Overexpression of GFP-MpD suppresses the formation of M3. (A–D) SR-SIM images of cells in which the native promotor of MPD was replaced with the MTT1 promoter to provide cadmium-inducible expression of GFP-MpD. Cells were grown for 6 h in the absence (A–A″) or presence (B–D) of 2.5 μg/ml cadmium chloride. Note that GFP-MpD is expressed at low level even without addition of cadmium ions due to the basal level of MTT1 promoter activity (A–A″). In the overproducing cells (B–B″), GFP-MpD accumulates in the subcortical region, around the micronucleus, and inside the macronucleus. (C and D) GFP-MpD–overproducing cells during divisional oral development. The cells in C is in stage 4b; note that scattered BBs are present in the posterior region of OP (yellow arrow), but they failed to form M3. The cell in D is in late stage 5f. The new OA has 2 M instead of 3 M rows. The old oral OA has a normal 3 M organization. Thus, the defect (inability to form M3) occurs during development. (E) The graph quantifies the 2 M row phenotype in cells in which either the WT (G939) or mutant GFP-MpD operates under the MTT1 promoter whose overexpression was either induced (cd) or not (−cd) by exposure to cadmium chloride (2.5 μg/ml, 6 h). The 2 M phenotype is only present in cells that overproduce a WT (G939) MpD. Mean ± SD. N = 3 experiments (100 cells scored per experiment). oa, mature oral apparatus; noa, new oral apparatus or OP; ac, apical crown; mi, micronucleus; ma, macronucleus. White arrows mark the M rows.