The Corn War

The true origin of corn is a question that's been debated for decades. Now a maverick geneticist says she may have the answer. But to get anyone to listen to her, she has to join a long-running academic food fight.

By Catherine Dold|Monday, December 01, 1997
RELATED TAGS: AGRICULTURE, GENETICS
It’s not a battle over grain supplies in distant lands. Nor a fight over corn oil prices or the fate of family farms. No, it’s a fight over something far more familiar, far more modest: the humble ear of corn itself and its mysterious origins.

Just where did corn come from? Botanists have repeatedly crossed scientific swords over proposed answers to this question, but for the past few years all has been rather quiet and the issue seemingly resolved. Now, however, an upstart anthropologist-turned-geneticist is entering the fray, and the Corn War is heating up again. Mary Eubanks, a postdoctoral researcher at Duke University, has bred a hybrid that produces ears resembling the world’s most ancient preserved corn--two-inch cobs, at least 3,600 years old, excavated from the dust of a cave near Tehuacán, Mexico. Moreover, an analysis of the hybrid’s DNA hints that Eubanks has experimentally resurrected one of corn’s long-lost ancestors, which may hold the key to breeding hardier descendants.

Some Corn War veterans are enraged, complaining that Eubanks’s work is nonsense. Other botanists flat out refuse to discuss her or her theory. And some say she just might be on to something.

Why the fuss? Well, for one thing, there are more than just academic egos on the line. In a world whose population is exploding while crop yields are stagnant, the secret to a better corn could be worth a lot. Remember, corn is big business. The plant was first domesticated in the Americas some 5,000 to 7,000 years ago; by the time Columbus arrived, about 300 different types of corn were flourishing here. Today U.S. farmers harvest 9 billion bushels of corn, worth about $30 billion, every year. Surprisingly, less than 2 percent of that harvest is consumed by humans as good old corn on the cob, cornbread, and other edibles. More than 50 percent is fed to farm animals. The rest is used in an astonishing array of products--everything from clothing and glue to aspirin and fireworks.

You are here not only because you had a mother and a father; you are also here because of corn, says Walton Galinat, a botanist at the University of Massachusetts who once bred a square ear of corn that wouldn’t roll off an airplane tray. Centuries ago, New World societies used corn not only for food but for art and religious inspiration. Today American farmers still seem to worship corn in their own way, erecting corn palaces and holding corn festivals at which people wander about dressed as ears of their favorite grain.

Mary Eubanks is harvesting kernels from the two dozen experimental plants she is growing in a greenhouse on campus. Some of her plants look distinctly cornlike: tall stalks adorned with large ears of kernels enclosed in papery shells. Others look more like plain grasses, with thin green shoots and tiny, unremarkable fruits. Nearly all the plants have paper bags covering some of their parts, protecting female ears from fertilization by pollen from the wrong male tassels. With these plants, Eubanks is attempting to move some desirable traits found in her hybrid, such as pest resistance, into modern races of corn.

Shouting above the roaring sheets of water that slide over the greenhouse roof to keep it cool in the North Carolina sun, Eubanks points out her hybrid, an unassuming plant standing in the corner that she has now patented under the name tripsacorn. It doesn’t look much like corn; but to a geneticist, looks aren’t everything.

Eubanks never intended to become a geneticist. She was trained as an archeologist and once spent time exploring corn motifs on pre-Columbian pottery. People in some ancient American cultures often pressed corncobs directly into wet clay, then used the resulting impression to create molds for exact replicas of corn. These clay cobs today provide an archeological record of the most popular races of corn as well as clues to cultural developments and trade routes. While still a graduate student at the University of North Carolina in the 1970s, however, Eubanks crossed paths with Paul Mangelsdorf, a distinguished biologist who had retired to North Carolina from Harvard’s Botanical Museum. Mangelsdorf encouraged her pottery studies, which benefited his own study of corn, and sparked in Eubanks an interest in genetics.

After completing her Ph.D. in anthropology and teaching for a few years in Cincinnati, Eubanks began working with Mangelsdorf on the question of corn’s ancestry. Then, in the mid-eighties, both her personal and professional lives took a turn. Following a divorce, she moved to Bloomington, Indiana, to do postdoctoral work in biology. She was studying the genetics of several American grasses when she noticed some odd similarities in the chromosomal structure of two species. Zea diploperennis, a rare relative of the wild grass teosinte, and Tripsacum dactyloides, a common grass, have knots of dense DNA--or knobs--only at the ends of their chromosomes. Modern corn (Zea mays) and many other domesticated crop plants, by contrast, have mid-chromosome knobs. The function of the knobs is not fully understood, says Eubanks, but the similarities in chromosome architecture made me think I might be able to cross-pollinate them.

The pollination worked and, much to Eubanks’s surprise, some of the fruits on the hybrid plants resembled the most primitive ears of preserved corn known. Though the puny ears of her hybrid corn and ancient corn didn’t look much like the robust golden ears of modern corn, they shared one fundamental trait: rows of exposed paired kernels.

