You say tomatoes, and these scientists say evolutionary puzzle

Biologists at the University of Massachusetts Amherst have found evidence of evolutionary “syndromes” — clusters of traits that occur together — that help explain how tomatoes first evolved their distinctive combination of color, sweetness, acidity and aroma.

The research, represented by a pair of research papers recently published in Plants, people, planet And American Journal of Botanynot only sheds light on how fruit evolved in the wild, but will also be valuable for crop improvement efforts aimed at breeding more nutritious and attractive varieties of fruit.

“Have you ever held a fresh tomato in your hand and wondered why it looked good, smelled good, and tasted delicious?” asks Jacob Barnett, a graduate student in organismal and evolutionary biology at the University of Massachusetts Amherst and the papers’ lead author. It turns out that juicy red tomatoes with their unique flavor have a long, indirect evolutionary history.

Barnett and his colleagues, including Ana Caicedo, a biology professor at the University of Massachusetts Amherst, turned to our modern tomato relatives, a group of several wild species that grow on the west coast of South America, from Chile to Ecuador, to explore this. a question. These wild species are not like what you find in your sandwich or salad today.

“For one thing, they’re small, about the size of a berry,” Barnett says. “Most of them are green when ripe. Many have the smell of apples, watermelon or even cucumbers, and a number of them taste terrible.”

So how did we go from a little green fruit with a bad tasting, smelly watermelon to the wonderful combination of color, sweetness, acidity and umami that makes tomatoes so beloved in pasta sauces, salads and pizza?

It turns out that fruits in the wild tend to have a group of traits that occur together, which biologists call syndromes. For example, many fruits are small, brightly colored, and high in sugar. But gathering evidence of evolutionary syndromes in wild tomatoes has been difficult, because no previous researcher has been able to grow all wild tomato species together at the same time.

“These two studies are the first to look at fruit traits across all species in the entire tomato group,” says Caicedo. “We were able to tell a comprehensive story of how wild tomatoes compare to each other and to our modern cultivated varieties.”

Part of this story involves the collecting efforts of Charles Rick, of UC Davis, who traveled across South America in the 1950s and 1960s collecting seeds from wild species and bringing them back to what later became C.M. Rick’s Tomato Genetics Resource Center. Barnett and Caicedo obtained seeds from 13 wild tomato species, as well as seeds from multiple variants within each species, and then planted them at the UMass Crop and Animal Research and Education Farm in South Deerfield, Massachusetts.

When the plants matured, they were “wild and fragile,” Caicedo says, and at one point Barnett had to hack his way through them with a machete on his way to collect their fruit and leaves. Back in the lab, the team scanned the fruits for color and shape, measured sugar and acid content, and analyzed DNA in leaf samples. With the help of co-author Dennis Tiemann, a research assistant professor at the University of Florida, Barnett measured and classified the volatile organic compounds in each sample — the chemicals responsible for tomato odor.

Not only did the team discover that smell, flavor and color are syndromes, they also discovered that there is what Barnett calls an “honest signal” – a match between the tomato’s external appearance and internal nutritional content. This correspondence supports the controversial hypothesis that animal preferences shaped the evolution of fruit syndromes, because animals would choose some fruits over others if they learned to associate the fruit’s appearance with its unique nutritional reward.

The small, green, melon-scented fruits may be a favorite of small mammals, while the sweet, colorful tomatoes are likely a favorite of birds. How humans prefer tomatoes that birds love is a mystery that has yet to be solved. In fact, as the authors point out, we need more field studies to ascertain which animals eat which fruits: “There are currently no systematic data on which animals eat wild tomato fruits.”