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Maize stalk rot reduces grain yield and quality. Information about the genetics of resistance to maize stalk rot could help breeders design effective breeding strategies for the trait. Genomic prediction may be a more effective breeding strategy for stalk-rot resistance than marker-assisted selection. We performed a genome-wide association study (GWAS) and genomic prediction of resistance in testcross hybrids of 677 inbred lines from the Tuxpeño and non-Tuxpeño heterotic pools grown in three environments and genotyped with 200,681 single-nucleotide polymorphisms (SNPs). Eighteen SNPs associated with stalk rot shared genomic regions with gene families previously associated with plant biotic and abiotic responses. More favorable SNP haplotypes traced to tropical than to temperate progenitors of the inbred lines. Incorporating genotype-by-environment (G x E) interaction increased genomic prediction accuracy. [ABSTRACT FROM AUTHOR]

Heat and drought stresses negatively affect maize (Zea mays L.) productivity. We aimed to identify the genetic basis of tolerance to heat stress (HS) and combined heat and drought stress (HS+DS) and compare how QTL and whole genome selection (GS) could be leveraged to improve tolerance to both stresses. A set of 97 testcross hybrids derived from a maize bi-parental doubled-haploid population was evaluated during the summer seasons of 2014, 2015, and 2016 in Ciudad Obregon, Sonora, Mexico, under HS and HS+DS. Grain yield (GY) reached 5.7 t ha−1 under HS and 3.0 t ha−1 under HS+DS. Twenty-six QTL were detected across six environments, with LOD scores ranging from 2.03 to 3.86; the QTL explained 8.6% to 18.6% of the observed phenotypic variation. Hyperspectral biomass and structural index (HBSI) had higher genetic correlation with GY for HS (r = 0.97) and HS+DS (r = 0.74), relative to the correlation with crop water mass or greenness indices. Genetic correlations between GY and canopy temperature for HS (r = −0.89) and HS+DS (r = −0.75) or vegetation indices, along with clusters of QTL in bins 1.02, 1.05, and 2.05, underline the importance of these genomic areas for secondary traits associated with general vigor and greenness. Prediction accuracy of the model used for GS had values below those found in previous studies. We found a high-yielding hybrid that was tolerant to HS and HS+DS. [ABSTRACT FROM AUTHOR]

Most maize production across the globe is rain‐fed, and production is set to be negatively impacted as duration and occurrence of droughts increases due to climate change. Development of water‐deficit tolerant maize germplasm has been a major focus for most breeding programmes. Here, we sought to assess the genetic gain for grain yield in two maize populations developed for drought tolerance at CIMMYT by evaluating their cycle progeny through hybrid performance. Inbreds derived from different cycles of the Drought Tolerant Population (DTP) and La Posta Sequia (LPS) were mated to a tester (CML550), and resulting hybrids were evaluated under managed water‐deficit stress and well‐watered conditions. The difference in yield between water‐deficit and well‐watered treatments was 27% and 36% for the DTP and LPS, respectively. Genetic gain for grain yield across cycles for the two populations was confirmed in the study. Genetic gain was observed for both treatments indicating that selection for water‐deficit stress tolerance simultaneously improves grain yield in well‐watered conditions. The DTP population had a genetic gain of 0.07 t ha−1 cycle−1, while the LPS had 0.16 t ha−1 cycle−1 under water‐deficit conditions. Significant genetic gain was also observed in the well‐watered treatments for both populations. Anthesis to silking interval was significantly reduced under water‐deficit stress conditions in both populations. Plant and ear height were reduced in the LPS population in both treatments, while no reductions were observed for the trait in the DTP population. Potential water‐deficit stress tolerance donor lines with yields comparable to commercial check varieties were identified. [ABSTRACT FROM AUTHOR]

