New Evidence For Corn's Ancestry Could Lead To Insect And Drought Resistant Crops

June 04, 1997

DURHAM, N.C. -- Scientists continue to debate the ancestry of domesticated corn. But Duke University researcher Mary Eubanks said she has mounting evidence that corn emerged from the interbreeding of two different wild American grasses.

Those grasses are Tripsacum dactyloides, also known as Eastern gamagrass, and Zea diploperennis, a perennial variety of teosinte. By successfully crossing them for the first time, Eubanks has produced fertile offspring that closely resemble the earliest known samples of primitive domesticated corn, she said in a recent interview.

Patented lines of her crosses are fertile over several generations and carry genetic traits that are "missing links" in corn's evolution, Eubanks adds in the latest issue of the journal Theoretical and Applied Genetics.

That article updates experiments originally described in the June, 1995 issue of Economic Botany. She will report on her work again at an August meeting of the American Institute of Biological Sciences in Montreal.

If Tripsacum is indeed a corn ancestor, then breeders could take advantage of some of the special traits of that hardy ubiquitous plant, the Duke botany research scientist said.

Indeed, Eubanks has crossed her lines with modern corn to produce hybrids that share Tripsacum's resistance to the corn rootworm. That's the root-eating juvenile -- or larval -- stage of an insect that costs corn farmers $1 billion a year in crop losses and pesticide expenses.

In preliminary experiments at Duke's Phytotron, a high-tech greenhouse where she rents space, Eubanks found far less root damage and far fewer larvae in her experimental hybrids than occurs in normal commercial corn.

"We're using the western corn rootworm, which is the most serious pest in the Midwest," she said. "It's an incredible problem and a big expense for corn growers. Because they don't know where they are going to have an outbreak, they now have to treat all their fields with extremely toxic chemicals."

Tripsacum is drought resistant too, and Eubanks plans to test her experimental hybrid for that trait as well.

Moreover, her Tripsacum-teosinte hybrids are perennials, and could conceivably be exploited to produce a perennial variety of corn, she added. All of today's commercial corn lines are annuals, meaning that their seeds must be planted anew each year.

Eubanks holds four patents on various Tripsacum-teosinte crosses developed during the past decade. She is also president of Sun Dance Genetics, a Durham, N.C. research company.

Corn is "an anomaly in the botanical kingdom," Eubanks said. "You have this tremendous ear. The ear has hundreds of kernels, held together on a rigid stalk and enclosed by a husk so they're easy to harvest and dry and store. It provided a wonderful food for the original inhabitants of the Americas once they began cultivating it."

But the earliest known versions of corn have far smaller ears that bear fewer and tinier kernels, providing silent testimony to the corn genome's great adaptability.

Scientists are in agreement on the ancestry of the Old World's simpler staple grains -- oats, rye and wheat. But the origins of the New World's major grain remain cloaked with mystery. That's true even though "this is one of the most well studied organisms." she said.

With a Ph.D. in anthropology, Eubanks would seem an unlikely person to pursue research into the molecular biology of corn genes. But, while still in graduate school at the University of North Carolina, she met Paul Mangelsdorf, a premier corn researcher who had retired from Harvard to surrounding Chapel Hill.

As an anthropologist, Eubanks was already investigating the corn images that pre-Columbian Central Americans frequently incorporated in their pottery. And she said that interested Mangelsdorf because native American pottery makers pressed real ears of corn into the wet clay to form botanically accurate molds.

That meant her pottery studies could give him vital clues about how domesticated corn looked many centuries before the Spanish conquest.

So her research trips to Mexico and Peru aided Mangelsdorf's continuing work. In turn, Mangelsdorf's passion quickly rubbed off on her. Eubanks began studying biology and performing botanical research at Duke, Indiana, North Carolina State and Vanderbilt universities.

Scientists, including herself, now agree that teosinte is an ancestor of corn, Eubanks said. Native to Mexico and Guatemala, it features corn-like leaves and a tall stalk crowned by corn-like tassels. It also has hard seeds that line up in a single row -- called a "spike."

However, Mangelsdorf once believed one of corn's true ancestors was Tripsacum, a grass that ranges throughout North and South America. "It's a pervasive weed around here," Eubanks said. "It's on roadsides, railroad tracks and bridges."

The Tripsacum plant resembles corn less than teosinte does. But both grasses grow single rows of grain on spikes. And both feature separate male and female flowers on the same plant -- just like corn.

