Genetic analysis of the relationships between the cell surface and the nuclei in Paramecium tetraurella.

In Paramecium tetraurelia, a number of mutations have been shown to affect simultaneously cortical organization (attachment of trichocysts to the cortex) and nuclear divisions (Ruiz et al. 1976). In order to analyze the genetic and physiological basis of this correlation, and we have isolated new mutations affecting the properties of the trichocysts and studied their genetic relationships with other previously known mutations. Of 24 to 28 loci controlling the biogenesis and properties of the trichocysts, mutations only in the 16 to 20 loci tht control trichocysts attachment to the cortex result in nuclear defects. Cytological observations show that all these mutants display the same set of nucler abnormalities: in particular, rounded shape of the resting macronucleus mispositioning and defective elongation of the dividing macronucleus and unequal repartition of the marco- and micronuclei. This common syndrome is independent of both the mutagenic origin and the mutated locus. Furthermore, by microinjection, it is possible to localize the site of action of the mutations in either the trichocyst compartment or the nontrichocyst compartment. It was found by this technique that the nuclear syndrome is also independent of the site of action of the mutation. All the genetic and physiological data support the conclusion that the nuclear defects are the consequence of the lack of trichocyst attachement to the cortex: in wild-type cells, trichocyst attachment would induce a membranar or perimembranar state necessary for correct nuclear positioning during cell division. In the absence of trichocyst attachment, the cortical control of nuclear division would be abolished. The possible involvement of cytoskeletal links between surface and nuclei is discussed.