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Identifying efficient testers for separating maize (Zea mays L.) inbred lines into heterotic groups can facilitate the development of superior hybrids. Striga-resistant yellow-maize inbred lines de...

Provitamin A-enriched maize (Zea mays L.) is an important complementary food staple for combating vitamin A deficiency (VAD) in high maize-producing and maize-consuming countries of sub-Saharan Afr...

Striga hermonthica, causes up to 100% yield loss in maize production in Sub-Saharan Africa. Developing Striga-resistant maize cultivars could be a major component of integrated Striga management strategies. This paper presents a comprehensive overview of maize breeding activities related to Striga resistance and its management. Scientific surveys have revealed that conventional breeding strategies have been used more than molecular breeding strategies in maize improvement for Striga resistance. Striga resistance genes are still under study in the International Institute for Tropical Agriculture (IITA) maize breeding programme. There is also a need to discover QTL and molecular markers associated with such genes to improve Striga resistance in maize. Marker Assistance Breeding is expected to increase maize breeding efficiency with complex traits such as resistance towards Striga because of the complex nature of the host-parasite relationship and its intersection with other environmental factors. Conventional alongside molecular tools and technical controls are promising methods to effectively assess Striga in Sub-Saharan Africa.

Several maize (Zea mays L.) hybrids have been developed and commercialized in the Nigeria savannas, but limited informationis available on how they perform at plant densities higher than the recomm...

Maize streak virus (MSV) continues to be a major biotic constraint for maize production throughout Africa. Concerning the quantitative nature of inheritance of resistance to MSV disease (MSVD), we sought to identify new loci for MSV resistance in maize using F2:3 population. The mapping population was artificially inoculated with viruliferous leafhoppers under screenhouse and evaluated for MSVD resistance. Using 948 DArT markers, we identified 18 quantitative trait loci (QTLs) associated with different components of MSVD resistance accounting for 3.1–21.4% of the phenotypic variance, suggesting that a total of eleven genomic regions covering chromosomes 1, 2, 3, 4, 5 and 7 are probably required for MSVD resistance. Two new genomic regions on chromosome 4 revealed the occurrence of co-localized QTLs for different parameters associated with MSVD resistance. Moreover, the consistent appearance of QTL on chromosome 7 for MSVD resistance is illustrating the need for fine-mapping of this locus. In conclusion, these QTLs could provide additional source for breeders to develop MSV resistance.

Striga hermonthica is a widespread, destructive parasitic plant that causes substantial yield loss to maize productivity in sub-Saharan Africa. Under severe Striga infestation, yield losses can range from 60 to 100% resulting in abandonment of farmers’ lands. Diverse methods have been proposed for Striga management; however, host plant resistance is considered the most effective and affordable to small-scale famers. Thus, conducting a genome-wide association study to identify quantitative trait nucleotides controlling S. hermonthica resistance and mining of relevant candidate genes will expedite the improvement of Striga resistance breeding through marker-assisted breeding. For this study, 150 diverse maize inbred lines were evaluated under Striga infested and non-infested conditions for two years and genotyped using the genotyping-by-sequencing platform. Heritability estimates of Striga damage ratings, emerged Striga plants and grain yield, hereafter referred to as Striga resistance-related traits, were high under Striga infested condition. The mixed linear model (MLM) identified thirty SNPs associated with the three Striga resistance-related traits based on the multi-locus approaches (mrMLM, FASTmrMLM, FASTmrEMMA and pLARmEB). These SNPs explained up to 14% of the total phenotypic variation. Under non-infested condition, four SNPs were associated with grain yield, and these SNPs explained up to 17% of the total phenotypic variation. Gene annotation of significant SNPs identified candidate genes (Leucine-rich repeats, putative disease resistance protein and VQ proteins) with functions related to plant growth, development, and defense mechanisms. The marker-effect prediction was able to identify alleles responsible for predicting high yield and low Striga damage rating in the breeding panel. This study provides valuable insight for marker validation and deployment for Striga resistance breeding in maize.

The development of maize varieties with increased concentration of Provitamin A (PVA) is an effective and affordable strategy to combat vitamin A deficiency in developing nations. However, the considerably high cost of carotene analysis poses a major challenge for maize PVA biofortification, prompting the use of marker-assisted selection. Presently, two types of genotyping with PVA trait-linked functional markers have been developed and extensively used in breeding programs. The two systems are low throughput gel-based genotyping and genotyping with Kompetitive Allele-Specific PCR (KASP) single nucleotide polymorphism (SNPs) markers. Although the KASP SNPs genotyping was developed to replace the gel-based genotyping, studies have not been conducted to compare the effectiveness of the KASP SNPs markers with the gel-based markers. This study was conducted to assess the carotenoid content of 64 tropical PVA biofortified maize inbred lines containing temperate germplasm in their genetic backgrounds and screen them with both gel-based and KASP markers of PSY1, LCYE and crtRB1 genes. Many of the 64 inbred lines had PVA concentrations surpassing the 15 µg/g provitamin A breeding target set by the HarvestPlus Challenge Program. Favorable alleles of crtRB1, crtRB1 and the KASP SNPs markers were detected in 25 inbred lines with high PVA concentrations. Inbred lines with the favorable alleles of LCYE had the highest concentrations of non-PVA carotenoids, whereas those with the favorable alleles of crtRB1 had high levels of PVA carotenoids. Data from the sequenced region of LCYE revealed one SNP in the first intron that clearly differentiated the high and low β-carotene maize inbred lines. The results of our study demonstrate that the automated KASP SNPs markers can replace the gel-based genotyping for screening a large number of early generation maize inbred lines for PVA content.

