The bipolar nature of the mitotic spindle is essential to the proper segregation of chromosomes. In large part, the structure is determined by the centrosomes which organize microtubules and form the poles of the spindle. To ensure bipolarity of the spindle, the centrosome must be duplicated once each cell cycle so that two centrosomes are present at mitosis. Defects in either the duplication or the microtubule organizing function of centrosomes can affect spindle structure, and ultimately, chromosome segregation. Despite the importance of the centrosome, much remains to be learned about how it functions and how its duplication is precisely controlled. Recently, we have identified two genes required for centrosome duplication and function in Caenorhabditis elegans. Mutations in the
zyg-1 gene block centrosome duplication in early embryos and lead to the formation of monopolar mitotic spindles. Curiously, some of the alleles also cause a strikingly different phenotype; fertilized eggs often contain multiple centrosomes, possibly as a result of defects in spermatogenesis. These observations suggest that
zyg-1 plays an essential, and perhaps complex, role in centrosome duplication. We have initiated attempts to clone
zyg-1 and have obtained rescue with the cosmid F59E12. Using the RNAi gene silencing technique, we have identified a predicted open reading frame that likely corresponds to
zyg-1. Determination of the sequence of several of the mutant alleles will be necessary to confirm the identity of this gene. We are also studying a second gene
spd-2, which is required for normal microtubule organization and spindle morphology.
spd-2 mutants possess abnormal arrays of microtubules. By immunofluorescence microscopy, centrosome-specific markers fail to localize properly suggesting a defect in centrosome function. As the
zyg-1 and
spd-2 genes appear to function in most tissues and likely encode key centrosomal proteins, continued analysis of these genes should provide a clearer understanding of centrosome duplication and function.