Application of Molecular Markers in Sugar Beet Breeding
Abstract: Around 1750 a German chemist discovered it to be possible to extract sugar crystals from beet juice. Since then, breeding and improved agricultural practices have increased the concentration of sucrose per root from around 4% to around 18%. Sugar content is still one of the most important characters in sugar beet breeding, but breeders of today are also faced with many more characters, many of a complex nature. The introduction of molecular marker techniques has added a very powerful tool to traditional breeding techniques. Through use of molecular markers it has become possible to breed with higher precision and with a higher degree of control. Molecular markers have also enabled complex traits to be dissected into some of their contributing factors. The studies included in the thesis have all been carried out for the purpose of finding and investigating the usefulness of molecular markers in sugar beet breeding. Large parts of the thesis is therefore concerned with describing how sugar beets are bred, how molecular markers can assist in this process and what should be taken account of in marker applications. The first three studies formed the basis for the use of markers in breeding applications, whereas the three studies thereafter are examples of marker applications. In study I, DNA sequences from a Beta vulgaris library were characterized to evaluate the usefulness of such sequences as markers in backcrossing and in comparative mapping. Since sugar beet is a crop of recent and narrow origin the variation within its breeding pool as compared with that of wild beets was of interest. This was investigated and found to be satisfactory in study II. A high-density RFLP linkage map of sugar beet was constructed in study III to create a basis for backcrossing and localisation of factors that influence important traits. In study IV, bulked segregant analysis was used to accumulate markers near a locus for resistance to beet cyst nematodes so as to generate a marker closer to the gene responsible for the trait. In study V, the inheritance of resistance to the fungal disease Cercospora leaf spot was investigated by means of QTL analysis and five factors influencing the trait were mapped. Finally, in study VI the genetic basis for the restoration of Owen cytoplasm male sterility was investigated by means of QTL analysis resulting in three mapped loci.
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