LOW-COPY-NUMBER MOLECULES ARE PRODUCED BY RECOMBINATION, ACTIVELY MAINTAINED AND CAN BE AMPLIFIED IN THE MITOCHONDRIAL GENOME OF BRASSICACEAE - RELATIONSHIP TO REVERSION OF THE MALE-STERILE PHENOTYPE IN SOME CYBRIDS

A PCR analysis of mitochondrial (mt) genomes of cybrid rapeseed plants revealed substoichiometric concentrations of molecules bearing differ ent configurations of the gene (orf138) responsible for Ogura cytoplas mic male sterility (CMS). These substoichiometric molecules are also p resent in plants bearing the unmodified Ogura cytoplasm. In one cybrid family, which shows reversion of the male sterile phenotype, we obser ved changes in the respective proportions of these molecules. The phen otypic (sterility-fertility) reversion occurs as a result of a modific ation of the equilibrium state between the different forms of the orf1 38 gene and is very probably determined by the level of expression of this gene. Stable situations are always characterized by one predomina nt form; the others, when present, exist in substoichiometric amounts. We report results indicating that the different forms of the orf138 g ene are continuously interconverted by recombination and that an activ e mechanism is involved in the maintenance of some substoichiometric m olecules.