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Maize (Zea mays L.) breeding programs are increasingly using molecular tools to enhance the efficiency and speed of developing productive cultivars. As a preliminary step toward using each other's germplasm, 20 elite inbred lines, 10 each from CIMMYT and IITA, were crossed according to a modified North Carolina Design II and were genotyped using 62 simple sequence repeat (SSR) markers. The design used six sets of 25 hybrids, including all combinations among groups of five lines each from CIMMYT and IITA's respective heterotic groups A and B. The 150 single-cross hybrids were evaluated in six environments in Zimbabwe and Nigeria during 2003 and 2004, with average grain yield ranging from 5.3 to 11.9 Mg [ha.sup.-1]. Combining ability analysis indicated that CML444 and CML312 can contribute to the breeding program of IITA, and that TZMI702 should be useful to CIMMYT. The SSR markers identified 209 alleles with an average 3.4 alleles [locus.sup.-1] and revealed moderate genetic distances among the parent lines. Genetic distances were not significantly associated with hybrid performance, specific combining ability effects, or midparent heterosis for grain yield, but they effectively grouped the lines according to known pedigree relationships and approximately according to heterotic patterns used by CIMMYT and IITA. Although SSR-determined genetic distances were not useful to predict hybrid grain yield, they could be helpful for preliminary classification and use of exotic or poorly characterized germplasm.

Maize (Zea mays L.) improvement involves formation, evaluation, selection, and recombination of genetically variable families or inbred lines, and because cultivars must combine many desirable traits, the process can be complicated and lengthy. This study compared 12 experimental open-pollinated varieties (OPVs) developed from maize population Pool 9A by two approaches, full-sib (FS) selection alone and FS selection combined with inbreeding with selection for Maize streakvirus (MSV) resistance. The experimental OPVs were evaluated in 13 environments in Africa, including three sites with artificial MSV inoculation. Full-sib selection combined with inbreeding and selection for MSV resistance [with mild selection for gray leaf spot resistance (GLS, caused by Cercospora zeae maydis Tehon & Daulels)], reduced ear height and reduced days to flowering] resulted in OPVs with similar grain yield but superior MSV resistance (P < 0.01), fewer days to anthesis (P < 0.01), and lower ear height (P < 0.05) than OPVs developed by FS selection alone. Each generation (from [S.sub.0] to [S.sub.3]) of inbreeding with selection resulted in OPVs that were 16, 8, 2, and 1% improved for MSV and GLS resistance, ear height, and days to anthesis, respectively. Our results demonstrate improvement of a maize population for MSV resistance and other traits by selection during inbreeding (from [S.sub.0] to [S.sub.3]), without negative impact on gains for grain yield achieved by evaluation and selection among the progenitor FS families., DEMAND for maize grain is projected to increase from 1995 levels by 50% globally by 2020, including 93% in sub-Saharan Africa, 92% in South Asia, 62% in Latin America, and [...]

Maize weevil, Sitophilus zeamais Motschulsky, is an important pest of stored maize (Zea mays L.) in the tropics. We used 14 southern African maize inbred lines to assess (i) combining ability for resistance to maize weevil in [F.sub.2] and [F.sub.2]-Syn 1 grain, (ii) the importance of maternal effects for resistance of [F.sub.2] and [F.sub.2]-Syn 1 grain to maize weevil, and (iii) combining ability for grain yield of [F.sub.1] hybrids. Maize weevil resistance of [F.sub.2] grain was evaluated in 2000 for a 14-parent diallel, and in 2002 for [F.sub.2] and [F.sub.2]-Syn 1 grain of a 10-parent subset diallel. Fifty-gram grain samples of each hybrid were infested with 32 weevils for a 10-d oviposition period, after which the samples were incubated in a laboratory. Grain yield was evaluated for the 14-parent diallel during the summer of 1999-2000 at four locations in Zimbabwe. General combining ability (GCA), specific combining ability (SCA), and reciprocal effects were highly significant (P < 0.01) for number of [F.sub.1] weevils emerged from [F.sub.2] grain samples for the combined 2000 and 2002 analysis for the 10-parent subset diallel. For weevil resistance of [F.sub.2]-Syn 1 grain, GCA and SCA effects were significant (P < 0.01), while reciprocal effects were not important. The GCA was more important than SCA for grain yield, indicating that yield was controlled mainly by additive gene action among these lines. There was no significant relationship between grain yield and weevil resistance. Although inheritance of weevil resistance was complex, our results suggest using inbred lines with good GCA for weevil resistance as female parents for hybrids or as components of synthetic (open-pollinated) cultivars for regions where farmers often store maize grain without chemical protection against weevils., MAIZE WEEVIL (Coleoptera: Curculionidae) is an important pest of stored maize in the tropics, particularly where maize is stored on-farm, with no control of moisture content and without chemical protectants. [...]

