Molecular genetic studies in flax (Linum usitatissimum L.)

In this thesis five molecular genetic studies on flax ( Linum usitatissimum L.) are described, of which two chapters aim to characterize the genetic structure and the amount of genetic diversity in the primary and secondary gene pool of the crop species. Three chapters describe the development of AFLP markers, linkage map construction and QTL analysis of resistance and quality traits.Genetic diversity in the primary gene pool was studied by AFLP fingerprinting 110 varieties representing linseed and fiber flax germplasm, the two cultivar groups in flax. Fiber flax germplasm is a narrow subset within linseed flax, and modern breeding tends to further narrow the genetic diversity. Forty two AFLP bands showed strong association with cultivar group. In a second diversity study 52 accessions of 27 Linum species could be differentiated in clearly distinct groups. These groups showed some resemblance to the current taxonomic grouping of species in sections of the genus Linum , but many aberrations were observed. In our view the sexually compatible wild species L.bienne and L. angustifolium should be excellent sources to broaden the genetic basis of L. usitatissimum .In view of the extremely low number of polymorphic markers in AFLP fingerprints, an entire study was dedicated to select primer combinations with the highest number of markers. Unselected primer combinations show on average 2.3 and 3.8 polymorphisms for two different parental combinations. Our selection offavourableprimer combinations has on average 7.2 and 10.7 markers, allowing a 50% reduction of the workload involved in linkage map construction. An unexpected side effect of this selection was the frequent occurrence of allelic codominant length polymorphisms among the resulting AFLP markers.Two linkage maps were constructed in mapping populations descending from a fiber x fiber and a fiber x linseed cross. These maps were integrated to obtain a linkage map of flax. Two different computer algorithms were used to calculate marker order, JoinMap and RECORD, which are complementary, and their joint use allowed reaching the most plausible marker order. For 18 different traits involved in disease resistance ( Fusarium wilt and scorch) and quality traits related to fiber yield and quality, as well as fatty acid composition we identified a total of 63 QTL positions. In practical breeding some of these QTL loci could be of high practical value. Phenotypic selection of these loci would require field trials of the highest quality, but genotypic selection of these loci is easily pursued with DNA diagnostics. In view of the results obtained with these five molecular genetic studies recommendations are made to implement Marker Assisted Breeding (MAB), to proceed with linkage disequilibrium studies and to broaden the genetic basis of fiber flax breeding, for example with the aid of a library of backcross introgression lines.