Moreover, Tripsacum occasionally produces paired kernels, a distinctive feature of corn linked to the evolution of corn's multiple rows. In contrast to teosinte, Tripsacum's kernels are also easy to remove from their hard fruitcases, making them accessible as a wild food. And "they are highly nutritious and delicious," Eubanks said.

Based on breeding experiments in the 1930s, Mangelsdorf and other researchers postulated that the earliest true corn was derived from a cross between Tripsacum and a now-extinct wild corn. Under that view, annually growing teosinte -- the only kind then known -- was a later offspring rather than an ancestor.

Archeological evidence in Central America backed up Tripsacum's role, Eubanks said. In the 1960s, Mangelsdorf and colleagues found the most ancient known preserved corn samples, believed to be more than 5,000 years old, within dry caves in Mexico's Valley of Tehuacán. The earliest samples of teosinte have been dated to 1800 B.C. And the earliest evidence for Tripsacum, found at Tamaulipas Mexico, predates that teosinte by about 500 years.

Nevertheless, Mangelsdorf joined the teosinte bandwagon in the 1980s after other researchers discovered Zea diploperennis growing on the threshold of extinction in the mountains of Jalisco, Mexico. Rescued from the wilds, lines of that perennial teosinte are now being maintained for research purposes.

Mangelsdorf's new hypothesis, which he held until his death, said annual teosinte resulted from a cross between perennial Zea diploperennis and an extinct line of early corn. Domesticated corn then arose from crosses between that corn and the new annual teosintes.

Most other experts believe natural genetic mutations actually caused the spikes of some wild teosinte plants to directly evolve into the first primitive ears of corn, Eubanks said. Under that hypothesis, Native Americans then improved the first corns through accidental or intentional breeding efforts.

But Eubanks maintains that Tripsacum is corn's other ancestor plant. And the discovery of perennial teosinte is helping her prove that hypothesis, she said.

Her initial breeding experiments showed that -- unlike any other teosinte -- Zea diploperennis can be successfully crossed with Tripsacum to produce a fertile hybrid. Previous attempts to breed Tripsacum with any other teosinte varieties had failed.

Similarly, previous attempts by breeders to cross Tripsacum with corn always resulted in sterile plants. But Eubanks found she could also successfully breed corn with some of her Tripsacum and Zea diploperennis crosses.

Since fertile offspring implies genetic similarities among plants, Eubanks believed her experiments rekindled the notion that Tripsacum was a forebear of corn.

Other scientists initially didn't accept her hypothesis. "It questioned accepted dogma," she said. "I had difficulty convincing the scientific community that I even had a hybrid between Tripsacum and the perennial teosinte. They said I just had teosinte that was contaminated with corn, and that it was impossible to cross teosinte with Tripsacum."

Eubanks said she continued in the face of criticism because she knew her work had already been validated by the U.S. Patent Office's rigorous documentation process. She also persisted because of her plants' potential contributions to agriculture and human welfare, she added.

Seeking further proof, Eubanks used molecular genetic analysis to show that traits from both plant types were present in the offspring. She worked with key heritable DNA markers that other scientists -- including her principal detractor -- had previously identified as being crucial in the evolution of domestic corn.

Her new report in Theoretical and Applied Genetics shows some of those markers have now been "stably inherited" for three generations, she said. Eubanks is now embarked in an expanded study of the complex genetic relationships between teosinte, Tripsacum, primitive corns and other related New World grasses.

The National Science Foundation's Small Business Innovation in Research program is funding Eubanks' research on corn rootworm resistance. She is conducting her evolutionary studies of corn and related grasses under an Andrew P. Mellon fellowship in plant systemics at Duke. She has also supported her work from earnings as a self-employed consultant, and by teaching advanced high school biology at the North Carolina School of Science and Mathematics and Durham Academy.

With funding from the National Geographic Society, she is now completing a book about the interdisciplinary archaeobotanical studies she did in the 1970s of corn impressions on Mexican and Peruvian pottery. The book, called Corn in Clay, will be published by the University of Florida Press.

"There are two Peruvian pieces in the collection here at the Duke Art Museum," she noted. "There is one that depicts a rat eating an ear of corn. Then there is one that represents a basket of corn."

The first piece is "somewhat problematic," Eubanks said. The corn casts it carries have more rows and larger kernels than any Latin American race of corn that exists today. "We think it may represent an extinct race," she added.

Duke University

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