The anaphase promoting complex/cyclosome is required during development for modified cell cycles

Animals and plants use modified cell cycles to achieve particular developmental strategies. In one common example, most animals and plants have tissues in which the cells become polyploid or polytene by means of an S–G cycle, but the mechanism by which mitosis is inhibited in the endo cycle is not understood. The Drosophila morula (mr) gene regulates variant cell cycles, because in addition to disrupting the archetypal cycle (G(1)–S–G(2)–M), mr mutations affect the rapid embryonic (S–M) divisions as well as the endo cycle (S–G) that produces polyploid cells. In dividing cells mr mutations cause a metaphase arrest, and endo cycling nurse cells inappropriately reenter mitosis in mr mutants. We show mr encodes the APC2 subunit of the anaphase promoting complex/cyclosome. This finding demonstrates that anaphase promoting complex/cyclosome is required not only in proliferating cells but also to block mitosis in some endo cycles. The mr mutants further indicate that transient mitotic functions in endo cycles change chromosome morphology from polytene to polyploid.