Horizontal transfer of genetic material among Saccharomyces yeasts

The genus Saccharomyces consists of several species divided into the sensu stricto and the sensu late groups. The genomes of these species differ in t he number and organization of nuclear chromosomes and in the size and organ ization of mitochondrial DNA (mtDNA). In the present experiments me examine d whether these yeasts can exchange DNA and thereby create novel combinatio ns of genetic material. Several putative haploid, heterothallic yeast strai ns were isolated from different Saccharomyces species. All of these strains secreted an a- or alpha-like pheromone recognized by S. cerevisiae tester strains. When interspecific crosses were performed by mass mating between t hese strains, hybrid zygotes were often detected. In general, the less rela ted the two parental species were, the fewer hybrids they gave. For some cr osses,viable hybrids could be obtained by selection on minimal medium and t heir nuclear chromosomes and mtDNA were examined. Often the frequency of vi able hybrids was very low. Sometimes putative hybrids could not be propagat ed 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 ease of sensu late hybrids, during genetic stabiliz ation 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 genet ically more or less stable when the genetic material originated from phylog enetically 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 fr agments of the other one, could be found in genetically stabilized hybrid l ines. The results obtained indicate that Saccharomyces yeasts have a potent ial to exchange genetic material. If Saccharomyces isolates could mate free ly in nature, horizontal transfer of genetic material could have occurred d uring the evolution of modern yeast species.