Defective spatial control in patterning of microtubular structures in mutants of the ciliate Paraurostyla: I....

The development of the primordia of ciliary structures was studied in a single-gene recessive mlm (multi-left-marginal) mutant of the hypotrich ciliate P. weissei. The modifications in morphogenesis consist of excessive production of basal bodies in the left half of the ciliate. In extreme cases, the formation of double sets of oral primordia (OP), production of multiple streaks of the left marginal cirri sometimes intermingled with the OP, and development of more than one streak in the left dorsal bristle rows is observed. All abnormalities in development of ciliary primordia are expressed most profoundly at the middle stages of morphogenesis. In late stages of morphogenesis, the excess basal bodies in the oral primordium are resorbed, and some of the ciliary primordia regress. Thus, the developmental phenotype of the mlm mutants involves a combination of broadening and overlapping of organellar domains in the left half of the ciliate surface layer, with occasional reversal of ciliary structures formed at the border between the two domains. The mlm mutation probably causes perturbations in the cell's ability to maintain discrete boundary conditions around the circumferential dimension.