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One of the greatest agricultural and evolutionary puzzles is the origin of
maize - and part of the answer may lie in a plot of corn on the western edge
of Madison, where a hybrid crop gives new life to ancient genetic material,
September 2005 by Katie Weber.

While many biologists argue that teosinte, a wild Mexican grass, is the
progenitor of maize, others believe that the differences between teosinte
and maize are too complex to have arisen through natural mutation or human
selection. One of the most significant inconsistencies between the two
plants is that teosinte kernels are locked in a hardened casing and have to
be cracked like walnuts, while maize kernels are exposed on the surface of
the ear.

However, a team led by a University of Wisconsin-Madison geneticist has
demonstrated that a single gene, called tga1, controls kernel casing. And
beyond implications for the study of maize evolution, the results are
evidence that modest alterations in single genes can cause dramatic changes
in the way traits are expressed, the team wrote in the August issue of the
journal Nature.

"What really interests me is how traits evolve," says John Doebley, the
professor of genetics who led the study. "How did changes in genes cause the
diversity of life to arise on earth? The corn and teosinte model is an
excellent system to investigate this question."

While Doebley has laboratory facilities on campus, he is equally at home in
the field plots at the West Madison Agricultural Research Station where he
raises second-generation hybrids of teosinte and maize. He studies the
inheritance of different traits in the hybrids and uses genetic tools to
identify the genes involved, applying highly advanced technology to an
ancient species.

The history of corn is closely intertwined with the history of humans in the
New World, says Doebley. "In the Americas, corn - which was first
domesticated insouthern Mexico - fueled the societies and the cultures that
developed. Without corn, the societies of the new world would have been
completely different."

However, the genetic changes that occurred during the domestication of corn
have been a controversial issue in the field of evolutionary biology,
Doebley says. "Some argue that evolution works like building up a sandstone
cliff with lots of tiny grains deposited over a very long time. Others
believe that there may be big boulders set into that cliff - or that
evolutionary changes may result from single genes with very big effects."

In fact, Doebley says that his team's findings "fit exactly with the idea
that a single gene change, or a small number of changes, could be sufficient
to make teosinte a useful food crop, and in a relatively short amount of
time." He adds that it is likely that a change in just one amino acid within
the gene was enough to cause the key event, and ultimately influence human
society in the new world.

The process of maize evolution might have begun with humans growing teosinte
as a food source - although an inefficient one, says Doebley. "And then in
the fields popped up a new mutation that changed the tga1 gene and reduced
the kernel casings," he says. "Humans then would have applied artificial
selection, and soon almost every plant they grew would have had the exposed
kernels."

Although the results published in Nature shed light on one great mystery,
there is much still to learn. Doebley and colleagues at six institutions are
almost halfway through a five-year project to study the molecular and
functional diversity of the maize genome. At $10 million, the National
Science Foundation grant that supports the project is one of the largest
awards ever given for plant research. Doebley is trying to identify which
genes ancient farmers selected for their crops. In a recent paper published
in the journal Science, the team presented analysis indicating that 2 to 4
percent of the genes in the maize genome experienced artificial selection.

Doebley's team for the Nature paper included Huai Wang, a current
postdoctoral student; Qiong Zhao, a current graduate student; Yves
Vigouroux, a former postdoctoral student; Kirsten Bomblies and Lewis Lukens,
former graduate students; Tina Nussbaum-Wagler, a research specialist; and
Bailin Li and Mariana Faller, researchers at the DuPont corporation.

The work is funded by the National Institutes of Health, NSF, the United
States Department of Agriculture and the state of Wisconsin.

[www.wisc.edu]

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