Dosage of the smallest chromosome affects both the yeast-hyphal transition and the white-opaque transition of Candida albicans WO-1.

The WO-1 strain of Candida albicans is capable of alternating between two highly distinct yeast cell types termed white and opaque (E. H. A. Rikkerrink, B. B. Magee, and P. T. Magee, J. Bacteriol. 170:895-899, 1988; B. Slutsky, M. Staebell, J. Anderson, L. Risen, M. Pfaller, and D. R. Soll, J. Bacteriol. 169:189-197, 1987). We have isolated WO-1 mutants that show a marked deficiency at being able to switch from the white form to the opaque form under conditions normally favorable for this transition. Pulsed-field electrophoresis demonstrated that one of the initial two spontaneous nonswitching mutants lacked the smallest chromosome that is normally present in WO-1. The availability of a WO-1 derivative whose only functional ADE2 gene is located on this small chromosome made possible, through the induction of chromosome nondisjunction, the isolation of numerous new mutants missing this chromosome as well as mutants containing two copies of the chromosome. Mutants missing the smallest chromosome showed a greatly diminished ability to produce opaque sectors and to produce germ tubes in the presence of human serum. Mutants containing two copies of the small chromosome showed an increased ability to produce germ tubes. These results indicate that this small chromosome carries one or more genes involved in both the white-opaque switch and the yeast-hyphal switch.