Ploidy, ascus formation and recombination in Torulaspora (Debaryomyces) hansenii.

X-ray inactivation studies on the type strain of Torulaspora hansenii carried out to determine ploidy, provided proof that the species has a haplontic life cycle, a fact which hitherto has only been presumed. Observations on the genesis of the ascus by light microscopy and transmission electron microscopy provide no evidence for, what some earlier workers in this field have presumed to be, heterogamous conjugation between a mother cell and its bud. They do, however, show that asci, bearing obliquely-attached, vestigal, bud-like appendages, arise from some cells to form single, non-abstricting, and frequently, recurving protuberances which enlarge. These could, conceivably, be responsible for the impression that abstricted buds are connected to the mother-cells by bent copulatory tubes. The formation during sporulation of elongated protuberances and the presence of a medial, electron-dense line within the electron-translucent layer of the walls of ascospores fixed with OsO4 preclude the possibility of using these features to differentiate between the genera Torulaspora and Debaryomyces. Furthermore, recombinant studies, which involved the use of auxotrophic mutants, indicated that during sporulation the fusion of independent cells accounted for only 0.03-0.6% of the asci formed. The conclusion was reached that somatogamous autogamy must be the main agency of diploidization and that the species is largely inbreeding.

Regulation of the potassium to sodium ratio and of the osmotic potential in relation to salt tolerance in yeasts.

By using the isotope pairs (22)Na-(24)Na and (42)K-(86)Rb, the uptake and retention of Na and K was studied in the salt-tolerant Debaryomyces hansenii and in the less tolerant Saccharomyces cerevisiae at NaCl levels of 4 mm and 0.68, 1.35, and 2.7 m in the medium. The ratio of K to Na is much higher in the cells than in the media, and higher in D. hansenii than in S. cerevisiae under comparable conditions. The difference between the two species is due to a better Na extrusion and a better uptake of K in D. hansenii. The kinetics of ion transport show that at about the time when extrusion of Na could be demonstrated in D. hansenii, K-Rb previously lost to an easily washable compartment of the cells was reabsorbed in both organisms. More H(+) was given off from S. cerevisiae than from D. hansenii in the course of these events. The findings fit the working hypothesis tested, which regards salt tolerance as partly dependent on the ability to mobilize energy to extrude Na from the cells and to take up K. The volume changes in S. cerevisiae are greater and are more slowly overcome than those in D. hansenii. The total salt level of the cells is not sufficient to counteract the osmotic potential of the medium, so that additional osmoregulatory mechanisms must be involved in determining halotolerance.