Mitochondrial genetics IX: A model for recombination and segregation of mitochondrial genomes in saccharomyces cerevisiae.

HE last few years have witnessed the accumulation of a considerable body experimental data dealing with the isolation of mitochondrial mutants in yeast conferring resistance to various inhibitors and with the results of two, three or four factor crosses using these mutants as genetic markers. All these results can be classified as belonging to two types of experimental approaches: one based essentially on an analysis of the progeny of an individual cell heterozygous for mitochondrial genetic markers and the second based essentially on an analysis of the progeny of a population of cells heterozygous for mitochondrial genetic markers. The first approach is exemplified by the work of LINNANE et al. 1968; THOMAS and WILKIE 1968; COEN et al. 1970; SAUNDERS et al. 1971 ; LUKINS et al. 1973; WILKIE and THOMAS 1973; the second by that of COEN et al. 1970; BOLOTIN et al. 1971; KLEESE, GROTBECK and SNYDER 1972 a and b; RANK and BECHHANSEN 1972; AVNER et al. 1973; AVNER and GRIFFITHS 1973; HOWELL et al. 1973; RANK 1973; WAKABAYASHI and KAMEI 1973; WOLF, DUJON and SLONIMSKI 1973; NETTER et al. 1974; COEN et al. (in preparation). At the very start of this period of mitochondrial genetics it became apparent that individual zygotic clones issued from a cross of two mitochondrially pure lines may behave quite differently from one another with respect to the types and proportions of the mitochondrial genotypes they give rise to (COEN et al. 1970). Pedigree analyses performed since have shown that not only are the proportions of different cell types variable from clone to clone but also their order of appearance (LUKINS et al. 1973; WILKIE 1973). Apart from the demonstration of interand intraclonal heterogeneity as one of the basic features of mitochondrial crosses, reminiscent of course of the heterogeneity in burst size and composition in bacteriophages crosses, this “individualistic” approach has not as yet yielded any substantial insights into the mechanism of mitochondrial genetics. The “population” approach is based on what COEN et al. (1970) have called a standard cross and has been adopted with no or only minor modifications by several authors (BOLOTIN et al. 1971; KLEESE, GROTBECK and SNYDER 1972 a and b; RANK and BECH-HANSEN 1972; SUDA and UCHIDA 1972; AVNER and GRIFFITHS 1973; AVNER et al. 1973; HOWELL et al. 1973; RANK 1973; TREMBATH et al. 1973; WAKABAYASHI and KAMEI 1973; WOLF, DUJON and SLONIMSKI 1973; NETTER et al. 1974 and COEN, in prep.). The standard cross is performed in such a way that (1) a sample of diploid cells randomly collected at the end of the cross should