MOLECULAR MECHANISM OF PLOIDY-SHIFT AND VARIATION OF COLONY MORPHOLOGY IN THE IMPERFECT YEASTS

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Some strains of Candida albicans poroduce a proportion of the cell population with a higher ploidy than the rest in normal diploid state. This phenotype was named as Pld^-by the author. Ploidy-shift was observed between two different ploidy states. Pld-STN21 (arg4, Pld^-) was isolated from a weakly growing sector of a half-sectored colony, after UV-irradiation of STN14, an arg4 derivative of C.albicans CBS5736 and from the other normally growing sector, STN22 (arg4, Pld^+) was isolated. STN22, as well as its progenitor, CBS5736, showed a homogeneus colony size when plated out, but STN21 showed a characteristic heterogeneity of colony size on YPD plates at an early stage of incubation : two types of colonies, small ones and large ones, were observed.The chromosomes of these strains were separated out by PFGE.Some culture of STN21 showed the single band of chromosome 7, suggesting due to the occurrence of chromosome non-disjunction. The other culture of STN21 gave an extrachromosomal band, the size of which was larger than that of chromosome 7, and the loss of one of the two bands of chromosome 7. Southern blot hybridization experiments showed the existence of a SfiI fragment 7F,a central portion of chromosome 7, on the extrachromosomal band but the absence of a terminal SfiI fragment 7C.Protoplast fusion experiments showed, at least, two complementation groups of PLD,the function of which may stabilize both ploidy state and chromosome fidelity.