Zugl.: München, Techn. Univ., Diss., 2009

No description available.

Pre-harvest aflatoxin contamination is a major limitation to maize production in Texas and the southern United States, causing major economic loss and severe health problems worldwide. Screening for resistance to aflatoxin accumulation is commonly conducted through inoculation with a highly concentrated solution of Aspergillus flavus FR: Link spores, a naturally occurring fungus which infects maize and produces a toxic metabolite (aflatoxin) to humans and animals consuming the grain. No commercial hybrids exist with full resistance to aflatoxin accumulation; however, sources exist to reduce susceptibility. These sources commonly lack good agronomic characteristics for use in commercial hybrids. Exotic germplasm with favorable traits for reduced aflatoxin accumulations are introgressed with temperate and locally adapted lines. This program utilizes only one isolate of A. flavus even though many isolates exist in the environment. The objectives of this thesis are i) to evaluate the progress of the Maize Breeding and Genetics Program’s accomplishments of breeding maize for the reduction in susceptibility of aflatoxin accumulation in yellow inbreds through analysis of hybrid and inbred per se trials and ii) to determine whether interaction exists between geneticallydifferent isolates of A. flavus and several genotypes of maize. Response to aflatoxin accumulation for hybrids and inbreds was measured at up to three environments across Texas. Significant differences were detected for most years and environments. Maize lines CML285, CML288, CML323, CML325, CML326, CML338, Tx601y and lines derived from Population 69 and from Tx772 crosses in hybrid combinations tended to accumulate less aflatoxin than commercial hybrid checks. Significant differences were detected at each environment aflatoxin accumulation was measured for inbred lines per se. Inbreds Tx772, Tx601y, CML289, CML294, CML323 and derived lines from Population 69 show reduced aflatoxin accumulations. Interaction between genetically different isolates of A. flavus and several genotypes of maize were not detected in hybrid or inbred trials at two or three environments, across locations and across years. Introgression of exotic germplasm into locally adapted germplasm has improved agronomic characteristics for use in the Southern U.S. and brought sources for decreased aflatoxin accumulation.

Aspergillus flavus is a fungus pathogen of maize that causes contamination of maize with aflatoxin. Aflatoxin is a carcinogenic toxin that can cause harm to human and animal health. Several management practices have been developed, such as cultural, chemical, biological and breeding, for host resistance. Development of host plant resistance has been the most desirable but this has been hampered by several factors, such as environmental influence, time consuming phenotyping and costly inoculation and field evaluations. Because of the problems associated with breeding for aflatoxin resistance, heritability estimates along with genetic correlations for aflatoxin and its secondary traits were estimated in this study. This experiment was conducted in two Texas locations (College Station and Weslaco) and phenotypic data were collected for aflatoxin concentration, maturity, endosperm texture, percentage of rotten ears and grain yield per ear. The heritability was moderate to high for aflatoxin and secondary traits such as endosperm texture and percentage of rotten ears. Aflatoxin was observed to be negatively correlated to grain yield and positively correlated to percentage of rotten ears. A population of recombinant inbred line derived from a cross between CML161 and B73o2 were evaluated in replicated trials in two environments for resistance to aflatoxin contamination. The families were genotyped using simple sequence repeat (SSR) markers. The mapping Recombinant Inbred Line population was used to detect and characterize Quantitative Trait Loci associated with aflatoxin accumulation. Alleles for reducing aflatoxin contamination came from both parents across the chromosomes. Thirty-eight epistatic interactions were detected for aflatoxin resistance. Several other QTLs were identified for other traits such as grain yield, endosperm texture, and percentage of rotten ears. The QTLs reported in this experiment are promising and need to be validated in other environment and genetic backgrounds for further use in marker assisted selection. Inheritance studies using generation mean analysis in six crosses showed additive and dominance effects to be mainly responsible for aflatoxin resistance. Two inbreds, CML176 and CML161, were identified as sources of resistance in breeding programs and use for selection.

One major problem facing maize producers in the southern US is contamination with the mycotoxin aflatoxin, produced by Aspergillus flavus (Link:fr). Aflatoxin is a serious threat to human and animal health, with no resistant commercial hybrid available. Development of resistance to aflatoxin production has several major limitations. Aflatoxin is highly variable both across and within environments, even under inoculation, requiring several locations and replications for breeding. Additionally, there is no screening method that is reliable, rapid, inexpensive, and allows for high throughput. Several secondary traits, such as kernel texture, kernel integrity, husk cover, and visible ear rot, have previously shown to be related to aflatoxin accumulation. These traits are easily characterized in the field and are candidates for indirect selection if they are correlated to aflatoxin concentration. Root lodging, a plant’s inability to maintain upright stature, is another complex characteristic of root related traits that traditionally is selected for indirectly. It can greatly reduce harvestable yield. It is affected by morphological traits and environmental conditions, but its genetic components are little understood. This dissertation comprises three studies presented in chapters II, III, and IV. Chapter II involved white and yellow hybrid maize trials as well as quality protein maize trials from several years across Texas environments. Data was analyzed both per and across location to determine repeatability of response to aflatoxin. Additionally, aflatoxin levels were correlated to several secondary characteristics (female flowering, endosperm texture, husk cover, and ear rot ratings) to determine usefulness in indirect selection. Chapter III was a phenotypic evaluation of a recombinant inbred line (RIL) mapping population, which was derived from divergent parental inbreds Tx811 and CML176. The trials were conducted in two Texas locations, and phenotypic data for aflatoxin concentration, kernel integrity, endosperm texture, female flowering date, and root lodging was collected. Variance components for these traits and genetic and phenotypic correlations were determined. Chapter IV was a genotypic evaluation of the Tx811/CML176 mapping population using simple sequence repeat markers. Genotypic and phenotypic data were combined to identify quantitative trait loci (QTL) and epistatic interactions for response to aflatoxin and for root lodging.

