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We mapped the dwarf Bzh gene in B. napus with RAPD and RFLP markers. Research of the linked markers proceeded in two ways: a random approach through the construction of a detailed genetic map and targeting of the dwarf gene using both near-isogenic lines (NILs) and the bulked segregant analysis (BSA) method. The BSA approach was the most efficient in finding DNA markers linked to Bzh, whereas the efficiency of the NILs approach was limited by a too great similarity of the genetic background between the dwarf donor parent and the recurrent lines. Eight RAPD markers were identified as linked to Bzh, the closest being at 0.8±0.7 cM. The random genetic mapping approach added markers and extended the linkage group containing Bzh. This work represents the first step towards a better understanding of the dwarf mutation, the development of marker-assisted selection, and the cloning of the underlying gene responsible for dwarfing.

We have undertaken the construction of a Brassica napus genetic map with isozyme (4%), RFLP (26.5%) and RAPD (68%) markers on a 152 lines of a doubled-haploid population. The map covers 1765 cM and comprises 254 markers including three PCR-specific markers and a morphological marker. They are assembled into 19 linkage groups, covering approximatively 71% of the rapeseed genome. Thirty five percent of the studied markers did not segregate according to the expected Mendelian ratio and tended to cluster in eight specific linkage groups. In this paper, the structure of the genetic map is described and the existence of non-Mendelian segregations in linkage analysis as well as the origins of the observed distortions, are discussed. The mapped RFLP loci corresponded to the cDNAs already used to construct B. napus maps. The first results of intraspecific comparative mapping are presented.

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