Amazingly Mobile Maize

How did a crop domesticated some 7,000 years ago from a humble Mexican grass called teosinte become the number-one food crop in Africa and Latin America and a major food, feed and industrial crop just about everywhere else?

The global migration of maize based on historical records (click on image for larger format)

The story of the spread of maize has been told in books (notably Maize and Grace: Africa's Encounter with a New World Crop 1500-2000, by James C. McCann), but there have always been lingering doubts and unanswered questions. If, for example, Christopher Columbus brought maize to Spain in 1493 following his pioneering visit to the Caribbean, as records show, how is it that reliable accounts have the crop being grown in 1539 in the very different climate of Germany? That's only 46 years later, far too soon for tropical maize to change its preferred temperature, humidity and day length. In another case, maize was assumed to have been brought to Nigeria by Portuguese colonists, but the local names for maize in that country derive from Arabic, suggesting that the crop was likely brought by Arabic-speaking traders.

Recent work by the International Maize and Wheat Improvement Center (CIMMYT) and partners sheds new light on maize's global migration. With support from the Generation Challenge Program of the CGIAR, and in collaboration with nine research institutes on four continents, scientists have used DNA markers (molecular signposts for genes of interest) and new approaches to analyze nearly 900 populations of maize and teosinte from around the world.

"What is emerging is a far clearer picture of the crop's global diversity and the pathways that led to it," says Marilyn Warburton, a CIMMYT molecular geneticist and the leader of the effort.

Phase I of the work, funded by the European maize consortium PROMAIS, focused on North America and Europe. The Generation Challenge Program commissioned Phase II, which expanded coverage worldwide and raised the number of maize populations studied to 580. In Phase III, partners are adding another 300 populations of maize and teosinte to plug the geographical gaps. A primary objective is to gather samples of landraces (local varieties developed through centuries of farmer selection) and ensure their conservation in genebanks. The diversity studies apply a method Warburton developed that uses DNA markers on bulk samples of individuals from large, heterogeneous populations like those typical of maize.

Among its findings, the study confirms that northern European maize originates from North American varieties brought to the continent some decades after Columbus returned, not from the tropical genotypes he carried.

"The two main modern divisions of maize arose about 3,000 years ago, as maize arrived in what is now the southwestern US and, at about the same time, on the islands of the Caribbean," says Warburton. "Temperate maize spread further north and east across North America, while tropical maize spread south. The temperate-tropical division remains today. What maintains it are differences in disease susceptibility and photosensitivity - essentially, how day length affects flowering time. The two maize types are now so different from each other that they do not cross well, and their hybrids are not well adapted anywhere."

The work continues and, in addition to elucidating the epic journey of maize, will help breeders to home in on, and more effectively use, traits like drought tolerance found in the vast gene pool of maize.

For more information, read a longer report entitled Tracing History's Maize in Generation's Partner and Product Highlights 2006 or contact Marilyn Warburton ( m.warburton@cgiar.org ) .