Two improved tropical maize composites, TZL COMP3 and TZL COMP4; representing complementary heterotic pools have been subjected to four cycles of reciprocal recurrent selection (RRS) for two decades to enhance varietal cross performance. The objectives of this study were to evaluate the effect of selection on genetic gain in heterosis for grain yield and other agronomic traits of these composites. Ten parental populations representing the C0 to C4 of each composite and their crosses plus a varietal check were evaluated in a trial at eight environments in Nigeria. Grain yield of the varietal crosses increased with selection by 3.1% cycle–1. Mean grain yields of the C4 × C4 varietal cross exceeded that of a popular improved reference variety by 23%. Selection also reduced anthesis-silking interval, improved ear characteristics, phenotypic appeal and resistance to foliar diseases. Mid-parent heterosis (MPH) increased from 4% at C0 × C0 to 24% at C4 × C4. The average rate of genetic gain in heterosis for grain yield in population crosses was 3.1% possibly because of presence of non-additive gene effects. The results of our study present the potential usefulness of the advanced selection cycle as sources of diverse inbred lines with improved combining ability as well as improved varietal crosses that can be multiplied and deployed in areas with limited market access.

A desirable tester that elicits greater genetic difference in Striga resistance among test crosses in a breeding program has not been reported. Therefore, this study was conducted to characterize 30 Striga resistant yellow endosperm maize inbred lines and three testers with varying resistance levels to Striga using DArTseq SNP markers and agronomic traits to identify a suitable tester for resistance hybrid breeding. Marker-based and agronomic trait-based genetic distances were estimated for yellow endosperm maize inbred lines and testers with varying resistance levels to Striga. The Marker-based cluster analysis separated the Striga resistant lines and testers into two distinct groups. Although the susceptible tester (T3) was the most distantly related to the 30 Striga resistant inbred lines, it exhibited a narrower range in genetic distance estimates and poor agronomic performance under Striga infestation in crosses with the resistant lines. In contrast, the resistant tester (T2) showed a broader range in genetic distance estimates in pairs with the 30 resistant lines. Also, it formed many high yielding hybrids with desirable traits under parasite pressure. Furthermore, the most significant positive association between agronomic trait-based and marker-based distance estimates (r = 0.389, P = 0.01) was observed when T2 has paired with the Striga resistant maize inbred lines. It thus appears that T2 may be used as a suitable tester to determine the breeding value of lines in hybrid maize resistance breeding programs. T2 was the most suitable tester, with a tolerant tester (T1) as an alternative tester to characterize the combining ability of Striga resistant maize inbred lines. This result can also encourage other breeders to investigate testers relative discriminating ability with varying levels of resistance in hybrid breeding for resistance to diseases, pests, and other parasitic plants.

Time lags in reporting to national surveillance systems represent a major barrier for the control of infectious diseases, preventing timely decision making and resource allocation. This issue is particularly acute for infectious diseases like malaria, which often impact rural and remote communities the hardest. In Guyana, a country located in South America, poor connectivity among remote malaria-endemic regions hampers surveillance efforts, making reporting delays a key challenge for elimination. Here, we analyze 13 years of malaria surveillance data, identifying key correlates of time lags between clinical cases occurring and being added to the central data system. We develop nowcasting methods that use historical patterns of reporting delays to estimate occurred-but-not-reported monthly malaria cases. To assess their performance, we implemented them retrospectively, using only information that would have been available at the time of estimation, and found that they substantially enhanced the estimates of malaria cases. Specifically, we found that the best performing models achieved up to two-fold improvements in accuracy (or error reduction) over known cases in selected regions. Our approach provides a simple, generalizable tool to improve malaria surveillance in endemic countries and is currently being implemented to help guide existing resource allocation and elimination efforts., Author summary Infectious disease monitoring systems worldwide are often threatened by severe reporting delays, hindering resource planning efforts, particularly for endemic diseases like malaria and in remote localities. In Guyana, a country located in South America, regions facing infrastructural barriers and consisting of highly mobile or socially disadvantaged populations oftentimes face the greatest delays in reporting local health facility data to the country’s central database. To better characterize and address this issue, we analyzed time and spatial trends in malaria reporting delays across administrative regions and implement methods to transform incomplete case data to more accurate current case counts. We first identify factors which significantly spatially overlap with areas marked by increased reporting delays. We then introduce methods which build on prior case information only available at the time of the target month of interest to anticipate the total number of cases to occur that month, a fraction of which are not known until the subsequent month(s). We compared our regional monthly estimates with eventually reported cases and found that they were up to two times more accurate than case counts known at the month of reporting. Our models provide a widely accessible and adaptable tool for improving on malaria surveillance efforts.

Striga hermonthica is a serious biotic stress limiting maize production in sub-Saharan Africa. The limited information on the patterns of genetic diversity among maize inbred lines derived from source germplasm with mixed genetic backgrounds limits the development of inbred lines, hybrids, and synthetics with durable resistance to S. hermonthica. This study was conducted to assess the level of genetic diversity in a panel of 150 diverse maize inbred lines using agronomic and molecular data and also to infer the population structure among the inbred lines. Ten Striga-resistance-related traits were used for the phenotypic characterization, and 16,735 high-quality single-nucleotide polymorphisms (SNPs), identified by genotyping-by-sequencing (GBS), were used for molecular diversity. The phenotypic and molecular hierarchical cluster analyses grouped the inbred lines into five clusters, respectively. However, the grouping patterns between the phenotypic and molecular hierarchical cluster analyses were inconsistent due to non-overlapping information between the phenotypic and molecular data. The correlation between the phenotypic and molecular diversity matrices was very low (0.001), which is in agreement with the inconsistencies observed between the clusters formed by the phenotypic and molecular diversity analyses. The joint phenotypic and genotypic diversity matrices grouped the inbred lines into three groups based on their reaction patterns to S. hermonthica, and this was able to exploit a broad estimate of the actual diversity among the inbred lines. The joint analysis shows an invaluable insight for measuring genetic diversity in the evaluated materials. The result indicates that wide genetic variability exists among the inbred lines and that the joint diversity analysis can be utilized to reliably assign the inbred lines into heterotic groups and also to enhance the level of resistance to Striga in new maize varieties.

