Gene mutations affecting nuclear behaviour in Paramecium tetraurelia

Three Paramecium tetraurelia cell lines with abnormal patterns of nuclear behaviour at cell division and at nuclear reorganization were recovered in separate experiments following nitrosoguanidine mutagenesis. Two of these lines carried non-allelic "tam" mutations, (tam A and tam G). Tarn is a series of recessive genes that exert a pleiotropic effect on macronuclear behaviour during cell division and on the migration and attachment of trichocysts during trichocyst morphogenesis (Ruiz, et al., 1976). At binary fission the macronuclei of homozygous "tam" cells fail to elongate and fail to migrate to the dorsal cortex. This leads to unequal division of the macronucleus or, in extreme cases, complete absence of division of the macronucleus. At cytokinesis, in the latter event, the entire prefission macronucleus is retained by one sister cell, usually the proter. At nuclear reorganization, missegregation of the macronuclear anlagen commonly occurs either at the first or at the second post-reorganizational cell division. Exautogamonts or exconjugants that do not receive a macronuclear anlage at cell division undergo macronuclear regeneration (Sonneborn, 1940; Berger, 1973a). Improper distribution of micronuclei occurs frequently during cell division in cells expressing the "tam" phenotype. The trichocysts of homozygous "tam" cells are abnormally shaped, are unattached and do not discharge. The third variant recovered is unable to complete conjugation owing to abnormalities in the micronuclear events.Science, Faculty ofZoology, Department ofGraduat