Decades of search for Cotton’s Birthplace

 Cotton’s birthplace revealed in Northwestern Yucatán after decades of research 

Cotton is so common that most people rarely stop to think about where it came from. It fills closets, homes, hospitals, and factories around the world. Genome sequencing of nearly 300 wild plants confirms exactly where modern cotton came from, and how much diversity was left behind when it got there. Jonathan Wendel keeps a cotton boll in his office at Iowa State, the kind that comes off a commercial field: a soft white burst of fiber exploding out of a capsule about the size of a golf ball. Next to it he sometimes sets a wild boll, and the contrast is almost comic. The wild capsule is smaller, harder, and the fiber inside is short, coarse, and the color of weak tea. Nobody looking at one would guess it’s related to the other. Wendel has spent forty years trying to explain how it happened anyway. The soft white fibers used in modern textiles look very different from the cotton that grows in the wild. Wild cotton plants produce much smaller bolls and shorter, darker fibers. Over thousands of years, people transformed those plants into one of the world’s most important crops. Scientists have long wanted to know exactly where that transformation began.

A new study has now provided the clearest answer yet. By combining newly collected wild cotton samples with advanced genetic analysis, researchers traced the origins of modern cotton to a diverse wild population in Mexico’s Yucatan Peninsula. A team led by Weixuan Ning and Corrinne Grover, with Wendel as senior author, sequenced 299 newly collected wild cotton plants from the Yucatán Peninsula and the coast of southwestern Florida and compared them against existing genomic data from domesticated cultivars, early landraces, and two related wild species. What they found does more than confirm a decades-old hypothesis. It sharpens it considerably. Upland cotton, Gossypium hirsutum, the species that accounts for roughly 90% of cotton grown today, traces its domestic lineage to one specific patch of coastal scrubland in northwestern Yucatán, a region which still carries more genetic diversity than anywhere else the species exists in the wild. The journey to solve this puzzle took far longer than the study itself.

Jonathan Wendel, a distinguished professor of ecology, evolution and organismal biology at Iowa State University, has spent much of his career studying cotton genetics. “This is my 40th year on faculty, and I came here with this project in mind. And it took 40 years to develop the resources, tools and technologies to solve the problem,” he said. Scientists had suspected for years that the Yucatan Peninsula was the birthplace of domesticated cotton. Earlier research pointed in that direction, but the evidence was not precise enough to settle the question. As the technology became faster and more affordable, researchers gained the ability to compare hundreds of cotton genomes in remarkable detail. The challenge was finding enough wild cotton plants to study. Wendel spent decades collecting specimens from herbarium shelves and natural habitats throughout the Caribbean region and beyond. “If everything you’re looking at has crazy new variation, you clearly haven’t reached saturation. But if the next 10 things look like the last thing you picked and everything’s forming a nice tight cluster, well, why bother to keep doubling up?” Wendel said. The project also relied on extensive field collections in known wild cotton populations across the Yucatan. Corrinne Grover, an Iowa State research scientist and assistant adjunct professor, led much of the sequencing work and data analysis. “Our collaborators did an amazing job sampling across the Yucatan strategically, and once we had that sequencing data it was very clear that’s where it came from,” she said.

Wild cotton has short, brown, and coarse fibers, while modern domesticated cotton has white, fine and abundant fibers. The hypothesis itself isn’t new. Wendel and colleagues proposed a Yucatán origin more than thirty years ago, using allozyme markers and RFLP data that, by today’s standards, amounted to squinting at the problem through a keyhole. The basic geography held up, but the methods couldn’t distinguish a broad regional origin from a precise one, and they couldn’t rule out the possibility that the “wild” cotton being sampled wasn’t actually feral, escaped cultivars gone rogue and readopting wild traits while still carrying the genomic fingerprints of domestication underneath. Telling truly wild cotton apart from feral cotton turns out to be one of the harder problems in this field, since a domesticated plant can drift back toward wild-looking morphology within a few generations while its genome quietly remembers where it came from.

After cotton spread beyond its birthplace, one species rose above the rest. The Mexican species, Gossypium hirsutum, commonly known as upland cotton, eventually became the dominant form grown around the globe. Other cotton varieties were domesticated independently in places such as South America, Africa and India. Yet upland cotton steadily expanded until it became the world’s primary source of natural textile fiber. Researchers found that this success did not come from a few dramatic genetic changes. Instead, it appears to have resulted from many generations of gradual improvement by farmers selecting plants with desirable traits. Researchers compared cotton genomes in several ways. They measured genetic differences between plants and examined which populations were most closely related. The results consistently pointed to the northwestern corner of the Yucatan Peninsula. Cotton domestication is believed to have started there about 5,000 years ago. “Essentially, we’re building huge data-powered genealogies of these plants, just like you could with people,” Wendel said.

Sorting that out required two things the earlier studies didn’t have: cheap whole-genome sequencing and an enormous number of plants collected in the right places. The team built a new reference genome from a Yucatán accession, assembled from PacBio and Hi-C data into 26 chromosomes, and used it as the scaffold for comparing 392 cotton genomes total, spanning Florida, Puerto Rico, Guadeloupe, the Yucatán, modern cultivars, two early landrace groups, and the related species. Domestication often creates a genetic bottleneck. Farmers choose plants with useful characteristics and repeatedly breed them. Over time, many other genetic traits disappear from the population. “When humans domesticate a plant, you pick from a big population and everything else is left behind. Do that for 1,000 generations, and you have a very narrow genetic base,” Wendel said. "So we’re very interested in that wild genetic diversity. We want to know what’s still out there.” The study found that two randomly selected wild cotton plants from northwestern Yucatan contain, on average, twice as many genetic differences as two randomly selected modern cotton varieties. “As it turns out, cultivated cotton was poured out of a very small genetic bottleneck,” Wendel said.

The value of these discoveries goes far beyond solving an ancient mystery. Wild plant populations often contain traits which help them survive harsh environments, resist diseases, tolerate salty soils or cope with changing conditions. Many of those traits may have been lost during thousands of years of selective breeding. Researchers observed that while domesticated cotton plants produce larger, fluffier bolls, wild plants frequently appear healthier overall. “We know there are genetic traits in wild populations that could be useful if we can figure out what they are and get them into domesticated cotton,” said Grover. “Now we have all this data from the Yucatan, and it’s ready to be mined.” As climate challenges grow and agricultural demands increase, those forgotten genes could become some of cotton’s most valuable resources. Understanding where cotton came from may help scientists build stronger, more resilient crops for the future.