Molecular-based characterization of open-pollinated varieties (OPVs) in maize is useful to define their unique profiles. A total of 58 SSR markers selected from a panel of 70 were used for genotyping three samples of 30, 50 and 100 plant bulks for each of 32 OPVs. The SSR markers detected a total of 253 alleles in the 32 maize OPVs across the three bulk samples. The number of alleles per marker varied from 2 to 7, with an overall mean of 4.36. The genetic distance among the OPVs varied from 0.17 to 0.93 with an average of 0.70 ± 0.007 for bulk samples of 30 plants, 0.20 to 0.91 with an overall mean of 0.70 ± 0.007 for bulk samples of 50 plants and from 0.30 to 0.95 with an average of 0.75 ± 0.006 for bulk samples of 100 plants. Cluster analysis separated the 32 maize OPVs into two major groups, which were further separated into two sub-groups for each type of bulk sample. The groupings of the OPVs into two major groups and their corresponding sub-groups was consistent with known breeding history (common parentage) and common target traits during development of the OPVs irrespective of the sample size used. However, in the bulk samples of 50 and 100 plants, some sub-groups were composed of OPVs with mixed maturity classes and diverse genetic backgrounds. Of the three independent bulk samples, the smallest sample size of 30 plants was found to be optimal for characterizing heterogeneous and heterozygous maize populations and OPVs owing to its cost-effectiveness and relative ease of sample processing.

Maize streak virus (MSV, genus Mastrevirus, family Geminiviridae), vectored by the leafhoppers (Cicadulina sp.), is the most economically important viral disease of maize endemic to Sub-Saharan Africa and its offshore islands. Yield losses due to MSV are mainly controlled through use of resistant varieties. ‘Recovery’ (ability of plants to reduce symptom severity), is one of the types of resistance being used to develop MSV resistant cultivars through breeding. This study was conducted to map Quantitative Trait Loci (QTLs) associated with recovery resistance to MSV in a mapping population comprising 250 S1 lines derived from a cross between two MSV resistant lines. The population was genotyped using single nucleotide polymorphism (SNP) markers at 269 loci. A genetic map of 11 linkage groups was constructed comprising 230 SNP markers. Four QTLs, two putative QTLs with significant effect on chromosome 3 and two other QTLs with reduced effect on chromosomes 7 and 9, were identified from the population. The two QTLs on chromosome 3 together accounted for 47 to 51% of the total phenotypic variance while the other two QTLs accounted for 28 to 32% of the total variation. These QTLs originated from the two parents of the mapping population had both additive and dominance effects but interaction among the four loci was not significant. Further validation of these QTLs associated with recovery resistance in other diverse populations will lead to the development of new genomic resources to enhance breeding for MSV resistant maize. Keywords: Mastrevirus, Maize breeding, Maize streak, Leafhoppers, QTL

The quest for precise and rapid phenotyping of germplasm is increasing the interest of breeders and physiologists in the application of remote sensing techniques in maize breeding. Twenty-four drought-tolerant maize inbred lines were crossed using a modified North Carolina II mating scheme to generate 96 single-cross hybrids. The parents and the hybrids were evaluated under full irrigation and drought stress conditions in the dry seasons of 2010 and 2011 at Ikenne, southwest Nigeria. Normalized difference vegetation index (NDVI) was recorded at 3- and 8-leaf growth stages. Hybrids differed significantly for NDVI. Both general (GCA) and specific (SCA) combining ability effects were significant for NDVI measured at 8-leaf stage under both irrigation regimes, with GCA accounting for 53% of the total variation under full irrigation. Both additive and non-additive genetic effects played significant roles in the inheritance of NDVI. The females GCA effects for grain yield was positively correlated with females ...

Genotype × environment interactions complicate selection of superior genotypes for narrow and wide adaptation. Eighteen tropically-adapted maize cultivars were evaluated at six locations in Nigeria for 2 yrs to (i) identify superior and stable cultivars across environments and (ii) assess relationships among test environments. Environment and genotype × environment interactions (GEI) were significant (P < 0·05) for grain yield. Environments accounted for 63.5% of the total variation in the sum of squares for grain yield, whereas the genotype accounted for 3.5% and GEI for 32.8%. Grain yield of the cultivars ranged from 2292 kg ha–1 for DTSTR-W SYN2 to 2892 kg ha−1 for TZL COMP4 C3 DT C2 with an average of 2555 kg ha−1. Cultivar DT SYN2-Y had the least additive main effect and multiplicative interaction (AMMI) stability value of 7.4 and hence the most stable but low-yielding across environments. AMMI biplot explained 90.5% and classified cultivars and environments into four groups each. IWD C3 SYN F3 was identified as the high-yielding and stable cultivar across environments. ZA15, ZA14, BK14, BK15 and IL15 had environment mean above the grand mean, while BG14, BG15, LE14, LE15, IL14, LA14 and LA15 had mean below the grand mean. ZA, BK, BG, LE and LA were found to be consistent in ranking the maize cultivars. However, Zaria, Birnin Kudu, and Ilorin were identified as the best test locations and could be used for selecting the superior maize cultivars. The identified high-yielding and stable cultivar could be further tested and promoted for adoption to contribute to food insecurity in Nigeria.

