Mo17 Population Research Articles

Comparative analyses of responses to exogenous and endogenous antiherbivore elicitors enable a forward genetics approach to identify maize gene candidates mediating sensitivity to herbivore-associated molecular patterns.

Crop damage by herbivorous insects remains a significant contributor to annual yield reductions. Following attack, maize (Zea mays) responds to herbivore-associated molecular patterns (HAMPs) and damage-associated molecular patterns (DAMPs), activating dynamic direct and indirect antiherbivore defense responses. To define underlying signaling processes, comparative analyses between plant elicitor peptide (Pep) DAMPs and fatty acid-amino acid conjugate (FAC) HAMPs were conducted. RNA sequencing analysis of early transcriptional changes following Pep and FAC treatments revealed quantitative differences in the strength of response yet a high degree of qualitative similarity, providing evidence for shared signaling pathways. In further comparisons of FAC and Pep responses across diverse maize inbred lines, we identified Mo17 as part of a small subset of lines displaying selective FAC insensitivity. Genetic mapping for FAC sensitivity using the intermated B73 × Mo17 population identified a single locus on chromosome 4 associated with FAC sensitivity. Pursuit of multiple fine-mapping approaches further narrowed the locus to 19 candidate genes. The top candidate gene identified, termed FAC SENSITIVITY ASSOCIATED (ZmFACS), encodes a leucine-rich repeat receptor-like kinase (LRR-RLK) that belongs to the same family as a rice (Oryza sativa) receptor gene previously associated with the activation of induced responses to diverse Lepidoptera. Consistent with reduced sensitivity, ZmFACS expression was significantly lower in Mo17 as compared to B73. Transient heterologous expression of ZmFACS in Nicotiana benthamiana resulted in a significantly increased FAC-elicited response. Together, our results provide useful resources for studying early elicitor-induced antiherbivore responses in maize and approaches to discover gene candidates underlying HAMP sensitivity in grain crops.

Meiotic crossovers characterized by haplotype-specific chromosome painting in maize

Meiotic crossovers (COs) play a critical role in generating genetic variation and maintaining faithful segregation of homologous chromosomes during meiosis. We develop a haplotype-specific fluorescence in situ hybridization (FISH) technique that allows visualization of COs directly on metaphase chromosomes. Oligonucleotides (oligos) specific to chromosome 10 of maize inbreds B73 and Mo17, respectively, are synthesized and labeled as FISH probes. The parental and recombinant chromosome 10 in B73 x Mo17 F1 hybrids and F2 progenies can be unambiguously identified by haplotype-specific FISH. Analysis of 58 F2 plants reveals lack of COs in the entire proximal half of chromosome 10. However, we detect COs located in regions very close to the centromere in recombinant inbred lines from an intermated B73 x Mo17 population, suggesting effective accumulation of COs in recombination-suppressed chromosomal regions through intermating and the potential to generate favorable allelic combinations of genes residing in these regions.

QTL mapping for maize starch content and candidate gene prediction combined with co-expression network analysis.

A major QTL Qsta9.1 was identified on chromosome 9, combined with GWAS, and co-expression network analysis showed that GRMZM2G110929 and GRMZM5G852704 are the potential candidates for association with maize kernel starch content. Increasing maize kernel starch content may not only lead to higher maize kernel yields and qualities, but also help meet industry demands. By using the intermated B73 × Mo17 population, QTLs were mapped for starch content in this study. A major QTL Qsta9.1 was detected in a 1.7Mb interval on chromosome 9 and validated by allele frequency analysis in extreme tails of a newly constructed segregating population. According to genome-wide association study (GWAS) based on genotyping of a natural population, we identified a significant SNP for starch content within the ORF region of GRMZM5G852704_T01 colocalized with QTL Qsta9.1. Co-expression network analysis was also conducted, and 28 modules were constructed during six seed developmental stages. Functional enrichment was performed for each module, and one module showed the most possibility for the association with carbohydrate-related processes. In this module, one transcripts GRMZM2G110929_T01 located in the Qsta9.1 assigned 1.7Mb interval encoding GLABRA2 expression modulator. Its expression level in B73 was lower than that in Mo17 across all seed developmental stages, implying the possibility for the candidate gene of Qsta9.1. Our studies combined GWAS, mRNA profiling, and traditional QTL analyses to identify a major locus for controlling seed starch content in maize.

