Quality Protein Maize Germplasm Research Articles

Temporal profiling of essential amino acids in developing maize kernel of normal, opaque-2 and QPM germplasm.

Maize, an important cereal crop, has apoor quality of endosperm protein due to the deficiency of essential amino acids, especially lysine and tryptophan. Discovery of mutants such as opaque-2 led to the development of nutritionally improved maize with a higher concentration of lysine and tryptophan. However, the pleiotropic effects associated with opaque-2 mutants necessitated the development of nutritionally improved hard kernel genotype, the present-day quality protein maize (QPM). The aim of present study was to analyze and compare the temporal profile of lysine and tryptophan in the developing maize kernel of normal, opaque-2 and QPM lines. A declining trend in protein along with tryptophan and lysine content was observed with increasing kernel maturity in the experimental genotypes. However, opaque-2 retained the maximum concentration of lysine (3.43) and tryptophan (1.09) at maturity as compared to QPM (lysine-3.05, tryptophan-0.99) and normal (lysine-1.99, tryptophan-0.45) lines. Opaque-2 mutation affects protein quality but has no effect on protein quantity. All maize types are nutritionally rich at early stages of kernel development indicating that early harvest for cattle feed would ensure a higher intake of lysine and tryptophan. Two promising lines (CML44 and HKI 1105) can be used for breeding high value corn for cattle feed or human food in order to fill the protein inadequacy gap. Variation in lysine and tryptophan content within QPM lines revealed that differential expression of endosperm modifiers with varying genetic background significantly affects nutritional quality, indicating that identification of alleles affecting amino acid composition can further facilitate QPM breeding program.

Genetic analysis of tropical quality protein maize (Zea mays L.) germplasm

Maize (Zea mays L.) is an important source of carbohydrates and protein in the diet in sub-Saharan Africa. The objectives of this study were to (i) estimate general (GCA) and specific combining abilities (SCA) of 13 new quality protein maize (QPM) lines in a diallel under stress and non-stress conditions, (ii) compare observed and predicted performance of QPM hybrids, (iii) characterize genetic diversity among the 13 QPM lines using single nucleotide polymorphism (SNP) markers and assess the relationship between genetic distance and hybrid performance, and (iv) assess diversity and population structure in 116 new QPM inbred lines as compared to eight older tropical QPM lines and 15 non-QPM lines. The GCA and SCA effects were significant for most traits under optimal conditions, indicating that both additive and non-additive genetic effects were important for inheritance of the traits. Additive genetic effects appeared to govern inheritance of most traits under optimal conditions and across environments. Non-additive genetic effects were more important for inheritance of grain yield but additive effects controlled most agronomic traits under drought stress conditions. Inbred lines CKL08056, CKL07292, and CKL07001 had desirable GCA effects for grain yield across drought stress and non-stress conditions. Prediction efficiency for grain yield was highest under optimal conditions. The classification of 139 inbred lines with 95 SNPs generated six clusters, four of which contained 10 or fewer lines, and 16 lines of mixed co-ancestry. There was good agreement between Neighbor Joining dendrogram and Structure classification. The QPM lines used in the diallel were nearly uniformly spread throughout the dendrogram. There was no relationship between genetic distance and grain yield in either the optimal or stressed environments in this study. The genetic diversity in mid-altitude maize germplasm is ample, and the addition of the QPM germplasm did not increase it measurably.

Combining Ability of Certain Agronomic Traits in Quality Protein Maize under Stress and Nonstress Environments in Eastern and Southern Africa

ABSTRACTGrowing maize (Zea mays L.) hybrids tolerant to drought and low‐nitrogen (N) stress would significantly reduce yield losses occurring in Africa. This study evaluated the performance of quality protein maize (QPM) F1 hybrids, and general (GCA) and specific combining ability (SCA) of QPM inbred lines for grain yield and other agronomic traits under stress and nonstress environments. A diallel cross of 15 QPM inbred lines was evaluated under drought and low‐N stresses and optimal conditions in a total of 17 environments in Eastern and Southern Africa. Significant variations were observed among the hybrids for all measured traits. TZMI703 × (6207QB/6207QA), GQL5 × (6207QB/6207QA), and CML511 × (6207QB/6207QA) were identified as the best single crosses across environments. The GCA and SCA mean squares were significant for all measured traits, indicating that additive and nonadditive genetic effects were important in this set of genotypes under all test environments. The GCA effects were more important under drought stress, and SCA effects were more important under low‐N and optimal conditions for grain yield. There was preponderance of GCA effects for most agronomic traits tested in all environmental conditions. Inbred lines CML159SR, GQL5, CML159, and CML312SRQ exhibited favorable GCA effects for grain yield under stress and optimal conditions, indicating that the genetic systems controlling a given trait under different conditions are at least partially similar. Cross combinations with favorable SCA effects for grain yield and other traits were also identified. Generally, this study provided evidence that good performance can be achieved under stress and nonstress conditions in QPM germplasm.

