Renowned geneticist Charles M. Rick spent 50 years collecting specimens of the stinky, sour-fruited South American vines from which all cultivated tomatoes arose.
Tomatoes are a wonderful example of how we humans created a modern fruit. Modern tomatoes (Solanum lycopersicum) are far removed from their wild ancestors in both taste and appearance, and they’re also remarkably homogeneous, despite the range of colors, shapes, and flavors heirloom tomatoes exhibit. The domesticated tomato has gone through several genetic bottlenecks as travelers carried tomato seeds from the Andes in South America to Mesoamerica, then to Europe, and from there to the rest of the world. Each time people carried the tomato over a long distance, only a handful of seeds from the source location made the trip. By the time travelers and explorers introduced the tomato to Europe, cultivated tomatoes had only 5 percent of the genetic variation of their wild relatives. And yet, the modern tomato sits atop the produce heap as the No. 1 fruit vegetable — produce that’s commonly considered a vegetable, although botanically it’s a fruit — produced in the world.
To better understand tomato genetics, and with particular interest in wild tomatoes’ resistance to pests and drought, the late botanist Charles M. Rick made a number of trips to the western coast of the Andes, where tomatoes originated. Once described as “Charles Darwin and Indiana Jones rolled into one,” Charles explored Peru, Ecuador, and Chile for wild populations of tomato species and other members of the nightshade family over a 50-year period, starting in the 1940s, and he brought his family along on many of his expeditions.
The western Andes are a rich genetic source for tomatoes, and a challenging place to collect samples. In the shadow of the Andes — running 4,700 miles from Panama in the north to the southern tip of South America — lies roughly 300 miles of foothills and coastal plains sloping down to the Pacific Ocean. More than 50 rivers drain off the steep western slopes, more or less straight into the Pacific Ocean. The river valleys are separated by miles of dry land, making the valleys “island” habitats, in geneticist terms. The intervening inhospitable ground effectively cuts off gene exchange among plants in different river valleys, which allows them to develop in vastly different ways without being influenced by other species. It is here, in the western Andes, that the story of domesticated tomatoes begins.
John Rick remembers his father as “Charley” Rick, a treasure hunter in many ways, but far from seeking ancient relics, he was after plant diversity in the form of particular wild relatives of modern tomatoes. Finding his treasures was a matter of locating plant populations by poring over old herbarium records and making educated guesses from local topography. His field excursions to collect and record wild tomatoes in their native environments provided crucial information in identifying the wild ancestors of modern cultivated tomatoes, and his research efforts led to his election to the National Academy of Sciences in 1967.
In 1956, when John was 6 years old and his sister, Susan, was 10, getting anywhere took a lot more time than it does now. Getting from San Francisco to Lima, Peru, for example, took several weeks by freighter. But John remembers such travel as part of a glorious adventure — no doubt a view of the world influenced by his father. John explains that Charley “knew how to get himself out of things; he had an excellent sense of do-it-yourself-ism.” To Charley, it didn’t matter what the situation was; it was all fun.
That cheerful approach to travel was essential, because the Rick family’s self-made expeditions out of Lima, Peru, invariably met with one challenge after another. They never knew exactly where they’d end up. Sometimes, when John asked where the family was going to sleep, Charley’s answer was, “We have sleeping bags, don’t we? We’ll just bunk down here.” When they had a hotel room, sometimes the hotel was nothing more than “a few walls which were less walls than holes.”
Another daily challenge was the rickety, rented Volkswagen minivan. It was reliably unreliable, but Charley’s view was simple: If a fan belt on the van broke — not to mention one of the many other ways the van could and did break down — it was nothing to worry about. Charley’s wife, Martha, and their children, John and Susan, would hitch a ride along the Pan-American Highway to the nearest town to get parts. Charley would stay with the van, typing field notes, drying plants, checking his rudimentary maps, and plotting the next route in pursuit of wild tomato plants. As the family picked up more Spanish through conversations with the locals, they were able to get clues from people describing the wild tomatoes they’d seen.
The Ricks collected wild tomatoes by driving along the rough, dusty roads of the arid Andean coastal plain while the family looked out the windows for the small, sprawling wild tomato and its sister species. Martha, in particular, became an expert at spotting these plants, even when the van was rumbling along at 45 miles per hour. “She’d say, ‘There’s a peruvianum, but I don’t think it has mature fruit,’ ” John recalls. “We’d turn around and check the plant, and she was right most of the time!”
