For decades, Tetrahymena thermophila, a unicellular ciliate organism, has been a favorite research subject for experimental biologists. Its study has led to new insights into the mechanisms involved in the cell cycle, the function of enzymes, and epigenetic histone acetylation.
Tetrahymena cannot reproduce by itself. But organisms of different mating types can come together and mate. And it’s been more than 50 years since scientists recognized that this organism actually had not 2, but up to 7 different mating types.
“You don’t see any shape or other differences to denote the mating type,” says Edward Orias, a biologist at UCSB. “But there are chemical differences on the cell surface that allow cells to contact one another and, if they are of different sexes, mate and reproduce.”
How sex might be determined in offspring was unknown. But Orias and colleagues had a suspicion DNA arrangement processes may be involved in some way. They decided to take a closer look at the cell’s germline nucleus, one of the organism’s two nuclei and where genetic information for progeny is stored, to see if that hypothesis held water.
“The first step to understanding sex determination was to genetically map the germline mating type locus—we were able to narrow down its location by a combination of linkage mapping and other genetic techniques,” says Orias. Then, using the germline genome sequence, the group discovered that the germline nucleus for each mating type contained a tandem array of six incomplete gene pairs. When the organisms mated, one of those incomplete pairs “matched” up in the new cell’s second nuclei, the somatic nucleus, while the remaining five pairs were deleted, giving the progeny exactly one gene pair—and exactly one sex.
“We could not imagine what a beautifully organized system this is,” says Orias. “What we saw was this very symmetrical process by which the ends of an incomplete gene pair are completed, and then the rest of the DNA information is discarded. It’s absolutely fascinating.”
Tetrahymena has already made numerous contributions to human medicine—the Nobel prize-winning discovery of telomere function being the most recent. But Orias says that understanding DNA arrangements like sex designation may provide insights into allorecognition—the ability of an organism to recognize its own tissue—as well as the development of certain cancers.
“By studying a system like this one where everything is programmed, where we know where to go look for different events, we can have better understanding of these DNA rearrangement processes,” he says. “It can help us investigate many different questions, and, as we learn more about the genes in Tetrahymena that correspond to genes in humans, we may see some important applications to human disease.”
1. Cervantes, M. D., E. P. Hamilton, J. Xiong, M. J. Lawson, D. Yuan, M. Hadjithomas, W. Miao, and E. Orias. 2013. Selecting one of several mating types through gene segment joining and deletion in tetrahymena thermophila. PLoS Biol 11(3):e1001518+.