Quality Protein Maize Germplasm Characterized for Amino Acid Profiles and Endosperm Opacity

ABSTRACTQuality protein maize (QPM) is improved over normal (non‐QPM) maize in grain concentrations of the essential amino acids lysine and tryptophan. Quality protein maize has a long history as tropical adapted germplasm, but little effort has been made to incorporate temperate or sub‐tropical germplasm for temperate adaptation and interactions between different modifier loci in these backgrounds are poorly understood. A design‐II mating scheme including new temperate and subtropical lines produced 69 hybrids. Large hybrid genetic variation components resulted in substantial broad‐sense heritability H2 estimates, specifically tryptophan (0.46) and endosperm opacity (0.82). A microbial assay for amino acid estimation proved robust across diverse environments with minimal genotype × environment (G×E) effects. Endosperm opacity had no G×E effects across both Texas and Iowa locations demonstrating stability for this trait. Endosperm opacity primarily followed an additive, midparent trend, with a few hybrids deviating from the trend (36%) suggesting a complex nature of multiple modifier loci across diverse germplasm. The top QPM hybrid outperformed the top commercial hybrid by 35 and 30% for lysine and tryptophan as a proportion of grain, respectively. QPM line Tx832 was a parent of top hybrids for lysine and tryptophan, and the highest noncommercial hybrids for methionine. Minimal correlations with yield and other traits suggest that future breeding should result in QPM hybrids with increasingly competitive yields.

Genetic Effects of Quality Protein Maize on Kernel Traits

To efficiently use tropical hard quality protein maize (QPM) germplasm in enriching temperate QPM germplasm, improving kernel traits, and developing hard QPM hybrids, the genetic effects and heterosis of 8 QPM inbreds with different en-dosperm traits and their F1 and F2 progenies were evaluated via Griffing I diallel mating design. The hundred-kernel weight (HKW), hundred-kernel volume (HKV), endosperm modification (EM), and kernel density (KD) showed positive heterosis in F1 and F2 generations. EM showed differences in F1 reciprocal crosses, indicating obvious maternal effect. Correlation analysis showed that values of HKW, HKV, EM, and KD in F2 generation had the biggest correlation coefficients with the average parental values. Maternal effect was the main factor on HKW, HKV, and KD, while direct genetic effect was mainly on EM. Gene actions for HKW, HKV, EM, and KD were dominant, dominant, additive, and additive dominant, respectively. Cytoplasmic effect was little for the above 4 kernel traits suggesting that kernel traits are mainly controlled by nuclear genes.

Phenotypic Characterization of Quality Protein Maize Endosperm Modification and Amino Acid Contents in a Segregating Recombinant Inbred Population

The protein quality of maize (Zea mays L.) can be improved by replacing normal Opaque2 alleles with nonfunctional recessive alleles (o2). Homozygous o2 kernels have increased levels of lysine and tryptophan. Unfortunately, the associated soft texture of the o2 kernels causes poor yield and susceptibility to diseases and insects. Breeding has resulted in the development of o2 genotypes with improved endosperm hardness; such genotypes are referred to as quality protein maize (QPM). Quality protein maize germplasm is utilized in breeding programs worldwide and has been competitive in yield trials. To understand the genetics of endosperm modification, a population of 146 recombinant inbred lines (S5 to S7) derived from a cross between the o2 inbred B73o2 and the QPM inbred CML161 was evaluated in two Texas locations from 2004 to 2006. The endosperm traits texture, opacity, and vitreousness were highly affected by inbred line genotype, were highly correlated with each other, and exhibited high broad‐sense heritability. Relative content of the essential amino acids lysine, tryptophan, and methionine were also highly affected by the inbred line genotype, and exhibited high broad‐sense heritability. Negative correlation was observed between endosperm texture traits and amino acid contents. Favorable responses to selection can be expected for both endosperm texture modification and relative content of the essential amino acids if they are efficiently monitored.

