ABSTRACT
We have studied the ultrastructure of interphase and developing cells of mutants (mlm and mlmlpl) of Paraurostyla weissei that overproduce basal bodies in certain cortical domains and bring about coordinate modifications of the overall pattern [8, 25, 26]. In interphase cells, basal bodies and accessory microtubular organelles appear normal. In developing cells, overgrowth of adorai primordia, multiplication of ciliary primordia, differentiation of dorsal bristles on the ventral surface, and reversal of the orientation of oral and cirral promordia occur in spite of apparently normal assembly events of all studied ciliary structures. The three kinds of basal body pairs (paroral, adorai and somatic) have the same ultrastructure as in wild-type cells. Abnormal orientation of pairs occurs during the first wave of basal body proliferation. In the second wave, the new basal bodies complete new cirri and membranelles regardless of their orientation. As in wt cells, there is no second wave of basal body proliferation in the paroral membranes. Early steps of elimination of one category of cirri (FC) involve small defects in basal body assembly during the first wave of basal body proliferation. Thus modifications in the ciliary pattern in the mlm/pl mutants result from overduplication, abnormal location and orientation of basal bodies taking place during the first half of the process of morphogenesis in this ciliate.
