Cytoskeletal interactions with post-mitotic migrating nuclei in the oyster mushroom fungus, <i>Pleurotus ostreatus</i>: evidence against a force-generating role for astral microtubules

Abstract

ABSTRACT Nuclei of Pleurotus ostreatus migrate in a highly predictable manner foflowing conjugate mitosis at clamp connections. Parameters were determined by observations in living hyphae and these data were used to predict the living behaviour of freeze-substituted nuclei. Three of four classes of nucleus migrate immediately after telophase and move at similar speeds. Freeze-substitution electron microscopy shows that these nuclei have prominent nucleus-associated organelles (NAOs) with large astral microtubule (MT) arrays. Although the NAOs do not have a consistent position with respect to the nuclear motion, they are preferentially located near the hyphal axis. The fourth class of nucleus remains in the developing clamp until it fuses with the main hypha, whereupon it migrates to its interphase position at a rate much faster than the other classes. This class of nucleus has a small NAO and no astral MTs. Treatment of hyphae with a MT-disrupting drug, MBC, reduced the astral arrays in the first three classes but did not slow their rate of movement. Moreover, serial section analysis of drug-treated nuclei whose migration rate at the time of fixation was known showed no relationship, positive or negative, between astral MT number and rate of movement. These data suggest that astral MTs neither generate nor transduce force for post-mitotic nuclear migration in P. ostreatus. The rofe of astral MTs and possible mechanisms of postmitotic nuclear migration are discussed.