A CYTOPLASMIC DYNEIN REQUIRED FOR MITOTIC ASTER FORMATION IN-VIVO

An astral pulling force helps to elongate the mitotic spindle in the f ilamentous ascomycete, Nectria haematococca, Evidence is mounting that dynein is required for the formation of mitotic spindles and asters, Obviously, this would be an important mitotic function of dynein, sinc e it would be a prerequisite for astral force to be applied to a spind le pole. Missing from the evidence for such a role of dynein in aster formation, however, has been a dynein mutant lacking mitotic asters. T o determine whether or not cytoplasmic dynein is involved in mitotic a ster formation in N. haematococca, a dynein-deficient mutant was made. Immunocytochemistry visualized few or no mitotic astral microtubules in the mutant cells, and studies of living cells confirmed the veracit y of this result by revealing the absence of mitotic aster functions i n vivo: intra-astral motility of membranous organelles was not apparen t; the rate and extent of spindle elongation during anaphase B were re duced; and spindle pole body separation almost stopped when the anapha se B spindle in the mutant was cut by a laser microbeam, demonstrating unequivocally that no astral pulling force was present. These unique results not only provide a demonstration that cytoplasmic dynein is re quired for the formation of mitotic asters in N. haematococca; they al so represent the first report of mitotic phenotypes in a dynein mutant of any filamentous fungus and the first cytoplasmic dynein mutant of any organism whose mitotic phenotypes demonstrate the requirement of c ytoplasmic dynein for aster formation in vivo.