The mitochondrial genome of an asymmetrically cell-fused rapeseed, Brassica napus, containing a radish-derived cytoplasmic male sterility-associated gene

Cytoplasmic male sterility (CMS) is an agronomically important trait whose causative genes are located in the mitochondrial genome. A CMS rapeseed, Brassica napus 'SW18', was made 25 years ago by an asymmetric (or "donor-recipient") cell fusion between B. napus Westar' and a CMS radish (Raphanus sativus 'Kosena'), in order to transfer the radish CMS-associated gene without disturbing the rapeseed features. Here, we determined the nucleotide sequences of the mitochondrial genomes of Kosena radish and SW18. SW18 has a recombinant mitochondrial genome, which includes the whole 222-kb genome of Westar (54 genes) and a total of 23 kb insertions of four fragments from Kosena radish (three genes: orf125, trnfM and atpl). All of the Kosena radish-derived fragments in the SW18 mitochondrial genome had sequences at their ends (ranging from 63 bp to 628 bp) that are identical to the sequences at the sites of insertion on the Westar rapeseed derived mitochondrial genome. This suggests that these insertions were mediated by homologous recombination. These results confirm at the nucleotide level that a desired CMS-associated gene (orf125) along with a few extraneous genes from radish were successfully transferred.