


A team of researchers at the University of Georgia has discovered a new way that genetic entities called transposable elements (TEs) can promote evolutionary change in plants.

The research, published Sept. 30 in the journal Nature, was led by Dr. Susan Wessler, a Distinguished Research Professor of plant biology at UGA.

The Wessler lab studies TEs, which are pieces of DNA that make copies of themselves that can then be inserted throughout the genome. The process can be highly efficient. Almost half of the human genome is derived from TEs and, this value can go to an astounding 95 percent or even higher for some plants, such as the lily.

"Normally transposable elements just copy themselves, said 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 entire 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 Dr. 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 3000 Pack-MULEs that contained over a thousand different rice gene fragments. Many of the Pack-MULEs have two or three gene fragments picked up from different genes but 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," said Wessler. "A second is the duplication of an existing genes 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 mutate into oblivion."

The discovery of thousands of Pack-MULEs in the rice genome indicates that this may be an important mechanism to create 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 and/or gene variants that help a population adapt to a changing environment, for example, or to inhabit a different niche.

Why are transposable elements so successful? Some think that they are simply "junk" that, much like viruses, they 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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