Browsing by Subject "DNA-Marker"

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Molecular and genetic analyses of aggressiveness in Fusarium graminearum populations and variation for Fusarium head blight resistance in durum wheat
(2011) Talas, Firas; Miedaner, Thomas
Fusarium head blight (FHB) is a devastating disease of wheat, barley and other cereals, which affects all wheat-growing areas of the world. The most prevalent species are Fusarium graminearum Schwabe (teleomorph: Gibberella zeae (Schweinitz) Petch) and Fusarium culmorum (W. G. Smith) Saccardo. Wheat breeding for FHB resistance has become the most effective and cost efficient strategy to combat this disease. Assisting long term stable breeding programs need a better understanding of the biology and dynamic changes of the population structure. Deoxyninalenol (DON) has the most economical impact among the other mycotoxin secreted by this fungus. Several chemotypes characterizes F. graminearum isolates. All chemotypes (3-ADON, 15-ADON, and NIV) were detected in Europe. The prevalent chemotype in Germany and UK is 15-ADON. Population structure is the result of evolutionary forces acting on the population in time and space together with mutation, recombination, and migration enhancing the genetic variance of a population, random drift and the selection reducing it. Aggressiveness in F. graminearum denotes the quantity of disease induced by a pathogenic isolate on a susceptible host in a non-race specific pathosystem, and is measured quantitatively. The quantitative traits such as aggressiveness and DON production mirror both the environmental changes and the genetic variation. Several genes are responsible for DON production; majority of these genes are grouped in TRI5 cluster. Few genes are known to be associated with F. graminearum aggressiveness such as MAP kinase genes, RAS2, and TRI14. Association between single nucleotide polymorphism and genetic variation of aggressiveness and DON production traits provide a clear identification of quantitative participation of different SNPs in expressing the trait. Also, this approach provides a good method to test the association between candidate genes and the traits. The objectives of this research were to (1) screen some durum wheat landraces for FHB resistance; (2) determine the genetic and chemotypic structure of natural population of F. graminearum in Germany; (3) determine the phenotypic variation in Aggressiveness and DON production, which come out one farmer wheat field; (4) compare the phenotypic variation and genetic variation occurring in one wheat field; and (5) associate the phenotypic traits with SNPs in candidate genes. Screening for FHB resistance was performed on sixty-eight entries form the Syrian landraces. The main characters of selection for resisting FHB disease are low mean value of infection and stability in different environments. Four genotypes (ICDW95842, ICDW92330, ICDW96165, Chahba) had small mean FHB value, small value of deviation form regression, and regression coefficient close to zero. These genotypes were considered as candidate resistant sources of FHB for further agronomic performance analysis through backcrossing generation. The causal agent of FHB in Germany is F. graminearum s.s. with a dominating rate of 64.9 % (out of 521 Fusarium spp. isolates). Nonetheless, the three chemotypes were detected in Germany and some times within one wheat field. The 15-ADON chemotype dominated the populations of F. graminearum s.s. in Germany followed by 3-ADON then NIV chemotype (92, 6.8, and 1.2%, respectively). High genetic diversity (Nei?s gene diversity ranged form 0.30 to 0.58) was detected on a single wheat field scale. Analysis of molecular variance (AMOVA) revealed a higher variance within populations (71.2%) than among populations (28.8%). Populations of F. graminearum s.s. in Germany display a tremendous genetic variation on a local scale with a restricted diversity among populations. Surprisingly the phenotypic variation of aggressiveness and DON production revealed a similar partitioning scale as the genetic variation. In other words, analyses of variance (ANOVA) revealed a higher variance within populations (72%) than between (28%) populations. The wide spectrum of aggressiveness (i.e., from 18 to 39%) and DON production (from 0.3 to 23 mg kg-1) within single wheat field simulate the global variation in both traits. Consequently, associating the observed variation of aggressiveness and DON production with detected single nucleotide polymorphism (SNPs) in some candidate genes revealed few but significant associations. According to Bonferroni-Holm adjustment, three SNPs were associated significantly with the aggressiveness, two in MetAP1 and one in Erf2 with explained proportions of genotypic variance (pG) of 25.6%, 0.5%, and 13.1%, respectively. One SNP in TRI1 was significantly associated with DON content on TRI1 (pG=4.4). The rapid decay of the LD facilitate a better high resolution of the association approach and is in turn suggest the need of higher number of SNP marker to facilitate a genome wide association study. The linkage disequilibrium between unlinked genes suggests the involvement of these genes in the same biosynthesis network. In conclusion, building wheat breeding program for FHB resistance depend initially on identifying sources of resistance among wheat varieties or wild relatives. Moreover, understanding the population structure of the pathogen and the selection forces causing genetic alteration of the population structure enable us employ a sufficient increase of the host resistance. Keeping such a balanced equilibrium between increasing host resistance and changes occur in genetic structure of F. graminearum population would insure no application of additional selection pressure. Further association of candidate genes with aggressiveness can provide effective information of the population development. Continuous observation of Fusarium population?s development is needed to insure a stable management of Fusarium head blight disease.
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.