Potential sexuality hidden in the imperfbct yeast

Abstract

Summary: The human pathogenic fungus Candida albicans has traditionally been classified as a diploid, asexual organism. In 1986, a phenomenon of ploidy shift was described by the author in this organism. A clinical isolate NUM51 contained both diploid and tetraploid cells and an electron microscopy of the culture showed that some diploid cells seemed to be undergoing endomitosis to shift up their ploidy level from diploid to tetraploid, while some tetraploids showed nuclear structures similar to those of meiosis II in S. cerevisiae, suggesting the occurrence of reductional nuclear division to shift down from tetraploid to diploid. The phenotype of these strains executing the ploidy-shift was named Sps-. Both high frequency of the occurrence of chromosome reorganization and of the loss of heterozygosity were found due to the existence of high-ploidy cells in the cell population of the C. albicans Sps-strains. Recently, a set of C. albicans MTL genes has been identified that corresponds to the master sexual cycle regulators al, alphal, and alpha2 of the Saccharomyces cerevisiae mating-type(MAT)locus and mating-competent forms of the organism were recently described that produced tetraploid mating products. The Sps-strain NUM51 was found heterozygous for the two mating types MTLa and MTLx by colony-PCR. When mutant strains containing only MTLx were constructed by the loss of one homolog of chromosome 5, such strains no longer showed the occurrence of ploidy shift. This finding suggests that the ploidy shift may be under the control of the heterozygous MTLa/MTLx locus.