No, this isn't something out of an Octavia Butler novel. It’s Tetrahymena thermophila — a single-celled organism that goes way beyond male and female. It has seven different sexes to choose from. Now a new study published in PLOS has finally made sense of its bizarrely complex and seemingly random sex life.
T. thermophila are egg-shaped unicellular eukaryotes that can be found in freshwater. But unlike their asexually reproducing single-celled brethren, these organisms have a rather unique sexual stage to their life cycle that works to increase their reproductive chances.
Here’s how it works.
First, any T. thermophila can mate with any other mating type except its own. So far so good.
But here’s where it starts to get a bit complicated. After two cells mate, the offspring can be one of seven different sexes. Each of these cells has two genomes, and each of them are contained within their own separate nucleus. The researchers, a team led by Marcella D. Cervantes and Eduardo Orias, liken this genome to our ovaries or testes. They contain all the genetic information required by the offspring, while the “working” genome controls the operation of the cell (including its sex).
It’s at this point that I’ll let Stephanie Pappas from LiveScience take over:
When two Tetrahymena fuse in their version of single-cell sex, they produce a gamete nucleus, which is the protozoan equivalent of a fertilized egg in humans. This fertilization nucleus starts making copies of itself, some of which are destined to become germline nuclei and some of which are somatic.
It is during this step that the mating type is chosen, the researchers found. Each germline nucleus holds an array of incomplete gene pairs ― one for each of the organism's seven sexes. The cell joins and completes one of these gene pairs randomly, thus setting the cell's mating type. The rest of the incomplete gene pairs are thrown out.
Which is really wild. So basically, each of the seven sexes is capable of “completing” any one of the other six sexes — and it does so purely by chance. According to the researchers, seven mating types makes it more likely for T. thermophila to run into a cell they can reproduce with when they meet in a pond.
The entire article is available for free at PLOS.