Two tropical maize composites were subjected to four cycles of reciprocal recurrent selection to develop divergent inbred lines with good combining ability. This study was conducted to examine the extent of genetic diversity, changes in allele composition and genetic structure, of 100 randomly selected S1 lines each from the original (C0) and advanced (C4) selection cycles of TZL COMP3 and TZL COMP4, genotyped using single nucleotide polymorphism (SNP) markers. Results revealed that the proportion of alleles at both low and high frequencies decreased from C0 to C4, whereas those at intermediate frequencies increased at C4 in the two composites. More unique and other alleles were lost at C4 in TZL COMP3 relative to those in TZL COMP4. The changes in different measures of genetic diversity were either small or negligible with selection in the two composites. The proportion of markers departing from Hardy–Weinberg equilibrium (HWE) decreased with selection, whereas the total number of pairs of markers in linkage disequilibrium increased with selection in the two composites. Examination of changes in population structures using a model-based approach as well as cluster and multivariate analyses found a high degree of concordance in stratifying the 400 S1 lines into four non-overlapping groups corresponding to the two selection cycles each within the reciprocal composites. The observed molecular-based divergence between cycles within the same composite and the clear differentiation between the complementary composites highlight the importance of reciprocal recurrent selection for preserving genetic diversity for long-term selection. This increases the potential of the advanced selection cycles to sustain genetic gain in productivity of hybrids adapted to the savannas in West and Central Africa.

The article [Factor analysis to investigate genotype and genotype × environment interaction effects on pro-vitamin A content and yield in maize synthetics], written by [Wende Mengesha, Abebe Menkir, Silvestro Meseka, Bunmi Bossey, Agbona Afolabi, Juan Burgueno and Jose Crossa], was originally published electronically on the publisher’s internet portal (currently SpringerLink).

Maize is a major source of food security and economic development in sub-Saharan Africa (SSA), Latin America, and the Caribbean, and is among the top three cereal crops in Asia. Yet, maize is deficient in certain essential amino acids, vitamins, and minerals. Biofortified maize cultivars enriched with essential minerals and vitamins could be particularly impactful in rural areas with limited access to diversified diet, dietary supplements, and fortified foods. Significant progress has been made in developing, testing, and deploying maize cultivars biofortified with quality protein maize (QPM), provitamin A, and kernel zinc. In this review, we outline the status and prospects of developing nutritionally enriched maize by successfully harnessing conventional and molecular marker-assisted breeding, highlighting the need for intensification of efforts to create greater impacts on malnutrition in maize-consuming populations, especially in the low- and middle-income countries. Molecular marker-assisted selection methods are particularly useful for improving nutritional traits since conventional breeding methods are relatively constrained by the cost and throughput of nutritional trait phenotyping.

Vitamin A Deficiency (VAD) is a major public health problem in Sub-Saharan Africa affecting 33 million preschool-age children. Enrichment of maize varieties with provitamin A could provide sustainable and affordable solution to VAD. This study was conducted to understand the extent of GEI effects on both grain yield and provitamin A content in 21 maize synthetics and identify synthetics combining stable performance with high level provitamin A content across diverse environments in West Africa. Combined analysis of variance found significant (p

Maize streak virus (MSV) is responsible for streak disease of maize and poses a serious threat to maize production in sub-Saharan Africa. Polygenic resistance to MSV has become an essential requirement in modern maize cultivars to mitigate yield losses. Many single nucleotide polymorphism (SNP) markers linked to putative MSV resistance loci have been identified for use in forward breeding. This study aimed to validate, using the high-throughput kompetitive allele specific PCR (KASP) assay, the diagnostic ability of the three SNP markers linked to the loci for the Msv1 resistance trait in 151 early generations inbred lines with diverse genetic backgrounds, together with nine MSV-resistant elite lines and a susceptible check (cv. Pool-16). The phenotypic responses were determined by MSV inoculation using viruliferous leafhoppers (Cicadulina triangular) under screenhouse conditions. Based on an established MSV disease rating system, the maize lines were categorized into resistant, moderately resistant, susceptible, and highly susceptible. The three SNPs associated with MSV resistance were detected in 133 lines, which were categorized as resistant (54), moderately resistant (76), and susceptible (1). The 18 early generation lines without these SNPs were classified as moderately resistant (10), susceptible (5), and highly susceptible (3). This study confirms the strong association of SNPs with MSV resistance and their usefulness for forward breeding in maize while emphasizing the need to identify additional markers to screen lines for MSV resistance without any ambiguity.

Drought occurs at high frequency in maize production zones in Africa. Carefully controlled moisture deficits have been imposed at anthesis and grain filling stages of maize to develop drought tolerant germplasm. The present study was conducted to examine the potential effects of drought tolerant parents selected under such managed drought stress on agronomic performance of their top-cross hybrids across variable growing conditions in the rainy season. Three independent trials consisting of top-cross hybrids involving drought tolerant parents and commercial hybrids were evaluated under manged drought stress (MDS) and well watered (WW) condition at Ikenne in Nigeria for 3 years during the dry season and across 30 stressful (SE) and 52 favourable (FE) rainfed environments in West Africa. The drought stress imposed in the dry season caused a 53–64 % annual average yield reductions in these trials. In spite of these, about 56 % of the top-crosses produced high grain yields under MDS and WW conditions. The rank orders for hybrid grain yields did not change markedly across different years under MDS and WW conditions. Also, nearly 78 % of the top-crosses produced high grain yields across SE and FE environments in the rainy season. Sixteen top-cross hybrids amongst these were present in the top yielding groups under the MDS environment. Some high yielding top-crosses with stable performance across variable rainfed environments were identified in the current study. The phenotypic and genetic correlations between grain yield under MDS and that under SE environment were positive and moderate. Also, the phenotypic and genetic correlations showed that the capacity of top-cross hybrids to produce high grain yields under MDS was independent of the yield potential under FE environment. These results suggest that selection of parents under manged drought stress promotes the development of top-cross hybrids broadly adapted to variable rainfed conditions.

