Modified recombination and transmission of mitochondrial genetic markers in rho minus mutants of Saccharomyces cerevisiae.

A large number of primary petite (rho-) clones were isolated after ethidium bromide mutagenesis of various grande (rho+) strains of S. cerevisiae that contained the mitochondrial genetic markers, CR, ER, OIR (or OIIR), and PR. From the frequency of coretention of markers in the petites, we have deduced a probable circular order of the markers in the grande mitochondrial genome. From these primary clones several series of pure and stable petite clones were obtained and analyzed genetically. (a) In general, the omega allele is retained or lost together with the region carrying both CR and ER markers. (b) The petites that have retained only the CR marker fall into two classes: some have kept the omega allele of the grande strain they issued from; others exhibit a new omega expression. (c) The proportion of diploid petites in petite X grande crosses is independent of the presence of the omega allele. (d) In most cases, the coordinated transmission of markers observed so far in all grande X grande nonpolar corsses does not exist anymore in petites.