Cytoplasmic reversion to fertility in cms-S without loss of linear mitochondrial plasmids

The S type of sterile cytoplasm in maize is characterized by the presence in the mitochondria of autonomously replicating linear plasmid-like DNAs, S1 and S2. These plasmids have not been found in any normal fertile maize mitochondria, nor previously in male-fertile cytoplasmic revertants. Recent findings in our laboratory, however, indicate that the retention or loss of S1 and S2 in association with cytoplasmic reversion to fertility may be primarily a function of the nuclear genetic background, rather than a characteristic of the S-type cytoplasm itself.

The cytoplasmic revertants studied previously (Levings et al., Science 209:1021-1023; Laughnan et al., Stadler Genet. Symp. 13:93-114) have the nuclear background of inbred line M825, which shows the highest frequency of spontaneous reversion to fertility (Gabay-Laughnan and Laughnan, Maydica 28: 251-263). So far we have examined 27 cytoplasmic revertant strains with the M825 nuclear background, and in all cases the reversion event was correlated with disappearance of both S1 and S2.

Five spontaneous cytoplasmic revertants recovered from WF9, four from RD-WF9 and one from ML-WF9, all retained free S1 and S2 plasmids in the mitochondria. The amount of the free plasmids remained undiminished relative to that of the sterile parent line, even after four generations of crossing to the maintainer line, thus discounting the possibility that the free plasmids are merely carry-over from the sterile parent.

Cytoplasmic revertants, recovered after RD-WF9 and ML-WF9 were crossed seven times with M825 (recurrent male parent), did not show the free plasmids S1 and S2. Since these revertants presumably differed from the above WF9 revertants only in nuclear genotype, this indicates that the nuclear genome has the major influence on retention or loss of free plasmids in the mitochondria during cytoplasmic reversion to fertility. Alternatively, changes in nuclear genotype may impose changes in organization of the mitochondrial genome prior to cytoplasmic reversion, so that the same reversion event in two strains with different nuclear genotypes might well produce revertants with different characteristics. In either case, these studies indicate that the cms-S sterility-fertility phenomenon is not directly dependent upon the status of the S1 and S2 plasmids.

Loida J. Escote, Susan Gabay-Laughnan and John R. Laughnan

Please Note: Notes submitted to the Maize Genetics Cooperation Newsletter may be cited only with consent of the authors.

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