Principles of Plant Genetics and Breeding

406 CHAPTER 22
1964 when researchers at Purdue University discovered
mutant genes, called opaque-2 and floury-2, which
increased the lysine content of the kernel. The patterns
of expression of the mutant genes differ slightly. The
opaque-2 gene has a recessive gene action, whereas the
floury-2 gene exhibits a dosage effect. The resulting
corn is called high lysine corn, and has a characteristic
soft and starchy endosperm. Consequently, the softer
endosperm predisposes high lysine kernels to breakage,
cracking, and rot. Generally, high lysine cultivars have
lower yields than their conventional counterparts.
Cross-pollination with normal dent corn reverses the
soft endosperm to normal dent endosperm. High lysine
corn production must be done in isolated fields. The
opaque-2 recessive gene increased the lysine content
of the kernel from about 0.26–0.30% to about
0.34–0.37%. High lysine has also been transferred into
sorghum.
Quality protein maize
Quality protein maize (QPM) may be described as an
extension of the improvement of high lysine maize. It is
Introduction
Twumasi Afriyie
a high lysine product because it uses the opaque-2 gene.
However, it is unlike the traditional high lysine maize
because it lacks all the undesirable attributes of high
lysine products (i.e., low yields, chalky-looking grain,
and susceptibility to diseases and insect pests). It looks
like regular maize but has about twice the levels of
lysine and tryptophan. QPM was developed by two
researchers, K. V. Vasal and E. Villegas over about three
decades. They used conventional breeding methods to
incorporate modifier genes to eliminate the undesirable
effects of the lysine gene. The two scientists were
rewarded with the World Food Prize in 2001 for their
efforts.
QPM has less of the indigestible prolamine-type
amino acids that predominate in the protein of normal
maize. Instead, QPM cultivars have about 40% of the
more digestible glutelins and a balanced leucine :
isoleucine ratio for enhanced niacin production upon
ingestion. Research also indicates that QPM has better
food and feed efficiency ratings (grain food intake/grain
weight gain) following feeding tests with animal (e.g.,
pigs, poultry). QPM cultivars have been released for
production in over 20 developing countries since 1997.
Industry highlights
QPM: enhancing protein nutrition in sub-Saharan Africa
International Maize and Wheat Improvement Center (CIMMYT), PO Box 5689, Addis Ababa, Ethiopia
Maize is a major staple in sub-Saharan Africa and also constitutes an important source of food for children in particular. For example,
children in Ghana grow well during the first 6 months of life but thereafter when breast milk ceases to be sufficient to sustain their
rapid growth, malnutrition becomes normal. This nutritional trend is explained by the pervasive use of a thin gruel porridge made
from maize or millet as the first weaning food fed to children. Few mothers supplement such cereal diets with other sources of
protein such as beans, fish, or milk due to ignorance about proper nutrition, high cost, or lack of time. The cereals alone do not
provide a balanced diet because they are low in lysine and tryptophan, essential amino acids, which cannot be synthesized by
monogastric animals including humans (National Research Council 1988). Normal maize, for example, has approximately 10%
protein but the full amount is not utilizable by monogastric animals because the protein is low in lysine and tryptophan. When
children are fed normal maize without any better-balanced protein supplement, they become malnourished and develop the
protein deficiency disease called kwashiorkor.
In 1963, Mertz and his coworkers at the University of Purdue discovered a recessive mutant maize gene, opaque-2, which
resulted in grain protein with approximately twice the quantities of lysine and tryptophan, the two limiting amino acids in ordinary
maize (Mertz et al. 1964). There was an immediate upsurge of worldwide interest to develop nutritionally improved maize
varieties. However, it was soon discovered that the gene conferring the improved nutritional quality also resulted in several undesirable
agronomic characters including low grain yield potential, unacceptable chalky grain type, high moisture at harvest, and
high susceptibility to insects and disease attacks (National Research Council 1988). When farmers rejected the early “high lysine”
hybrids quickly released to them, the research in opaque-2 or high lysine maize waned markedly worldwide. However, unrelenting
research continued for some 30 years at the CIMMYT, Mexico, resulting in the development of maize germplasm combining