Biology Reference
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intensity in yellow maize grains, and reduc-
tion in kernel weight and density. Thus, despite
the nutritional superiority of
o2
maize, it did
not become popular with farmers or consumers
mainly because of reduced grain yield, chalky
and dull kernel appearance, and susceptibility
to ear rot and stored grain pests. Accordingly,
CIMMYT, the International Maize and Wheat
Improvement Center, undertook to improve the
phenotype of
o2
kernels in order to facili-
tate greater acceptability, by developing hard
endosperm grain types with the protein qual-
ity of chalky
o2
strains. CIMMYT received
funding support beginning in 1965 from the
United Nations Development Program and intro-
duced gene modifiers that changed the soft,
starchy endosperm to a vitreous type preferred
by farmers and consumers whilst retaining the
elevated levels of lysine and tryptophan. CIM-
MYT has subsequently developed a range of
hard endosperm
o2
genotypes with better pro-
tein quality through genetic selection, and these
are popularly known as quality protein maize
(QPM). Today's QPM is essentially interchange-
able with normal maize in both cultivation and
agronomic characteristics and is competitive in
terms of yield, lodging, disease and pest resis-
tance, and moisture level, while retaining the
superior lysine and tryptophan content (Vasal
2001). In 2005, QPM was planted on 695,200
hectares across 24 developing countries.
There are various breeding options for devel-
oping QPM that is competitive in agronomic
performance and market acceptance. Among the
several approaches tested in CIMMYT, the most
successful and rewarding option exploited com-
bined use of
o2
with the associated endosperm
and amino acid modifier genes. Intrapopu-
lation selection for genetic modifiers in
o2
backgrounds exhibiting a higher frequency of
modified
o2
kernels and recombination of supe-
rior hard endosperm
o2
families resulted in the
development of good quality QPM donor stocks
with a high degree of endosperm modification.
This was followed by the large-scale develop-
ment of QPM germplasm with a wide range
of genetic backgrounds, representing tropical,
subtropical, and highland maize germplasm and
involving different maturities, grain color, and
texture. An innovative breeding procedure des-
ignated as “modified backcross cum recurrent
selection” was designed to enable rapid and effi-
cient conversion programs (Vasal 1980; Prasanna
et al. 2001). More recently, pedigree backcross-
ing schemes have been used to convert elite QPM
lines to maize streak virus (MSV) resistant ver-
sions for deployment in Africa, as well as for
conversion of elite African lines to QPM ver-
sions (Krivanek et al. 2007).
QPM hybrid breeding efforts were initiated
at CIMMYT in 1985, as the QPM hybrid prod-
uct has several advantages over open-pollinated
QPM cultivars: (1) higher yield potential compa-
rable to the best normal hybrids, (2) assured seed
purity, (3) more uniform and stable endosperm
modification, and (4) less monitoring of protein
quality required during seed production. Sev-
eral QPM hybrid combinations were derived and
tested through international trial series at mul-
tiple CIMMYT and NARS locations in Asia,
Africa, and Latin America. Current QPM breed-
ing strategies at CIMMYT focus on pedigree
breeding wherein the best performing inbred
lines with complementary traits are crossed to
establish new segregating families. Both QPM
×
QPM and QPM
non-QPM crosses are made,
depending on the specific requirements of the
breeding project. In addition, backcross conver-
sion is also followed to develop QPM versions
of parental lines of popular hybrid cultivars that
are widely grown in CIMMYT's target regions.
Inbred lines developed through this process are
then used in the formation of QPM hybrids and
QPM synthetics (Krivanek et al. 2007; Atlin et al.
2011).
The breeding of QPM involves manipula-
tion of three distinct genetic systems (Bjarna-
son and Vasal 1992; Krivanek et al. 2007):
(1) the recessive mutant allele of the
O2
gene,
(2) the endosperm hardness modifier genes; and
(3) the amino acid modifiers/genes influencing
free amino acid content in the endosperm. The
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