A novel F1 screen for mutants in cell cycle checkpoints. E.A. Silva , L.S. Caceres , R. Patel , D. Renouf , B.R. Vilay , H.Y. Yu , S.D. Campbell. Biological Sciences, University of Alberta, Edmonton, Alberta, Canada.

   Proper regulation of the cell cycle is necessary for the development and survival of all organisms. Failure to do so can result in a number of abnormalities, from birth defects to tumor formation. Signaling pathways called checkpoints, which are responsive to environmental and developmental cues, ensure that cell cycle events are properly coordinated and prevent cells with damaged or incompletely replicated genomes from entering mitosis. A number of genes involved in cell cycle checkpoints are well characterized in yeast, however their functions are not well understood in multicellular organisms. We have undertaken a screen, using EMS mutagenesis, to identify mutations in genes required for cell cycle checkpoints that are responsive to blocks in DNA replication in Drosophila melanogaster. We have begun with examination of three of the five large chromosome arms using a screen adapted from Stowers and Schwarz (1999)¹, which utilizes the FLP/FRT system to generate eye specific mitotic clones. In this screen, recombination between homologous chromosomes is induced only in the precursor eye cells of the F1 progeny. The presence of eye promoter-driven cell death genes on the non-mutagenized chromosomes ablates all but the genotype of interest, resulting in the selective proliferation of the homozygous mutant cells in the eye. Mutations in checkpoint genes should result in phenotypically visible eye defects when the G2/M or the S/M checkpoint is induced, for example by the presence of a block to DNA replication or by DNA damage. Various chemical blocks to replication were used to induce this phenotype.
   1. Stowers RS and Schwarz TL. A genetic method for generating Drosophila eyes composed exclusively of mitotic clones of a single genotype. Genetics 152: 1631-1639. August 1999.