The minute I saw the ears, I knew, Eubanks recalls. There, for the first time, was experimental proof. We had reproduced ears with the basic characteristics of corn. It was very exciting.

Current dogma holds that corn arose from annual teosinte, a Central American grass domesticated by native Americans. There is an undeniable logic to the assumption--teosinte is corn’s closest known relative. Yet fundamental questions persist. Teosinte doesn’t produce anything that resembles the modern corncob. Where corn produces the familiar multiple-rowed ear of kernels, teosinte grasses bear a single row of five to seven small, hard kernels. Mature corn kernels remain on the cob and are protected by a husk, so they are easily harvested, but teosinte kernels are enclosed in hard fruit cases that shatter on maturity, scattering the kernels to the ground. Corn kernels are paired, with two kernels growing in a cuplike holder that sits on the cob, while teosinte has just one kernel per cupule. Finally, teosinte--with its few kernels and rigid fruit cases--would have proved a difficult harvest for hungry hunter- gatherers. So how, with all these shortcomings, could a skinny local reed like teosinte ever have made the evolutionary leap to the bountiful, easily harvested, many-kerneled, many-eared plant that now grows nearly everywhere on Earth?

Eubanks’s experimental cross might answer that question. In breeding tripsacorn, she demonstrated that in just one hybridization event you can get a dramatic change in ear structure. Tripsacorn not only had rows of exposed paired kernels, but those kernels were also attached to a central rachis, a supporting structure that is much more like a corncob than a rigid grass fruit case. That innovation, Eubanks suspected, was the legacy of the Tripsacum parent; Tripsacum doesn’t look much like corn, but it does bear accessible kernels that occasionally occur in pairs.

If a tripsacorn-like hybrid once occurred naturally, then, says Eubanks, the evolutionary puzzle disappears--and with it, teosinte’s starring role in the story of corn. If hunter-gatherers saw this hybrid with kernels that popped out easier and tasted good, they would choose it. And maybe cultivate it and protect it.

Eubanks speculates that the wild hybrid plant, once it arose, was tended by humans and crossed with close kin. The result of such crosses over time, she thinks, yielded both annual teosinte and corn. In other words, annual teosinte, the plant thought to be the mother of corn, is really more like a cousin. But could Eubanks’s hybrid occur in nature? The parent Zea diploperennis is now found only in parts of Mexico, but it was probably once more widespread, says Eubanks, growing in the same areas of North and Central America as Tripsacum. And although the two plants don’t normally bloom and get fertilized at the same time, she theorizes that a natural event such as ash in the air from a volcanic eruption could have shortened the days, causing changes in blooming cycles.

Eubanks is not the first to suspect that corn has a Tripsacum ancestor. The size of the corn genome alone is one strong clue that the plant probably has a diverse parentage. Indeed, back in the 1930s, Eubanks’s mentor, Paul Mangelsdorf, had hypothesized that corn could not have evolved by way of a few mutations in teosinte, as the prominent biologist George Beadle had argued. Instead, Mangelsdorf contended that teosinte was the result of a cross between an extinct wild form of corn and Tripsacum. Only then did teosinte backcross with its wild corn parent, he argued, leading to modern corn. By the 1960s, Mangelsdorf’s hypothesis was considered the law of the land.

The corn pendulum, however, swung back toward Beadle’s view at the end of that decade. Hugh Iltis, a botanist at the University of Wisconsin, performed studies of corn structure that supported Beadle’s theory. This view--that teosinte is the mother of corn--is still the most popular scientific account of corn evolution. And Iltis and his colleagues are not much inclined to consider any other theories. Mention Mary Eubanks and Iltis simply bellows, She’s crazy!

More than a decade passed before Eubanks published her theory. After creating her hybrid in 1984, she left Indiana and enrolled at Vanderbilt University, in Tennessee, to pursue advanced studies in biological sciences. In 1987 she wrote her master’s thesis on her theory. Eubanks then returned to North Carolina. She had three children and no time or financial support to pursue corn genetics. But she continued to maintain her plants, growing them in her own backyard. Eventually she struck up a friendship with Dick White, then dean of Trinity College at Duke, who was familiar with the studies of the by then deceased Mangelsdorf. He arranged library and greenhouse privileges for her at Duke, and Eubanks returned to her plant studies and set about writing up her theory.

Her first article was turned down. The journal’s editor said she had no proof that her plant was a true hybrid. So she began sitting in on molecular systematics courses at Duke, grew some more hybrids, and harvested their leaves to investigate their genetics. Finally, in 1995, 11 years after she first crossed Tripsacum and Zea diploperennis, Eubanks published her own first volley in the Corn War in the journal Economic Botany.

The molecular evidence showed there were definitely Tripsacum genes in the hybrid, says Eubanks. For her study, she used a standard DNA fingerprinting technique: restriction fragment length polymorphism (rflp) analysis. The DNA from the plants was cut up using restriction enzymes that could identify key stretches of DNA. These stretches then had to be sorted by size and examined for specific genetic regions. To do so, Eubanks used molecular probes that would bind to those areas and, when exposed to a photographic plate, leave behind characteristic banding patterns. Numerous bands unique to the Tripsacum parent did indeed show up in the hybrid’s banding pattern. And that pattern, she explains, is evidence that those genetic regions were transferred during the cross-pollination. Eubanks’s plant was a real hybrid.

