The basis for cytoplasmic instability in S cytoplasm

The S group is one of several sources of cytoplasmic male sterility (cms) in maize. It is distinguished from the other cms groups by its fertility restoration pattern. Furthermore, two plasmid-like DNAs are uniquely associated with mitochondrial preparations from cms-S maize (D. R. Pring, C. S. Levings III, W. W. L. Hu, and D. H. Timothy, PNAS 74:2904-2908, 1977). These DNAs have molecular weights of 4.1 and 3.5 x 106 daltons and occur in addition to the usual high molecular weight mitochondrial DNAs (mtDNA). So far these plasmid-like DNAs have been observed in every source of the S cytoplasm studied. Conversely, these small DNA species have not been detected in normal (fertile), T or C cytoplasms. Unlike T and C, S cytoplasm has proven to be unstable (J. R. Laughnan and S. H. Gabay-Laughnan, 1978, Maize Breeding and Genetics, D. B. Walden, ed.). Hundreds of cases have been observed in which cms-S plants have reverted to male fertility. The majority of these reversions have been shown genetically to have arisen by cytoplasmic change rather than nuclear change, although this has recently been shown to be primarily determined by the nuclear genome (see contribution this News Letter, Laughnan and Gabay-Laughnan, University of Illinois). This report deals with biochemical studies of the cytoplasmic revertants.

MtDNA preparations were made from the various cytoplasmic types and fractionated by gel electrophoresis. The results of these analyses are presented diagramatically in Fig. 1 where the mtDNA preparations are from the following types: lane 1, normal cytoplasm which is male-fertile; lane 2, S cytoplasm which is male-sterile; lane 3, Vg (M825 version) cytoplasm which is male sterile; lane 4, Vg (revertant) cytoplasm which has reverted from the male-sterile to the male-fertile condition. The Vg cytoplasm is a member of the S group of cytoplasms. In the M825 inbred, Vg cytoplasm is highly unstable; about 10% of plants exhibit reversion, and over 90% of such reversions occur at the cytoplasmic level.

All four cytoplasmic types contain a main band of high molecular weight mtDNAs as well as a fast migrating small (1.2 x 106) molecular weight DNA. The S cytoplasm, lane 2, contains the two plasmid-like DNAs, S-S and S-F, in equimolar amounts. The S-S and S-F DNAs are not found in normal cytoplasm, lane 1. In lane 3, the Vg cytoplasm (M825 source) has the two plasmid-like DNAs, but they do not occur in equimolar amounts; the S-F DNA species is present in reduced quantities. Lane 4 contains the mtDNA preparation from a cytoplasmic revertant which has changed from the male-sterile to the male-fertile condition. In this case, the two plasmid-like DNAs, S-S and S-F, are no longer observed.

Seemingly, the loss of the S-S and S-F DNAs is correlated with the cytoplasmic reversion from the male-sterile to the male-fertile condition. To date, five cytoplasmic revertants have been investigated, and in each case the plasmid-like DNAs were absent. This result constitutes compelling evidence that the genetic basis for cms-S resides in the mitochondrion, more specifically, that the S-type of male sterility is associated with the S-S and S-F DNAs.

Finally, it is clear that the unstable Vg (M825 version) cytoplasm is not a typical cms-S. It seems likely that the reduction in the amount of the S-F DNA species observed in mitochondrial preparations from this source may be related to the cytoplasmic reversions that occur so frequently in this strain.

Figure 1.

C. S. Levings, III, D. R. Pring, M. F. Conde, J. R. Laughnan and S. J. Gabay-Laughnan


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