Some tomato lovers learn all there is to know about growing the perfect tomato plant. Or they devote themselves to coddling prizewinning behemoths. Still others are variety collectors in search of, say, the best slicer, the best for making sauce, the most beautiful or the most prolific. And then there's Dr. Charles Rick, who has built a 50-year career around knowing Lycopersicon esculentum in all its genetic diversity.
Rick is a plant geneticist and professor emeritus at the University of California at Davis, where he serves as director of the Tomato Genetics Resource Center (TGRC). The ambitious mission of the center is to maintain its 3,000 different strains of tomatoes -- many of which Rick helped collect -- and to disseminate seed to others who study tomatoes.
About a third of the specimens in TGRC's collection are wild tomatoes. Truth is, they look more like weeds than "real" tomatoes. The collection includes all nine wild species of Lycopersicon, and a few closely related Solanum species. The majority of these plants have fruit that is tiny and green.
Only three types show any yellow or red in their fruit. Of the remaining 2,000 or so tomatoes in the collection, about half of them carry a single mutant gene -- for example, the gene for determinate growth, which was first isolated in 1914, has become an ancestor of virtually all bush-tomato varieties. The remainder fall into 10 subcategories -- prebreds (first steps in bringing wild genes into commercial hybrids), primitive cultivars and others. Only a handful of the varieties in Rick's collection are what we would call heirloom tomatoes.
It's not that Dr. Rick isn't interested in heirlooms. In fact, he says almost any variants are worth saving, but groups such as the Seed Savers Exchange, are doing a good job collecting and maintaining old European and North American tomato varieties. And while heirloom tomatoes exhibit an amazing range of fruit colors and shapes, they don't represent deep genetic diversity.
"We know from our measurements of molecular markers that in tomato varieties from before the 1940s, the gene pool is terribly lacking in variability," Rick says. "And that's what you would expect, if you think about all the bottlenecks that occurred as the tomato's ancestors moved away from their place of origin."
Tomatoes have their genetic roots in the Andes Mountain region: Chile, Peru, Colombia and Ecuador. That's where Rick and other researchers find the richest diversity, including all the wild relatives of tomatoes. But there is no evidence of ancient cultivated tomatoes in the Andean region, claims Rick. People began to domesticate tomatoes farther north, in Mexico. In fact, our word tomato is derived from the Mexican Indian word tamatl. By the time the plant was first taken to Europe about 400 years ago, it had reached an advanced state of domestication. Cortez was in Mexico in 1515, and by 1544 there is written record of tomatoes in an Italian herbal.
"Now imagine," says Rick, "what happened when the wild cherry tomato began its migration away from its native region. The seed was probably carried -- perhaps by animals or birds, and later by humans. Each time a geographic jump occurred, the new population would tend to be just a small number of plants. The seed might be from a single plant, or even a single fruit. Then much later, how many seeds were carried to Europe? And how many plants were grown in botanical gardens as the gardeners reproduced their seed? Again, the numbers would tend to be relatively small.
"Events in this chain would favor a reduction in variability at each step, not an increase, until you would have a population of great uniformity. And that's just what we see in the tomatoes that have come back to America from Europe, that is, the typical heirloom tomatoes. They are genetically much more similar to primitive cultivars and wild types from Mexico than to wild tomatoes from the Andes.
"On the other hand, if you look at varieties produced since the 1940s, when American breeders began introducing genes from the wild species, you begin to see all sorts of things. To obtain disease-resistance, higher-soluble solids or consistent-bearing plants, breeders turn to the wild species."
Rick has played an important role in expanding our knowledge of the range of wild tomato varieties and in bringing a diverse collection of them back to the United States. He has been on more than a dozen major collecting trips in the course of his career.
During his latest trip, in October, Rick discovered an interesting population of L. chmielewskii, a small green-fruited plant that has proven to be a good source of high-soluble solids. Its principal sugar is sucrose, which provides a sweeter taste and may boost soluble solids. The sugars in cultivated tomatoes are typically glucose, fructose and other monosaccharides. Does that mean the wild one is sweet and delicious? Not really. "You could taste one," Rick says, "but you would never take a second bite."
After 50 years of study, Rick's interest is far more than academic. He is a tomato lover at heart. His favorite lunch is a sandwich of sliced tomatoes and mayonnaise. And he dries large numbers of plum tomatoes each season. When asked if he still grows many tomatoes at home, he responds dryly, "No, not really. Only about three dozen."
His list of proven performers is surprisingly ordinary. For one thing, any tomato that is going to make it in his garden must be resistant to tobacco mosaic virus. And so he grows 'Celebrity' ("That's the one my field crew likes, too"), 'Caligrande' and 'Sweet Cherry'. For drying he uses 'Enchantment', a plum tomato variety.
Avoiding the mosaic virus means that Rick can't grow heirlooms himself, but he thinks many are worth saving. Heirlooms may not represent much genetic diversity, but they are "wonderful novelties." Most of them resulted from either interesting mutations or cross-pollination by insects -- rare events in cultivated tomatoes. And it is unlikely that major tomato breeders would pay much attention to heirloom traits.
"Gardeners are always looking for interesting variants," says Rick. "For them, it doesn't matter that a variety isn't highly productive or is so tender that it could never make it in the marketplace. If people find a tomato they like, they should save it, by all means."