GENETIC MOSAICS OF ncl-1 AND ZYGOTIC LETHAL MUTATIONS.

Abstract

We have been characterizing spontaneous somatic duplication losses in animals of genotype dpy-l ncl-l; sDp3(IIIo[dpy-l (+) ncl-l (+)]. Six amphidial neurons (ASKL, ADLL, ASIL, ASKR, ADLR and ASIR) of non- Dpy animals were screened for the presence of enlarged nucleoli caused by the recessive, cell-autonomous ncl-l mutation. When a Ncl cell was found, related cells were scored in order to specify the precise nature of duplication loss. Among a total of 100 events, 93 appeared to be simple duplication losses; in the remaining 7 cases, duplication loss by one cell was followed by duplication loss by a niece of that cell, as if a failure in the replication or segregation of the duplication was repeated in a succeeding cell cycle. The frequency of duplication loss was approximately constant throughout the lineage at about 1/440 per cell division at 20 C. We have also been conducting mosaic analyses of lethal mutations, using animals of genotype dpy-l ncl-l let; sDp3,where the duplication carries let(+). Both pat-3(rhS4) and let(rhS9) appear to be lethal when all body muscle cells are mutant, but for both mutations sizeable mutant clones can be found in viable animals. For example, an rhS4 mosaic in which the body muscle cells derived from D are mutant has a right-handed helical twist in that part of the body where the mutant body muscle cells are situated; the mutant muscle cells are viable but fail to spread and form a myofilament lattice. Other cell autonomous defects observed in rhS4 mosaics are in the anal depressor muscle ( leading to defecation defects), the sex myoblasts, the somatic gonad, and the excretory cell (which fails to extend its canals along the hypodermis but instead retains convoluted canals within the cell body). AB.p(-) mosaics of rhS9 are severely uncoordinated (but fertile). Mutant neuron cell bodies are rounded, less adhesive than normal, and have many cytoplasmic granularities. PLM axons are normal in trajectory but much thinner than normal. Some AB.p(-) mosaics were sectioned for electron microscopy; mutant neuron somata exhibited characteristic membrane inclusions that may be associated with the lysosomal system and suggest an analogy to neuron storage diseases. We have been using gamma rays to promote the fusion of sDp3 and nonhomologous free duplications, with the aim of exploiting the virtues of ncl-l as a cell marker for mosaic analysis of unlinked genes. The fusions that have been generated so far appear to be inconveniently stable mitotically, probably because of their relatively large size; we are therefore making smaller derivatives of sDp3 and fused duplications.