Characteristics of cms-S reveversion to male fertility in maize : (cytoplasmic male sterility, male fertility restoration S-type cytoplasm, mitochondrial DNA)

Abstract

The association of cytoplasmic reversion of cms-S male-sterile strains to male fertility with disappearance of the S1 and S2 mtDNA plasmids as discrete molecules has been established for all 23 cytoplasmic revertant strains that have been studied so far. This correspondence between mutational step and molecular event provides the first unequivocal evidence that the genetic determination of cms-S male-sterility; male-fertility expression is located in mtDNA. When cytoplasmic reversion of cms-S strains to the male-fertile condition occurs, at least some, perhaps all, of the S1 and S2 plasmid sequences are transposed and integrated into the main high molecular weight mitochondrial DNA. These findings are consistent with the model proposed earlier for cytoplasmic reversion that connected it with "fixation" at the cytoplasmic level of an episomal fertility element carried by reversion-prone cms-S strains. Whether a corresponding transposition and integration of S1 and/or S2 mtDNA sequences into nuclear chromosomes is involved in the nuclear reversions, as called for by the model, is uncertain. If such a correlation were established it would provide the first example in higher plants of inter-organellar transposition of a naturally occurring genetic element. The S1 and S2 mtDNA plasmids, like the IS elements of bacterial transposons, have terminal inverted repeat sequences that probably equip them for integration into high molecular weight genomes. The possibilities for use of S1 and S2, and other mtDNA plasmids that have been identified in maize, as vehicles in interorganellar gene transfer are discussed briefly.JOHN R. LAUGHNAN, SUSAN GABAY-LAUGHNAN and JOHN E. CARLSON, Department of Genetics and Development, University of Illinois Urbana, IL