Browsing by Subject "Maiszünsler"

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Resistance breeding in maize (Zea mays L.) against the European corn borer (Ostrinia nubilalis Hübner) and the use of DNA-markers for marker-assisted selection
(2005) Papst, Christine; Melchinger, Albrecht E.
The European corn borer (Ostrinia nubilalis Hb., ECB) is an important pest in maize production. Feeding of ECB larvae causes grain yield losses of up to 30% and promotes ear and stalk rots caused by Fusarium spp.. Maize cultivars carrying the Bt gene are highly resistant to ECB larvae feeding. However, the use of transgenic cultivars is controversially discussed. In contrast, the natural host plant resistance (HPR) is regarded as more durable. The main objective of this study was to identify quantitative trait loci (QTL) for HPR against ECB and to draw conclusions about their usefulness in marker-assisted selection (MAS). The specific research questions were: (1) Where are QTL for ECB resistance and related agronomic traits located in the maize genome and what are their genetic effects? (2) How consistent are QTL detected across unrelated populations? (3) How consistent are QTL detected for line per se and testcross performance? (4) Which physiological mechanisms underlie the resistance against ECB larvae feeding? (5) What is the association between ECB resistance and mycotoxin concentrations in grain maize? Two unrelated dent populations (A and B) were developed. For Experiment 1 the F2:3 families were evaluated for line per se performance for ECB resistance. All F2:3 families of Population B were testcrossed with a susceptible tester line and also evaluated for ECB resistance (Experiment 2). Two sets of F2:3 families from Population B, each comprising the most resistant and the most susceptible lines, were selected (Experiment 3). In Experiment 4, 10 maize cultivars consisting of four pairs of transgenic hybrids and their isogenic counterparts were used to determine the association between mycotoxin concentration and ECB resistance. All entries in Experiment 4 were analyzed for mycotoxin concentration of deoxynivalenol (DON), fumonisin (FUM), fusarenon-X (FUS), moniliformin (MON) and nivalenol (NIV) in grain samples. In all four experiments, resistance to ECB larvae feeding was evaluated using manual infestation with ECB larvae. Furthermore agronomic and quality traits were recorded. In Experiment 1, two QTL for resistance were detected in Population A, both explaining about 25% of the genotypic variance. No common QTL for resistance traits was found across Populations A and B. Possible explanations for the low consistency of QTL across populations are a low power of QTL detection caused by small population sizes, sampling, and environmental effects. Furthermore, population-specific QTL regions cannot be ruled out. In Experiment 2, six QTL for resistance explaining 27% of the genotypic variance were found for testcross performance. Three common QTL for resistance were detected for line per se and testcross performance. Phenotypic as well as genotypic correlations between line per se and testcross performance were low for resistance, indicating a moderate consistency across the different types of progeny. The low consistency across both types of progeny is presumably attributable to the low power of QTL detection in TC progenies caused by a decreased genotypic variance and masking effects of the tester allele. Despite the low consistency of QTL across populations and progenies in the present study, a comparison with other reports from the literature revealed that most of the QTL occurred in clusters. Given the low percentage of genotypic variance explained by QTL-marker associations, we conclude that MAS will not be efficient for resistance breeding against ECB with the current molecular marker techniques. In Experiment 3, significant correlations were observed between resistance and quality traits, such as digestibility and stalk strength. These findings confirm the importance of increased cell-wall fortification for resistance against ECB larvae feeding, and support the hypothesis that candidate genes for resistance are involved in lignin biosynthesis. The analyses of mycotoxin concentrations in Experiment 4 showed that DON, FUM, and MON were the most prevalent mycotoxins in maize kernels. Differences between protected and infested plots were only significant for DON and FUM. Transgenic Bt hybrids showed lower mycotoxin concentrations in kernels than the other hybrids. However, only low correlations were found between ECB resistance and mycotoxin concentrations across all 10 hybrids. Therefore, selection for ECB resistance does not necessarily reduce mycotoxin concentration, suggesting that each complex of characters must be improved simultaneously by breeding. Even if MAS for resistance against the ECB does not seem promising at the moment, the information about QTL regions may be a first step for further research on possible candidate genes, e.g., brown midrib genes located in the common QTL regions with effects on the lignin biosynthesis. Genotypes with an improved digestibility, without impairing ECB resistance by reduced cell-wall strength, would be most promising.
Resistance of Maize (Zea mays L.) Against the European Corn Borer (Ostrinia nubilalis Hb.) and its Association with Mycotoxins Produced by Fusarium spp.
