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.