Browsing by Subject "Diallel"

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Effiziente Verfahren für die Züchtung neuer Erdbeersorten (Fragaria x ananassa Duch.)
(2015) Bestfleisch, Markus; Wünsche, Jens Norbert
In this thesis, different approaches aimed on efficient breeding of new strawberry cultivars were investigated that comprise horticultural traits such as earliness and yield potential as well as resistance traits against two of the most important pathogens in strawberry cultivation, Botrytis cinerea and Xanthomonas fragariae. High yield potential and an extended ripening period in combination with disease resistance form the basis for economic success and contribute to a competitive and sustainable strawberry cultivation in Germany. In a traditional diallel cross breeding experiment, a set of 13 strawberry cultivars were crossed in a reciprocal way. The crossings resulted in a total of 144 F1-populations which were evaluated in a field trial over two years with regards to their horticultural traits in order to investigate the general and specific combining abilities of the parental cultivars concerning the traits earliness and marketable yield. Within this thesis, a statistical model was developed to explain the components of genetic variance and the breeding value of the parental cultivars based on their calculated combining abilities. It was demonstrated that there is no reciprocal effect on the progeny and it is practically irrelevant whether a cultivar is used as mother or father in the crossing experiment. The genetic variance in the breeding experiment is mainly based on the general combining ability (GCA) of the parental cultivars (additive effects). Specific and reciprocal combining abilities (non-additive effects) appeared less relevant. These findings are taken into account for the further development of the breeding approach which has also a main focus on resistance towards the grey mold disease and the angular leaf spot disease. The grey mold disease, caused by the necrotrophic fungus Botrytis cinerea Pers. [teleomorph Botryotinia fuckeliana (de Bary) Whetzel], is the most important fungal disease in strawberries and requires frequent applications of chemical plant protection products. The angular leafspot disease, caused by Xanthomonas fragariae Kennedy & King, is the most important bacterial disease in strawberry cultivation worldwide and there are currently no plant protection products available within the EU for an effective control of the disease. In a successful control strategy, the cultivation of resistant cultivars is of fundamental importance and the results of this thesis will contribute to the development of a targeted breeding approach. Initially, resistance tests were established for both diseases. Reproducible results concerning the resistance characteristics of different cultivars were achieved in artificial inoculation experiments and adapted evaluation scales. With this approach, more than 100 strawberry cultivars, wild-types and breeding clones from the collection of the German Fruit Gene Bank of the JKI in Dresden-Pillnitz were evaluated in three years of testing. Concerning B. cinerea, a total of five genotypes were identified as partially resistant. In parallel, six partially resistant genotypes were found towards X. fragariae. No completely resistant genotyped were identified until now. Additionally to the resistance test against X. fragariae, the systemic dispersal of the bacteria within the plant was further investigated with molecular-biological methods. On the one hand, bacterial DNA was detected by a sensitive nested-PCR method in different plant tissues of inoculated plants from partially resistant and susceptible cultivars at different time points. Additionally, a GFP-tagged virulent X. fragariae strain was produced and used for inoculation experiments in different strawberry genotypes as well. The systemic dispersal of the bacteria was visualized under the fluorescent microscope and the results were confirmed by the nested-PCR detection method. Already three days after the inoculation, X. fragariae spreads systemically throughout the entire host plant and can be detected in all plant tissue samples. The bacteria were detected in all plant parts of partially resistant cultivars after three days post-inoculation as well. The results from the resistance tests lead to direct recommendations for the choice of cultivars on the one hand and on the other hand, the identified partially resistant genotypes can be used in further targeted breeding approaches. The results of this thesis show new ways and strategies for the improvement of strawberry breeding programs. They increase the success for targeted breeding of new cultivars for strawberry cultivation in Germany in order to maintain a competitive and sustainable strawberry production in the future.
Mixed modelling for phenotypic data from plant breeding
(2011) Möhring, Jens; Piepho, Hans-Peter
Phenotypic selection and genetic studies require an efficient and valid analysis of phenotypic plant breeding data. Therefore, the analysis must take the mating design, the field design and the genetic structure of tested genotypes into account. In Chapter 2 unbalanced multi-environment trials (METs) in maize using a factorial design are analysed. The dataset from 30 years is subdivided in periods of up to three years. Variance component estimates for general and specific combining ability are calculated for each period. While mean grain yield increased with ongoing inter-pool selection, no changes for the mean of dry matter yield or for variance component estimate ratios were found. The continuous preponderance of general combining ability variance allows a hybrid selection based on general combining effects. The analysis of large datasets is often performed in stage-wise fashion by analysing each trial or location separately and estimating adjusted genotype means per trial or location. These means are then submitted to a mixed model to calculate genotype main effects across trials or locations. Chapter 3 studies the influence of stage-wise analysis on genotype main effect estimates for models which take account of the typical genetic structure of genotype effects within plant breeding data. For comparison, the genetic effects were assumed both fixed and random. The performance of several weighting methods for the stage-wise analysis are analysed by correlating the two-stage estimates with results of one-stage analysis and by calculating the mean square error (MSE) between both types of estimate. In case of random genetic effects, the genetic structure is modelled in one of three ways, either by using the numerator relationship matrix, a marker-based kinship matrix or by using crossed and nested genetic effects. It was found that stage-wise analysis results in comparable genotype main effect estimates for all weighting methods and for the assumption of random or fixed genetic effect if the model for analysis is valid. In case of choosing invalid models, e.g., if the missing data pattern is informative, both analyses are invalid and the results can differ. Informative missing data pattern can result from ignoring information either used for selecting the analysed genotypes or for selecting the test environments of genotypes, if not all genotypes are tested in all environments. While correlated information from relatives is rarely directly used for analysis of plant breeding data, it is often used implicitly by the breeder for selection decisions, e.g. by looking at the performance of a genotype and the average performance of the underlying cross. Chapter 4 proposed a model with a joint variance-covariance structure for related genotypes in analysis of diallels. This model is compared to other diallel models based on assumptions regarding the inheritance of several independent genes, i.e. on genetic models with more restrictive assumptions on the relationship between relatives. The proposed diallel model using a joint variance-covariance structure for parents and parental effects in crosses is shown to be a general model subsuming other more specialized diallel models, as these latter models can be obtained from the general model by adding restrictions on the variance-covariance structure. If no a priori information about the genetic model is available the proposed general model can outperform the more restrictive models. Using restrictive models can result in biased variance component estimates, if restrictions are not fulfilled by the data analysed. Chapter 5 evaluates, whether a subdivision of 21 triticale genotypes into heterotic pools is preferable. Subdividing genotypes into heterotic pools implies a factorial mating design between heterotic pools and a diallel mating design within each heterotic pool. For two (or more) heterotic pools the model is extended by assuming a joint variance-covariance structure for parental effects and general combing ability effects within the diallel and within the factorials. It is shown that a model with two heterotic pools has the best model fit. The variance component estimates for the general combing ability decrease within the heterotic pools and increase between heterotic pools. The results in Chapter 2 to 5 show, that an efficient and valid analysis of phenotypic plant breeding data is an essential part of the plant breeding process. The analysis can be performed in one or two stages. The used mixed models recognizing the field and mating design and the genetic structure can be used for answering questions about the genetic variance in cultivar populations under selection and of the number of heterotic pools. The proposed general diallel model using a joint variance-covariance structure between related effects can further be modified for factorials and other mating designs with related genotypes.