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In his office at Iowa State University, Jonathan Wendel slides open a small box to reveal fluffy white fibers spilling out of a seed pod. The distinguished professor of ecology, evolution and organismal biology explains this is Gossypium hirsutum – more commonly known as upland cotton.
The species accounts for 90% of the cotton grown today. Manufacturers turn its fibers into t-shirts, paper money and medical dressings while its byproducts are used in makeup, salad dressing and livestock feed.
Wendel opens a second box with fibers that are shorter, coarser and tan. Researchers collected this and other seed pods from wild cotton in the eastern Yucatán Peninsula in Mexico. Wendel describes the plants as woody perennial shrubs with few flowers, which typically grow near sand dunes.
“[Humans] took this wild, rangy-looking, not very useful plant, and transformed it into this high-yielding, abundant, most important fiber plant in the world,” Wendel said.
Wendel has been studying the origins of upland cotton for 40 years. His prior work suggested the Yucatán was a likely candidate for the center of domestication. But this was confirmed for the first time in a recent paper that Wendel co-authored with 19 other researchers in the U.S., Mexico, Europe and China.
The researchers estimated the initial domestication began roughly 5,000 years ago.
Over time, upland cotton would largely replace three other species humans developed independently around the world. Upland cotton is highly productive and easy to grow as an annual, meaning it can be harvested in one year, Wendel said.
But the global cotton crop gene pool has an “astonishing” lack of genetic diversity compared to the wild populations in the Yucatán, he said. The researchers found two random wild cotton plants had on average twice as many genetic differences as two random modern cultivars of cotton.
“An unintended consequence of domestication is that you lose some of your genetic diversity, and the stuff that you lose might be stuff that you might want in the future,” said Corrinne Grover, another evolutionary biologist at Iowa State University who led the research team’s genomics work.
Genetic losses could include a plant’s natural resistance to a certain disease or high salt concentrations in the soil, she said.
“If we can recover some of those genes out of the wild population, then it can benefit the domesticated varieties,” Grover explained.
David Stelly, a geneticist and cotton breeder at Texas A&M University who did not contribute to the paper, said the research “solidifies past interpretations about the origins of cotton” and “adds quite a bit to our toolset.”
“As someone who's long been interested in capturing value from genetic resources outside of the cultivated species, this [paper] is really a great interest,” Stelly said.
Cotton breeders are often looking for traits that can increase yields and resistance to pests and diseases, Stelly said. Farmers also need cultivars that can handle rising temperatures and less water.
NASA satellite images reflect widespread crop losses this year in the Texas High Plains, the largest cotton-producing region in the U.S. Along with severe drought, groundwater irrigation is out-of-reach or limited due to the long-term drying-up of the Ogallala Aquifer — the region’s main water source.
“Cotton is relatively drought stress and heat stress tolerant, which is why we grow it in Texas,” Stelly said. “That doesn't mean that further improvements can't be made.”
Unlocking wild plants
Wendel said previous work had already sequenced cotton genomes from the important cultivated pools around the world, along with “a smattering of wild forms.” But their latest study significantly added to the collection and provided an advanced analysis of the data.
“We sequenced hundreds of samples and compared them to thousands of existing samples, and what we're basically looking for is changes in the DNA that tell us what groups everything belongs to,” Grover said. “We can find groups for the early cultivars, the ones that would have been in the Central American region and those that would have been in the Caribbean.”
Wendel gave the analogy of companies that provide genetic testing services for people wanting to learn about their ancestry.
“Imagine you have a whole bunch of unknown people,” Wendel said. “If you sequence their DNA and you compare it to the whole library of DNAs from human populations from around the world, you can pinpoint their origins, and that's what we did with these cotton samples.”
The lead researchers say their findings highlight the value of protecting biodiversity and warn that wild cotton is under threat from human activities. Grover said their breakthrough about the origins of upland cotton would not have been possible if wild populations had been destroyed by development.
“While it's very important for us to be able to have places to live and have farms to grow our food, it's also important to make those spaces to preserve the diversity that's out there,” Grover said. “Because we don't know what we've lost, and we don't know what we're going to gain until we look for it.”
This story was produced in partnership with Harvest Public Media, a collaboration of public media newsrooms in the Midwest and Great Plains. It reports on food systems, agriculture and rural issues.
