Cell-cycle control in Caenorhabditis elegans:: how the worm moves from G1 to S

The nematode Caenorhabditis elegans offers a powerful model system to study cell division control during animal development. Progress from the one- cell zygote to adult stage follows a nearly invariant pattern of divisions. This, combined with a transparent body and efficient genetics, allows for sensitive identification and quantitative analysis of cell- cycle mutants. Nearly all G1 control genes identified in C. elegans have mammalian homologs. Examples include a D- type cyclin and CDK4/ 6- related kinase, a member of the retinoblastoma protein family and CDK inhibitors of the Cip/ Kip family. Genetic studies have placed the currently known G1 regulators into pathways similar to those in mammals. Together, this validates the use of C. elegans in identifying additional regulators of cell- cycle entry and exit. For instance, we recently found that the CDC- 14 phosphatase promotes maintenance of the quiescent state. Here, we describe cell-cycle control as an integral part of C. elegans development, summarize current knowledge of G1 control genes in the worm, compare the results with those obtained in other species, and discuss the possible implications of cellcycle studies in C. elegans for higher organisms, including humans.