Two male sterile cytoplasms, nap and pol, are found in the oilseed rape spe
cies Brassica napus. As with many other forms of cytoplasmic male sterility
(CMS), the mtDNA regions implicated in specifying the two forms of male st
erility contain novel open reading frames (ORFs). Unlike other CMS-associat
ed ORFs, however, a high level of sequence similarity extends over the enti
re length of the nap and pol CMS-associated ORFs. Moreover, in other plant
species where more than one form of CMS is found, the nuclear genes that re
store fertility to various male sterile cytoplasms represent distinct genes
that map to different nuclear loci. The restorers for the nap and pol cyto
plasms (Rfn and Rfp, respectively), however, represent different alleles or
haplotypes of a single nuclear locus. Both alleles specify factors that in
fluence mtRNA processing events, but the specific processing events conditi
oned by the two alleles are different, suggesting that the factors encoded
by these genes recognize distinct RNA structural features. Finally, unlike
other nuclear genes that affect mitochondrial gene expression, Rfn is capab
le of modifying the expression of multiple mtDNA regions, some of which are
not associated with CMS. Findings made based on this observation are consi
stent with the view that new alleles of a restorer locus arise in response
to natural selective pressures generated by the appearance of new male ster
ile cytoplasms.