On the Tomato Trail: In Search of Ancestral Roots | Barry Estabrook

from Gastronomica 10:2

A Chilean soldier was guarding a lonely garrison in the Attacama Desert near the Peruvian border when American geneticist Roger Chetelat and his field research team arrived there in 2005. The soldier obligingly provided what should have been straightforward directions to their destination: Follow the road beside the railroad tracks. As an afterthought, the sentry quietly suggested that they stay on the road, adding with a knowing nod, “landmines.”

Chetelat, an athletic fifty-two-year-old, could be mistaken for a high-school gym teacher. In fact he is the director of the prestigious C.M. Rick Tomato Genetics Resource Center at the University of California, Davis, the world’s foremost repository of wild tomato plants and their seeds. The Center houses a collection of tough, versatile organisms that have evolved disease resistance and tolerance to extreme environmental conditions—genetic traits that researchers can incorporate into cultivated tomatoes, a feeble, inbred lot that, like some royal families in the Middle Ages and certain dog breeds that have become too popular, need all the genetic help they can get. The Center acts like a lending library, nurturing and preserving its 3,600-specimen collection but also making it readily available to scholars and plant breeders worldwide who want to “check out” seeds for their own experiments. “If it wasn’t for the genes of these wild species, you wouldn’t be able to grow tomatoes in a lot of areas,” explains Chetelat. “I don’t think there is a cultivated plant for which the wild relatives have been more critical.”

Drop by nearly any farmer’s market on a summer Saturday, and displays of cultivated tomatoes all but scream out the word diversity. Their descriptive names say it all: Big Beef, Orange Blossom, Pruden’s Purple, Striped German, Green Zebra, Great White, Yellow Pear, Red Grape, White Cherry, Black Cherry, Matt’s Wild Cherry. But all that variety is literally only skin deep. Botanists have but one name for all those oddball cultivated tomatoes: Solanum lycopersicum. “Most of the variation you are seeing is from a few genes that control color, shape, and size,” says Chetelat. “There is very little genetic variation.”

On that day in the desert five years ago, Chetelat and his group, which included scientists from the Universidad de Chile in Santiago, had been retracing a trail that had been cold for fifty years, its route filed away in the records of a Chilean herbarium. With luck—lots of it—the stale information might lead them to a few remote clumps of a wild tomato species called S. chilense. If the team was successful, seeds from those plants, which had never before been collected in that area, would become a valuable addition to the Center’s collection.

Luis Faundez collects a wild tomato plant, Solanum chilense, near Talabre, Chile, where the tomatoes were heavily damaged by goats and sheep. The few survivors grow in inaccessible places. Photograph by Carl Jones © 2005.

But that was a big “if.” First, there were questions concerning the accuracy of the pre-gps location, given as simply “kilometer 106–108” on the cog railway that switch-backed through the Andes between Chile’s port city Arica and La Paz, Bolivia. The notation had been scrawled in the journal of a plant collector sometime in the 1950s. Even if the directions were valid, a lot can happen in a half-century to an isolated cluster of plants. Roads get built. Gas pipelines go through. Settlements grow. Fields expand. Animals browse. Facing the distinct possibility that they were on botany’s answer to the wild goose chase, the researchers had been driving across the desert since dawn and had yet to see anything resembling a wild tomato.

The Attacama Desert makes up the southernmost part of the geographic range of modern tomatoes’ wild ancestors, which still grow in parts of western Chile, Peru, and Ecuador (and the Galapagos Islands, home to two errant species). It is a testament to the adaptability of the tomato that it can survive in the Attacama, one of the most inhospitable places on earth. The gravelly, boulder-strewn landscape is fifty times as dry as California’s Death Valley. Some parts have not received a drop of rainfall in recorded history. Chetelat has driven across its surface for an entire day without seeing a single living thing. Most of the plants that survive there are low and scrubby and, during the driest months, brown and to all appearances dead.

Chetelat was further discouraged when the road they had been told to follow diverged from the rail line several kilometers before they reached their goal. Frustrated but still determined, the driver veered onto the tracks, which were still occasionally used, and bounced and jolted along until that became too uncomfortable. Still well short of the marker, the scientists set out on foot, even though it was getting late in the day and no one wanted to bivouac in a semi-militarized no-man’s land. It didn’t help that they had not seen a tomato.

Until they arrived at kilometer 108, that is. There, just as described, with yellow flowers glowing in the afternoon light, were S. chilense, descendants of the plants seen by the 1950s collector. The researchers’ reward for a long, uncomfortable session in the field was a handful of seeds. Chetelat considered it a good day.

I met Chetelat and his chilense one cool, misty afternoon this past January inside a greenhouse belonging to the Rick Center, which is named after its founder, the late Charles M. Rick. Charley, as his associates called him, worked at the facility until shortly before his death in 2002 at age eighty-seven. He was a legendary plant science professor, a pioneer in discovering and preserving the seventeen species of wild tomatoes, and the world’s foremost authority on the genetics and evolution of the tomato. The Rick Center raises wild tomatoes (as well as other genetically interesting tomato lines) and saves their seeds in climate-controlled vaults until they are needed.

