Mapping Quantitative Trait Loci for Resistance to Fall Armyworm (Lepidoptera: Noctuidae) Leaf-Feeding Damage in Maize Inbred Mp705.

The fall armyworm, Spodoptera frugiperda (J. E. Smith), is an agronomically important pest that severely limits maize (Zea mays (Linnaeus) [Poales: Poaceae]) production. This migrant insect devastates maize plants in many countries threatening the livelihood of millions. Quantitative trait loci (QTL) were mapped to identify chromosomal regions that control resistance to fall armyworm leaf-feeding and to identify molecular markers linked to the target loci for use in marker-assisted selection (MAS). A bi-parental mapping population, comprising 243 F2:3 families from the cross Mp705 (resistant) × Mp719 (susceptible), was evaluated for fall armyworm leaf-feeding damage under artificial infestation over 3 yr. A linkage map comprised of 1,276 single-nucleotide polymorphism and simple sequence repeat molecular markers was constructed. Quantitative trait loci analyses identified two major QTL in bins 4.06 and 9.03 that when combined, explained 35.7% of the phenotypic variance over all environments. Mp705 was responsible for the leaf-feeding damage reducing alleles for both large effect QTL and most of the small effect QTL identified in this study. The QTL identified in bin 9.03 co-locates with a previously identified QTL that controls resistance to leaf-feeding damage in maize by fall armyworm and other lepidopteran insects. The QTL in bin 4.06 is a new source of resistance identified in this study. Beneficial alleles derived from Mp705 for the application of an integrated QTL-MAS approach could accelerate breeding efforts to minimize fall armyworm leaf-feeding in maize.

A comparison of simple sequence repeat and single nucleotide polymorphism marker technologies for the genotypic analysis of maize (Zea mays L.).

We report on the comparative utilities of simple sequence repeat (SSR) and single nucleotide polymorphism (SNP) markers for characterizing maize germplasm in terms of their informativeness, levels of missing data, repeatability and the ability to detect expected alleles in hybrids and DNA pools. Two different SNP chemistries were compared; single-base extension detected by Sequenom MassARRAY, and invasive cleavage detected by Invader chemistry with PCR. A total of 58 maize inbreds and four hybrids were genotyped with 80 SSR markers, 69 Invader SNP markers and 118 MassARRAY SNP markers, with 64 SNP loci being common to the two SNP marker chemistries. Average expected heterozygosity values were 0.62 for SSRs, 0.43 for SNPs (pre-selected for their high level of polymorphism) and 0.63 for the underlying sequence haplotypes. All individual SNP markers within the same set of sequences had an average expected heterozygosity value of 0.26. SNP marker data had more than a fourfold lower level of missing data (2.1-3.1%) compared with SSRs (13.8%). Data repeatability was higher for SNPs (98.1% for MassARRAY SNPs and 99.3% for Invader) than for SSRs (91.7%). Parental alleles were observed in hybrid genotypes in 97.0% of the cases for MassARRAY SNPs, 95.5% for Invader SNPs and 81.9% for SSRs. In pooled samples with mixtures of alleles, SSRs, MassARRAY SNPs and Invader SNPs were equally capable of detecting alleles at mid to high frequencies. However, at low frequencies, alleles were least likely to be detected using Invader SNP markers, and this technology had the highest level of missing data. Collectively, these results showed that SNP technologies can provide increased marker data quality and quantity compared with SSRs. The relative loss in polymorphism compared with SSRs can be compensated by increasing SNP numbers and by using SNP haplotypes. Determining the most appropriate SNP chemistry will be dependent upon matching the technical features of the method within the context of application, particularly in consideration of whether genotypic samples will be pooled or assayed individually.

An integrated SSR and RFLP linkage map of Sorghum bicolor (L.) Moench.

