What's a potato? A 9-million-year-old tomato

As a foodstuff, the potato can be baked, boiled, mashed, smashed, hashed, roasted, scalloped, fried and more. As a crop, it is among the world's most important, with more than 350 million tons produced annually. Its efficiency -- it requires less land than wheat or rice -- and its ability to grow in a variety of environments has made it essential to global food security.

For all that, the plant's origins have remained obscure. Everybody eats potatoes, it seems, but nobody can say where they came from.

Now scientists can, and the answer is: tomatoes. According to a study published Thursday, potatoes may have arisen 9 million years ago through the combining of genetic material from Etuberosum, a group of potato-like plants from South America, and wild tomato plants. According to the study, this hybridization event led to the origin of the potato plant's distinctive feature, the tuber, an underground structure that stores nutrients and, as humans eventually discovered, is edible.

"A potato is the child of tomato and Etuberosum," said Zhiyang Zhang, a researcher at the Chinese Academy of Agricultural Sciences and the lead author of the study, which was published in the journal Cell. "We did this analysis and we found, 'Oh, he's a child of two plants.'"

Scientists have long noted that, aboveground, modern potato plants closely resemble the subgroup of South American species called Etuberosum. But Etuberosum plants do not bear tubers. And genetically, potatoes appear to be more closely related to tomatoes; both fall under the shared genus Solanum. This was confounding: Why did potatoes resemble one plant but share kinship with another?

To solve this enigma, a team of international scientists analyzed 128 genomes from the three sister lineages (tomatoes, Etuberosum, and potato plants and their wild relatives), plus three eggplant species as an outside group. The researchers found that the modern spud had a mixed ancestry, which arose from a hybrid tomato and Etuberosum lineages 8 million to 9 million years ago and led to the origin of tubers. This hybridisation may have enabled subsequent potato species -- there are more than 100 today -- to diversify and expand their range across the high Andes, where colder climates prevailed.

"It was a very well-done study," said Esther van der Knaap, a plant geneticist at University of Georgia who was not involved in the research. "It provides a model of how this could happen in many other cases."

At first, the combination of two different plants may not have yielded anything noteworthy. "There's some ancient mixing of genomes, and there's some miserable plants coming out of that," van der Knaap said. But over time -- tens of thousands to perhaps millions of years -- natural selection led to "a whole new species complex," she said.

The study indicates that the genes responsible for tuber formation were a combination of the genetic material from each evolutionary parent. The gene that tells the plant when to start making tubers, called SP6A, originated with the tomato, whereas the gene that controls growth of the underground stems that develop into tubers, known as IT1, came from Etuberosum.

Pingxian Zhang, another lead author from the Chinese Academy for Agricultural Sciences, expressed excitement at the possibilities for future studies. Only a handful of potato species are cultivated, and improving on them has posed challenges: As a crop, they are typically propagated through cloning, which limits their genetic variety and makes them more vulnerable to diseases. Engineering potatoes with genetic material from tomatoes could be a promising way forward, Zhang said.

In the last few years, researchers have been able to analyse genomes at a much larger scale than before and infer previously opaque evolutionary connections. These studies have "revolutionized our understanding of what might have occurred in the past," said Leonie Moyle, an evolutionary biologist at Indiana University, Bloomington, who was not involved in the new research. Regarding the new results, she said, "if the inferences are correct, they could be exciting."

Richard Veilleux, a horticulturist at Virginia Tech who has traced the origins of the cultivated potato back thousands of years, described the study as "a creative use of genomics."

"One of the difficulties with evolutionary studies, of course, is that the species that existed 8 million years ago don't exist anymore," he said. "Now we know where potatoes came from a little bit better than we did before."