Topology of microtubules and actin in the life cycle of Xanthophyllomyces dendrorhous (Phaffia rhodozyma)

The morphology of budding and conjugating cells and associated changes in m icrotubules and actin distribution were studied in the yeast Xanthophyllomy ces dendrorhous (Phaffia rhodozyma) by phase-contrast and fluorescence micr oscopy. The non-budding interphase cell showed a nucleus situated in the ce ntral position and bundles of cytoplasmic microtubules either stretching pa rallel to the longitudinal cell axis or randomly distributed in the cell; n one of these, however, had a character of astral microtubules. During mitos is, the nucleus divided in the daughter cell, cytoplasmic microtubules disa ppeared and were replaced by a spindle. The cytoplasmic microtubules reappe ared after mitosis had finished. Actin patches were present both in the bud and the mother cell. Cells were induced to mate by transfer to ribitol- co ntaining medium without nitrogen. Partner cells fused by conjugation projec tions where actin patches had been accumulated. Cell fusion resulted in a z ygote that produced a basidium with parallel bundles of microtubules extend ed along its axis and with actin patches concentrated at the apex. The fuse d nucleus moved towards the tip of the basidium. During this movement, nucl ear division was taking place; the nuclei were eventually distributed to ba sidiospores. Mitochondria appeared as vesicles of various sizes; their larg e amounts were found, often lying adjacent to microtubules, in the subcorti cal cytoplasm of both vegetative cells and zygotes.