The endocycle controls nurse cell polytene chromosome structure during Drosophila oogenesis

Polytene chromosomes exhibit intricate higher order chromatin structure tha t is easily visualized due to their precisely aligned component strands. Ho wever, it remains unclear if the same factors determine chromatin organizat ion in polyploid and diploid cells. We have analyzed one such factor, the c ell cycle, by studying changes in Drosophila nurse cell chromosomes through out the 10 to 12 endocycles of oogenesis. We find that nurse cells undergo three distinct types of endocycle whose parameters are correlated with chro mosome behavior. The first four endocycles support complete DNA replication ; poorly banded polytene euchromatin progressively condenses during the lat e S phases to produce blob-like chromosomes. During the unique fifth endocy cle, an incomplete late S phase is followed by a mitosis-like state during which the 64C chromosomes dissociate into 32 chromatid pairs held together by unreplicated regions. All the subsequent endocycles lack any late S phas e; during these cycles a new polytene chromosome grows from each 2C chromat id pair to generate 32-ploid polytene nuclei. These observations suggest th at euchromatin begins to condense during late S phase and that nurse cell p olytene chromosome structure is controlled by regulating whether events cha racteristic of late S and M phase are incorporated or skipped within a give n endocycle.