Brazil has two growing seasons for maize (Zea mays L.) production: summer and winter. The additional maize produced in the winter and the high-yielding opportunities in the summer season make it important to understand responses of maize hybrids to row spacing and plant population across the two annual Brazilian growing seasons. In the 2015 to 2016 summer season and the 2016 winter season, maize response to plant population, ranging from 60,000 to 90,000 plants ha -1 , was evaluated for three hybrids in 0.45-, 0.60-, 0.75-, and 0.90-m row spacing. Plant architecture traits, grain yield and yield components did not differ with row spacing, and their responses to plant population were not affected by hybrid type and/or row spacing. The DKB390PRO2 hybrid (single cross) demonstrated better grain yield performance in the summer, whereas the BG7049YH hybrid (three-way cross) had the highest yield in the winter. Plant and ear height increased linearly in response to plant population in the summer season, whereas other plant architecture traits, ear leaf chlorophyll concentration, and grain yield components decreased linearly with increasing plant populations in both seasons. There was a quadratic response in maize grain yield to plant population, and it was maximized with 78,688 plants ha -1 in the summer, and 71,206 plants ha -1 in the winter. Our results show preliminary evidence that regardless of row spacing, maize grain yield can be maximized with DKB390 hybrid (single cross) with 78,500 plants ha -1 in the summer, and BG7049YH hybrid (three-way) with 71,000 plants ha -1 in the winter season.

Improvement in nitrogen-use efficiency (NUE) on maize is among the best strategies to mitigate the problems of poor soil fertility in tropical conditions. The objectives of this study were: i) to quantify the genetic variability for NUE-components and agronomic traits in a set of tropical maize inbred lines; ii) to study the genetic divergence among tropical maize inbred lines under contrasting nitrogen (N) levels; iii) to identify the secondary traits associated with NUE in tropical maize inbred lines; and iv) to identify maize inbred lines efficient in NUE and its components. Sixty-four tropical maize inbred lines were evaluated in the field under low- and high-N conditions for NUE-components and agronomic traits. Genetic variability for NUE-components and agronomic traits was found; lines in eight different groups for each N condition were allocated, and N-efficient inbred lines were identified in different groups. Furthermore, we suggest flowering time traits and kernel number as great secondary traits for selecting tropical maize inbred lines for NUE under both N conditions, and chlorophyll content for selecting for NUE under N stress.

Major locus for spontaneous haploid genome doubling detected by a case-control GWAS in exotic maize germplasm Key messageA major locus for spontaneous haploid genome doubling was detected by a case-control GWAS in an exotic maize germplasm. The combination of double haploid breeding method with this locus leads to segregation distortion on genomic regions of chromosome five.

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Background The characterization of genetic diversity and population differentiation for maize inbred lines from breeding programs is of great value in assisting breeders in maintaining and potentially increasing the rate of genetic gain. In our study, we characterized a set of 187 tropical maize inbred lines from the public breeding program of the Universidade Federal de Viçosa (UFV) in Brazil based on 18 agronomic traits and 3,083 single nucleotide polymorphisms (SNP) markers to evaluate whether this set of inbred lines represents a panel of tropical maize inbred lines for association mapping analysis and investigate the population structure and patterns of relationships among the inbred lines from UFV for better exploitation in our maize breeding program. Results Our results showed that there was large phenotypic and genotypic variation in the set of tropical maize inbred lines from the UFV maize breeding program. We also found high genetic diversity (GD = 0.34) and low pairwise kinship coefficients among the maize inbred lines (only approximately 4.00 % of the pairwise relative kinship was above 0.50) in the set of inbred lines. The LD decay distance over all ten chromosomes in the entire set of maize lines with r2 = 0.1 was 276,237 kb. Concerning the population structure, our results from the model-based STRUCTURE and principal component analysis methods distinguished the inbred lines into three subpopulations, with high consistency maintained between both results. Additionally, the clustering analysis based on phenotypic and molecular data grouped the inbred lines into 14 and 22 genetic divergence clusters, respectively. Conclusions Our results indicate that the set of tropical maize inbred lines from UFV maize breeding programs can comprise a panel of tropical maize inbred lines suitable for a genome-wide association study to dissect the variation of complex quantitative traits in maize, mainly in tropical environments. In addition, our results will be very useful for assisting us in the assignment of heterotic groups and the selection of the best parental combinations for new breeding crosses, mapping populations, mapping synthetic populations, guiding crosses that target highly heterotic and yielding hybrids, and predicting untested hybrids in the public breeding program UFV.

The identification of genomic regions associated with root traits and the genomic prediction of untested genotypes can increase the rate of genetic gain in maize breeding programs targeting roots traits. Here, we combined two maize association panels with different genetic backgrounds to identify SNPs significantly associated with root traits and through of genome-wide association study (GWAS) and to assess the potential of genomic prediction these traits in maize. For this, we evaluated 377 lines from the Ames and 302 from the BGEM (Backcrossed Germplasm Enhancement of Maize) panels in a Combined panel of 679 lines. The lines were genotyped with 232,460 SNPs, and four root traits were collected from 14-day old seedlings. We identified 30 SNPs significantly associated with root traits in the Combined panel, whereas only two and six SNPs were detected in the Ames and BGEM panels, respectively. Those 38 SNPs were in linkage disequilibrium with 35 candidate genes. In addition, we found higher prediction accuracy in the Combined panel than in the Ames or BGEM panels. We concluded that combining association panels appear to be a useful strategy to identify candidate genes associated with root traits in maize and improve the efficiency of the genomic prediction.

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Adapted exotic maize (Zea mays L.) germplasm, such as BS39, provides a unique opportunity for broadening the genetic base of U.S. Corn Belt germplasm. In vivo doubled haploid (DH) technology has been used to efficiently exploit exotic germplasm. It can help to purge deleterious recessive alleles. The objectives of this study were to determine the usefulness of BS39-derived inbred lines using both SSD and DH methods, to determine the impact of spontaneous as compared to artificial haploid genome doubling on genetic variance among BS39-derived DH lines, and to identify SNP markers associated with agronomic traits among BS39 inbreds monitored at testcross level. We developed two sets of inbred lines directly from BS39 by DH and SSD methods, named BS39_DH and BS39_SSD. Additionally, two sets were derived from a cross between BS39 and A427 (SHGD donor) by DH and SSD methods, named BS39×A427_DH and BS39×A427_SSD, respectively. Grain yield, moisture, plant height, ear height, stalk lodging, and root lodging were measured to estimate genetic parameters. For genome-wide association (GWAS) analysis, inbred lines were genotyped using Genotype-by-Sequencing (GBS) and Diversity Array Technology Sequencing (DArTSeq). Some BS39-derived inbred lines performed better than elite germplasm inbreds and all sets showed significant genetic variance. The presence of spontaneous haploid genome doubling genes did not affect performance of inbred lines. Five SNPs were significant and three of them located within genes related to plant development or abiotic stresses. These results demonstrate the potential of BS39 to add novel alleles to temperate elite germplasm.