(2004) Magg, Thomas; Melchinger, Albrecht E.
The European corn borer (ECB, Ostrinia nubilalis Hübner) is a major pest of maize (Zea mays L.) in Europe and continues to spread to northern maize growing regions. The ECB severely affects commercial maize production by decreasing yield stability. In addition, damaged plants often show an increased susceptibility to secondary infections caused by Fusarium spp.. Information about the potential of Bt hybrids (event 176, MON810) to reduce yield losses and mycotoxin contamination under Central European growing conditions is still lacking. However, such monogenic resistances with a strong negative effect on the ECB will break down rapidly. Improving the natural host plant resistance of maize could provide an economical and ecological tool for an integrated pest management system. The overall goal of this study was to evaluate alternative breeding strategies for improving resistance of maize against ECB damage and Fusarium spp.. The objectives were to (1) initiate a selection experiment in the early maturing European flint pool and evaluate a breeding program for ECB resistance in the European dent pool, (2) compare the efficiency of host plant resistance vs. Bt resistance in maize, (3) determine Fusarium-caused mycotoxin contamination of maize genotypes with improved host plant resistance to ECB, and (4) study the association between important agronomic traits, ECB resistance traits, and mycotoxin concentration in early European maize germplasm. The goal of the Hohenheim ECB breeding program, initiated in 1992, was to select lines with improved per se and testcross performance for multiple agronomic traits and ECB resistance. In the standard breeding scheme, line development started from a segregating S1 population. Genotypes were evaluated for their line per se ECB resistance in generations S1, S3, and S5. Lines from the S2, S4, and S5 generations were testcrossed and evaluated for their agronomic performance. Selection was based on ECB resistance and TC performance for grain yield and maturity. In order to compare transgenic Bt maize hybrids carrying event 176 or MON810 with their isogenic counterparts and commercial hybrids or experimental hybrids, field trials in multiple environments were conducted in 1998 to 2000. Furthermore, a laboratory bioassay with neonate ECB larvae was performed to assess mortality and subsequently the level of Bt antibiosis present in the used hybrids of 1998. Resistance traits such as damage rating of stalks, number of damaged plants, and number of larvae per plant were assessed exclusively in manually ECB infested plots. Grain yield, grain dry matter content and plant height were determined in the insecticide protected and the ECB infested main plots. In addition, grain samples from each subplot were drawn at random and analyzed separately for Fusarium mycotoxins such as type B trichothecenes (DON, NIV), Zearalenon (ZEN), Fumonisins (FUM), and Moniliformin (MON). The inbred lines displayed a significant genotypic variance for all ECB resistance traits evaluated. However, in the further course of selection and topcross testing, most dent and flint lines, especially those displaying improved resistance to ECB larvae feeding, were discarded because of their poor agronomic performance. Negative correlations between grain yield, early maturity and the damage rating of stalks were identified. However, three dent lines (P028, P029, P030) with moderate resistance to ECB were developed. In all experiments, Bt hybrids were superior to other hybrids in the control of ECB larvae. Non-Bt hybrids displayed a significant genotypic variance for all evaluated resistance traits; grain yield reductions ranged from 8.6 to 21.8% under manual infestation of ECB. All evaluated resistance traits were highly significantly correlated with each other and showed significant negative correlations to grain yield reduction. Bt hybrids did not differ from their isogenic counterparts for most agronomic traits. Highly significant location and genotype × location interactions were identified for all mycotoxins evaluated, except MON. MON concentration doubled under manual infestation of ECB compared to insecticide protected conditions and a similar trend was found for FUM. Bt hybrids displayed significantly lower MON concentrations than non-Bt hybrids and significantly lower DON concentrations than their isogenic counterparts under ECB infestation. Highly significant correlations between ECB resistance traits and MON were found. However, a significant genotypic variance was observed for DON, 15-A-DON, FUM, and MON concentrations, suggesting variation for resistance against Fusarium spp. in current elite hybrids. By combining different sources of monogenic Bt resistance and quantitatively inherited resistances to ECB, it may be possible to develop hybrids with multiple resistance by pyramiding the underlying genes in one genotype. Therefore, further research is required to identify new sources of ECB resistance and new breeding strategies should be developed. Furthermore, there is indication that an improved resistance against Fusarium spp. possesses a greater potential for reducing mycotoxin contamination of maize kernels than a high level of ECB resistance. Since resistance to ECB and resistance to Fusarium spp. are inherited fairly independently, simultaneous improvement of both resistances seems to be necessary for improving the stability and quality of future maize hybrids.