Potatoes Evolved From … Tomatoes?

The Atlantic

2 days ago

The annals of evolutionary history are full of ill-fated unions. Many plants and animals can and do sometimes reproduce outside of their own species, but their offspring—if they come to be at all—may incur serious costs. Mules and hinnies, for instance, are almost always sterile; so, too, are crosses between the two main subspecies of cultivated rice. When lions and tigers mate in zoos, their liger cubs have suffered heart failure and other health problems (and the males seem uniformly infertile).
For decades, evolutionary biologists pointed to such examples to cast hybridization as hapless—'rare, very unsuccessful, and not an important evolutionary force,' Sandra Knapp, a plant taxonomist at the Natural History Museum in London, told me. But recently, researchers have begun to revise that dour view. With the right blend of genetic material, hybrids can sometimes be fertile and spawn species of their own; they can acquire new abilities that help them succeed in ways their parents never could. Which, as Knapp and her colleagues have found in a new study, appears to be the case for the world's third-most important staple crop: The 8-to-9-million-year-old lineage that begat the modern potato may have arisen from a chance encounter between a flowering plant from a group called Etuberosum and … an ancient tomato.
Tomatoes, in other words, can now justifiably be described as the mother of potatoes. The plant experts I interviewed about the finding almost uniformly described it as remarkable, and not only because dipping fries into ketchup just got a little more mind-bending. Potatoes represent more than the product of an improbable union; they mark a radical feat of evolution. Neither of the first potato's parents could form the underground nutrient-storage organs we call tubers and eat in the form of sweet potatoes, yams, and potatoes. And yet, the potato predecessor that they produced could. Tubers allowed the proto-potato plant to flourish in environments where tomatoes and Etuberosum could not, and to branch out into more than 100 species that are still around today, including the cultivated potato. It's as if a liger weren't just fertile but also grew a brand-new organ that enabled it to thrive on a vegan diet.
Scientists have spent decades puzzling over potatoes' origin story, in large part because the plants' genetics are a bit of a mess, Ek Han Tan, a plant geneticist at the University of Maine who wasn't involved in the study, told me. Researchers have struggled to piece together the relationships among the 100-plus potato species found in the wild; they cannot even agree on exactly how many exist. And when they have tried to orient the potato in its larger family, the nightshades—which includes tomatoes, eggplants, peppers, and Etuberosum—they have found mixed clues. Some evidence has seemed to point to the potato being a tomato derivative: Large stretches of their genomes resemble each other, and the two crops are similar enough that they can be grafted together into a plant that produces both foods. But other patches of the potato genome look more similar to that of Etuberosum, which bears flowers and underground stems that are far more potato-esque than anything that the tomato sports. 'We couldn't resolve the contradiction for a long time,' Zhiyang Zhang, a biologist at the Chinese Academy of Agricultural Sciences, and one of the paper's lead authors, told me.
To settle the potato paradox, Zhang and his colleagues amassed more than 120 genomes from dozens of species spanning the potato, tomato, and Etuberosum groups and tried to piece together a narrative. One explanation for all of the shared genes, for instance, might have been that the potato lineage originally split off from the tomato one, then crossbred with Etuberosum later on. If that were the case, the genomes of more ancient potato species would be expected to look more tomato-like, and more modern ones should carry more of Etuberosum's genetic baggage. Instead, the researchers found that all of the potato genomes they sequenced had about the same tomato-Etuberosum split. That points to a possibility that potato researchers hadn't really considered before, Helen Tai, a plant geneticist with the Canadian government's agricultural department, told me. The entire potato lineage must have sprung from the same ancient source: a fusion between tomato and Etuberosum that persists, in a multitude of forms, into the modern day.
The key to that success seems to have been the hybrid's newfound ability to tuberize, a feat that neither of its parents managed, because each lacked the necessary genetic accoutrement. Only the proto-potato had the proper combination: underground stems from Etuberosum that provided a structural scaffold for the tubers, and a genomic switch from the tomato that told the tubers to grow there. Many hybrids struggle to sexually reproduce, but the proto-potato one didn't have to: The plant's underground storage organs (that is, the potatoes) allowed it to propagate asexually. (Potatoes can still be cloned today—just bury bits of one in the ground—but sometime in the past 8 to 9 million years, the plants gained the ability to reproduce sexually, too, a shift that scientists are still puzzling through.) Ancient tomatoes and Etuberosum were native to different stretches of the western coast of South America. But the proto-potato was able to colonize colder, higher, drier environments, allowing it to spread as far north as Arizona and west, out to the coasts of Argentina, Uruguay, and parts of Brazil. 'That's what a tuber does for you—it allows you to survive better in stressful conditions,' C. Robin Buell, a plant-genomics expert at the University of Georgia who wasn't involved in the study, told me.
Hybridization in nature still, more often than not, ends in tragedy—'offspring that are sterile, inviable, maladapted, or mixed up in some negative way,' Robin Hopkins, an evolutionary biologist at Harvard who wasn't involved in the research, told me. But through the sheer power of mixing genes into new combinations, the risky gamble of interspecies pairings has also sometimes majorly paid off. Hybridization among East African cichlids seems to have triggered an explosion in the diversity of certain genes important for eyesight, helping the animals navigate waters of varying murkiness and depth. Certain frogs have been documented soliciting mates outside of their own species to up the chances that their offspring will survive periods of drought. Our own ancestors mingled with Denisovans and Neanderthals, equipping modern humans with traits that may have helped us adapt to new environments. Today, farmers frequently breed different species of crops together to boost yield or hardiness against extreme weather and disease. The potato's innovations, though, are still exceptional. Rather than just collapsing its parents' various traits together, this ancient hybrid struck out on its own evolutionary path.
Although that proto-potato is long gone, understanding its origins could still keep fries and hashbrowns on modern tables. Cultivated potatoes are prone to disease, and—thanks to their four-copy genomes—a pain to breed and genetically manipulate. Some scientists are trying to address those issues by developing a two-copy-genome potato. But the past could offer another avenue toward sustainable spuds, Yiyuan Ding, a biologist at Huazhong Agricultural University and one of the paper's lead authors, told me. Perhaps, with some genetic help from Etuberosum, scientists might someday coax tomato plants into producing edible underground tubers of their own.