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

Species: Tetrahymena thermophila

Habitat: fresh water around the world, having way more sex than you

Finding someone to have sex with can be a trial. There are plenty of humans in the world, but the proportion who are desirable, live nearby and – crucially – are willing to have sex with you can be prohibitively small. Most of us make the quest still harder by ruling out half the population before we even start looking.


At first glance it looks like the single-celled organism Tetrahymena thermophila has cracked this problem in spectacular fashion. It has not two but seven sexes, and each one can mate with any of the others, which opens up the field considerably. Unfortunately, they all look alike. What’s more, the different sexes are not equally common – thanks to the peculiar way each individual’s sex is determined.

Fuzzy sex

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. Other species use different systems, but they are all clear cut when it comes to sex determination and mating.

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. They are divided into two major groups called A and B: A alleles produce every sex except IV and VII, while B alleles produce everything except I.

Skewed sex

As if that weren’t enough, sex itself is different for this animal. Most cells have a single nucleus that contains all their DNA, but Tetrahymena has two: a large macronucleus and a small micronucleus. The macronucleus controls the everyday functions of the cell, while the micronucleus deals with its complicated sex life. Mating is called conjugation, and involves swapping genes from the micronuclei. Each animal’s rejigged micronucleus then builds a new macronucleus.

With all this going on it should come as no surprise that Tetrahymena populations look a little weird. Unlike many animals, the sexes are not equally common. According to Rebecca Zufall and colleagues at the University of Houston in Texas, that is all down to the fuzzy way it chooses its sex.

They built mathematical models of populations of animals with different kinds of sex determination. So long as the populations were no bigger than about 1000, fuzzy sex determination always led to skewed sex ratios. This was true even if the different alleles complemented each other – one of them boosting sexes I, II and III, say, while another boosted IV, V, VI and VII.

Their model also showed that alleles supporting several sexes outcompeted alleles that only supported one, because they coped better with wild swings in the sex ratio caused by mass deaths and the like. Zufall says there are probably more animals with fuzzy sex determination than anyone suspects.

Journal reference: Evolution, DOI: 10.1111/j.1558-5646.2011.01266.x

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