Centrosome duplication is a critical step in assembly of the bipolar mitotic spindle, but the molecular mechanisms regulating this process during the cell cycle and during animal development are poorly understood. Here, we report that the zyg-1 gene of Caenorhabditis elegans is an essential regulator of centrosome duplication. ZYG-1 is a protein kinase specifically required for daughter centriole formation that localizes transiently to centrosomes and acts at least one cell cycle prior to each spindle assembly event. In the embryo, ZYG-1 participates in a unique regulatory scheme whereby paternal ZYG-1 regulates duplication and bipolar spindle assembly during the first cell cycle, and maternal ZYG-1 regulates these processes thereafter. ZYG-1 is therefore a key molecular component of the centrosome/centriole duplication process.

Centrosomes are major determinants of mitotic spindle structure, but the mechanisms regulating their behavior remain poorly understood. The spd-2 gene of C. elegans is required for centrosome assembly or "maturation." Here we show that spd-2 encodes a coiled-coil protein that localizes within pericentriolar material (PCM) and in the immediate vicinity of centrioles. During maturation, SPD-2 gradually accumulates at the centrosome in a manner that is partially dependent on Aurora-A kinase and cytoplasmic dynein. Interestingly, SPD-2 interacts genetically with dynein heavy chain and SPD-5, another coiled-coil protein required for centrosome maturation. SPD-2 and SPD-5 are codependent for localization to the PCM, but SPD-2 localizes to centrioles independently of SPD-5. Surprisingly, we also find that SPD-2 is required for centrosome duplication and genetically interacts with ZYG-1, a kinase required for duplication. Thus, we have identified SPD-2 as a factor critical for the two basic functions of the centrosome-microtubule organization and duplication.

We propose a limiting component hypothesis, in which the volume of the cell sets centrosome size by limiting the total amount of centrosome components. This idea could be a general mechanism for setting the size of intracellular organelles during development.