A selection of heirloom tomatoes.
STU SPIVACK

Editor's Note: This is the first in a series of six features on the science of food, running daily from March 30 through April 6, 2009.

Famous for their taste, color and, well, homeliness, heirloom tomatoes tug at the heartstrings of gardeners and advocates of locally grown foods. The tomato aficionado might conclude that, given the immense varieties—which go by such fanciful names as Aunt Gertie's Gold and the Green Zebra—heirlooms must have a more diverse and superior set of genes than their grocery store cousins, those run-of-the-mill hybrid varieties such as beefsteak, cherry and plum.

No matter how you slice it, however, their seeming diversity is only skin-deep: heirlooms are actually feeble and inbred—the defective product of breeding experiments that began during the Enlightenment and exploded thanks to enthusiastic backyard gardeners from Victorian England to Depression-era West Virginia. Heirlooms are the tomato equivalent of the pug—that "purebred" dog with the convoluted nose that snorts and hacks when it tries to catch a breath.

"The irony of all this," says Steven Tanksley, a geneticist at Cornell University, "is all that diversity of heirlooms can be accounted for by a handful of genes. There's probably no more than 10 mutant genes that create the diversity of heirlooms you see." But rather than simply debunking a myth about the heirloom’s diversity, Tanksley's deconstruction of the tomato genome, along with work by others, is showing how an unassuming berry from the Andes became one of the world's top crops. Genetics work will also point the way to sturdier, more flavorful tomatoes—albeit hybrid varieties whose seeds cannot be passed down from generation to generation but must be purchased anew by growers each season.*

New World Discovery
The cultivated tomato is a member of the nightshade family that includes New World crops such as potato and chili pepper, which spread around the world after Christopher Columbus brought them back to Spain in the 15th century.* But whereas scientists have uncovered a wealth of archaeological evidence—including microscopic starches on pottery shards that point to the taming of many crops from the Americas as far back as 10,000 years ago—the record is blank when it comes to the tomato.

Known scientifically as Solanum lycopersicum, the modern tomato seems to have its wild origins in the Peruvian Andes and may have been domesticated in Vera Cruz, Mexico—an agricultural hot spot. Primitive varieties still grow throughout the Americas. All told, botanists call some 13 species "tomatoes" and consider an additional four to be close allies. One might assume that one of these known wild species became today's cultivated crop, but that's not the case: the Mother Tomato has never been found. The closest relative is the currant tomato—Solanum pimpinellifolium—which, based on genetic comparisons, split from today's tomato some 1.4 million years ago.

So researchers like Tanksley have to work backward, crossing tomato varieties and species in order to understand how various genes influence shape and size. Once isolated, Tanksley later inserts those genes into other tomato varieties to make his case with a dramatic transformation.