Aust can replace Borr in S2 cells. S2 cells were incubated with dsRNA corresponding to regions of the UTR of borr for 5 d before fixing and staining. (A) Borr is a chromosomal passenger in S2 cells. (B) Cells treated with borr dsRNA lose staining for Borr and show defects in spindle formation, chromosome alignment, chromosome segregation, and cytokinesis. (C and D) Cultures treated with borr dsRNA in the presence of pAFMW-aust (C) or pAWR-aust (D). Cells do not show defects in mitosis. Although Borr staining is lost, RFP-Aust localizes as a passenger protein. (E) Histogram showing the frequency of polyploid binucleated cells in cultures treated with borr dsRNA, borr dsRNA and borr transgenes (pAWR-borr and pAFMW-borr), or borr-dsRNA and aust transgenes (pAWR-aust and pAFMW-aust). RNAi for borr led to ∼80% polyploid binucleated cells. Coexpression of either borr or aust transgenes rescues the phenotype with similar efficiency. Results were obtained from three independent experiments. Error bars represent SEM. (F and G) Phenotype of borr dsRNA cells. Control cells show mostly regular cell size and shape (F). After 5 d in the presence of borr dsRNA, many cells are polyploidy, indicating multiple failures of cytokinesis. Bars, 10 μm.