Simple sequence repeat polymorphism in Quality Protein Maize (QPM) lines

A set of 23 Quality Protein Maize (QPM) lines, including 13 lines developed in India and 10 lines at CIMMYT (International Maize and Wheat Improvement Center), Mexico, was analyzed using microsatellite or simple sequence repeat (SSR) markers. Polymorphic profiles for 36 SSR loci have aided in differentiating the QPM inbred lines. The study resulted in identification of SSR markers, such as bnlg439, phi037, bnlg125, dupssr34 andbnlg105, with high polymorphism information content in the selected QPM genotypes. Detection of 30 unique/rare SSR alleles could contribute to effective differentiation of 14 of the 23 QPM inbreds. An opaque2-specific microsatellite marker, phi057, also facilitated differentiation of opaque2-carrying QPM inbreds from the non-opaque genotypes. Analysis using SSR markers indicated high levels of heterozygosity in majority of the Indian QPM lines and in one CIMMYT QPM inbred, CML188. Cluster analysis using SSR data, followed by canonical discriminant analysis, clearly distinguished the Indian QPM inbreds from those developed at CIMMYT. The cluster patterns were largely in congruence with the available pedigree information of the QPM inbreds studied. The study demonstrates the effectiveness of SSR markers in QPM genotype discrimination and analysis of genetic relationships, and could potentially contribute towards effective utilization of the elite QPM germplasm in Indian maize breeding programmes.

The Quality Protein Maize Story

The paper describes the sequence of breeding stages that led to the development of acceptable quality protein maize (QPM) germplasm at the Centro Internacional de Mejoramiento de Maiz y Trigo ( CIMMYT). Initial emphasis was on the development of soft opaque-2 (o2) maize varieties, but these had undesirable consumer characteristics, among other problems. Research then shifted to the development of hard-textured QPM germplasm. Several breeding approaches were explored and tested in early 1970. The combined use of two genetic systems involving the o2 gene and the genetic modifiers of the o2 locus appeared to be the most promising approach. This strategy first involved the development of donor stocks from which a large volume of QPM germplasm was generated through a modified backcross programme and various recurrent selection procedures. Later merging and reorganization of QPM germplasm was undertaken, which resulted in a definite number of QPM populations and pools to meet germplasm needs for various agroclimatic conditions. This was a turning point in the breeding strategy that permitted work with homozygous o2 genetic backgrounds. In the mid-1980s, a hybrid development initiative was started. Basic information on combining ability and heterotic pattern(s) of QPM germplasm was generated. Later emphasis was shifted to development efforts for inbred and hybrid QPM. Several superior QPM germplasm products are now spreading commercially in several developing countries of Asia, Central and South America, and Africa

Heterosis and Combining Ability of CIMMYT's Quality Protein Maize Germplasm: I. Lowland Tropical

CIMMYT has developed a wide array of hard endosperm opaque‐2 (o2) maize (Zea mays L.) germplasm, through the combined use of two genetic systems involving the o2 gene for improving protein quality and the genetic modifiers of the o2 locus for improving kernel phenotype and ameliorating other o2‐associated defects. The objective of this study was to determine the heterotic patterns and combining ability of tropical quality protein maize (QPM) germplasm, and identify superior germplasm suitable for hybrid development. Ten parents (four QPM pools, five QPM populations, and experimental variety PR 7737) were used in a diallel study in eight environments. Data on grain yield, time to silk, plant height, and endosperm hardness were recorded. General combining ability (GCA) effects were highly significant for all traits; specific combining ability (SCA) effects were signififant for time to silk and plant height. Genotype × environment interactions and their partitions were significant for grain yield and endosperm hardness. Pool 24 QPM was the highest‐yielding parent (6.48 Mg ha‐1) and Pool 24 QPM × Population 63 was the highest‐yielding croos (6.56 Mg ha‐1) among hard endosperm parents. PR 7737, a soft endosperm opaque‐2, was low yielding as a parent, but performed better in crosses with other hard endosperm parents and showed high heterosis. High‐parent heterosis for grain yield was generally low in all crosses except those involving PR 7737 as a parent. Endosperm hardness ratings were intermediate relative to the parents, suggesting polygenic control. Crosses among white endosperm parents generally performed better than crosses among yellow endosperm parents. Crosses among dents and dent × flints yielded higher than flint crosses, but flint parents and their crosses had a superior endosperm modification. compared with dents. Populations 62, 63, and PR 7737 showed significant positive GCA effects for grain yield; Pool 23 QPM, Pool 25 QPM, and Populations 62, 64, and 65 had significant negative GCA effects for endosperm hardness. Populations 62 and 63 among white endosperm materials and Population 65 among yellow endosperm could be used for initiating hybrid development work. The broad genetic base provides opportunities for developing intrapopulation interline hybrids.