Aflatoxin accumulation in maize (Zea mays L.) kernels is a serious economic and health problem that reduces grain quality and nutritional values and causes death to livestock and humans. Understanding the genetic parameters and heterotic responses of exotic maize inbred lines can facilitate their use for developing aflatoxin resistant parents of hybrids in Africa. This study was designed to (1) determine the heterotic affinities of aflatoxin resistant exotic lines, (2) identify exotic inbreds with good combining ability, and (3) determine the mode of inheritance of resistance to aflatoxin contamination in these lines. A line × tester mating design was used to determine combining ability of 12 yellow and 13 white inbreds and classify them into heterotic groups. The inbreds were crossed to two adapted testers representing two African heterotic groups and the resulting testcrosses along with hybrid checks were evaluated in separate trials at two locations for 2 years in Nigeria. General combining ability (GCA) effects were more important than specific combining ability effects for aflatoxin and grain yield. Among 15 exotic inbred lines having negative GCA effects for aflatoxin and 13 with positive GCA effects for grain yield, six combined the two desired traits. Five white and six yellow endosperm testcrosses were found to be good specific combiners for the two desired traits. The exotic lines with negative GCA effects for aflatoxin accumulation will be used as donor parents to develop backcross populations for generating new inbred lines with much higher levels of resistance to aflatoxin accumulation.

The discovery and use of genetic markers associated with carotenoid levels can help to exploit the genetic potential of maize for provitamin A accumulation more effectively. Provitamin A carotenoids are classes of carotenoids that are precursors of vitamin A, an essential micronutrient in humans. Vitamin A deficiency is a global public health problem affecting millions of people, especially in developing countries. Maize is one of the most important staple crops targeted for provitamin A biofortification to help alleviate vitamin A deficiency in developing countries. A genome-wide association study (GWAS) of maize endosperm carotenoids was conducted using a panel of 130 diverse yellow maize tropical inbred lines genotyped with Genotyping by Sequencing (GBS) SNP markers. Numerous significant association signals co-localizing with the known carotenoid biosynthesis genes crtRB1, lcyE and ZEP1 were identified. The GWAS confirmed previously reported large effects of the two major carotenoid biosynthesis genes lcyE and crtRB1. In addition, significant novel associations were detected for several transcription factors (e.g., RING zinc finger domain and HLH DNA-binding domain super family proteins) that may be involved in regulation of carotenoid biosynthesis in maize. When the GWAS was re-conducted by including the major effects of lcyE and crtRB1 genes as covariates, a SNP in a gene coding for an auxin response factor 20 transcription factor was identified which displayed an association with β-carotene and provitamin A levels. Our study provides a foundation for design and implementation of genomics-assisted selection strategies for provitamin A maize breeding in tropical regions, and advances efforts toward identification of additional genes (and allelic variants) involved in the regulation of carotenoid biosynthesis in plants.

The nutritional value and health benefits derived from carotenoids have prompted increased investment in breeding crop varieties with elevated carotenoid levels for areas where crops like maize with low nutrient density are consumed in large quantities. Twelve exotic donor lines of high β-carotene were crossed to seven elite tropical yellow or orange lines and the F1s were backcrossed to the same or different elite line. Ninety-eight lines derived from these backcrosses (BC), seven recurrent parents, and 24 adapted lines derived from other source populations were included in a trial grown at Ibadan in Nigeria for three years. Carotenoid analyses of samples harvested from this trial found significant differences in accumulating provitamin-A and other carotenoids among lines, which were consistently expressed across years. The lines also displayed distinct carotenoid profiles. Among the best 25 inbred lines containing 5.0–16.6 μg g−1 β-carotene and 8.0–17.4 μg g−1 pro-vitamin A, 24 were BC-derived lines and one was derived from a bi-parental cross of tropical yellow elite lines. The best BC-derived lines accumulated 23–313 % more β-carotene and 32–190 % more provitamin-A than the recurrent parents. These BC-derived lines may be valuable resources for favorable alleles to develop maize varieties with high provitamin-A and for genetic studies to understand the underlying mechanisms regulating carotenoid biosynthesis. These results illustrate the effectiveness of exploiting natural allelic diversity existing in exotic lines through backcrossing combined with visual selection for bright yellow to orange kernel color with semi-flint to flint kernel texture in boosting provitamin-A to a new level in tropical maize.

Adoption of improved maize (Zea mays L.) varieties in the rainforest agro-ecology of Nigeria largely depends on yielding potential and resistance to common foliar diseases. The present study aims at assessing the combining ability of a set of adapted and exotic drought-tolerant maize inbred lines under irrigated and rainfed conditions in Nigeria. Ninety-six hybrids generated using a North Carolina Design II scheme, and four checks were evaluated in a 10 × 10 triple lattice with three replications under full irrigation in the dry seasons of 2010 and 2011 and under natural disease inoculation in the rainy seasons of 2011 and 2012. Hybrids differed significantly for all measured traits. Both general combining ability (GCA) and specific combining ability (SCA) were mostly highly significant (P 70%). EXL03, EXL06, EXL16, ADL33, and AD...

Maize has been targeted as one of the major crops for provitamin enrichment and delivery because it is an inexpensive and easily available source of food for millions of people in sub-Saharan Africa. Although tropical-adapted yellow maize contains provitamin-A carotenoids that can be converted into vitamin A in the human body, they represent less than 25% of the total carotenoids in most widely grown and consumed maize cultivars in Africa. Novel genes conditioning high concentration of β-carotene and other carotenoids were then continually introduced from the temperate zone and tropics to boost provitamin A in tropical-adapted maize. Several promising inbred lines developed from backcrosses involving diverse exotic donor lines displayed provitamin A concentrations that match or surpass the current breeding target of 15 μg g−1. Some of these lines attained high provitamin A content by accumulating mainly high β-carotene while others contained high provitamin A by promoting accumulation of high levels of both carotenes and xanthophylls. Several inbred lines with intermediate to high levels of provitamin A have already been used to develop hybrids and synthetics without compromising grain yield and other adaptive traits that are required to profitably cultivate maize by farmers in West and Central Africa.