Maize (Zea mays L), the third largest crop produced in the world is susceptible to pre and post contamination of aflatoxin. Aflatoxin is a secondary carcinogenic metabolite produced by Aspergillus flavus, an opportunistic fungus of maize that causes ear-rot and subsequent production of aflatoxin. Host plant resistance is one of the promising long term solution to combat aflatoxin contamination in maize grains. Resistance to aflatoxin is heritable and resistant germplasms that can be used to reduce aflatoxin resistance has been developed. Creating quantitative trait loci (QTL) mapping populations to identify regions within maize chromosomes contributing to aflatoxin resistance help identify markers that can be used for successful introgression of the QTL into desirable backgrounds. To further confirm and validate QTL detected in a mapping population, creating near isogenic lines (NILs) with identified QTL is important. Furthermore, stacking of validated QTL through recurrent selection (RS) will help to improve aflatoxin resistance in maize. In this study, an F2:3 mapping population was derived from crossing CML69, an aflatoxin resistant inbred genotype unrelated to other genotypes used in previous mapping population and Va35, an aflatoxin susceptible inbred germplasm adapted to southeastern U.S. The F2:3 families were phenotyped for aflatoxin at Mississippi State, MS and Lubbock TX in 2016 and 2017 and genotyped using 1331 polymorphic markers. Composite interval mapping (CIM) identified 16 significant QTL in all four environments on chromosomes 1, 2, 3, 4, 5, and 7, and these QTL explains between 4% - 18% of the phenotypic variation observed in the population. Fourteen of the 15 QTL co-locate with previously identified QTL, suggesting that they will be stable in different genetic backgrounds and environments, and one novel QTL will provide additional resistance. Sixteen single nucleotide polymorphism (SNP) markers linked to QTL identified in a previous genome wide association mapping (GWAS) study were also used to create Near Isogenic Lines (NILs) to validate their effect on aflatoxin resistance. Finally, recurrent selection (RS) populations were created to increase the frequency of aflatoxin-resistant alleles from an 8-way cross derived from 8 aflatoxin resistant inbred maize lines.

No description available.

Maize (Zea mays L.) is grown in a wide range of environments and altitudes worldwide. Maize has transitioned from open pollinated varieties to single cross hybrids over the last century. While maize production and genetic gain has increased, genetic diversity among U.S. maize hybrids has narrowed. Problems, such as insect pressure, diseases, and mycotoxins, present obstacles for breeders. One approach is to use exotic germplasm in breeding programs to provide useful, novel alleles for productivity, grain quality, and disease resistance. Little exotic germplasm has been used, because of lack of agronomic adaptation and problems with lodging, earliness, and tall plants in more temperate areas. Using exotic elite materials and evaluating them in targeted regions might increase success. Objectives of this research were: to characterize and evaluate agronomic adaptation and performance of Argentine commercial hybrids in the U.S., to evaluate semi-exotic testcrosses developed from semi adapted 100% tropical lines and elite U.S. inbred LH195, and to estimate response to aflatoxin contamination of Argentine hybrids and semi-exotic testcrosses under inoculation with Aspergillus flavus. Agronomic data was collected during 2004 in eleven Texas environments for Argentine hybrids, and eight Texas environments for semi-exotic testcrosses. Response to aflatoxin was measured in three southern Texas environments. U.S. commercial hybrids were used as checks. Significant differences among hybrids were observed for most environments and traits. In general, Argentine hybrids yielded lower, had lower 1000 kernel weights, and greater test weights than U.S. hybrids. Hybrids AX889, AX882MG, and AX890MG were competitive with U.S. hybrids for grain yield and were stable across environments. Semi-exotic testcrosses had similar lodging and grain moisture percentages, heavier test weights and competitive grain yields compared with U.S. hybrids. Hybrid TX-LAMA2002-9-2-B/lH195 had the highest overall grain yield mean for semi-exotic testcrosses and yielded better than two U.S. hybrids. Argentine hybrids had lower aflatoxin concentration than U.S. hybrids; several hybrids had less than 50 ng g-1 aflatoxin. Semi-exotic testcrosses had reduced aflatoxin compared to U.S. hybrids, with several hybrids under 35 ng g-1. These elite, exotic materials show promise for breeding programs, with competitiveness for grain yield, kernel traits, and reduced aflatoxin levels.

No description available.

Aspergillus flavus is a pathogenic fungus causing maize ear rot disease and producing aflatoxins that are carcinogenic to humans and animals. Characterizing maize host resistance mechanisms and prioritizing candidate resistance genes are important to the development of resistant maize germplasm. This study investigated transcriptomics approaches and statistics methods on the identification of maize host resistance genes. Full-length cDNA libraries were also constructed and evaluated for gene cloning and functional analysis. This study established important sources for functional studies on differentially expressed genes and for identification of allelic gene forms to characterize gene polymorphisms and facilitate the development of DNA markers.