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Mutational analysis of cell development in Paramecium tetraurelia Jones, Donald


Temperature-sensitive mutants have been used in this study to examine development in Paramecium tetraurelia. Ten mutations affecting development were described. Since two of the mutants were allelic, the effects of nine genes on Paramecium development were studied. Of these, two affect formation of the fission-zone (called dfz or defective fission-zone mutants), five affect constriction of the fission-furrow (called dc or defective constriction mutants), and two produce a reduction in cell size (called sm or small mutants). Morphometric measurements were made on inter-fission and dividing wild-type and mutant cells to examine two aspects of Paramecium development: changes in cell shape and size which preceed and accompany cell division and positioning of new structures on the cell surface during cell division. This analysis suggested the following hypotheses: 1. The shape and size changes which preceed and accompany cell division in Paramecium are causally related to cell division. Defective constriction mutants undergo exaggerated contraction prior to fission-furrow formation. This contraction appears to interfere with the decrease in cell width which ordinarily occurs during division. Although the mutant cells are able to make a normal amount of furrow surface, the abnormal cell width prevents furrow completion. Premature contractions seen in dfz mutants similarly interfere with furrow formation. 2. Surface growth in Paramecium is dependent on prior basal body proliferation. Basal bodies appear to act as organizing centres for surface growth. Reduced basal body proliferation in mutant cells was always associated with reduced surface growth: There was a consistent relationship between the number of new basal bodies produced proceeding cell division and the amount of surface growth which occurred. The order of the causal relationship was suggested by the observation that basal body proliferation was completed prior to the beginning of surface growth. 3. The positioning of new structures during cell division in Paramecium can be affected by the pre-existing cell shape, size, or structure. This model, called mechanical guidance, was based on observations of the movement and positioning of the vestibule (the opening leading to the mouth) in wild-type and mutant cells. This model was discussed in relation to other developmental mechanisms proposed to account for protozoan morphogenesis.

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