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Pack MULEs | |
| UGA scientists discover that some transposable elements in rice often carry fragments of other genes when they reproduce themselves | ||
| By Phil Williams |
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A team of UGA scientists has discovered
a new way that genetic entities called transposable elements—TEs—can
promote evolutionary change in plants. The research, published earlier
this fall in the journal Nature,
was led by Susan Wessler, a Distinguished Research Professor of Plant
Biology at the university.
Almost half of the human genome is derived from TEs and the proportion of TEs can rise to an astounding 95 percent or even higher for some plants, such as the lily. “Normally transposable elements just copy themselves,” says Wessler. “But there were a few anecdotal reports of plant TEs that contained fragments of plant genes that the TE had apparently captured while it was copying itself. The fact that these instances were so rare suggested that this was not an important process.” In analyzing the TE content of the complete rice genome, Ning Jiang and Xiaoyu Zhang, two postdoctoral fellows in the Wessler lab, along with Zhirong Bao, a graduate student in the lab of Sean Eddy of Washington University in St. Louis, discovered that capturing rice gene fragments is a way of life for one type of TE, called MULEs. MULEs with captured gene fragments were called pack-MULEs. The study identified more than 3,000 pack-MULEs, containing more than a thousand different rice gene fragments. Many of the pack-MULEs had two or three gene fragments picked up from different genes and now fused together into a new gene combination. “There are only a few mechanisms known for evolving new genes, and one is genetic recombination, which can bring fragments of different genes next to each other,” says Wessler. “A second is the duplication of an existing gene followed by mutation of one of the pair until it evolves into another function, though this is not the usual fate, because the duplicate copy usually mutates into oblivion.” The discovery of thousands of pack-MULEs in the rice genome indicates that this may be an important mechanism for creating new genes and new functions in rice and in other plants where MULEs are known to flourish. Recent studies indicate that species evolve through the generation of new genes or gene variants that help a population to adapt to a changing environment, for example, or to inhabit a different niche. Why are transposable elements so successful? Some scientists think that TEs are simply “junk” that, much like viruses, can make lots of copies but do little to help the host. There is mounting evidence, however, that TEs help organisms evolve, by making it easier to generate the sort of genetic novelty that is necessary for them to cope with a changing world. Thus, instead of being beasts of burden, pack-MULEs may serve rice as a tool of evolutionary change. |
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