RESTORER GENES FOR DIFFERENT FORMS OF BRASSICA CYTOPLASMIC MALE-STERILITY MAP TO A SINGLE NUCLEAR LOCUS THAT MODIFIES TRANSCRIPTS OF SEVERAL MITOCHONDRIAL GENES

The oilseed rape plant, Brassica napus, possesses two endogenous male sterile cytoplasms, nap and pal. Previous studies have shown that nucl ear restoration of pol cytoplasmic male sterility (CMS) is conditioned by a gene, Rfp, that is; also involved in modifying transcripts of th e pol CMS-associated orf224/atp6 mtDNA region. We now find that the na p nuclear restorer gene Rfn apparently is identical to Mmt, a gene tha t conditions the modification of transcripts from several different mt DNA regions, including one that is associated with nap CMS and contain s orf222, a chimeric gene related to orf224. Mmt, in turn, is found to be allelic to Rfp, suggesting that restorer genes for the two cytopla sms represent different alleles or haplotypes of a single nuclear locu s. This view is supported by restriction fragment length polymorphism mapping studies that indicate that Rfn and Rfp map to the same chromos omal position. Thus, in contrast to CMS in other species, different fo rms of Brassica CMS are restored by alleles of a single nuclear locus, and the restoration properties of these alleles reflect their involve ment in the modification of transcripts of corresponding CMS-associate d mtDNA regions. A survey of 51 varieties from 8 Brassica and Sinapis species failed to find evidence of Rfn(Mmt) in other than fertility-re stored, nap cytoplasm B. napus. This suggests that Rfn (Mmt) arose in Brassica with nap cytoplasm and that the necessity for fertility resto ration may have provided the selective pressure for its origin and mai ntenance.