Linkage group alignment from four independent Brassica oleracea RFLP maps

A Brassica oleracea linkage map was constructed from an F2 population of 69 individuals with sequences previously mapped independently in three linkage maps of this species. These were the maps published by Kianian and Quiros (1992), Landry et al. (1992), and Camargo et al. (1997). The base map developed in this study consisted of 167 RFLP loci in nine linkage groups, plus eight markers in four linkage pairs, covering 1738 cM. Linkage group alignment was also possible with a fourth map published by Ramsay et al. (1996), that contained loci in common with the map of Camargo et al. (1997). Common sequences across the mapping populations served to align most of the linkage groups of the independently developed maps. In general, consistent linear order among markers was maintained, although often the distances between markers varied from map to map. A linkage group in the map of Landry et al. carrying a clubroot resistance QTL and consisting of markers from two other linkage groups, was found to be rearranged. This was not surprising, considering that the resistance gene was introgressed from Brassica napus. The extensively duplicated nature of the C genome was revealed by 19 sequences detecting duplicated loci within chromosomes and 17 sequences detecting duplicated loci between chromosomes. The variation in mapping distances between linked loci pairs on different chromosomes demonstrated that sequence rearrangement is a distinct feature of this genome. Although the consolidation of all linkage groups in the four B. oleracea maps compared was not possible, the present work served to add a considerable number of markers to corresponding linkage groups. Some of the chromosome segments in particular, were enriched with many markers that may be useful for future gene tagging or cloning. It will be possible in the future to complete the consolidation of all four maps as new loci are added to each map.Key words: cole crops, Cruciferae, molecular markers, linkage maps.