Evolution of mitochondrial genomes in yeast: a study of mitochondrial divergence in two closely related species, Saccharomyces douglasii and Saccharomyces cerevisiae.

The informational content of the mitochondrial (mt) genome of yeasts is remarkably constant, although the size of the mtDNA and the overall organization of genes can vary among species belonging to the same genus (Clark-Walker, McArthur and Sriprakash 1983). A variability even between strains of the same species is observed in the internal organization of mosaic genes like the CYTb and LSU-rRNA genes (Clark-Walker 1992). The high degree of plasticity of yeast mtDNA thus offers a favorable framework for tracing events that underlie mt gene rearrangement. To this end we compared the mt genome organization in the two closely related yeast species Saccharomyces cerevisiae (S.C.) and Saccharomyces douglasii (S.d.); in particular, we focused our attention on the structure of the intergenic regions that might have been involved in gene rearrangements between the two genomes. S.C. and S.d. are thought to have diverged between 50 and 80 MYA based on the nucleotide sequence analysis of their nuclear genes (Herbert et al. 1988a, 1988b). Their mt genomes are of similar size and contain the same genes; exon sequences are highly conserved, with a frequency of nucleotide substitutions lower than 2%. The order of the genes, which otherwise has been found to be highly variable in yeast mt genomes, is conserved, except for the different location of a DNA segment of 15 kb spanning the COXZZZ and SW-rRNA genes and the inversion of a part of this segment (Tian et al. 1991). Other important differences lie in the number, structure, and position of both the introns and the short G+C-rich sequences known as G+C-rich clusters. In this work we analyze the endpoint structure of the regions which are differently located in the two genomes. We made use of the following strains: the S. dougZasii wild-type strain 4707/22D (aaa HO GAL SUC mal ura3 his4 adel Zeu2); the SD12 cytoductant containing the respiratory competent nuclear genome of S.C. strain CKOl (a Zeul karl canR rho(‘) and the respiratory competent mt genome of S.d. (Kotylak, Lazowska, and Slonimski 1985); the rhostrain KS 158/3-2ZBIE2 obtained by ethidium bromide mutagenesis from strain KS 158/32, a diuron-resistant derivative of SD12 (Lazowska,