Of course, then Charley would pull off the road to collect the fruits and make notes about the plants and their habitat, which could take hours. The children were instructed not to eat the fruits. “Not that one would be tempted,” John says. “The plants are very stinky.” The musk of wild tomato species is much stronger than that of modern tomato cultivars. Wild tomatoes are sticky, hairy, and generally unappealing. Years later, John remarked that the wild Peruvian currant tomatoes (Solanum pimpinellifolium) he grew in California “are frankly a lot of work for very little fruit, and the flavor is no great shakes.” Nonetheless, in the field, Charley would pop a few marble-sized fruits in his mouth — sometimes the fruits had a sweetness reminiscent of domesticated tomatoes.
The Rick family also booked passage to the Galápagos Islands, about 525 miles west of Ecuador. It took months to get permits and book passage on freighters for the entire family. John describes his father “disappearing over the horizon” while his family waited months for a better, but still rustic, vessel to carry them to the Galápagos to join Charley.
In the Galápagos, a fisherman helped Charley in his search for ever more samples, dropping him off on one side of an island early every morning, and picking him up on the other side every evening. Charley discovered that the wild tomatoes on the Galápagos have an unusually high level of traits that differ from those of their mainland ancestors. The populations were very small, and the islands lacked insect pollinators. Each plant could only breed with itself or close neighbors, which allowed mutant traits to surface. Check out the Summer 2018 Seed Shop to find a source for Galápagos wild tomato seeds.
Charley’s persistence in finding populations of wild tomatoes and their relatives, gathering seeds, and carefully describing his specimens and their habitats gave rise to a seed bank at the University of California, Davis, that now serves as the main source for disease-resistant genes to infuse into modern tomato cultivars. Charley and his colleagues also used his collection to figure out how thousands of wild tomato species and other nightshades are related, revealing more clues to the modern tomato’s origins and extremely restricted genetic diversity.
Charley realized that the remarkable homogeneity of domesticated tomatoes (S. lycopersicum) outside western South America, compared with their wild relatives, was due to a combination of factors that promoted changes in the frequency of gene variants in the population, known as “genetic drift.”
The winnowing of gene variability in tomatoes began with the transport of some wild tomato seeds from South America to Central America. Once there, the ancestors of the Aztec people began cultivating tomatoes. Centuries later, Bernardino de Sahagún, a friar who arrived in the Aztec capital of Tenochtitlan in 1529, reported that the marketplace offered a wide array of colors, sizes, and shapes of tomatoes. They were most commonly yellow or red, usually with deep vertical furrows in the fruit, rather than the smooth surface preferred in modern tomatoes. These tomatoes were sour and were used mainly as a condiment.
The next genetic bottleneck for tomatoes came with the Spanish conquistadors carrying seeds home. The conquistadors transported relatively few tomato seeds to Europe, and of that tiny sample of the Mesoamerican diversity of tomatoes, only a fraction survived. Those few survivors form the entire genetic background of modern heirloom and hybrid tomatoes. As a result of this compounded genetic drift, modern tomatoes carry only a tiny percentage of the total genetic variation within the Solanum genus to which they belong.
Since Charley Rick’s family field trips to gather wild nightshades more than 60 years ago, many wild tomato populations have gone extinct in their native habitats in the western Andes because of intensified agriculture, widespread use of herbicides, and mining activity. Time is running out to stock up on the genetic reserves still hidden in the tomato’s homeland — one more reason seed banks, such as the Tomato Genetics Resource Center, that houses Charley Rick’s wild tomato collection, continue to be of paramount importance.
Italian herbalist Pietro Andrea Mattioli earned fame in 16th-century Europe with his Latin translations of Pedanius Dioscorides’ herbal, Materia Medica. Mattioli’s 1554 edition includes what is believed to be the first printed evidence of tomatoes in Europe (see slideshow). For roughly 200 years after South American tomatoes arrived, European herbals emphasized the connection to tomatoes’ more toxic cousins in the Solanaceae family, such as nightshades. Tomatoes’ deadly reputation wasn’t completely unwarranted: The pewter tableware used by many wealthy Europeans would release lead when high-acidity tomatoes were placed on it, and many illnesses and deaths resulted.
The University of California, Davis, maintains one of the world’s most comprehensive collections of genetic material for wild tomato species and their relatives. Charles M. Rick’s collection of wild tomato seeds and samples are the basis of the collection, along with material from other sources. You can learn more about the TGRC collection and programs on the Tomato Genetics Research Center website.
Nancy Stamp is a biologist who has conducted research on tomatoes and their insect pests at the State University of New York, Binghamton. For this piece, she interviewed John Rick about traveling through South America in the 1950s with his father, Charles M. Rick, to collect wild tomato specimens.
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