CYTOCHALASIN-D BLOCKS CHROMOSOMAL ATTACHMENT TO THE SPINDLE IN THE GREEN-ALGA OEDOGONIUM

Mitosis in living cells of Oedogonium observed by time-lapse, was bloc ked by cytochalasin D (CD; 25-100 mu g/ml). Normal prometaphase to ana phase takes 10-15 min; blockage of entry into anaphase by CD was rever sible up to 2-2.5 h in CD and washout was followed within 10-20 min by normal anaphase and cytokinesis. After 3-6 h in CD, unseparated chrom atids segregated randomly into two groups as the spindle slowly elonga ted considerably, becoming distorted and twisted. During this ''pseudo anaphase'', chromatids sometimes split irregularly and this was stimul ated by late washout of CD. CD affected chromosomal attachment to the spindle. If applied al prophase and prometaphase, spindle fibres enter ed the nucleus; chromosomes moved vigorously and irregularly. A few ac hieved metaphase only briefly. Treatment at metaphase caused chromosom es to irregularly release and after random movement, all slowly gather ed at either pole. Upon removal of CD, chromosomes rapidly achieved me taphase and anaphase A and B soon followed. If CD took effect during a naphase, chromatids detaching from the spindle oscillated rapidly alon g it; anaphase and cytokinesis (phycoplast formation) were delayed as the cell attempted to correct for abnormal chromosomal behaviour. Thus , CD prevents normal kinetochore attachment to the spindle and actin m ay be the target for this response.