We report the development, testing, and use (for genetic mapping) of a large number of polymerase chain reaction (PCR) primer sets that amplify DNA simple sequence repeat (SSR) loci of Sorghum bicolor (L.) Moench. Most of the primer sets were developed from clones isolated from two sorghum bacterial artificial chromosome (BAC) libraries and three enriched sorghum genomic-DNA (gDNA) libraries. A few were developed from sorghum DNA sequences present in public databases. The libraries were probed with radiolabeled di- and trinucleotide oligomers, the BAC libraries with four and six oligomers, respectively, and the enriched gDNA libraries with four and three oligomers, respectively. Both types of libraries were markedly enriched for SSRs relative to a size-fractionated gDNA library studied earlier. However, only 2% of the sequenced clones obtained from the size-fractionated gDNA library lacked a SSR, whereas 13% and 17% of the sequenced clones obtained from the BAC and enriched gDNA libraries, respectively, lacked a SSR. Primer sets were produced for 313 SSR loci. Two-hundred sixty-six (85%) of the loci were amplified and 165 (53%) of the loci were found to be polymorphic in a population composed of 18 diverse sorghum lines. (AG/TC)n and (AC/TG)n repeats comprised 91% of the dinucleotide SSRs and 52% of all of the SSRs at the polymorphic loci, whereas four types of repeats comprised 66% of the trinucleotide SSRs at the loci. Primer sequences are reported for the 165 polymorphic loci and for eight monomorphic loci that have a high degree of homology to genes. Also reported are the genetic map locations of 113 novel SSR loci (including four SSR-containing gene loci) and a linkage map composed of 147 SSR loci and 323 RFLP (restriction fragment length polymorphism) loci. The number of SSR loci per linkage group ranges from 8 to 30. The SSR loci are distributed relatively evenly throughout approximately 75% of the 1406-cM linkage map, but segments of five linkage groups comprising about 25% of the map either lack or contain few SSR loci. Mapping of SSR loci isolated from BAC clones located to these segments is likely to be the most efficient method for placing SSR loci in the segments.

Heat-stress response of maize mitochondria.

We have identified maize (Zea mays L. inbred B73) mitochondrial homologs of the Escherichia coli molecular chaperones DnaK (HSP70) and GroEL (cpn60) using two-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunoblots. During heat stress (42 degrees C for 4 h), levels of HSP70 and cpn60 proteins did not change significantly. In contrast, levels of two 22-kD proteins increased dramatically (HSP22). Monoclonal antibodies were developed to maize HSP70, cpn60, and HSP22. The monoclonal antibodies were characterized with regard to their cross-reactivity to chloroplastic, cytosolic, and mitochondrial fractions, and to different plant species. Expression of mitochondrial HSP22 was evaluated with regard to induction temperature, time required for induction, and time required for degradation upon relief of stress. Maximal HSP22 expression occurred in etiolated seedling mitochondria after 5 h of a +13 degrees C heat stress. Upon relief of heat stress, the HSP22 proteins disappeared with a half-life of about 4 h and were undetectable after 21 h of recovery. Under continuous heat-stress conditions, the level of HSP22 remained high. A cDNA for maize mitochondrial HSP22 was cloned and extended to full length with sequences from an expressed sequence tag database. Sequence analysis indicated that HSP22 is a member of the plant small heat-shock protein superfamily.

Nucleotide sequence analysis of a novel globulin1 null allele from the Illinois high protein strain of maize.

The highly polymorphic maize globulin1 (glb1) gene encodes an abundant embryo storage protein. The present study extends the analysis of glb1 variants to further explore the nature of polymorphism at this locus. The null allele Glb1-N1Hb, derived from the Illinois High Protein (IHP) strain of maize was characterized at the molecular level by nucleotide sequence analysis. Among other differences, a single-base insertion leading to a premature termination codon in the carboxyl-terminal half of the otherwise normal protein was observed. The likely reasons for the absence of GLB1 protein accumulation in the IHP strain of maize are discussed.