Estimates of combining abilities and heterosis of inbred lines are imperative for selection of suitable parents of maize hybrids. This study examined the combining ability of 24 drought-tolerant maize inbred lines, 12 each from International Centre for Maize and Wheat Improvement (CIMMYT) and International Institute of Tropical Agriculture (IITA). The lines were allotted into six groups each comprising four lines. The four lines in one group were used as females and crossed to the four lines in another group as males in six different sets using a North Carolina Design II mating scheme to generate 96 hybrids. The hybrids were evaluated together with four checks across six environments in the rainforest and savannah agro-ecologies of Nigeria between 2011 and 2012. The parental inbred lines were also evaluated in separate trial in each location. Significant hybrids × environment interaction was observed for grain yield and other measured traits. GCA effects accounted for 83.3% of the variation for grain yield at Bagauda, 78.1% at Saminaka, and 77.7% at Ikenne. GCA also contributed 91.1 and 80.0% to the variation observed for plant height and ear aspect, respectively, across the environments. Significant SCA × environment interaction detected for grain yield suggests that hybrids were not stable across test environments. Prediction of grain yield in hybrids using midparent values resulted in a R 2 value of 0.13. Midparent heterosis for grain yield varied from 80 to 411%, with the top 36 hybrids recording >200%. Four CIMMYT (EXL02, EXL06, EXL04 and EXL16) and three IITA (ADL33, ADL41, and ADL32) inbred lines had positive and significant GCA effects for grain yield across environments. The novel alleles present in the CIMMYT lines will improve the adapted IITA germplasm in a new population for extracting new set of more productive inbred lines for developing adapted high yielding drought-tolerant maize hybrids.

Yield losses of maize due to drought stress (DS) in environments with limited rains can reach 100 %. Introgression of favorable alleles from diverse landraces into adapted elite varieties will improve performance, hence reducing losses. The objective of this study was to assess the performance of BC populations under DS and multiple locations (ML) after introgression of alleles from diverse maize landraces into an elite variety. Twelve backcross (BC1F2) populations together with their recurrent parent (TZLCOMP4C1) and a commercial hybrid (Oba Super1) were evaluated under DS, well watered (WW) conditions during the dry season, and in seven ML trials in Nigeria. Significant differences were observed among genotypes for all traits except for ear aspect score under DS conditions. Grain yield and most agronomic traits measured were highly significant (p

Maize (Zea mays L.) landraces in the northern Guinea savanna and Sudan savanna in West and Central Africa appear to have some drought-adaptive traits. This study was initiated to assess the level of improvement in yield potential and other agronomic traits achieved under drought stress (DS) and in multiple locations (ML) after introgression of alleles from maize landraces into an elite maize variety (AK9443-DMRSR) via backcrossing. Six backcross (BC) populations together with recurrent parent (AK9443-DMRSR), a commercial hybrid (Oba Super-II), and an improved variety (TZLCOMP4C1) were evaluated under controlled DS and full irrigation (FI) during the dry seasons of 1999 and 2000, as well as in seven ML trials. No significant differences were observed among genotypes for grain yield and most of the traits measured under DS and FI. Significant differences were recorded among genotypes for grain yield and other agronomic traits measured in ML and across 11 environments. Drought stress reduced grain yields of ...

Background Molecular characterization is important for efficient utilization of germplasm and development of improved varieties. In the present study, we investigated the genetic purity, relatedness and population structure of 265 maize inbred lines from the Ethiopian Institute of Agricultural Research (EIAR), the International Maize and Wheat Improvement Centre (CIMMYT) and the International Institute of Tropical Agriculture (IITA) using 220,878 single nucleotide polymorphic (SNP) markers obtained using genotyping by sequencing (GBS). Results Only 22% of the inbred lines were considered pure with

Improving maize productivity in the savannas of West Africa will depend on improving soil fertility and ensuring the efficient use of inputs and the relevant maize varieties tolerant to low nitrogen (N) that have been developed for this region. Several cycles of full-sib family selection that combine agronomic performance under low N with yield under high N to maximize gains under both environments have been used to improve three such developed populations. Evaluation of different cycles of selection revealed that desirable changes in gene frequencies had taken place after three cycles of selection in two of the three populations, leading to a yield gain of over 200 kg/ha/cycle under 30 kg N/ha and over 270 kg/ha/cycle under 90 kg N/ha. Trends in performance of the third population under low N simply indicated that additional cycles of selection would be required to shift gene frequencies significantly in the direction of increased yield production. This yield gain did not adversely affect other agronomic...

The majority of farmers in sub-Saharan Africa (SSA) depend on maize (Zea mays L.), which is mostly grown in drought-prone areas. Understanding the inheritance of drought tolerance would help in developing drought-tolerant maize hybrids that would help increase maize production under erratic rainfalls. Twenty-four maize inbred lines were subjected to NC design II to generate 96 single-cross hybrids. The 24 inbred lines and their 96 hybrids were evaluated under managed drought-stress conditions between 2002 and 2005 at Ikenne, Nigeria. The objectives of this study were to determine the effects of drought-susceptible and drought-tolerant inbred lines on performance of their hybrids, investigate mode of inheritance of drought tolerance, and determine the relationship between parental lines and their hybrids. Most crosses with at least one drought-tolerant parent produced tolerant hybrids, and the crosses between susceptible inbred lines produced susceptible hybrids. General combining ability (GCA) accounted f...

