Molecular Linkage Maps: Strategies, Resources and Achievements
The fi rst genetic linkage associations of genes controlling morphological traits in Brassica were detected over 80 years ago (Pease 1926). Further research describing linkage groups for the Brassica species concerned only a limited number of morphological and isozyme markers (Yarnell 1956; Stringam 1977; Sampson 1978; Arus and Orton 1983). Despite identifi cation and rough mapping of genes for morphological characters and isozyme markers, little was known about their organization in the Brassica genomes (James and Williams 1980). Development of molecular biology techniques and a high level of DNA polymorphisms disclosed both between various Brassica accessions and subspecies allowed construction of comprehensive Brassica genetic maps (Figdore et al. 1988; Song et al. 1988; McGrath et al. 1990). The fi rst detailed restriction fragment length polymorphism (RFLP) maps of Brassica have been published in the beginning of the 1990s (McGrath et al. 1990; Song et al. 1991; for a review see Babula et al. 2007). Many maps of Brassica have been constructed on the basis of the F2 or doubled haploid (DH) lines derived from different species, subspecies or varieties. This facilitated mapping of the genes or quantitative trait loci (QTLs) of economically important traits, identifi cation of introgressions between different genomes, marker-assisted breeding and gene pyramiding aimed at crop improvement. The presence of the same markers on different Brassica maps allows comparison of their localization, giving insight into the structure and evolution of genomes of the diploid and amphidiploid species.