Mitosis in Heterobasidiomycetous Yeasts

Abstract

Leucosporidium scottii, for many years considered to be an asporogenous yeast in the genus Candida, is now known to have a heterobasidiomycetous life-cycle. Although morphologically similar to ascomycetous yeasts such as Saccharomyces cerevisiae, budding cells of L. scottii have a strikingly different type of mitosis. In L. scottii, mitosis is not intranuclear as it is in S. cerevisiae. Neither does the nucleus constrict and separate into daughter nuclei in the neck region between mother cell and bud. In L. scottii the chromatin-containing portion of the nucleus moves into the bud before division. There is partial breakdown of the nuclear envelope and the chromatin divides along a spindle apparatus formed inside the bud within remnants of nuclear envelope. A portion of the nucleus containing the nucleolus is left behind in the mother cell and disintegrates there when the envelope breaks down. After division, the nuclear envelope reforms around daughter nuclei and one daughter moves back to the mother cell. These events have been established on the basis of 3 types of observation: on living cells with phase-contrast microscopy; on cells stained for chromatin and studied with light microscopy; and on glutaraldehyde-fixed cells studied with electron microscopy. As in L. scottii, breakdown of nuclear envelope during division has been reported in several mycelial basidiomycetes. The sequence of chromatin movement into the bud of L. Scottii division inside the bud, and return of one daughter nucleus to the mother cell, is closely comparable to the movements of chromatin in and out of clamp connexions known to occur in dikaryotic hyphae of Polystictus versicolor. These mitotic similarities are in accord with the taxonomic relationship of L. scottii to the basidiomycetes. Observations on a cytoplasmic organelle located outside interphase nuclei of L. scottii have convinced us of its considerable importance during mitosis. This structure consists of 2 globular electron-dense ends joined by a bridge-like middle piece. It accompanies the advancing tips of nuclei which enter the bud. After breakdown of the nuclear envelope, the 2 spherical components of this organelle are seen at opposite poles of the mitotic spindle. Therefore we refer to it as the ‘microtubule organizing centre’ (MTOC). We have also speculated about a possible role for the organelle in the regulation of nuclear envelope growth, breakdown, and reformation.