  In spring turnip rape (Brassica rapa L. spp. oleifera) the most promising F1
hybrid system would be the Ogu-INRA CMS/Rf system. A Kosena fertility restorer
gene Rfk1, homologue of the Ogura restorer gene Rfo, was successfully
transferred from oilseed rape into turnip rape and that restored the fertility
in female lines carrying Ogura cms. The trait was, however, unstable in
subsequent generations. The physical localization of the radish chromosomal
region carrying the Rfk1 gene was investigated using 8 GISH (genomic in situ
hybridization) and BAC-FISH (bacterial artificial chromosome fluorescence in
situ hybridization) methods. The metaphase chromosomes were hybridized using
radish DNA as the genomic probe and BAC64 probe, which is linked with Rfo gene.
Both probes showed a signal in the chromosome spreads of the restorer line
4021-2 Rfk of turnip rape but not in the negative control line 4021B. The GISH
analyses clearly showed that the turnip rape restorer plants were either
monosomic (2n=2x=20+1R) or disomic (2n=2x=20+2R) addition lines with one or two
copies of a single alien chromosome region originating from radish. In the
BAC-FISH analysis, double dot signals were detected in sub-terminal parts of
the radish chromosome arms showing that the fertility restorer gene Rfk1 was
located in this additional radish chromosome. Detected disomic addition lines
were found to be unstable for turnip rape hybrid production. Using the BAC-FISH
analysis, weak signals were sometimes visible in two chromosomes of turnip rape
and a homologous region of Rfk1 in chromosome 9 of the B. rapa A genome was
verified with BLAST analysis. In the future this homologous area in A genome
could be substituted with radish chromosome area carrying the Rfk1 gene.