Had Chetelat not been my guide, I would never have recognized the greenhouse plants as being remotely related to the plump, pinkish-red Brandywines I harvest each summer from my garden. These were perennials with solid, semi-woody stems, not the one-season wonders I know. Some plants crept along like thyme. Others climbed until they nearly touched the glass roof and then doubled back toward the floor. Foliage came in all shapes, sizes, textures, and very powerful smells. The round, scalloped leaves of one plant were covered with what seemed like a bad infestation of white, gnat-sized flies. When I touched a leaf, my finger stuck to its surface, a natural version of flypaper that had entrapped the would-be pests (and left me with gummy fingertips). Another plant bore leaves that were tough, wrinkled, and leathery looking, as if they had fallen from an ancient walnut tree. One could have been mistaken for Italian parsley, except its leaves were stiff and covered in a waxy substance to prevent water loss. When particularly dry, they folded in half. The plant across the aisle from the parsley look-alike had leaves covered in fine hairs like those on a prepubescent boy’s upper lip. When crushed between my fingers, they gave off a powerful piney smell mingled with hints of celery.

The fruits hanging from the vines seemed like a haphazard collection of miniature marbles, the biggest not much larger than my little fingernail. They came in an array of colors I did not associate with the tomato clan: black, yellow, purple, green with white stripes, green with a purplish blush.

Overcome by all this tomato diversity, I plucked a yellowish-green fruit from a plant Chetelat identified as S. arcanum. I squeezed it, and a slimy green substance containing dozens of seeds no bigger than pinheads squirted into my palm. I slurped it. The first sensation to assault my mouth was the distinct taste of soap, followed immediately by a dry, burning bitterness that lasted … and lasted. Could this inedible fruit really be a close relative of a plant central to culinary cultures around the world? The zesty yet sweet base of countless soups, sauces, salsas, and condiments? A treat savored unadorned and out of hand on a warm midsummer afternoon, by far the United States’ most popular garden crop, according to a Harris poll conducted in January 2009 for the National Gardening Association?

One lone plant in the greenhouse seemed to have a suggestion of gustatory promise. It bore a single fruit about the size of a pea, but at least it possessed the telltale red tomato hue. Because it had been sprayed, I couldn’t sample it, but Chetelat assured me that eating one would have been much more pleasant than my experience with the arcanum. “Mild and slightly sweet,” he said. “It’s called pimpinellifolium. They grow wild in Peru and Ecuador.” Inauspicious and easily overlooked, the tiny, round-fruited pimpinellifolium is actually the ancestor of all domesticated tomatoes.

An array of fruit samples from the wild tomato,
Solanum peruvianum, collected at Caleta Vitor, Chile. Each fruit cluster was taken from a different plant of the same population, revealing a high degree of genetic variation. Photograph by Carl Jones © 2005.

Chetelat speculates that some prehistoric forager or farmer noticed an unusual plant. Because of genetic mutations, it produced larger than usual fruits and instead of having only two interior cells filled with seeds, it had many. These “deformed” plants were probably domesticated somewhere in southern Mexico or northern Central America, more than one thousand miles from the home range of their wild kin. How they made this journey is lost in the shadows of prehistory. But the result was that this small mutant group became cut off from its ancestral roots, literally. As early farmers saved seed from year to year with no crossbreeding with other species, the population became increasingly inbred, a process geneticists call a “bottleneck effect.” Chetelat draws an example from human migration to explain this effect. “Imagine a handful of people settling a new continent. They represent only a small part of the genetic diversity that was within the continent they left behind. If there’s no more migration, then the diversity is even further reduced by inbreeding.”

The problem of inbreeding is exacerbated in cultivated tomatoes because they are self-pollinated. A single plant can “breed” with itself, and the resulting seeds produce offspring that are basically clones—identical to the parent plant. Not going to the bother of connecting with a mate is a rapid, surefire way to reproduce, but it further decreases genetic diversity. As a result, all the varieties of cultivated tomatoes that have ever been bred contain less than 5 percent of the genetic material in the overall tomato gene pool. “They seem diverse,” says Chetelat. “But at a dna level they are very similar.”

Domestic tomatoes, for example, had virtually no innate resistance to common tomato diseases and pests until breeders began crossing them with wild species in the 1940s. “They were a fairly defenseless lot,” explains Chetelat. Wild tomatoes, on the other hand, are more robust: “We know of at least forty-four pathogens for which resistance has been found in wild species.” Commercial seed companies have bred traits into domestic varieties to combat about half of those pests and diseases. (If you buy from a seed catalogue, the maladies that a tomato resists are usually represented by a series of letters following the name.) These include such notorious plant killers as stem canker, spotted wilt virus, fusarium wilt, grey leaf spot, nematodes, tobacco mosaic, and verticillium wilt.