Heterosis and Combining Ability of CIMMYT's Quality Protein Maize Germplasm: II. Subtropical

Information on the combining ability and heterotic patterns of CIMMYT's maize (Zea mays L.) germ plasm is of great value to maize breeders worldwide, for hybrid development. The objective of this study was to determine the heterosis and combining ability of CIMMYT's subtropical quality protein maize (QPM) germplasm, and to identify suitable germplasm for hybrid development. Six QPM gene pools and four QPM populations were used to make a 10‐parent diallei. Parents and the 45 crosses were tested in three environments in Mexico and three in the USA. Highly significant (P < 0.01) differences were observed among entries, parents, and crosses for grain yield, time to silk, plant height, and endosperm hardness in Mexican environments, and for grain yield, ear height, and time to silk in U.S. environments. General combining ability (GCA) effects were highly significant for all traits, while specific combining ability (SCA) effects were nonsignificant (P > 0.05). Population 68 QPM was the highestyielding parent (6.99 Mg ha‐1) in Mexican environments, and Pool 31 QPM was the highest yielding (4.65 Mg ha‐1) in U.S. environments. The highest‐yielding crosses were Pool 32 QPM × Population 68 QPM (7.05 Mg ha‐1) in Mexican and Pool 29 QPM × Population 70 QPM (5.12 Mg ha‐1) in U.S. environments. Under Mexican environments, Population 68 QPM and Pool 32 QPM among whites and Population 69 QPM and Pool 34 QPM among yellows were the best general combiners for grain yield. For endosperm hardness, the best combiners were Pool 33 QPM, Population 69 QPM, and Pool 31 QPM, all of which have flint grain type. Under temperate U.S. conditions, the two early‐maturity gene pools (Pool 27 QPM and Pool 29 QPM) and Population 70 QPM were the best combiners for grain yield. For subtropical conditions, Population 68 QPM, Pool 32 QPM, Population 69 QPM, and Pool 34 QPM hold potential for hybrid development. These are hard endosperm opaque‐2 (o2) parents (except for Pool 34 QPM, a partially modified parent). For temperate breeding programs in the USA, Pool 27 QPM, Pool 29 QPM, and Population 70 QPM could be used as exotic germplasm sources for introgression into U.S. Corn Belt QPM materials and for developing hard endosperm QPM hybrids.

Genetic and agronomic evaluation of short-season Quality Protein Maize

Development of early-maturing hybrid cultivars of Quality Protein Maize (QPM), a hard-endosperm high-lysine maize (Zea mays L.), could provide a balanced-protein alternative feed crop for northern temperate areas. A set of inbred lines derived from QPM germplasm was used as the base population for three experiments conducted in eastern Canada: an eight-parent one-half diallel analysis, an evaluation of inbred performance, and an evaluation of resistance to Fusarium ear rot. In the diallel analysis, general combining ability effects were significant for grain yield, moisture at harvest, and kernel opacity. Specific combining ability effects were also significant and accounted for 20 and 13% of the genotypic variability for grain yield and moisture at harvest, respectively. Some QPM hybrids yielded well, but they had relatively high levels of grain moisture at harvest, indicating a need to breed for improved adaptation. Within the base population of QPM inbreds, the variation and repeatability for most agronomic traits and for resistance to Fusarium ear rot appeared to be adequate to allow development of agronomically superior QPM inbreds and hybrids for northern temperate maize growing regions.Key words: Quality Protein Maize, opaque-2, repeatability, diallel, agronomic potential, Fusarium graminearum