Answers to these botanical mysteries could help a climate-stressed world

The unparalleled genetic diversity found at critical crops’ origin sites can help breeders create drought and heat-resilient varieties.

Whether you’re munching spicy kimchi on a Seoul street corner, tucking into a hearty turnip stew to get through a northern European winter, or enjoying crispy collards in a Southern soul food joint, your meal is anchored by one of just two species: Brassica rapa or Brassica oleracea. Eat anything fried in canola oil, as millions do daily, and you're benefiting from a hybrid between the two: Brassica napus.

For over a century, scientists have puzzled over the origin stories of these globally important food plants. Now, researchers have analyzed DNA from hundreds of wild brassicas around the world and declared the mysteries solved: Brassica rapa—cultivated as turnips, bok choi, napa cabbage, and more—came from mountains near the Afghanistan-Pakistan border, and Brassica oleracea—better known as broccoli, cauliflower, collards, kale, and Brussels sprouts, among others—originated in the eastern Mediterranean.

The findings should set off immediate efforts to collect and conserve plants from these regions, researchers say. As the world speeds toward a hotter, more volatile future, crops such as brassicas—as well as corn, wheat, and other crops that underpin global food security—could face unprecedented challenges from heat, drought, and disease. The unparalleled genetic diversity found at crops’ origin sites can help breeders create new, resilient varieties to feed a hungry, climate-stressed world.

“I really believe that these types of studies are crucial,” says Michael Purugganan, a biologist at New York University who was not involved in the research. “They set the foundation for a more intelligent approach to try to look for genes involved in adaptation.”

‘Dogs of the plant world’

Brassicas have long amazed, thrilled, and confounded biologists and food growers alike. They have been called the dogs of the plant world for the stunning variety of vegetables breeders have coaxed out of them: starchy roots; massive broccoli and cauliflower florets; the collard greens cultivated in Africa and later the American South; the panoply of Asian greens. Brimming with vitamins and nutrients, at least $14 billion worth of brassicas are sold annually around the world.

Cabbage

Illustrations by Andi Kur

That same diversity makes pinpointing the brassica origin story especially difficult. Cultivated brassicas easily “jump the fence” and start growing wild again, like a domesticated dog turning feral, says botanist Alex McAlvay at the New York Botanical Garden in the Bronx. From the Americas to far east Asia, yellow-flowered brassicas sprout in coastal meadows, along roadsides, and in farm fields, where farmers often encourage them, to add variety to their diets.

Many people have claimed, or at least speculated, that their brassicas were the first, from western Europe to east Asia and many places in between. Charles Darwin himself wondered if wild cabbages along the English coast were the ancestors of Brassica oleracea.

Brassica genetics, says Chris Pires, an evolutionary biologist at the University of Missouri in Columbia, are “a lovely mess.”

Geneticist Makenzie Mabry, of the Florida Museum of Natural History in Gainesville, agrees. “Brassica is infamous for being a difficult plant” to study, she says. “They all like to share their pollen with each other”—meaning that wild-growing brassicas are often mutts with multiple ancestors.

Detective work

To solve the mysteries, two teams of researchers rounded up seeds from seed banks and collections around the world. One team, led by McAlvay and including Pires and Mabry, used modern genomics technology to sequence parts of the genomes of more than 400 samples of Brassica rapa—far more than had been sequenced in any previous study of the crop’s origin—and environmental modeling to determine where brassicas could grow.

The researchers also enlisted the help of linguist and archaeologist colleagues to bring in other kinds of evidence, such as references in ancient literature to turnips and other brassica crops and findings from ancient village sites. “It’s detectives all working on different aspects of the same story,” says McAlvay.

They found that the genetic sequences and literary references converged on the Hindu Kush, a rugged mountainous region in Afghanistan near the border with Pakistan and Tajikistan. The first vegetable that was domesticated was the turnip, between 3,500 and 6,000 years ago. Only later did breeders create leafy varieties such as tatsoi, bok choi, and broccoli rabe, as well as seeds such as turnip rape, used for oil, and sarson mustard, used in Indian cooking. The researchers reported their findings in the journal Molecular Biology and Evolution.

For Brassica oleracea, a similar analysis led by Mabry and Pires, and including McAlvay and others, of more than 200 samples pointed strongly to islands in and around the Aegean Sea between Greece and Turkey as places of origin.

That study will be published later this month in the same journal; a preprint is available on bioRxiv.

The research “advances our understanding in a major way,” says Purugganan. “This was the first really systematic analysis at the level of population genomics for these species.”

But, he adds, “I don’t think it closes the book.” More analysis of plants collected from the origin sites are needed to confirm the findings, he says.

Diverse origins

The findings come at a tough time for global farmers and eaters. Hot weather and more droughts and floods are already hammering crop yields in some areas, and the number of hungry people has started to tick upward after decades of decline. Brassicas are mostly adapted to colder climates, making them potentially vulnerable in places where they’re a pillar of the diet. For example, Korean researchers have found that Chinese cabbage, the foundation of Korea’s national condiment, kimchi, is vulnerable to both heat and drought.

Rapeseed, the hybrid of Brassica rapa and Brassica oleracea grown globally for canola oil, may be most critical to the global food supply. Commercial rapeseed varieties have little genetic diversity, giving growers few resources for breeding in climate resilience. “There’s definitely a lot of room for improvement,” says Annaliese Mason, a plant breeding researcher at the University of Bonn in Germany.

The new findings could help, researchers say. Crops’ origin sites contain far more genetic diversity than elsewhere. Plant breeders often seek new genes from such sites to improve traits like disease resistance, better flavor, and drought and heat tolerance. Breeders, for example, have used genes from wild potatoes to defend against the devastating blight that caused the Irish potato famine. Genes found in wild wheat helped breed resistance to rust fungus into major wheat varieties that spurred the Green Revolution, which drastically reduced hunger and malnutrition in South Asia in the 1960s and 1970s and is credited with saving millions of lives.

It’s now urgent, researchers say, to collect and conserve seeds from the newly-identified brassica origin sites before they’re wiped out by human or natural threats. McAlvay worries that few Brassica rapa seeds from the Hindu Kush show up in global seed banks, and that warming could force the plant to migrate up mountainsides. It's a perilous fate facing many mountain-dwelling species, since they will reach the peak eventually and suffer ever-shrinking growing area in the meantime. Small populations on islands, where Brassica oleracea likely originated, can be especially vulnerable, Mabry adds.

The Brassica oleracea team had planned to travel to Mediterranean islands such as Crete and Cyprus to collect seeds before the COVID-19 pandemic halted travel; they hope to make the trip in 2022.

In addition to original ancestors, feral plants in other diversity hotspots should also be collected and conserved, adds Eve Emshwiller, an ethnobotanist at the University of Wisconsin-Madison who coauthored the Brassica rapa study. Wild-growing varieties are often seen as weeds, and farmers are sometimes advised to eradicate them.

“Whatever the future will throw at those crops, we really need to keep all the pieces,” Emshwiller says. “All the varieties of the crops, the diversity of alleles and genes in them, and to keep wild relatives as well from going extinct.”

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