This marks the third of the monk's seven genes that researchers have precisely identified, and the first since the late 1990s, before the genome sequencing boom.
"It's extremely gratifying," says plant geneticist Ian Armstead of the Institute of Grassland and Environmental Research in Aberystwyth, Wales, lead author of a report on the findings in this week's Science. "Many of the loci that Mendel looked at haven't been characterized biochemically before, and it's just interesting to have discovered one of them."
If you've ever taken a biology class, you may recall seeing a portrait of Mendel next to a picture of pea plants that vary in traits such as their height and the color and shape of their seeds (round or wrinkled; green or yellow). By counting the proportions of these traits in several generations of pea plants, the inquisitive monk concluded that these features must derive from pairs of what we now call genes, which he discovered were randomly divided between offspring.
But researchers had never managed to sequence Mendel's gene for seed color, and the pea genome is too huge to go fishing for it, says co-author Norman Weeden, a pea researcher at Montana State University. Luckily, along came Armstead and his colleagues, who were working to precisely locate the sequence of a gene called staygreen (sgr) in the meadow grass Festuca pratensis, some variants of which remain green in drought and other unfavorable conditions because they are unable to break down a green pigment.
The key forward was the genetic similarity between Festuca and rice, which has had its genome sequenced. The group compared genetic markers specific to the sgr region of the grass's chromosome with the markers on the corresponding portion of the rice genome.
The rice chromosome contained 30 potential genes in that area, including one similar to other pigment-metabolizing proteins. To confirm the gene's function, the researchers turned to another lab plant, the thale cress Arabidopsis thaliana, in which they could deactivate the corresponding gene; they found that the resulting plants stayed green longer.
To find out if the equivalent pea sgr was Mendel's gene, they picked out the location of its sequence from pea plants that varied in their seed color. Sure enough, the pea version of sgr was always found in the same tiny part of the chromosome as the old monk's seed color gene.
"We still don't know exactly how it does what it does," Armstead says, "but now we have the gene and we can begin to study it."
As for the identities of Mendel's other four genes, Weeden says he expects them to be revealed in the next few years as more plant genomes give up their sequences. "I was hoping they'd go a little faster," he says.