Dissection of cell division processes in the one cell stage Caenorhabditiselegans embryo by mutational analysis

To identify novel components required for cell division processes in comple x eukaryotes, we have undertaken an extensive mutational analysis in the on e cell stage Caenorhabditis elegans embryo. The large size and optical prop erties of this cell permit observation of cell division processes with grea t detail in live specimens by simple differential interference contrast (DI C) microscopy. We have screened an extensive collection of maternal-effect embryonic lethal mutations on chromosome III with time-lapse DIC video micr oscopy. Using this assay, we have identified 48 mutations in 34 loci which are required for specific cell division processes in the one cell stage emb ryo. We show that mutations fall into distinct phenotypic classes which cor respond, among others, to the processes of pronuclear migration, rotation o f centrosomes ansi associated pronuclei, spin die assembly, chromosome segr egation, anaphase spindle positioning, and cytokinesis. We have further ana lyzed pronuclear migration mutants by indirect immunofluorescence microscop y using antibodies against tubulin and ZYG-9, a centrosomal marker. This an alysis revealed that two pronuclear migration loci are required for generat ing normal microtubule arrays and four for centrosome separation. All 34 lo ci have been mapped by deficiencies to distinct regions of chromosome III, thus paving the way for their rapid molecular characterization. Our work co ntributes to establishing the one cell stage C. elegans embryo as a powerfu l metazoan model system for dissecting cell division processes.