Maize weevil (Sitophilus zeamais Motschulsky) is an important pest of maize (Zea mays L.) in the tropics, causing serious losses for many resource-poor farmers who store grain on-farm for use as food and seed. This study evaluated whether weevil resistance of six maize populations could be divergently changed by [S.sub.1] and [S.sub.2] selection, and assessed the importance of replicating grain samples when screening for resistance. Weevil resistance was evaluated for unreplicated [S.sub.1] and for replicated and unreplicated [S.sub.2] lines by infesting 50-g grain samples with 32 young adult weevils, and then incubating the samples in a controlled temperature and relative humidity (CTH) laboratory. Divergent synthetics were formed by recombining the most resistant 10% and the most susceptible 10% of at least 100 lines screened for weevil resistance for each maize population. Replicated [S.sub.2] selection was successful for both populations where it was applied, resulting in an average of 16% (P < 0.01), 49% (P < 0.05), and 20% (P < 0.01) difference between divergent synthetics for weevil progeny emerged, grain weight loss, and the Dobie index of susceptibility, respectively. [S.sub.1] unreplicated selection was successful for two of the six populations, while [S.sub.2] unreplicated selection was not successful. Reciprocal effects were significant (P < 0.01) for weevil resistance of [F.sub.1] varietal crosses among the divergently selected synthetics, indicating the influence of maternal effects. Nevertheless, the most resistant crosses were those among the most resistant synthetics, confirming that additive gene action for weevil resistance was important. Our results provide practical insights regarding methodologies and demonstrate that it is possible to improve weevil resistance of maize populations., MAIZE WEEVIL (Coleoptera: Curculionidae) is an important pest of stored maize in the tropics, particularly in the lowland and mid-altitude, hot, and humid environments (Longstaff, 1981; DeVries and Toenniessen, 2001; [...]

We studied the interrelation among time to anthesis, physiological maturity, and grain yield in tropical maize (Zea mays L.) facing terminal drought and nitrogen deficiency. Sixty-one hybrids were grown under optimum, nitrogen-deficient (LN), and drought-stressed (DS) conditions. Grain yield was negatively correlated with time to anthesis under DS (r = −0.4*) and positively correlated with the grain-filling period (optimum: r = 0.54**; LN: r = 0.61***; DS: r = 0.69***) across treatments. These results suggest that coselecting for grain yield, early anthesis, and a long grain-filling period within a maturity group could help increase grain yield. A second experiment involving eight hybrids contrasting for grain yield and time to anthesis was conducted under similar stresses to identify secondary traits associated with grain yield and time to physiological maturity: early vigor (measured as normalized differential vegetation index) correlated with grain yield under optimum (r = 0.77*) and LN conditions (r = 0.72*). Time to physiological maturity was associated with senescence across treatments (optimum: r = −0.97***; LN: r = −0.82*; DS: r = −0.81*), making senescence suitable to approximate physiological maturity. Repeatability for secondary traits (r2 = 0.79 on average) was higher than for grain yield (r2 = 0.68) across treatments. In light of the association of these traits with grain yield and their high repeatability, grain yield could be increased within a maturity group by simultaneously selecting for high normalized differential vegetation index during vegetative growth and early anthesis and low senescence during grain filling, in addition to grain yield. [ABSTRACT FROM AUTHOR]

