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May, 2008
Source: E-newsletter vol
5 no
5
- May 2008
CIMMYT is adapting an advanced
technology—the doubled haploid approach—to develop inbred lines
of tropical maize for sub-Saharan Africa. It promises to reduce
costs and speed the arrival of better-adapted maize for
resource-poor farmers in the world’s toughest environments.
CIMMYT scientists have begun developing drought tolerant
varieties of tropical maize for places like sub-Saharan Africa
using a high-tech approach—known as doubled haploids—previously
applied principally by commercial seed companies working mostly
on temperate maize.
“Haploid” refers to the number of chromosomes in a reproductive
cell, like sperm or ovum. In grasses like maize, the
reproductive cells—pollen and ovules—contain half the
chromosomes of a full-grown individual. Fertilization joins the
genetic information from the two parents, and offspring carry
paired sets of chromosomes, reflecting the diversity of each
parent.
“Maize breeders working on hybrids—the most productive type of
maize variety and the one marketed by most seed companies—must
at some point create genetically-stable and pure lines of
desirable, individual plants, for use as parents of hybrids,”
says CIMMYT maize physiologist
Jose Luis Araus.
Conventionally, breeders get the lines by repeatedly fertilizing
selected, individual maize plants with the plant’s own pollen.
The process requires expensive field space, labor, and
time—normally, seven or more generations, which represents at
least three years, even in settings where it’s possible to grow
two crops per season.
Purer, faster, cheaper
In the latter part of the 20th century, crop scientists
developed a quicker, cheaper path to genetically-uniform parent
lines—though a technically intricate method. The first step
involves crossing normal maize with special maize types called
“inducers,” whose pollen causes the normal maize to produce seed
containing haploid embryos. The haploid embryo carries a single
set of its own chromosomes, rather than the normal paired sets.
The embryos are planted, and subsequent treatment of the
seedlings with a particular chemical causes them to make
“photocopies” of their haploid chromosomes, resulting in a
fertile plant endowed with a doubled set of identical
chromosomes and able to produce seed of 100% genetic purity.
“The actual treatment, as well as getting from the embryo to a
reasonable amount of seed of the pure line, is very
complicated,” says Ciro Sánchez Rodríguez, CIMMYT technician in
charge of doubled haploid field trials, “but when the process is
perfected, it only takes two generations—about one year—and the
logistical advantages are tremendous.”
First extensive use in the tropics
 CIMMYT
is implementing the doubled haploid technology on a research
station in Mexico, using drought tolerant plants adapted to
sub-Saharan Africa. “CIMMYT’s use of the practice is another
example of how we put advanced technologies at the service of
disadvantaged, small-scale farmers,” says Araus. “Among other
things, this represents a significant opportunity to increase
the availability of improved, drought tolerant maize varieties
for sub-Saharan Africa,” he says.
Commercial seed companies in Europe and North America have been
the main users of the doubled haploid technology, and the
inducer genotypes available are of temperate adaptation. “The
inducers perform very poorly in the tropical conditions of our
Mexico stations,” says Vanessa Prigge, a PhD student from the
University of Hohenheim working at CIMMYT to perfect the
technique. To generate inducers that work better in tropical
settings, Prigge and colleagues are crossing temperate inducers
from Hohenheim with CIMMYT maize from Mexico, Kenya, and
Zimbabwe. “We expect to have tropical versions of the inducers
in a couple years,” she says.
Reaching farmers’ fields
Maize lines from this work will be used initially in
the Drought
Tolerant Maize for Africa (DTMA) and
the Water Efficient Maize for Africa (WEMA) projects.
“This is a very exciting technology,” says Aida Kebede (photo),
an Ethiopian PhD student from Hohenheim helping to establish the
doubled haploid technology at CIMMYT. “It holds the key to
addressing more quickly the persistent problems of African maize
growers: drought, disease pressure, and low productivity. I’m
happy to contribute!”
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