Tomatoland (4 page)

Today’s tomatoes may be big, juicy, and smooth skinned, but on their circuitous journey from the arid hillsides and rocky canyons of coastal
South America to our dinner tables, they lost many of the genetic traits that were once critical to their survival. The pea-size
S. pimpinellifolium
and the other wild relatives of modern tomatoes that Chetelat and his team seek out and attempt to preserve are 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 and certain overpopularized dog breeds, need all the genetic help they can get.

Drop by nearly any farmers’ market on a summer Saturday, and displays of cultivated tomatoes all but scream out the word
diversity
. Small cherry tomatoes, grape tomatoes, pear-shaped salad tomatoes, soft ball–size beefsteak tomatoes the color of fire trucks, plum tomatoes, tomatoes that are ribbed like pumpkins, tomatoes that are as perfectly spherical as a billiard ball, tomatoes that are lobed and lumpy, tomatoes that mature ninety days after being transplanted, tomatoes that require only sixty days, tomatoes that when ripe are red, pink, orange, yellow, purple, green, or any combination thereof. But all that variety is literally only skin deep. Botanists have but one name for all those oddball cultivated tomatoes:
S. lycopersicum.
“Most of the variation you are seeing is from a few genes that control color, shape, and size,” said Chetelat. “Other than that, there is very little genetic variation.”

The mutant plants that the
Mayans domesticated were literally cut off from their ancestral roots, living in isolation more than one thousand miles away from other plants of the same species. As early farmers saved seeds from offspring of the original few plants from year to year, the population became increasingly inbred, a process geneticists call a “
bottleneck effect.” Chetelat draws an example from human migration to explain this phenomenon. “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.” Tomatoes went through a series of such bottlenecks in their prehistoric journey from Peru to Mexico, losing genetic diversity each time, and then went through another series of bottlenecks when conquistadores took them from Mexico to Europe.

The problem of inbreeding is exacerbated in cultivated tomatoes because, unlike their wild brethren who must receive pollen from another plant to produce fertile seeds, they are self-pollinated.
A single domesticated 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, producing generation after generation of plants with the same traits—or lack of them. 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,” said Chetelat. “But at a DNA level they are very similar. If it wasn’t for the genes of these wild species, you wouldn’t be able to grow tomatoes in a lot of areas. I don’t think there is a cultivated plant for which the wild relatives have been more critical.”

I met Chetelat and some of the offspring of those Chilean
S. chilense
one cool, misty January afternoon inside a greenhouse belonging to the Rick Center at the University of California Davis, which is named after its founder, the late Charles M. Rick Jr. Charlie, 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.

Born in Reading
, Pennsylvania, Rick developed a love of
horticulture by working in apple orchards as a boy. After getting a PhD from Harvard, he moved to Davis, where he became a professor of plant genetics. Receiving a Guggenheim Fellowship in 1948, he spent a year in Peru, the first of fifteen field trips he would eventually make to South America to collect seven hundred samples of seeds and other genetic material from populations of wild relatives of tomatoes, many of which have since gone extinct in their native habitat and live on today in the collections of the Rick Center. He combined a photographic memory and an indefatigable work ethic with a puckish character and a natural flair for storytelling. Associates say he possessed attributes of Charles Darwin, Mark Twain, and Indiana Jones.
Until the end of his life, he was an easily recognized character on the U. C. Davis campus, mounted on an ancient, thick-tired bicycle with his full white beard, granny glasses, and floppy cotton fishing hats.

More than anything, however, Rick was big hearted and generous in an academic field where professional relationships are often marked by secrecy and competitiveness. It was Rick who instituted the center’s policy of giving seeds away. The Rick 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.

Today, those seeds are kept in a vault that resembles a restaurant’s walk-in refrigerator. Chetelat ushered me inside. A roaring compressor kept the air at a chilly forty-five degrees and the humidity at a dry 25 percent. The space was jammed with shelves holding trays that were filled with small manila envelopes containing seeds. Periodically, supplies in the vault are augmented by seeds from plants grown in a greenhouse like the one where I met Chetelat.

