MpH localizes to M rows and forms gradients that reflect the time of BB maturation. (A–H) SR-SIM images of cells expressing MpH-GFP-mNeonGreen (in the native locus, green), stained with the anti-centrin (magenta) and DAPI (blue). The MpH-GFP-mNeonGreen fluorescence is detected directly (green). Insets show either the mature OA (A) or the OP region (B–H) at a higher magnification. (A) Interphase. Note the presence of MpH in the 3 M rows, not in the UM. (B) Stage 1a. (C) Early stage 1b. MpH-GFP-mNeonGreen foci are enriched near BBs on the left side of the OP. (D) Stage 2. BB pairs align into M rows, with M1 partly formed. MpH-GFP-mNeonGreen presents as rings corresponding to the position of the older (posterior) BB in the pair. The centrin foci are at positions consistent with the anterior (younger) BB of the pair. Note the gradient of MpH-GFP-mNeonGreen intensity between the M rows, with M1 at the gradient’s high point. (E) Stage 4b. Within the M rows, MpH-GFP-mNeonGreen is abundant in the first (oldest and most posterior) and second (intermediate) subrows but weakly present or not detectable in the third (youngest) subrow. The M1-M2-M3 gradient across the oral field is also apparent (see inset). (F) Early stage 5f. (G) Intermediate stage 5f. MPH-GFP-mNeonGreen present in all three subrows. Note a reversed gradient of intensity M3>M2>M1 also seen in the old OA in interphase (see panel A). (H) Late stage 5f. m1, m2, and m3; right ends of M rows marked in insets; ac, apical crown. The arrowheads in panels B–D mark MpH-GFP-mNeonGreen foci at the apical cell.