GENETIC AND MOLECULAR ANALYSIS OF HYBRIDS IN THE GENUS SACCHAROMYCES INVOLVING SACCHAROMYCES-CEREVISIAE, S-UVARUM AND A NEW SPECIES, S-DOUGLASII

We have studied the phenomenon of infertility of yeast hybrids obtaine d 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 hyb rids from crosses between these species sporulate well but are essenti ally infertile. The rare viable spores, one per 10(4) to 10(5) asci, t hat have been examined carry a complete genome comprised of chromosome s 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 pa iring and disjunction of homologous chromosomes which in most cases re sults in spores with an incomplete set of chromosomes. This apparent l ack of pairing of 'homeologous' chromosomes in meiosis I was analysed in most detail with S. cerevisine/S. nouglasii hybrids. As a genetic t ool we studied frequencies of recombination, taking advantage of an S. douglasii breeding stock of some 50 identified mutations in non-switc hing haploids. Recombination, although markedly reduced, could be obse rved at both the chromosomal and allelic levels, implying a sporadic p airing in meiosis to allow genetic exchange. Meiotic recombination fre quencies were studied for 14 gene pairs and generally found to be redu ced ten-fold. Heteroallelic recombination (gene conversion) frequencie s were measured at 22 loci and were judged to be reduced at least two- to 100-fold. DNA hybridization experiments with S. cerevisiae gene pr obes gave results consistent with low DNA sequence homologies between S. cerevisiae and S. douglasii. Moreover, by chance, our experiments d isclosed another Saccharomyces strain (CBS2908, originally classified as S. cerevisiae) with hybridization patterns identical to S. douglasi i except for the hybridization with the Ty transposon probes. Crosses between CBS2908 and S. douglasii yielded diploid hybrids with 80-90% s pore viability, thus establishing a second member of the S. douglasii species.