Examination of the mitochondrial genome of revertant progeny from S cms maize with cloned S-1 and S-2 hybridization probes.

We have examined the molecular rearrangement of mitochondrial DNAs in each of several fertile revertants that arose spontaneously from S-type cytoplasmically male-sterile maize. Cloned segments of S-1 and S-2 DNAs (plasmid-like DNAs characteristic of the mitochondrial DNAs of S-type lines) were hybridized to untreated and restriction endonuclease-treated mitochondrial DNAs from fertile plants carrying normal cytoplasm, from cytoplasmically male-sterile plants, and from plants that had cytoplasmically reverted to fertility. The relative intensity of hybridization with S-1 and S-2 probes was different among the fertile, sterile, and revertant lines. The sizes of some restriction endonuclease fragments from the fertile revertant lines that hybridize with the S-2 probe differ from those of the sterile parental lines. Preferential synthesis of high molecular weight components of the mitochondrial genome carrying S-1 and S-2 sequences, concomitant with cessation in apparent autonomous replication of discrete S-1 and S-2 DNAs and their replicative intermediates (described here), could accommodate the hybridization data. The results suggest but do not prove that S-2 sequences are transposed coincident with the sterile plant's reversion to fertility. The inserted segments could arise from sequences already present in the high molecular weight DNA or from the lower molecular weight linear components of the mitochondrial genome. Putative target sites of insertion of S-1 and S-2 sequences would be multiple and separate for each. Reversion of S-type cytoplasmically male-sterile plants to fertility does not restore the organization of the mitochondrial genome to that of a normal fertile plant.