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Browsing by Person "Papst, Christine"

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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.