Mexico
June 1, 2007
Source:
CIMMYT E-News, vol 4 no. 5,
May 2007
No need to dig for ancient seeds
to discover how and when maize moved from its ancestral home in
Mesoamerica to become one of the world’s most widely-sown and
popular food crops. New work by gene sleuths from CIMMYT and
numerous maize growing countries solves the puzzle using DNA of
present-day 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 incredible story of maize has been told in books, but there
have always been lingering doubts, unanswered questions. If, for
example, as records show, in 1493 Columbus brought maize to
Spain from his visit to the warm climes and long days of the
Caribbean, how is it that reliable accounts have the crop being
grown in 1539 in the cold, short daylengths of Germany? That’s
only 46 years later, and far too soon for such a radical
adaptation in tropical maize. In another case, maize was
supposedly brought to African countries like Nigeria by
Portuguese colonists, but the local names for maize in that
country are of Arabic derivation, suggesting that the crop
likely arrived via Arabic-speaking traders.
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The
global migration of maize, based on historical records. |
Deciphering the history in
genes
Recent work by CIMMYT and partners sheds new light on maize’s
global migration. With support from Generation, a Challenge
Program of the Consultative Group on International Agricultural
Research, 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 CIMMYT
molecular geneticist and leader of the effort,
Marilyn Warburton.
Phase I of the work was funded by PROMAIS, a European maize
consortium, and focused on North America and Europe. The
Generation Challenge Program commissioned Phase II, which
featured global coverage and brought the number of maize
populations studied to 580. In Phase III, partners are adding
another 300 populations of maize and teosinte, to fill any
geographical gaps. A primary objective is to gather samples of
landraces—local varieties developed through centuries of farmer
selection—and ensure their conservation in germplasm banks. The
diversity studies apply a method developed by Warburton for
using DNA markers on bulk samples of individuals from large,
heterogeneous populations like those typical for maize.
The great divide: Temperate vs tropical maize
Among other things, the studies corroborate the notion that
northern European maize originates from North American varieties
brought to the continent several decades after Columbus’
returned, and definitely not from tropical genotypes. “The two
main modern divisions of maize arose about 3,000 years ago,”
says Warburton, “as maize arrived in what is now the
southwestern US and, at about the same time, on the islands of
the Caribbean. 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 daylength 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 from the vast gene
pool of maize.
The above report is largely based on a longer description of
this work, “Tracing history’s maize,” that appears in
Generation’s “Partner and Product Highlights 2006.” |
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