Linkage maps in studies of genetic variation - examples from Beta vulgaris

Sammanfattning: Today, most studies of genetic variation utilise molecular markers, making it possible to detect variation at the DNA level. This has greatly facilitated studies of genetic variation in population and evolutionary genetics, systematics and transmission genetics. By studying the inheritance of molecular markers in crosses, it is possible to map the markers, i.e. to determine their genomic positions. Utilising mapped markers can make studies of genetic variation much more efficient. The present thesis explores several different applications of molecular markers requiring that the genomic positions be known, using beet (Beta vulgaris) as a model. Marker maps are often the basis for mapping genes of interest. Gene mapping studies usually focus on qualitative variation in traits determined by a single gene. Such genes are fairly easy to map in crosses segregating for the trait, although the resolution for the map position may be poor unless very large samples are studied. One method to increase the resolution is to use linkage disequilibrium mapping, which has been successfully employed in mapping several human disease genes. I have explored the possibility of using this method in plants as well, taking the gene for annuality in beets as an example. Mapping the genes responsible for variation in quantitative traits is much more complicated, since the genotypes in the genes cannot be observed directly. An example of the mapping of a quantitative trait, namely Cercospora resistance in beet, is included in the thesis. Utilising a high-density AFLP map, five genes were detected and mapped. Two different topics in population genetics that require mapped markers are presented. First, the distribution of linkage disequilibrium in both wild and cultivated beets is investigated. It is shown that linkage disequilibrium is only present between markers that are tightly linked. This result has important implications for an understanding both of the genetics underlying evolution and of how to conduct linkage disequilibrium mapping in beets. Secondly, I have studied the relation between levels of genetic variation and of recombination in natural populations of sea beets. The neutral theory predicts that genomic regions of high and low recombination should be equally variable. Results of empirical investigations in Drosophila are not in line with this prediction. Several non-neutral explanations have been invoked. Our study is one of the first on this topic in any other organism than Drosophila. The results show that the same positive correlation between levels of genetic variation and of recombination as in Drosophila is found in beets.