Aspergillus flavus infection of maize and subsequent contamination with carcinogenic aflatoxins poses serious health concerns, especially in developing countries. Maize lines resistant to A. flavus infection have been identified; however, the development of commercially-useful aflatoxin-resistant maize lines has been hindered due to a lack of breeding markers. To identify maize resistance-associated proteins (RAPs) as potential markers for breeding, 52 BC1S4 lines developed from crosses between five African maize inbreds and five temperate aflatoxin-resistant lines were screened using the kernel screening assay. Five pairs of closely-related lines that had 75–94% genetic similarity within each pair and which varied within each pair in aflatoxin accumulation were selected for proteomic investigation. Kernel embryo and endosperm protein profile differences within the pair and across pairs were compared using two-dimensional polyacrylamide gel electrophoresis. Differentially expressed (≥1.5-fold) RAPs were sequenced through tandem mass spectrometry and were identified as antifungal, stress-related, storage or regulatory proteins. Sequence homology analysis highlighted several proteins in maize that confer resistance to A. flavus infection and/or aflatoxin production.

Maize (Zea mays) is a staple food crop in sub-Saharan Africa, but its production is threatened by Striga hermonthica (Del.) Benth. Transfer of resistance genes from wild relatives may increase resistance to S. hermonthica in tropical maize. The objective of this study was to determine the combining ability of resistance to S. hermonthica among lines containing Zea diploperennis and tropical germplasm. Forty-five diallel crosses of 10 inbred lines were evaluated in an alpha-lattice design with and without artificial Striga infestation at two locations each in the Republic of Benin and Nigeria for 3 years. Results of analyses showed that only general combining ability (GCA) mean square was significant (P = 0.01) for number of emerged Striga plants (NESPP), while both GCA and specific combining ability (SCA) mean squares were significant for host damage score (HDS) and grain yield under Striga infestation. The ratio of GCA to SCA mean squares for the three traits varied from 3.5 to 57.5. Although GCA x environment interaction was significant for the three traits, two inbred lines containing Z. diploperennis (ZD 551) and tropical (TZL TC 87) germplasm had negative and significant GCA effects for NESPP and HDS and positive GCA effects for grain yield under Striga infestation in the two countries. Correlation between NESPP and HDS was strong and significant (r = 0.87, P = 0.01). Our results highlight the importance of harnessing useful genes from wild relatives to improve resistance to S. hermonthica in adapted maize germplasm.

In drought-affected maize production zones with short growing periods, the development and use of early maturing drought-tolerant cultivars can stabilize maize production. We evaluated 10 improved and 25 farmers' early maturing maize varieties under moisture deficit and well-watered conditions for 2 years to identify suitable genetic materials for breeding drought-tolerant cultivars. The varieties exhibited significant differences in grain yield and other traits under both moisture deficit and well-watered conditions. Changes in the rank order of the varieties for grain yield was not significant across the different levels of moisture supply in this study. Grain yield was significantly correlated with days to anthesis, days to silking, plant height, ear height, ear number and anthesis–silking interval (ASI) under the two irrigation treatments and with leaf death scores under moisture deficit, suggesting that the common traits were beneficial in maximizing grain yield under both sufficient water supply and moisture deficit. Grain yield and the traits significantly correlated with it differentiated the early maturing maize varieties into two distinct groups under well-watered condition and moisture deficit. The improved varieties were superior to the farmers' varieties in grain yield and other traits under moisture deficit, possibly due to selection of their progenitors for improved performance in multiple locations. We found some farmers' and improved varieties with similar yield potential and flowering time under well-watered conditions but with marked differences in grain yield and other traits under moisture deficit. Use of such promising landraces that would also be invaluable sources of desirable farmers-preferred end-use quality traits in combination with promising improved varieties as breeding materials could enhance the genetic grain from selection for drought tolerance in early maize.

Aflatoxins are carcinogenic secondary metabolites produced mainly by Aspergillus flavus Link ex. Fries, and A. prarasiticus Speare during infection of susceptible crops, such as maize, cottonseed, peanuts and tree nuts. This paper will review research efforts in identifying aflatoxin resistance-related proteins/genes in maize. Similar strategies may be useful in peanut. For maize, although genotypes resistant to A. flavus infection or aflatoxin production have been identified, the incorporation of resistance into commercial lines has been slow due to the lack of selectable markers and poor understanding of host resistance mechanisms. Recently, resistance-associated proteins (RAPs) were identified through proteomic comparison of constitutive protein profiles between resistant and susceptible maize genotypes. These proteins belong to three major groups based on their peptide sequence homologies: storage proteins, stress-related proteins, and antifungal proteins. Preliminary characterization of some of these RAPs suggest that they play a direct role in host resistance, such as pathogenesis-related protein 10 (PR10), or an indirect role, such as glyoxalase I (GLX I), through enhancing the host stress tolerance. To verify whether these RAPs play a role in host resistance, RNA interference (RNAi) gene silencing technique was used to silence the expression of these genes in maize. RNAi vectors (glx I RNAi and pr10 RNAi) were constructed using Gateway technology, and then transformed into immature maize embryos using both bombardment and Agrobacterium infection. The extent of gene silencing in transgenic callus tissues ranged from 20% to over 99%. The RNAi silenced transgenic maize seeds have also been obtained from plants regenerated from Agrobacterium transformed callus lines. Kernel screen assay of the transgenic maize kernels demonstrated a significant increase in susceptibility to A. flavus colonization and aflatoxin production in some of the silenced transgenic lines compared with non-silenced control kernels, suggesting the direct involvement of these two proteins in aflatoxin resistance in maize.

