Diversity in organization and the origin of gene orders in the mitochondrial DNA molecules of the genus Saccharomyces

Sequencing of the Saccharomyces cerevisiae nuclear and mitochondrial genome s provided a new background for studies on the evolution of the genomes. In this study, mitochondrial genomes of a number of Saccharomyces yeasts were mapped by restriction enzyme analysis, the orders of the genes were determ ined, and two of the genes were sequenced. The genome organization, i.e., t he size, presence of intergenic sequences, and gene order, as well as polym orphism within the coding regions, indicate that Saccharomyces mtDNA molecu les are dynamic structures and have undergone numerous changes during their evolution. Since the separation and sexual isolation of different yeast li neages, the coding parts have been accumulating point mutations, presumably in a linear manner with the passage of time. However, the accumulation of other changes may not have been a simple function of time. Larger mtDNA mol ecules belonging to Saccharomyces sensu stricto yeasts have acquired extens ive intergenic sequences, including guanosine-cytosine-rich clusters, and a pparently have rearranged the gene order at higher rates than smaller mtDNA s belonging to the Saccharomyces sensu late yeasts, While within the sensu stricto group transposition has been a predominant mechanism for the creati on of novel gene orders, the sensu late yeasts could have used both transpo sition- and inversion-based mechanisms.