Genomics and Applied Biology 2019, Vol.10, No.2, 10-19
14
modifications among available germplasm stocks (Paez et al., 1969). The mechanism underlying change of the
grain structure from chalky to vitreous in modified opaque-2 mutants (
mo
2
) by the endosperm modifiers is yet not
clear. But, breeders can accumulate above modifier complexes (QTLs) using marker assisted selection to assist
QPM breeding programme (Dudley and Lambert, 2004). Vasal et al. (1980) succeeded to combine the
opaque 2
allele with QTLs for genetic modifiers for development of modified version of QPM germplasm with hard kernel
and much higher quantity of lysine and tryptophan.
8 Identification of Molecular Markers
Identification of molecular markers that co-inherit with the
opaque 2
phenotype is a crucial step for their use in
marker assisted breeding. Until recent years, several polymorphic markers have been identified in QPM lines
using RAPD (Nkongolo et al., 2011; Hemavathy, 2015), ISSR (Nkongolo et al., 2011; Lenka et al., 2015), SSR
(Bantte and Prasanna, 2003) and SNP (Semagn et al., 2012) primers. Bantte and Prasanna (2003) reported a few
SSR markers e.g., bnlg 105, bnlg 125, bnlg 439, phi 037 and dupssr 34 with high polymorphic information
content to differentiate QPM lines. But, none of these was efficient to differentiate between QPM and Normal
maize inbreds. CIMMYT designed
o
2
-gene specific SSR primers viz., phi 057, phi 112 and umc 1066 which are
located as internal repetitive elements within
opaque 2
gene on short arm of chromosome 7
(
). Allelic polymorphism among QPM and normal maize inbreds was surveyed by
several workers using the above SSR primers (Babu et al., 2005; Jompuk et al., 2006; Magulama and Sales, 2009;
Gupta et al., 2013). Bantte and Prasanna (2003) detected 30 unique SSR alleles to differentiate QPM inbreds.
Besides, they identified the SSR primer phi 057 to detect QPM inbreds carrying
opaque 2
mutation. Phi 057
amplified a 169 bp band in QPM donors and 159 bp amplicon in normal maize inbreds (Magulama and Sales,
2009). phi 057 and Umc 1066 are reported to be co-dominant, while phi 112 is a dominant marker (Magulama and
Sales, 2009). Kata et al. (1994) used RFLP technique using Hind III digestion of genomic DNA and
opaque 2
locus specific cDNA probe to detect
O
2
/
O
2
,
O
2
/
o
2
, and
o
2
/
o
2
genotypes of individual plants in breeding populations.
Besides, Zhang et al. (2010) reported use of molecular marker umc1141 to trace the inheritance of the erstwhile
mentioned
opaque 16
mutant allele that led to elevated synthesis of lysine and tryptophan content.
9 Breeding for Quality Protein Maize (QPM)
Maize is a cross pollinated crop. The major breeding approach for increasing productivity is production of hybrids
using heterosis breeding. The success from this method depends on development and identification of suitable
inbred lines using an appropriate mating design; and selection of most promising heterotic normal maize hybrid.
For production of QPM hybrid, the ultimate aim is to combine the advantage of heterosis along with amelioration
of amino acid composition using QPM donors. To achieve this, conversion of either of the parental non-QPM
inbreds to QPM status is the first step to develop a heterotic QPM hybrid. A number of reliable QPM donors are
made available at CIMMYT by selection for modified grain texture in QPM backgrounds using various selection
schemes. Initially, four tropical hard endosperm populations (composite K, Ver 181-Ant gp venezula-1, Thai
composite and PD 9MS6) and one highland composite (Composite 1) were developed by intra-population
selection of genetic modifiers in
o
2
background followed by grouping of modified
o
2
sources into pools. These
were later recombined to maintain modified QPM status and were used for large scale conversion of non-QPM
inbreds to their QPM version. The inbred lines e.g., CML 176 and CML 186 are reported to be potential QPM
donors (Manna et al., 2005; Danson et al., 2006) for introgression of
o
2
allele to non-QPM maize. Vivek et al.
(2008) reported tryptophan content more than 0.60% and lysine content more than 2.6% in a set of CIMMYT
QPM inbreds. Besides, Ortega and Villegas (1988) reported on an average 0.8% tryptophan and 3.1% lysine
content among a set of QPM inbreds as against 0.4% and 1.6% respectively in normal maize lines.
CIMMYT adopted a conservative approach to develop modified
opaque 2
(
mo
2
) genotypes to strike a balance
between proteins levels and grain quality and competitive yield levels. The modifier complex is polygenic in
nature. Therefore, molecular marker based screening for QPM status coupled with phenotypic selection for
improving endosperm characteristics seems to be an appropriate strategy for development of QPM introgression
lines. The erstwhile mentioned gene-specific co-dominant markers phi 057 (Manna et al., 2005; Danson et al.,
