Organism with Seven Sexes? Scientist Discover Gender Selection Process

A single-celled organism called the Tetrahymena thermophila has long been known to have up to seven sexes of them, but now scientists have discovered how each cell's sex, or mating type, is determined, according to a new study.

By identifying Tetrahymena's long-unknown mating-type genes, a team of UC Santa Barbara biologists, with research colleagues in the Institute of Hydrobiology of the Chinese Academy of Sciences, and in the J. Craig Venter Institute, also uncovered the unusual process of DNA rearrangements needed for sex determination in this organism.

Scientists learnt that in this multi-sexed, single-celled organism, the sex of the offspring is randomly determined by a series of "cut and paste" genome events that bring together one complete gene pair and delete all others. 

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 deleted.

"We found a pair of genes that have a specific sequence, which is different for each mating type," professor emeritus Eduardo Orias of UC Santa Barbara said in a press release. "They are very similar genes - clearly related to one another, going back probably to a common ancestor - but they have become different. And each is different in a specific way that determines the mating type of the cell."

"It's completely random, as if they had roulette wheel with six numbers and wherever the marble ends up is what they get," Orias noted.

"Tetrahymena has about as many genes as the human genome. For thousands of those genes you can recognize the sequence similarity to corresponding genes in the human genome with the same biological function. That's what makes it a valuable organism to investigate important biological questions."

The new findings are published 26 March in the journal PLOS Biology.