Towards positional cloning in Brassica napus: generation and analysis of doubled haploid B. rapa possessing the B. napus pol CMS and Rfp nuclear restorer gene.

The Polima (pol) system of cytoplasmic male sterility (CMS) and its fertility restorer gene Rfp are used in hybrid rapeseed production in Brassica napus. To facilitate map-based cloning of the Rfp gene, we have successfully transferred the pol cytoplasm and Rfp from the amphidiploid B. napus to the diploid species B. rapa and generated a doubled haploid pol cytoplasm B. rapa population that segregates for the Rfp gene. This was achieved through interspecific crosses, in vitro rescue of hybrid embryos, backcrosses, and microspore culture. Male fertility conditioned by Rfp was shown to co-segregate in this population with Rfp-specific mitochondrial transcript modifications and with DNA markers previously shown to be linked to Rfp in B. napus. The selfed-progeny of one doubled haploid plant were confirmed to be characteristic B. rapa diploids by cytogenetic analysis. Clones recovered from a genomic library derived from this plant line using the RFLP probe cRF1 fell into several distinct physical contigs, one of which contained Rfp-linked polymorphic restriction fragments detected by this probe. This indicates that chromosomal DNA segments anchored in the Rfp region can be recovered from this library and that the library may therefore prove to be a useful resource for the eventual isolation of the Rfp gene.