How to pick from seven sexes

Meet Tetrahymena thermophila, a one-celled eukaryote with seven different sexes (aka mating types). A cell of one mating type can successfully mate with a cell from any of the other six mating types. And that’s not even the most interesting part of this tiny creature’s sex life. It turns out that the mating types are assigned stochastically. According to first author Marcella Cervantes of the University of California, Santa Barbara and her colleagues, each cell’s mating type is effectively determined by random chance rather than by genetic history.

Our story begins with the observation that T. thermophila have two nuclei rather than one. One nucleus contains the ‘somatic’ genome, which is responsible for the everyday running of the cell. The other, smaller nucleus contains the inactive ‘germline’ genome. We keep our germline genomes in our ovaries and testes. Within the germline genome are all seven pairs of sex determination genes, one for each of the possible mating types. The somatic genome contains only a single set of mating type genes, and it is this set that determines what that cell’s sex will be.

This means that while each T. thermophila cell contains all the genetic material necessary to become any mating type, six of those genes are only found in the quiescent germline genome. Only one set of mating type genes finds its way into the actively expressed somatic genome. Here’s how that happens.

When two T. thermophila mate, they exchange genetic material with each other (unlike with our unions, no third individual is created). Both cells subsequently destroy their own somatic genomes. They then use material from their germline genomes to construct new somatic genomes. But remember, the germline genome includes genes for all seven sexes. During the process of rebuilding the somatic genome, six of those mating types genes are excised so that only a single one remains. There is an equal probability that any one of the seven sets of mating type genes could be the last one standing.

This means that the sex of the cell after a conjugation event is in no way related to the sex of that same cell before that event. Meanwhile, the ‘reborn’ cell retains all the mating type genes within its own germline genome so that it can continue the tradition.

You might be wondering why an organism would need seven different sexes. Note that T. thermophila only mate under conditions of extreme duress. When you're in danger of starving to death, it’s best not to waste too much time looking for a suitable mate, especially when you’re only 40 or so microns long and can only travel so far. If you can successfully mate with six out of every seven potential partners you meet, so much the better.

Image top left: Tetrahymena thermophila 
Credit: Robinson, R. (2006). Ciliate Genome Sequence Reveals Unique Features of a Model Eukaryote PLoS Biology, 4 (9) DOI: 10.1371/journal.pbio.0040304

Article: Cervantes, M., Hamilton, E., Xiong, J., Lawson, M., Yuan, D., Hadjithomas, M., Miao, W., & Orias, E. (2013). Selecting One of Several Mating Types through Gene Segment Joining and Deletion in Tetrahymena thermophila PLoS Biology, 11 (3) DOI: 10.1371/journal.pbio.1001518.