Fine mapping of resistance against root aphids in sugar beet

Abstract: Sugar beet, Beta vulgaris, is an important crop for the global sugar industry being one of the two major crops cultivated for sugar production. In the USA, the sugar beet root aphid, Pemphigus betae, is a regionally occurring pest which damages sugar beets by feeding on their secondary roots which results in reduced biomass and hence a lower yield. To date, the best alternative to control the aphids is by use of resistant varieties. To find such resistance and to introduce it in plant material by plant breeding it is necessary to have a precise and fast selection method for the resistance. In modern plant breeding more and more of useful plant characteristics are selected for via molecular markers. A locus on sugar beet chromosome I is known to harbor resistance to the root aphid, but the markers flanking the locus were too far from the actual gene(s) to be really useful for selections. The aim of this master thesis was to fine map the resistance locus by means of sequencing to enable identification of SNPs in close proximity of the locus and by conducting a phenotypic test under greenhouse conditions. The phenotypic test showed varying results, but was still a useful complement to the marker analysis. Several lines scored fairly consistent, while other lines had plants which scored anything between susceptible and resistant. However, in most cases the results from the phenotypic test were comparable to earlier phenotypic data from a field test, and part of the variation was because there still was segregation among sugar beet offspring but most likely also due to escapes from infestation. Since the phenotypic test was a pilot test there were some aspects that could be improved, for instance the selection of soil type. The results from the sequencing showed that nearly 50% of the DNA targets were polymorphic. The other half of the DNA targets turned out to be either monomorphic or failed due to technical reasons or difficulties in amplification of the locus. From the fine mapping, five SNP markers were found in the vicinity of the interval where the resistance locus was previously mapped, making it possible to narrow down the resistance locus. The recombination frequency of the two markers closest to the resistance locus was 1.6%. Although the new markers can be utilized in marker-assisted breeding for the sugar beet root aphid resistance, recombinant events are still present between the two markers suggesting that further narrowing-down of the interval would be feasible.