Organism With Seven Sexes Plays DNA Roulette

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It’s been more than 50 years since scientists discovered that the single-celled organism Tetrahymena thermophila has seven sexes. But researchers only recently determined how each cell’s sex, or mating type, is determined.

The seven sexes, as it turns out, are randomly determined by a series of cut and paste genetic recombination events. These assemble one complete gene pair and delete all others, according to the paper, published in PLoS Biology.

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“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.”

The discovery doesn’t just shed light on Tetrahymena. Better understanding of DNA rearrangements could have potential human health implications ranging from tissue transplantation to cancer.

One potential benefit is the ability of an organism to distinguish its own tissues from those of another. That ability is a first-line defense against infection and illness, but it can also cause problems, like rejection of transplanted organs.

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As a single-celled organism, the “parents” associated with Tetrahymena reproduction are actually two nuclei: one that contains the genetic information for progeny cells, and another that’s referred to as the “working copy.”

“It’s completely random, as if they had roulette wheel with six numbers and wherever the marble ends up is what they get,” Orias said.

“Tetrahymena has about as many genes as the human genome,” Orias said. “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.”

Image: Tetrahymena thermophila. Wikimedia Commons

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