Figure 1.

A cell group forms abnormally at the anterior pole of step RNAi embryos. (A) Staining for the plasma membrane marker Discs large (Dlg) shows a cell group formed at the anterior of a step RNAi embryo in contrast to control (arrow) at blastoderm cellularization. The germ plasm marker Vasa shows that this abnormal cell group lacks germ plasm in contrast to the posterior PGCs. For each embryo, three separate images were stitched together, and signal intensities were adjusted to be equal across the composite image. (B) Live phase-contrast imaging showing the budding and formation of abnormal anterior cells in step RNAi embryos with the same timing as PGCs in step RNAi and control embryos. The anterior (left) and posterior (right) ends of the same embryos at nuclear cycles 9 and 10 are shown (frames from File S1 and File S2). (C) Imaging of a step RNAi embryo showing that the abnormal anterior cells lack full basal membranes at nuclear cycle 10 (hollow arrow), similar to some cells at the posterior (hollow arrow) although other posterior cells are fully separated from the soma (solid arrow). (D) Confocal sectioning shows that by nuclear cycle 12 the abnormal anterior cells of step RNAi embryos are fully enclosed by plasma membrane (stained with Dlg). (E) Phosphohistone H3 staining of mitotic chromosomes shows the asynchronous cell cycling of the abnormal anterior cells of step RNAi embryos, in contrast to the synchronous divisions of wild-type syncytial somatic nuclei, left. (F) A 3D rendering of Anillin staining shows that the abnormal anterior cells of step RNAi embryos form individual cytokinetic rings (arrows). The cortex on either side of the Anillin-positive ring is indicated with dots for one dividing cell.