Maize (Zea mays L.) cultivars with increased concentrations of provitamin A (proVA) carotenoids can improve the health of millions of consumers who are vitamin A deficient and rely on maize as a staple food. Three open-pollinated maize cultivars (OPC) were subjected to three cycles of S1 recurrent selection for increased proVA concentration. Agronomic performance of Cycles 0, 1, 2, and 3 for each OP C was evaluated using three replications at 10 locations, and changes in proVA concentration were assessed for hand-pollinated grain produced at two sites. Selection resulted in significant (P < 0.01 for 11, and P < 0.05 for 1 occurrence) linear increases of 25 to 67% per cycle for total proVA, 28 to 60% for β-carotene, 18 to 70% for β-cryptoxanthin, and 11 to 46% for zeaxanthin. These findings are especially significant because, in contrast to recent trends, they demonstrate the feasibility of developing proVAenhanced maize while meeting nutritionists' recommendations not to sacrifice β-cryptoxanthin and zeaxanthin to increase β-carotene concentration in grain. Grain yield increased in one but decreased (P < 0.01) in two of the OP Cs, and we hypothesize that linkage drag associated with proVA-enhancing genes from exotic donor lines may be responsible for the negative trends. We conclude that breeding proVA-enriched maize without sacrificing β-cryptoxanthin and zeaxanthin concentrations is feasible, but that (i) it remains unknown whether such approaches can achieve the high concentrations of proVA reported elsewhere by using marker-assisted selection for genes that favor b-carotene accumulation, and (ii) that grain yield and agronomic performance should be simultaneously selected if useful cultivars are desired. [ABSTRACT FROM AUTHOR]

Analysis of the sequences of 74 randomly selected BACs demonstrated that the maize nuclear genome contains ≈37,000 candidate genes with homologues in other plant species. An additional ≈5,500 predicted genes are severely truncated and probably pseudogenes. The distribution of genes is uneven, with ≈30% of BACs containing no genes. BAC gene density varies from 0 to 7.9 per 100 kb, whereas most gene islands contain only one gene. The average number of genes per gene island is 1.7. Only 72% of these genes show collinearity with the rice genome. Particular LTR retrotransposon families (e.g., Gyma) are enriched on gene-free BACs, most of which do not come from pericentromeres or other large heterochromatic regions. Gene-containing BACs are relatively enriched in different families of LTR retrotransposons (e.g., Ji). Two major bursts of LTR retrotransposon activity in the last 2 million years are responsible for the large size of the maize genome, but only the more recent of these is well represented in gene-containing BACs, suggesting that LTR retrotransposons are more efficiently removed in these domains. The results demonstrate that sample sequencing and careful annotation of a few randomly selected BACs can provide a robust description of a complex plant genome. [ABSTRACT FROM AUTHOR]

Despite the breeding efforts by many institutions, maize (Zea mays L.) productivity in sub‐Saharan Africa is still low. A limited number of productive maize hybrids have been developed partly due to a lack of knowledge on the diversity and heterotic relationship of the germplasm, especially in public breeding programs. Understanding the extent of diversity, structure, and heterotic grouping of available maize germplasm originating from different breeding programs is important to enhance long‐term genetic gain in hybrid maize breeding programs by optimizing heterotic pools using modern breeding tools. Information about the genetic structure of the available germplasm could help breeders design effective breeding strategies to improve yield. This study was conducted to determine the genetic diversity, population structure, and heterotic alignment among 187 elite maize inbred lines from the IITA (International Institute of Tropical Agriculture) and CIMMYT (International Maize and Wheat Improvement Center) maize breeding programs. The inbred lines were genotyped with 9857 Diversity Array Technology sequencing based single nucleotide polymorphism markers. Hierarchical clustering revealed three major groups, with some subgroups consistent with the selection history, and pedigree of the inbred lines. Three broad groups were detected: two consisting of CIMMYT lines and a mixed group consisting of both CIMMYT and IITA inbred lines. The STRUCTURE analysis revealed six subpopulations (fixation index value = 0.58), which depicts a moderate genetic diversity among the materials. Population 2 comprises the highest number of genotypes (102) from both programs. More than 89% of the elite lines had homozygosity exceeding 95%, with the remaining lines requiring further inbreeding through repeated self‐pollination. There was inconsistency in the predetermined heterotic groups' alignment between CIMMYT and IITA elite inbred lines. Analysis of molecular variance revealed that 96% of the total variation was accounted for by differences within groups, with the remaining 4% representing the variation between groups. This suggests that the two programs can benefit from germplasm exchange for the improvement of maize productivity. [ABSTRACT FROM AUTHOR]