Seven sexes of an organism

Tetrahymena thermophila.

A CILIATED protozoa, or unicellular organism, called Tetrahymena thermophila, has been found to have seven sexes that can mate with each other! In a paper published in PLoS Biology, a team of scientists has described the intricate process of DNA editing and rearrangement which determines the sex of a new T. thermophila.

T. thermophila is covered with short, hair-like cilia. Although ciliates are single-celled, unlike bacteria, they house their DNA in a nucleus the same way that plants, animals and fungi do.

In fact, ciliates are remarkable for having two nuclei, a larger one and a smaller one. While multicellular animals have special cells for reproduction (sperm and eggs), ciliates, being unicellular, have multiple nuclei instead. The larger nucleus (macronucleus) is used for the day-to-day running of the cell, while the smaller micronucleus is used only for sexual reproduction. When two ciliates reproduce, the macronucleus disappears from both partners, while the micronuclei undergo meiosis and then come together to form a new macronucleus. It is during this process that the sex of the new T. thermophila gets determined.

The researchers used modern sequencing techniques to track down the sex-determination genes. In T. thermophila, the sex-determination genes are active during mating, when they control whether two T. thermophila cells can get together. Actually, ciliates, fungi, and similar organisms have “mating types” instead of sexes. Unlike sexes, mating types look the same—they are just incompatible when it comes to mating. Cells which express different sex-determination genes can mate, while those with the same gene cannot. While T. thermophila has seven different mating types, some kinds of fungi have thousands of different mating types!

The researchers identified a pair of genes, dubbed MTA and MTB, which are responsible for the sex of T. thermophila. The proteins made by these genes are probably expressed on the surface of the cell, where they can interact with the MTA and MTB proteins of other T. thermophila cells to find out whether they are different. By studying the macronucleus and micronucleus, the researchers discovered that MTA and MTB, which are always located side by side, come in different versions corresponding to the different sexes. The sex of the new T. thermophila cell depends on which copy of MTA and MTB it gets, which seems to be randomly decided.