The genus Saccharomyces consists of several species divided into the sensu stricto and the sensu lato groups. The genomes of these species differ in the number and organization of nuclear chromosomes and in the size and organization of mitochondrial DNA (mtDNA). In the present experiments we examined whether these yeasts can exchange DNA and thereby create novel combinations of genetic material. Several putative haploid, heterothallic yeast strains were isolated from different Saccharomyces species. All of these strains secreted an a- or α-like pheromone recognized by S. cerevisiae tester strains. When interspecific crosses were performed by mass mating between these strains, hybrid zygotes were often detected. In general, the less related the two parental species were, the fewer hybrids they gave. For some crosses, viable hybrids could be obtained by selection on minimal medium and their nuclear chromosomes and mtDNA were examined. Often the frequency of viable hybrids was very low. Sometimes putative hybrids could not be propagated at all. In the case of sensu stricto yeasts, stable viable hybrids were obtained. These contained both parental sets of chromosomes but mtDNA from only one parent. In the case of sensu lato hybrids, during genetic stabilization one set of the parental chromosomes was partially or completely lost and the stable mtDNA originated from the same parent as the majority of the nuclear chromosomes. Apparently, the interspecific hybrid genome was genetically more or less stable when the genetic material originated from phylogenetically relatively closely related parents; both sets of nuclear genetic material could be transmitted and preserved in the progeny. In the case of more distantly related parents, only one parental set, and perhaps some fragments of the other one, could be found in genetically stabilized hybrid lines. The results obtained indicate that Saccharomyces yeasts have a potential to exchange genetic material. If Saccharomyces isolates could mate freely in nature, horizontal transfer of genetic material could have occurred during the evolution of modern yeast species.

~~Several yeast species/isolates belonging to the genus Saccharomyces were examined for the organization of their mtDNAs and ability to generate petite mutants. A general characteristic for all of the mtDNAs tested was that they were very A+T-rich. However, restriction patterns and inducibility of petite mutations revealed a great diversity in the organization and genetic behaviour of mtDNAs. One group of yeasts, Saccharomyces sensu stricto, contains mtDNA ranging in size from 64 to 85 kb. mtDNAs from these yeasts contain a high number of restriction sites that are recognized by the enzymes Haelll and Mspl, which cut specifically in G+C clusters. There are three to nine o r i h p sequences per genome. These yeasts spontaneously generate respiration deficient mutants. Ethidium bromide (Et-Br), a t low concentrations, induces a majority of cells to give rise to petites. A second group of yeasts, Saccharomyces sensu lato, contains smaller mtDNAs, ranging in size from 23 to 48 kb, and probably only a f e w intergenic G+C clusters and no ori/rep sequences. These yeasts also generate petite clones spontaneously, but Et-Br, even when present a t high concentrations, does not substantially increase the frequency of petites. In most petite clones from these yeasts only a small fragment of the wild-type molecule is retained and apparently multiplied. A third group, represented by Saccharomyces kluyveri, does not give rise to petite mutants either spontaneously or after induction.Keywords : yeast, mitochondrial genome, petite mutation, intergenic sequences, taxonomy INTRODUCTIONA remarkable aspect of mitochondrial genomes of all organisms is that they contain a very similar set of genes. On the other hand, mtDNA molecules among diverse species are highly variable in size and organization (5,36). The yeast Saccharomyces cerevisiae has played the central role in studies of mtDNA heredity (for reviews, see 9, 28). This is due mainly to the property that this yeast is a facultative anaerobe; i.e. it can survive without active mitochondria. The average cell of S. cerevisiae contains as many as 50 mtDNA molecules, but the number varies J. Piikur and others almost half of the genome, and they are usually separated from each other by over 200 G + C clusters (38). A large fraction of G + C clusters contains recognition sites for restriction enzymes splitting target sequences which contain only G and C residues, i.e.HaeIII and HpaIIIMspI (38). A special class of G + C clusters are orilrep sequences which are about 300 bp long and are present in eight copies scattered around the genome (4,40). G + C clusters can be grouped into eight families which presumably originated from a proto-G+ C cluster (38). Therefore, mtDNA contains a variety of short duplications which potentially can be involved in intramolecular recombination.S. cerevisiae spontaneously produces mutants, petites, which are deficient in the ability to respire aerobically. The spontaneous frequency is about 1 %, but upon induction with chemical mutagens, e.g. ethidium bro...

An improved pulsed-f ield electrophoresis program was developed to study differently sized chromosomes within the genus Saccharomyces. The number of chromosomes in the type strains was shown to be nine in Saccharomyces castellii and Saccharomyces dairenensis, 12 in Saccharomyces servazzii and Saccharomyces unisporus, 16 in Saccharomyces exiguus and seven in Saccharomyces kluyveri. The sizes of individual chromosomes were resolved and the approximate genome sizes were determined by the addition of individual chromosomes of the karyotypes. Apparently, the genome of 5. exiguus, which is the only Saccharomyces sensu lato yeast to contain small chromosomes, is larger than that of Saccharomyces cerevisiae. On the other hand, other species exhibited genome sizes that were 10-25% smaller than that of 5. cerevisiae. Well-defined karyotypes represent the basis for future genome mapping and sequencing projects, as well as studies of the origin of the modern genomes.