BEHAVIOR OF MITOCHONDRIA, MICROTUBULES, AND ACTIN IN THE TRIANGULAR YEAST TRIGONOPSIS-VARIABILIS

Dimorphic yeast Trigonopsis variabilis is a unique species that can fo rm either an ellipsoidal or a triangular cell depending upon nutrition al conditions. This fluorescence microscopic study was intended to cor relate morphological changes of mitochondria in the triangular cells w ith the distribution of the cytoskeleton. In addition, unique features in the behavior of the cyloskeleton were also examined during triangu lar cell formation. In log-phase cells stained with 4',6-diamidino-2-p henylindole, mitochondrial nucleoids appeared as a string of beads thr oughout the vegetative growth. The profile of mitochondria stained by 3,3'-dihexyloxacarbocyanine iodide showed a network corresponding to t he fluorescence images of mitochondrial nucleoids in both mother and d aughter cells. Cell-cycle-dependent fragmentation of mitochondria was not discerned. As the culture reached stationary phase, a network of m itochondria gradually changed to form unique rings that were located n ear the angles of triangular cells. When examined by immunofluorescenc e microscopy with anti-tubulin antibody, microtubules were found to be well developed along the sides of cells in the cytoplasm of T. variab ilis interphase cells. Although distributions of microtubules and mito chondria are different during cell cycle as a whole, cytoplasmic micro tubules frequently extended along a part of the mitochondria in budded cells, suggesting correlation of microtubules and mitochondria. Rhoda mine-phalloidin staining revealed both actin patches and cables. Actin cables elongated from mother cells into the buds and showed close pro ximity to mitochondria, although complete overlapping of both structur es was rare. Moreover, actin patches localized on the mitochondrial ne twork at a frequency of 65%. These results suggested that actin cables and patches, as well as microtubules, participated in the distributio n of mitochondria. The localization of actin patches separated towards opposite ends at a bud tip when the bud grew to medium size. The uniq ue localization of actin patches is responsible for bi-directional gro wth of the bud, forming triangular cells.