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Corn genome results reveal maze of diversity: hints on hybrid vigor among genetic findings about maize.

Cue the corny jokes.

Researchers have completed a draft of the maize genome. And while people may quip about the "amaizing" achievement, scientists say the genetic blueprint of one strain of corn reveals serious amounts of genetic diversity and some weighty biology lessons that could lead to improvements in the economically important crop plant.

Papers in the Nov. 20 Science and in the online journal PLoS Genetics report the draft genome and analyses of the plant's genetic makeup. The work, conducted by many institutions with funding from the National Science Foundation, reveals that corn has an unusual ability to make new genes, lose others, alter activity of its genes and withstand radical genome remodeling. And surprising differences between two strains of corn may provide clues about why hybrids sometimes grow or yield better than parent strains.

Scientists have known that different strains of maize can vary widely in genetic makeup. Although corn was domesticated only 8,000 to 10,000 years ago from the grass teosinte, the genetic diversity between any two strains of corn exceeds that between humans and chimpanzees, species separated by millions of years of evolution. For instance, DNA of the strain B73, the agriculturally important and commonly studied variety decoded by the maize genome project, contains 2.3 billion bases. The genome of a strain of popcorn decoded by researchers in Mexico is 22 percent smaller than B73's.


"You could fit a whole rice genome in the difference between those two strains of corn," says molecular biologist Virginia Walbot of Stanford University.

Much of that difference is due to transposable elements, also known as jumping genes. Transposable elements (transposons for short) are mobile pieces of genetic material that hop around the genome, sometimes taking genes or pieces of genes with them. The genome project discovered new families of transposons, revealing a total of 1,300 such families in maize, says Patrick Schnable, a maize geneticist at Iowa State University in Ames. Transposons and other repeated bits of DNA make up about 85 percent of the maize genome in the B73 strain.

That's an unusually high volume of repetitive DNA, making the decoding effort challenging, says Richard K. Wilson of Washington University School of Medicine in St. Louis. "This was a really tough one," he says.

The large number of repeated stretches meant the popular shotgun approach--relying on computers to assemble a scattershot of DNA fragments from all over the genome into a coherent picture--wouldn't work for corn's code.

Instead, scientists used a slower method similar to that of the Human Genome Project, first putting the pages of corn's genetic instruction book in order and then finding the DNA letters on each page. The $30 million maize project is not entirely finished but has already provided a wealth of knowledge.

B73's genome contains about 32,000 genes, one team reports in Science. Transposons have also left pieces of genes scattered throughout the genome. Some of those fragments are active and may help control activity of other genes, Schnable says. The researchers also uncovered evidence that maize strains are creating new genes and losing others. At least 180 of B73's genes are missing from another strain of corn known as Missouri 17, for example. In fact, thousands of segments of DNA found in one strain are completely missing from the other, Schnable and colleagues report in one of the 10 companion articles published in PLoS Genetics.

The researchers also describe variation in the number of copies of genes the two strains carry. Some of those gains and losses of genes and other sequences might contribute to hybrid vigor, a condition in which offspring are heartier and better yielding than either parent. Scientists don't know the source of hybrid vigor, but the genome sequence may make it easier to trace, Schnable says.

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Title Annotation:Genes & Cells
Author:Saey, Tina Hesman
Publication:Science News
Geographic Code:1USA
Date:Dec 19, 2009
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