Identification of Quantitative Trait Loci for Goss's Wilt of Maize

Since its discovery in 1969, Goss's wilt, a foliar blight and vascular wilt disease caused by the Gram‐positive bacterium Clavibacter michiganensis (Smith) Davis et al. subsp. nebraskensis (Vidaver & Mandel) Davis et al. (Cmn), has emerged as one of the top four diseases of maize (Zea mays L.) in the United States and Ontario, Canada. No source of complete resistance has been described for Goss's wilt, and little is known about the genetic and mechanistic basis of host resistance to Cmn. Our objective was to perform linkage mapping on three populations to uncover the genomic regions associated with Goss's wilt resistance. We evaluated the intermated B73 × Mo17 population and two corresponding disease‐resistant introgression line populations: B73(4) × Mo17 and Mo17(4) × B73. We identified putative quantitative trait loci (QTLs) in bins 1.05 to 1.06, 2.06, 7.01 to 7.02, 8.05, and 10.04, both confirming previous findings and identifying novel resistance QTLs. The QTL on chromosome 1, designated qGW1.06, was identified in multiple environments and overlaps with a known multiple disease resistance locus. The QTL in bin 8.05 represents a novel region associated with Goss's wilt. Using the data from this study and previous studies, we found that Goss's wilt resistance was correlated with northern leaf blight [Setosphaeria turcica (Luttr.) K.J. Leonard & Suggs], but not gray leaf spot (Cercospora spp.) or southern leaf blight [Cochliobolus heterostrophus (Drechsler) Drechsler]. These results offer a deeper understanding of the genetic basis of resistance to Goss's wilt in maize that may facilitate breeding for resistance, and qGW1.06 is a strong candidate for further characterization and use.

Prospective Targeted Recombination and Genetic Gains for Quantitative Traits in Maize.

Advances in clustered regularly interspaced short palindromic repeats (CRISPR) technology have allowed targeted recombination in specific DNA sequences in yeast (). My objective was to determine if the selection gains from targeted recombination are large enough to warrant the development of targeted recombination technology in plants. Genomewide marker effects for quantitative traits in two maize ( L.) experiments were used to identify targeted recombination points that would maximize the per-chromosome genetic gains in a given cross. With nontargeted recombination in the intermated B73 × Mo17 population, selecting the best out of 180 recombinant inbreds led to a 7.1% gain for testcross yield. Having one targeted recombination on each of the 10 maize chromosomes led to a predicted gain of 15.3% for yield. Targeted recombination therefore led to a predicted relative efficiency () of (0.153 ÷ 0.071) = 212% of targeted recombination compared with nontargeted recombination. For the five other traits in the intermated B73 × Mo17 population and for four traits in 45 other maize crosses, the values ranged from 105 to 600%. The targeted recombination points differed among traits and crosses. Predicted gains increased when the number of targeted recombinations per chromosome increased from one to two. Overall, the results suggested that targeted recombination could double the selection gains for quantitative traits in maize and that the development of targeted recombination technology is worthwhile. Empirical experiments with current marker-assisted breeding procedures are needed to validate the per-chromosome predicted gains.

