One gene may mark the difference in shape between round supermarket tomatoes and some of the $5-a-pound heirlooms that grace farmers markets each August. A team of botanists has pinpointed the mutation that endows varieties like Howard German, spitz, and Opalka with their elongated shapes.

Most tomatoes—including the puny wild breed from which all others descend—have just one copy of a gene called SUN, says Esther van der Knaap of Ohio State University in Wooster. She and a team of botanists discovered that some heirlooms have an extra copy of SUN lurking on chromosome 7. Adding this second copy to wild plants makes their fruit grow long and odd-shaped, while shutting off the gene rounds out oval-shaped fruit, the researchers report in the March 14 Science.

Van der Knaap's team is currently working out how the SUN gene slims down tomatoes. She says the gene might control auxin, a hormone linked to fruit growth.

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