Identification and Characterization of Quantitative Trait Loci (QTL) for Resistance to the Cyst Nematode (Globodera pallida) in the Potato (Solanum tuberosum)
This work focused on the identification and characterization of the genetic basis of important agronomic traits in the potato with main interest centered on resistance to the potato cyst nematode, Globodera pallida. A high level of genetic resistance to nematodes is an important aspect in breeding new potato varieties. The first step towards that is the localization of genetic factors controlling resistance on the potato map. We made use of information obtained from experimental diploid mapping populations to evaluate tetraploid breeding material for the applied research. We explored the genetic resistances present in the crop, which show a continuous variation and are assumed to be controlled by several loci which act collectively on the expression of resistance. These loci are referred to as quantitative trait loci (QTL) (Meyer et al. 1998). The number of QTL for resistance was determined through an association mapping approach. Two tetraploid populations were genotyped with several markers either within or physically linked to candidate genes. Phenotypic data was supplied by the two breeding companies, Saka-Ragis and Böhm-Nordkartoffel. Significant associations with quantitative resistance were found for all marker types. Single nucleotide polymorphisms (SNPs) were shown to be the most efficient marker type to detect significant associations between markers and the resistance phenotype. Two of them served as basis for developing a PCR-based marker highly diagnostic for potato varieties with high resistance to Globodera pallida (Sattarzadeh et al. 2006). Based on an InDel in the sequence we designed an allele-specific CAPS marker for the quantitative detection of maturity corrected resistance to the oomycete Phytophthora infestans. Using this marker we provide solutions for the selection of superior potato genotypes through the development of easy-to-use DNA based molecular markers. Fine mapping of a QTL for resistance to the nematode located on chromosome V was done by a combination of association tests and physical mapping to gain more insight in the genomic organisation of a QTL. We were able to delimit the size to a genetic region of 2 cM. Fluorescent in situ hybridization (FISH) was applied to obtain valuable complementary information regarding size and chromosomal position of this QTL. The region was estimated to span less than 1 Mb and is located in the euchromatic region of the long arm of chromosome V. These findings will assist future map based cloning efforts in this region. The detection of numerous loci in tetraploid breeding material associated with important agronomic traits provides valuable data which can be used to design additional markers not just for resistance against G. pallida but also for other traits, such as resistance to Phytophthora infestans, maturity, yield, and starch content