Drought Tolerance in Maize: Indirect Selection through Secondary Traits versus Genomewide Selection

ABSTRACTPhenotyping maize (Zea mays L.) for drought tolerance is costly and time consuming. Our objectives were to determine (i) the heritability, genetic variance, and genetic correlations for grain yield and secondary traits in maize under drought and (ii) the efficiency of indirect selection through secondary traits versus genomewide selection. Testcrosses of 238 recombinant inbreds from the intermated B73 × Mo17 population were evaluated in multilocation trials under managed drought and nondrought (control) conditions in Minnesota in 2009 and 2010. Mean grain yield under drought was 52% of the mean grain yield in the control experiments. Heritability for grain yield was 0.37 ± 0.08 under drought and 0.60 ± 0.04 in the control experiments. Indirect selection based on anthesis‐silking interval, leaf senescence, leaf chlorophyll content, or grain yield in control conditions was not predicted to be more efficient than direct selection for grain yield under drought. Genomewide selection (with 998 markers) for grain yield under drought had a predicted relative efficiency of 1.24. Genetic correlations estimated from genomewide marker effects agreed well with correlations estimated from genetic covariances. Given that multiple cycles of marker‐based selection can be done per year in maize and that genotyping is cheaper than phenotyping for drought tolerance, our results suggest that genomewide selection could increase genetic gains per unit time for grain yield under drought.

Corn Performance under Managed Drought Stress and in a Kura Clover Living Mulch Intercropping System

A corn (Zea mays L.) and kura clover (Trifolium ambiguum M. Bieb.) intercropping system provides ecological services, but competition for water between the grain crop and the living mulch crop often reduces corn yields. Our objectives were to determine (i) if corn that performs well under managed drought (i.e., drought‐tolerant corn) can minimize the grain yield losses incurred when corn is intercropped with kura clover, and (ii) if strong suppression of the kura clover living mulch minimizes the loss in yield of drought‐tolerant and drought‐susceptible corn. From drought trials in Minnesota in 2009 and 2010, we identified five drought‐tolerant and five drought‐susceptible hybrids from the intermated B73 × Mo17 population. Under drought conditions, mean grain yields were 9.66 Mg ha−1 for the drought‐tolerant hybrids and 5.46 Mg ha−1 for the drought‐susceptible hybrids. The 10 hybrids were evaluated in Minnesota and Wisconsin in 2011 in a kura clover intercropping system. Drought‐tolerant hybrids had statistically equal mean yields of 12.90 Mg ha−1 in the killed mulch treatment and 13.69 Mg ha−1 in the living mulch treatment. In contrast, mean yields of drought‐susceptible hybrids were significantly reduced from 4.53 Mg ha−1 in the killed mulch treatment to 3.48 Mg ha−1 in the living mulch treatment. For the drought‐tolerant hybrids, mulch recovery at corn harvest was significantly lower with killed kura clover (41%) than with living kura clover (50%). Our results indicate that the yield of drought‐tolerant corn was not reduced by growing in a living mulch of kura clover.

Ionomic Characterization of Maize Kernels in the Intermated B73 × Mo17 Population

Dietary mineral deficiencies affect nearly half of the people on our planet, largely due to poverty. Enhancing nutritional quality—or biofortification—represents an efficient and sustainable potential solution to this massive public health problem. To create biofortified crops, one must understand the genetic and environmental factors that influence the ionome, or collection of mineral nutrients, in the target organism and tissue. We describe the use of quantitative trait loci (QTL) mapping to characterize the maize (Zea mays L.) grain ionome illustrated by the intermated B73 × Mo17 (IBM) recombinant inbred population. Ionomic profiling was applied to field grown materials from Florida, North Carolina, and New York. Twenty‐seven QTL were detected for 10 traits derived from the North Carolina and New York grown maize explaining between 4 and 46% of the variance observed. For biofortification to succeed, QTL effective in multiple environments need to be the targets for improvement efforts. Florida grown maize were sampled shallowly to provide a low cost dataset to evaluate the models based on the combined North Carolina and New York traits. Twenty‐five QTL were detected as significant in two or more locations using ANOVA on the original single location and/or site data; 12 QTL were found to be significant in Florida. While this strategy may have not detected every potential QTL from our data, we suggest these QTL effective in multiple environments represent a starting position for the biofortification of maize grain.

