Fire blight resistance of the wild apple species Malus fusca
Erwinia amylovora is the pathogen responsible for inciting fire blight - the most dreaded bacterial disease of apple (Malus × domestica) and other members of the Rosaceae family. The disease is very destructive as is difficult to control. Even though fire blight was first observed over two centuries ago in America, no sustainable control measure is known till date. Disease management practices such as pruning of affected tissues as well as the application of copper and antibiotics, for example, streptomycin, are to minimize the population of E. amylovora in an orchard. However, the use of streptomycin is not allowed in many European countries as a consequence of its environmental risks and the issue of raising antibiotic resistant E. amylovora populations. Therefore, natural resistance is thought to be the most sustainable approach to manage fire blight. Genetic resistance has been investigated in Malus leading to the detection of several quantitative trait loci (QTLs) in apple cultivars and apple wild species accessions. Nevertheless, only one functionally characterized fire blight resistance gene has been isolated till date. This situation, coupled with the proof that resistance is strain specific, reinforces the need to detect more donors that could be used to establish durable resistance against fire blight. For this reason, the works described in this thesis aimed at investigating fire blight resistance in another apple wild species – Malus fusca. Different accessions of M. fusca were phenotyped in JKI, Dresden, Germany to ascertain the accession with a high resistance level. Accession MAL0045, found as having a very high resistance level was then crossed with the very susceptible apple cultivar ‘Idared’ to establish a segregating F1 population of 134 individuals. To facilitate the development of a genetic map of M. fusca, molecular markers such as DArT (Diversity Arrays Technology), SNPs (Single Nucleotide Polymorphisms) and SSRs (Simple Sequence Repeats) were developed, sourced, tested and polymorphic ones applied to the 134 individuals and then mapped. The phenotypic and genotypic data were employed for QTL analyses which resulted in the identification of a major quantitative trait locus which is located on linkage group 10 (LG10) of the apple genome and could explain about 66 % of the phenotypic variation; the second highest effect of all QTLs previously detected in Malus. Furthermore, this thesis also describes the stability and validation of the M. fusca fire blight resistance locus (Mfu10) after another phenotypic evaluation of the F1 population with a highly virulent E. amylovora isolate Ea3049 originating from Canada. Moreover, the fine mapping of the resistance region was undertaken via chromosome walking approach with the development of closely linked SSR markers suitable for marker assisted selection (MAS). For this purpose, the population was substantially increased to 1,336 individuals from an additional cross of M. fusca × ‘Idared’ and a reciprocal cross of ‘Idared’ × M. fusca. Genotyping of the whole population allowed for the identification of individuals showing recombination events within the interval of the QTL region. Phenotyping of recombinant individuals ensured that the exact position of the QTL was well defined. The first steps towards uncovering the underlying gene(s) responsible for the resistance of fire blight in M. fusca have been achieved with the development and screening of a M. fusca bacterial artificial chromosome (BAC) library with SSR markers closely linked to Mfu10 and the identification of some BAC clones in the QTL interval. This is the first report of a major quantitative trait locus for resistance to fire blight in this wild relative of apple. The implications of the results obtained in these research works in respect to breeding for resistance against the very destructive fire blight disease of Malus are discussed extensively.
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