During development of multicellular organisms, cell divisions need to be coordinated with the developmental program of the entire organism. Although the mechanisms that drive cells through the division cycle are well understood, very little is known about the pathways that link extracellular signals to the cell-intrinsic cell-cycle machinery. We used the nematode Caenorhabditis elegans as a model system to study the control of cell-cycle progression during development of a multicellular organism. C. elegans provides a unique animal model in which developmental controls of cell-division can be identified genetically. Cell divisions can be followed in vivo by light microscopy, and the nearly invariant cell-lineage of C. elegans has been completely described. In addition, redundancy between genes is limited in C. elegans. Finally, powerful genetic techniques are available in C. elegans to study gene function.\ud To use C. elegans as a model system to identify developmental regulators of cell-cycle progression, we first needed to identify the basic cell-cycle machinery in C. elegans. We demonstrated that ncc-1 CDK1 is required specifically for entry into mitosis and progression through meiosis. In addition, an ortholog of CDK4/CDK6 kinases (cdk-4), and a D-type cyclin (cyd-1) are required for G1 progression of postembryonic somatic cells. Furthermore, a homolog of the mammalian Cip/Kip kinase inhibitors, cki-1, was shown to negatively regulate cell-cycle progression. Finally, we determined the role of lin-35 Rb, efl-1 E2F and dpl-1 DP in G1 progression and found that lin-35 and efl-1 are negative regulators of G1 progression while dpl-1 has aspects of both a positive regulator and a negative regulator. In each case, the C. elegans cell-cycle gene acts in a similar fashion as its mammalian counterpart.\ud These results established C. elegans as an attractive genetic system in which to study cell-cycle regulation, as results obtained in C. elegans will likely be applicable to cell-cycle regulation in mammals. The major strength of studies in C. elegans is the ability to use genetic techniques to identify novel genes. We performed several screens to identify novel regulators of G1 progression. \ud The cell-cycle regulators lin-35 Rb, efl-1 E2F and dpl-1 Dp are also members of the synthetic Multivulva (synMuv) gene family, which regulates vulval cell fate specification. Using a reverse genetic approach, we identified a role for three additional synMuv genes (lin-9 Aly, lin-15B and lin-36) as negative regulators of G1 progression. These genes all encode proteins for which no function in G1 regulation had previously been described.\ud In addition to this reverse getic approach, we performed classical genetic screens to identify novel targets of cyd-1 and genes that cooperate with lin-35 Rb during development of C. elegans. One of the identified mutations likely defines a gene that acts downstream of cyd-1. In addition, we have already identified mutations whose phenotype is rescued by loss of lin-35, and mutations that are lethal only when combined with inactivation of lin-35. These preliminary results show that these screens have great potential to identify novel cell-cycle regulators
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