Mapping of quantitative trait loci/locus conferring resistance to foxtail mosaic virus in maize using the intermated B73 × Mo17 population

With 2 tables AbstractFoxtail mosaic virus (FoMV) has been reported to have a broad range of hosts, including millet, sorghum, barley, and maize. FoMV‐H93, a new strain of FoMV which was isolated from sorghum in 1993, was used to inoculate the Intermated B73 × Mo17 (IBM) syn4 population for quantitative trait loci/locus (QTL) analysis. The parents of the IBM population, B73 and Mo17, are susceptible and resistant, respectively, to this virus isolate. A panel of 94 recombinant inbred lines was used as the mapping population in our study. Eight different QTL responsible for resistance at three different stages on chromosomes 1, 2, 3, 5 and 7 were detected. The QTL provide new resources in both maize and closely related crops such as sorghum for breeding resistant plants.

High-Throughput Genetic Mapping of Mutants via Quantitative Single Nucleotide Polymorphism Typing

Advances in next-generation sequencing technology have facilitated the discovery of single nucleotide polymorphisms (SNPs). Sequenom-based SNP-typing assays were developed for 1359 maize SNPs identified via comparative next-generation transcriptomic sequencing. Approximately 75% of these SNPs were successfully converted into genetic markers that can be scored reliably and used to generate a SNP-based genetic map by genotyping recombinant inbred lines from the intermated B73 x Mo17 population. The quantitative nature of Sequenom-based SNP assays led to the development of a time- and cost-efficient strategy to genetically map mutants via quantitative bulked segregant analysis. This strategy was used to rapidly map the loci associated with several dozen recessive mutants. Because a mutant can be mapped using as few as eight multiplexed sets of SNP assays on a bulk of as few as 20 mutant F(2) individuals, this strategy is expected to be widely adopted for mapping in many species.

Mapping of QTL for resistance to the Mediterranean corn borer attack using the intermated B73 × Mo17 (IBM) population of maize

The Mediterranean corn borer or pink stem borer (MCB, Sesamia nonagrioides Lefebvre) causes important yield losses as a consequence of stalk tunneling and direct kernel damage. B73 and Mo17 are the source of the most commercial valuable maize inbred lines in temperate zones, while the intermated B73 x Mo17 (IBM) population is an invaluable source for QTL identification. However, no or few experiments have been carried out to detect QTL for corn borer resistance in the B73 x Mo17 population. The objective of this work was to locate QTL for resistance to stem tunneling and kernel damage by MCB in the IBM population. We detected a QTL for kernel damage at bin 8.05, although the effect was small and two QTL for stalk tunneling at bins 1.06 and 9.04 in which the additive effects were 4 cm, approximately. The two QTL detected for MCB resistance were close to other QTL consistently found for European corn borer (ECB, Ostrinia nubilalis Hübner) resistance, indicating mechanisms of resistance common to both pests or gene clusters controlling resistance to different plagues. The precise mapping achieved with the IBM population will facilitate the QTL pyramiding and the positional cloning of the detected QTL.

Quantitative Trait Loci Controlling Leaf and Tassel Traits in a B73 × Mo17 Population of Maize

ABSTRACTLight penetration into the canopy of maize (Zea mays L.) production fields is an important determinant of grain yield. Factors affecting light penetration include agricultural practices such as planting density and plant morphological factors such as leaf angle, leaf size, and tassel size. The objectives of this experiment were to identify genomic regions controlling the inheritance of leaf angle and tassel morphology in a B73 × Mo17 recombinant inbred population. Three quantitative trait loci (QTL) for tassel branch angle were detected which explained 35.6% of the phenotypic variation. Six QTL were detected for tassel branch number with three of these QTL on chromosome 2. Nine QTL were detected for leaf angle in one or more environments. Significant phenotypic correlations were detected between tassel branch angle and tassel branch number and between tassel branch number and leaf angle. Overlapping support intervals were identified between QTL detected for leaf angle and for tassel branch number on chromosome 2 near marker umc53a. Additionally, a QTL near marker bnl6.10 on chromosome 5 identified for tassel branch angle was in the same region as a QTL identified for leaf angle. The results of this study indicate that common genetic relationships exist between tassel traits and leaf angle.