And more potential remains untapped. Any grower in the Northeast last summer who had to dig up and either bury or burn every wilted, blackened tomato vine in the garden is familiar with the ravages of a late blight epidemic. Chetelat told me that there are wild species quite tolerant to the disease waiting for the attention of a future plant breeder. Currently, the Center is working with researchers from India who hope to incorporate from a wild species into domesticated varieties resistance to tomato yellow-leaf curl virus, a devastating disease that limits tomato production around the world.

Wild tomatoes might even help fight disease in humans. Chetelat and his associates have conducted experiments showing that it is feasible to boost the levels of ascorbic acid, lycopene, beta-carotenes, and other healthful antioxidants by introducing genes from wild tomatoes into domestic varieties. Because tomatoes and tomato products are a major source of nutrients worldwide, higher antioxidant levels could have enormous health benefits.

The possibilities of using wild traits to improve cultivated tomatoes seem almost limitless. Some wild species grow at chilly altitudes 3,500 meters up in the Andes, tolerating low temperatures that would cause other tomatoes to shrivel and die. Others thrive in humid rainforests. A few can eke out existences in deserts. They have adapted to scant rain and intense heat, potentially useful for commercial crops in warm, dry areas like California’s Central Valley during a time of irregular rainfall and global warming. With advances in the technologies of working with dna, new areas are opening up for breeders. Better methods will allow scientists to routinely address more complex traits, such as the elusive matter of taste, which is controlled by multiple genes. Chetelat believes it is a time of opportunity.

But, unfortunately, time may be running out for the wild populations upon which future discoveries may depend. Modern agricultural practices and urban sprawl eliminate habitat for wild tomatoes. Herds of goats, llamas, alpacas, and other domestic animals eat and trample them. Even though the Rick Center can produce seed from previously gathered wild specimens, thereby maintaining genetic lines, Chetelat insists that collections preserved by humans, however carefully, are no substitute for what he calls “in situ” plants, meaning ones that grow in their native environments without human interference. The most obvious difference between the two is that Chetelat and his team grow their wild tomatoes artificially in a greenhouse with adequate water, optimum lighting, and no competition. Pests and diseases are chemically controlled. “You’re really changing the environment,” he says. “And that causes genetic shifts from one generation to the next. It’s artificial selection.” There are other potential problems. If growers are not careful, pollen can flow between two distinct populations of a species being raised in the same greenhouse. A harried technician might simply mislabel seeds or mishandle them, allowing one variety to mingle with another. “We wouldn’t have a problem if we could store seeds forever and if we had an infinite number of seeds to fulfill researchers’ requests,” notes Chetelat. “Of course, that’s not the way it works.”

So he and his associates must still pack their collection equipment and head back out into the field. Their latest trip to northern Peru in 2009 illustrated the severity of the conservation challenge. When Rick and others had visited the area decades earlier, they had made detailed records of where they had observed native populations. Chetelat intended to return to the same sites to reexamine those populations. Most of them were gone.

Since the mid 1990s intensive sugarcane agriculture has come to low-lying valleys north of Lima, vast monoculture fields carpeting the valley floors from one mountain range to the other. Chetelat talked to farm supervisors and laborers, who formerly nibbled the ubiquitous little tomatoes as snacks, as North Americans might pick a few wild blackberries in the fencerow beside a pasture. They told him that when the sugarcane came, accompanied by the usual arsenal of herbicides and other agricultural chemicals, the tomatoes disappeared.

Everywhere Chetelat went, the story was the same. It was only when his group ventured higher into the mountain valleys, above one thousand meters, where conditions were too rough and available spaces too small for sugar estates, that they began to find wild tomatoes growing in rocky areas and out of cracks in fieldstone walls. “That area is the center of diversity for one of the immediate ancestors of the cultivated tomato,” he says. “And now most of those populations are gone.”

In an effort to more thoroughly evaluate the situation, Chetelat is hoping to return to the region sometime in the next few years. Despite the modern advances in genetics and dna mapping, this expedition will be more in the spirit of early plant collectors like Dr. Rick, the Center’s founder. “I can’t overemphasize how much of a debt we owe to them,” Chetelat told me. His goal is to secure funding from the National Science Foundation to work closely with a team of Peruvian graduate students. Like the old-time botanists, they are going to scour the landscape and count individual plants at differing elevations. After a thorough, scientific evaluation of the remaining wild populations, he hopes to convince officials to take steps to preserve these tomatoes before they, too, are bulldozed or blasted with herbicide.

As we left the balmy greenhouse and stepped back into the chilly mist, Chetelat paused before locking the door. Nodding to the vines behind the glass, he said pointedly, “There may be a chance that fifty years from now, someone will find something really important in something that’s growing in there. That is what this is all about.”