A CDC2 DEPENDENT CHECKPOINT MAINTAINS DIPLOIDY IN DROSOPHILA

DNA replication in G2 does not normally occur due to the checkpoint co ntrol, To elucidate its mechanism, the functions of the escargot and D mcdc2 genes of Drosophila were studied, When escargot function was eli minated, diploid imaginal cells that were arrested in G2 lost Cyclin A , a regulatory subunit of G2/M cdk, and entered an endocycle, escargot genetically interacted with Dmcdc2 which encodes a catalytic subunit of G2/M cdk, The mutant phenotypes of Dmcdc2 itself was similar to tho se of escargot: many diploid cells in imaginal discs, salivary glands and the central nervous system entered an endocycle and sometimes form ed polytene chromosomes, Since mitotically quiescent abdominal histobl asts still required Dmcdc2 to remain diploid, the inhibitory activity of G2/M cdk on DNA replication appeared to be separable from its activ ity as the mitosis promoting factor, These results suggest that in G2, escargot is required to maintain a high level of G2/M cdk that active ly inhibits the entry into S phase.