Six tropical maize (Zea mays L.) germplasm lines, TZAR101 (Reg. No. GP-568, PI 654048), TZAR102 (Reg. No. GP-569, PI 654049), TZAR103 (Reg. No. GP-570, PI 654050), TZAR104 (Reg. No. GP-571, PI 654051), TZAR105 (Reg. No. GP-572, PI 654052), and TZAR106 (Reg. No. GP-573, PI 654053), with resistance to afl atoxin contamination were developed by the International Institute of Tropical Agriculture through a collaborative breeding project with Southern Regional Research Center of the USDA-ARS. The lines were derived from biparental crosses and backcross populations involving afl atoxinresistant tropical elite and temperate inbred lines as parents. These lines had afl atoxin levels similar to or lower than a resistant U.S. inbred check, MI82, in both preliminary and confi rmation tests conducted in the laboratory using a kernelbased screening assay. Further fi eld tests of the six lines under artifi cial inoculation with an African strain of Aspergillus fl avus in Nigeria revealed that these lines had lower levels of afl atoxin compared with elite tropical commercial inbred lines used as checks. These lines also had good agronomic traits and resistance to important diseases in the lowlands, including southern corn leaf blight [caused by Bipolaris maydis (Nisikado & Miyake) Shoemaker], southern corn rust (caused by Puccinia polysora Underw.), and ear rot.

A major problem that usually confronts the breeder is ensuring that there is sufficient genetic variability in a population for the trait being improved because this determines the breeding schemes to use for improvement and whether or not sufficient progress could be made. Three hundred full-sib families from cycle 3 of the early maturing yellow endosperm maize (Zea mays L.) population, TZE-Y Pop DT STR, were evaluated under artificial Striga hermonthica infestation at Mokwa and Abuja, Nigeria in 2003. The objectives of this study were to investigate the type of gene action involved in the inheritance of S. hermonthica resistance in TZE-Y Pop DT STR C3, determine the extent of genetic variability available in the population after three cycles of S1 recurrent selection, and the phenotypic and genetic correlation coefficients among the traits used for selecting for resistance to S. hermonthica. Estimates of dominance variances were larger than additive genetic variances for grain yield, plant heig...

Introgression from exotic maize (Zea mays L.) into adapted breeding pools can broaden and diversify the genetic base of adapted germplasm. The first objective of this study was to determine the agronomic performance of white maize lines derived from adapted × exotic backcrosses in tropical environments. Six exotic white maize inbred lines were crossed to an adapted white line (1368) and the F1s were backcrossed to 1368. Forty-one BC1F4 lines derived from these backcrosses and the recurrent parent were crossed with a common inbred tester (9071) and the progeny were evaluated at eight environments in Nigeria. The testcrosses × environment interaction mean square was not significant for all agronomic traits. The BC-derived lines containing exotic germplasm contributed significantly to the variation in testcross mean grain yields and other agronomic traits. Among the 41 testcrosses, only 5 yielded significantly less than 1368 × 9071, with the 7 best testcrosses producing between 304 and 867 kg ha−1 more grain than 1368 × 9071. The second objective of this study was to assess the genetic divergence of the BC-derived white lines from their adapted recurrent parent. Thirty-nine BC-derived white lines along with 1368 and 9071 were genotyped with ten AFLP primer pairs that generated 506 polymorphic fragments. The average allelic diversity of the lines was 31 ± 0.07. The genetic distance (GD) estimates of each BC-derived line from 1368 varied from 0.76 to 0.84, with a mean of 0.80 ± 0.003. The average GD for all pairs of the BC-derived lines was 0.63 ± 0.005, varying from 0.31 to 0.89. The observed significant amount of variation among the BC-derived white lines suggests that they can contribute new alleles for expanding the genetic base of tropical maize and for developing high-yielding hybrids.

Striga hermonthica is the most widespread and destructive obligate root parasite infecting maize and other cereals in Africa. Maize inbred lines supporting reduced S. hermonthica emergence can form an important basis for developing Striga-resistant maize cultivars. Twenty new inbred lines selected for field resistance to S. hermonthica, and five inbred checks with known resitance, tolerance and susceptibility reactions to S. hermonthica were evaluated in pots, greenhouse and field experiments under artificial Striga infestation for 3 years. The experiments were conducted to determine the extent of variation in parasite attachment to the roots of these lines and its relationship with emerged Striga plants and other traits. Significant differences (P < 0.0001) were detected among the inbred lines for the numbers of attached and emerged Striga plants and the results were consistent across test environments. Also, the lines exhibited significant differences for Striga damage symptom ratings and other traits recorded in the field. Parasite attachment to the roots was significantly correlated with emerged Striga count in the screenhouse (r = 0.67-0.68, P < 0.001) and in the field (r = 0.82-0.84, P < 0.0001) and with levels of grain yield reduction due to Striga (r = 0.71, P < 0.0001). Regression analysis of the numbers of attached parasites on the first principal component axis scores that integrated several traits recorded in the field was significant (P < 0.0001) and accounted for 62% of the total variation in numbers of attached parasites. The new inbred lines and the resistant inbred check were the least affected by S. hermonthica and exhibited yield losses of 0-37% compared with the yields of the tolerant and the susceptible inbred checks, which were reduced by 40-85%. Sixteen new inbred lines supported significantly fewer attached parasites compared with the susceptible inbred check. Some of these lines also supported significantly fewer emerged parasites and sustained lower damage symptoms and percentage yield losses due to Striga compared with the susceptible inbred check. These inbred lines would be useful in breeding programmes for developing resistant maize cultivars.

Full-sib families derived from Pool 16 DT, a tropical maize ( Zea mays L.) population, were evaluated in the 1995/96 and 1997/98 dry seasons, using two sites each season in Cote d’Ivoire. In all sites, the crop was irrigated from planting to about 2 weeks before anthesis, irrigation was discontinued thereafter for the rest of the season in one site in 1995 and both sites in 1997. Irrigation was continued till maturity in the second site in 1995. The means and ranges showed that the induced stress environments produced significantly lower grain yield, fewer ears per plant (EPP) and lower grain moisture percentage than the non-stressed site. The coefficients of variation (CVs) associated with the stressed environment were consistently larger than those associated with the non-stressed environment. Similarly, the coefficients of determination (R 2 ) values were generally higher for the non-stressed than the stressed site. Only 7 families were common to the highest-yielding 20 % of the 90 families in each eva...