Genetic and molecular analysis of hybrids in the genus Saccharomyces involving S. cerevisiae, S. uvarum and a new species, S. douglasii

Abstract

We have studied the phenomenon of infertility of yeast hybrids obtained with physiological conditions under the control of compatible mating systems. The yeasts investigated are three Saccharomyces species: S. cerevisiae, S. uvarum and a new species, S. douglasii. The diploid hybrids from crosses between these species sporulate well but are essentially infertile. The rare viable spores, one per 10(4) to 10(5) asci, that have been examined carry a complete genome comprised of chromosomes contributed by both parents but invariably have extra chromosomes, i.e. they are generally disomic for at least two or three chromosomes. This observation is consistent with a failure, in meiosis I, of the pairing and disjunction of homologous chromosomes which in most cases results in spores with an incomplete set of chromosomes. This apparent lack of pairing of 'homeologous' chromosomes in meiosis I was analysed in most detail with S. cerevisiae/S. douglasii hybrids. As a genetic tool we studied frequencies of recombination, taking advantage of an S. douglasii breeding stock of some 50 identified mutations in non-switching haploids. Recombination, although markedly reduced, could be observed at both the chromosomal and allelic levels, implying a sporadic pairing in meiosis to allow genetic exchange. Meiotic recombination frequencies were studied for 14 gene pairs and generally found to be reduced ten-fold. Heteroallelic recombination (gene conversion) frequencies were measured at 22 loci and were judged to be reduced at least two- to 100-fold. DNA hybridization experiments with S. cerevisiae gene probes gave results consistent with low DNA sequence homologies between S. cerevisiae and S. douglasii. Moreover, by change, our experiments disclosed another Saccharomyces strain (CBS2908, originally classified as S. cerevisiae) with hybridization patterns identical to S. douglasii except for the hybridization with the Ty transposon probes. Crosses between CBS2908 and S. douglasii yielded diploid hybrids with 80-90% spore viability, thus establishing a second member of the S. douglasii species.

Spatial organisation and behaviour of the parental chromosome sets in the nuclei of Saccharomyces cerevisiae × S. paradoxus hybrids

“…In the two yeast species analysed, DNA reassociation kinetics had suggested the nuclear DNA heterology to be as high as ~50% (Vaughan Martini, 1989). On the basis of DNA hybridisations the sequence divergence between S. cerevisiae and S. douglasii (which is regarded as a variety of S. paradoxus) was estimated to be 30-40% (Hawthorne and Philippsen, 1994). By contrast, the ARG4 and the YSD83 coding regions of S. paradoxus differ from their S. cerevisiae homologs only by 8.1% and 12.5%, respectively.…”

“…S. douglasii) (Naumov and Naumova, 1990) are closely related yeast species with an estimated genome divergence of about 8-20% determined by DNA sequence comparison of certain coding and noncoding sequences (Herbert et al, 1988;Adjiri et al, 1994;Chambers et al, 1996). Natural and artificial hybrids are viable, but practically sterile (Hawthorne and Philippsen, 1994;. Nevertheless, rare progeny must occur since natural introgression was observed (Naumov et al, 1997).…”

Lager Yeast Comes of Age

“…Based on the lack of apparent (or effective) prezygotic barriers, it has been proposed that speciation in Saccharomyces is ensured by postzygotic barriers preventing sporulation or the generation of viable spores. One such barrier is sequence divergence that interferes with recombination during meiosis (114)(115)(116). On the other hand, even single chromosomal translocations can contribute to reproductive isolation (117).…”

Evolutionary Role of Interspecies Hybridization and Genetic Exchanges in Yeasts

“…Molecular characterization of wine, beer, or cider yeasts revealed numerous independently formed hybrids between S. cerevisiae and S. kudriavzevii (8,9,22,57,63,64,120,121,164,198), between S. cerevisiae and S. bayanus (60,78,106,133,134,141,146,175,198), and even (triple hybrids) between S. cerevisiae, S. kudriavzevii, and S. bayanus (18,32,62). Such hybrids often exhibit more robust characteristics than the parental strains, such as tolerance to various stresses induced by artificial fermentations (11,174,175,216).…”

“…Hybrids have also been observed in natural environments (78,164,238). Indeed, it appears so universal between Saccharomyces yeasts that even some type strains, normally used to define the species, may eventually prove to be hybrids themselves after more extensive analysis.…”