thermophila is a single cell covered with a coat of hairs called cilia. The cilia wave back and forth, powering it through the water.

Its seven sexes are rather prosaically named I, II, III, IV, V, VI and VII. An individual of a given sex can mate with individuals of any except its own, so there are 21 possible orientations.

In most animals, what sex you are is straightforward. A human with two X chromosomes is female, while someone with an X and a Y is male...

Not so for Tetrahymena. Its sex is controlled by a gene called mat, but it is not as simple as one version of the gene encoding one sex. Instead, each allele of the gene sets out a series of probabilities. For instance, an individual born with the mat2 allele has zero chance of being type I, a 0.15 chance of being type II, a 0.09 chance of being type II, and so on.

There are at least 14 of these alleles, each offering a different set of probabilities...

Although not a prediction, sf writers often explore unusual, well, circumstances. In his sardonic 1949 story Venus and the Seven Sexes, writer William Tenn tells us about a rather complicated arrangement - the seven sexes of the Plookh.

"...I am a representative of the seventh sex - nzred."

"A nzred, huh? What do you do?"

"I coordinate... You see, a mlenb is primarily interested in winning the affections of a likely strob and finding a tkan whom he can love. A tkan merely courts a mlenb and is attracted to a good guur. I am responsible for getting a complete chain of these individuals in operation, a chain of compatibility where perfect amity runs in a complete circle - a chain which will produce offspring of maximum variability. Then, after the matrimonial convention, when the chain is established, each sex begins to secrete in its original germ with the full forty-nine chromosomes. A busy time for the nzredd!

"The nzred... fits himself in at any point in the chain which the exigencies of the situation seem to demand. He may receive the sextuple supergamete from the tkan and transmit the original single gamete to the guur, he may be between the flin and blap, the blap and srob, whatever is required."

Via NewScientist.

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