But were the hybrid’s key cornlike traits--the paired kernels and so forth--inherited from Tripsacum genes? Such evidence would strongly suggest that the same traits in modern corn could have come from Tripsacum and not from mutations in teosinte genes.

Again she used rflp analysis. This time she selected molecular probes that, based on the genetics of modern corn, can pick up areas encoding the characteristic corn traits. Once again, several sections of unique Tripsacum DNA turned up in the hybrid. Moreover, Eubanks showed that the Tripsacum DNA occurs in the same region of the chromosomes where the crucial traits are also found in modern corn’s DNA. This is stronger evidence that it really requires hybridization with Tripsacum to derive the ear of corn, as opposed to strictly a mutation in teosinte genes, says Eubanks.

She is talking nonsense, snorts Iltis. He claims her hybrid is not a true hybrid but a corn-teosinte mix, produced somehow by cross- contamination. It’s just typical Mangelsdorfian fiction, he adds. He had one crazy idea after another. Others are a bit more charitable. Walton Galinat, himself a member of the teosinte camp, says Eubanks’s work is no major breakthrough, but he is encouraging her to continue her research. Major Goodman, a professor of crop science at North Carolina State University, says, I don’t think Eubanks has any followers in the origin- of-corn community. But he hints that perhaps the Corn War is not quite as settled as Iltis believes. Whether corn and teosinte arose from a common ancestor or corn arose from a teosinte . . . There are those of us who are not convinced that the evidence is all in. The earliest archeological evidence of cultivated corn, he adds, does not look at all like teosinte.

At least one botanist thinks Eubanks might be on the right track. She’s shown that it’s entirely possible that Tripsacum played a role in the origin of corn, says Bruce Baldwin, an evolutionary biologist at the University of California at Berkeley. I’ve seen her data and I think her interpretations are perfectly legitimate. Baldwin adds that Eubanks is now getting a lot more respect from other botanists and has changed the minds of some real skeptics.

Eubanks would have given up long ago, except, she says, that her evidence offers a way to build a better corn plant. I was being rejected by academic science and validated by the patent office. They told me that I must not give up because this could be so important to agriculture. I think if it were basil or something, I would have given up.

Tripsacum’s got good genes, she adds. My hybrid is literally a genetic bridge to move Tripsacum genes into corn. Tripsacum and, consequently, tripsacorn seem to be naturally resistant to corn rootworm, a pervasive pest that attacks the roots of a corn plant, causing it to keel over. The tiny bug costs farmers more than $1 billion every year, says Eubanks. Other researchers have tried to transfer rootworm resistance by crossing Tripsacum with corn, but the offspring were usually sterile. Eubanks is now attempting to develop fertile, resistant plants by crossing her hybrid with various races of corn. If such crosses work--and early results look good--it could eliminate the need for some pesticides.

The more we know about the wild relatives of any crop, the more we’ll be able to tap the gene pools of those relatives for crop improvements, says Eubanks. Plants that survive in the wild are much hardier, more adaptable. They carry genes that can be so beneficial--the kinds of things we need to become a sustainable agricultural community and less dependent on chemicals.

Holding out a palm filled with kernels freshly harvested from one of those crosses, Eubanks says, This seed is like gold. It’s probably more valuable than gold. It has the resistance to rootworm. The same plants are also drought-tolerant, she says. You might be able to grow a really good protein plant in a marginal environment. That might help in regions like Africa, where hunger is such a problem. It might even be possible, she adds, to breed a perennial corn. A perennial’s strategy is to produce many small ears, she explains, so it wouldn’t be of much value to commercial agriculture. But it might be a boon in regions where farmers are just scraping by.

Eubanks is also gathering more ammunition for her theory. She is analyzing even more closely the genetic contributions of Tripsacum and Zea diploperennis to the hybrid, probing their DNA for more markers that correspond to characteristic corn traits. She is hoping to isolate DNA sequences unique to corn that are not found in teosinte--evidence, she says, that there is no way you could have gotten corn from teosinte.

She is also planning to compare the DNA of modern plants with ancient remains from archeological sites in the United States, Mexico, and Peru. Looking at known genetic regions and the changes that occur in them in different types of corn should allow her to construct their relationship to one another. If that weren’t enough to keep her busy, Eubanks is also finishing a book on corn motifs in pottery. She has been taking measurements of the corn impressions found on ancient pottery samples and using them to trace which races were used when and where.

Corn is an incredible puzzle, she says. The story is just unfolding.
Comment on this article
ADVERTISEMENT

Discover's Newsletter

Sign up to get the latest science news delivered weekly right to your inbox!

ADVERTISEMENT
ADVERTISEMENT
Collapse bottom bar
DSCJulyAugCover
+

Log in to your account

X
Email address:
Password:
Remember me
Forgot your password?
No problem. Click here to have it emailed to you.

Not registered yet?

Register now for FREE. It takes only a few seconds to complete. Register now »