Had he not been my guide, I would never have recognized the plants that surrounded us in the Davis greenhouse as being even remotely related to the plump, red tomatoes in the produce section. These varieties were perennials with solid, semiwoody stems, not the one-season wonders of my garden. Some plants were almost mosslike, creeping along the soil like thyme. One, called
S. ochranthum
, climbed until it touched the glass roof twelve feet overhead and then doubled back toward the floor. Chetelat told me that its vines can grow fifty feet tall, completely covering small barns and outbuildings in its native Peru.

Foliage came in all shapes, sizes, textures. Some of the flowers were odorless, but others perfumed the air with the aromas of jasmine and honey. The round, scalloped leaves of
S. pennielli
were covered with what seemed like a bad infestation of white, gnat-size flies. When I touched a leaf, my finger stuck to its surface, a natural version of flypaper that entraps would-be pests (and left me with
gummy fingertips). Another plant,
S. juglandifolium
, bore leaves that were tough, wrinkled, and leathery looking, as if they had fallen from an ancient walnut tree. When rubbed, they gave off piney notes.
S. sitiens
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 fold themselves in half to preserve moisture. 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. Crushed between my fingers, the leaves exuded 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: black, yellow, purple, green with white stripes, green with a purplish blush. One plant,
S. habrochaites
, a native of the high Andes, bore tiny, furry fruits that smelled like Vicks VapoRub. Chetelat said that the red color we so closely associate with tomatoes was a one-time genetic event carried by a single member of the tomato family,
S. pimpinellifolium
.

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 distinct taste of soap assaulted my mouth, followed immediately by a dry, burning bitterness that lasted… and lasted. “You’re the first visitor here who has been brave enough to eat one of those things,” Chetelat said nonchalantly. “Hopefully you won’t die.” 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?

But the Rick collection is not really about taste. Domestic tomatoes had virtually no innate
resistance to common tomato diseases and pests until breeders began crossing them with wild species in
the 1940s. “They were fairly a defenseless lot,” explained 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 catalog, 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 (the disease that wiped out tomatoes in Florida in the early 1900s),
grey leaf spot, nematodes (microscopic worms),
tobacco mosaic, and
verticillium wilt. “Most of the efforts so far have been focused
on disease,” said Chetelat. “It’s been the first target, because disease resistance often determines whether or not you can grow a tomato, period. But secondly, on a genetic basis, disease resistances tend to be simply inherited. For the most part, you are talking about single, dominant genes that are fairly easy for geneticists and breeders to work with, whereas breeding for something like increased yield or improved flavor involves multiple genes, so it is harder for researchers to get a handle on those issues. Disease resistance is the best justification for this facility.”

Chetelat pointed to an example from his own backyard tomato patch. “I happen to live in an area where the soil is infested with nematodes, which are so much of a problem that I cannot grow heirlooms. They immediately get sick, develop root galls, and collapse before I get much fruit. So I grow nematode-resistant hybrids. That resistance comes from a wild species called
S. peruvianum
. It’s native to Peru and bears small green fruits that are inedible—they’d probably make you sick to your stomach. But it is resistant to nematodes. That resistance has been bred into hybrids.”

And more potential remains untapped. Any grower in the Northeast in the summer of 2009 who had to dig up and either bury or burn every wilted, blackened tomato vine in the garden is familiar with the ravages of
Phytophthora
, commonly known as
late blight, the
same mold that killed the potato crop in Ireland in the 1840s, causing the Great Famine. Chetelat told me that there are wild species that are 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-carotene, 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 thirty-five hundred 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 an existence in the desert. 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 said he viewed it as a time of opportunity.

But, unfortunately, time could 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 insisted 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 greenhouses with adequate water, optimum lighting, and no competition. Pests and diseases are chemically controlled. “You’re really changing the environment,” he said. “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,” said Chetelat. “Of course, that’s not the way it works.”