Variation among Maize Inbred Lines and Detection of Quantitative Trait Loci for Growth at Low Phosphorus and Responsiveness to Arbuscular Mycorrhizal Fungi

Maize (Zea mays L.) growth at low soil P levels is affected both by inherent physiological factors as well as interactions with soil microbes. The objectives of this study were (i) to quantify differences among maize inbred lines for growth at low P and response to mycorrhizal fungi, and (ii) to identify quantitative trait loci (QTL) controlling these traits in a B73 × Mo17 recombinant inbred population. Shoot dry weight and root volume were measured in the greenhouse after 6 wk of growth in a factorial experiment of 28 inbred maize lines using treatments of low vs. high P and mycorrhizal vs. nonmycorrhizal treatments. Shoot dry weight for the low P treatment in the absence of mycorrhizae ranged from 0.56 to 3.15 g. Mycorrhizal responsiveness based on shoot dry weight ranged from 106 to 800%. Shoot dry weight in the low P–nonmycorrhizal treatment was highly negatively correlated with mycorrhizal responsiveness. Plants grown at high P in the presence of mycorrhizae accumulated only 88% of the biomass of plants grown at high P in the absence of mycorrhizae, indicating that mycorrhizae can reduce plant growth when not contributing to the symbiosis. Percentage of root colonization was not correlated with mycorrhizal responsiveness. B73 and Mo17 were among the extremes for growth at low P and mycorrhizal responsiveness, and a B73 × Mo17 population of 197 recombinant inbred lines was used to detect QTL for growth at low P and mycorrhizal responsiveness. Three QTL were identified which controlled growth at low P in the absence of mycorrhizae based on shoot weight and one QTL which controlled mycorrhizal responsiveness. This study indicates that there is substantial variation among maize lines for growth at low P and response to mycorrhizal fungi. This variation could be harnessed to develop cultivars for regions of the world with P deficiency and for reduced‐input production systems.

Loci Controlling Resistance to High Plains Virus and Wheat Streak Mosaic Virus in a B73 × Mo17 Population of Maize

High Plains disease has the potential to cause significant yield loss in susceptible corn (Zea mays L.) and wheat (Triticum aestivum L.) genotypes, especially in the central and western USA. The primary causal agent, High Plains virus (HPV), is vectored by wheat curl mite (WCM; Aceria tossicheila Keifer), which is also the vector of wheat streak mosaic virus (WSMV). In general, the two diseases occur together as a mixed infection in the field. The objective of this research was to characterize the inheritance of HPV and WSMV resistance using B73 (resistant to HPV and WSMV) × Mo17 (moderately susceptible to HPV and WSMV) recombinant inbred lines. A population of 129 recombinant inbred lines scored for 167 molecular markers was used to evaluate resistance to WSMV and to a mixed infection of WSMV and HPV. Loci conferring resistance to systemic movement of WSMV in plants mapped to chromosomes 3, 6, and 10, consistent with the map position of wsm2, wsm1, and wsm3, respectively. Major genes for resistance to systemic spread of HPV in doubly infected plants mapped to chromosomes 3 and 6, coincident or tightly linked with the WSMV resistance loci. Analysis of doubly infected plants revealed that chromosome 6 had a major effect on HPV resistance, consistent with our previous analysis of B73 × W64A and B73 × Wf9 populations. Quantitative trait loci (QTL) affecting resistance to localized symptom development mapped to chromosomes 4 (umc66), 5 (bnl5.40), and 6 (umc85), and accounted for 24% of the phenotypic variation. Localized symptoms may reflect the amount of mite feeding or the extent of virus spread at the point of infection. Identification of cosegregating markers may facilitate selection for HPV and WSMV resistance in corn breeding programs.