Fig. 1. Subcellular localisation of Orbit/Mast during germline cyst divisions. Wild-type germaria were stained with anti-Orbit/Mast (red), anti-HtsF (green) antibodies and TOTO3 to reveal DNA (not shown). (A) Orbit/Mast locates on the spindle remnant (sr) during stem cell cytokinesis. The HtsF associates with the nascent fusome plug (fp), which is situated at the mid-zone of the spindle remnant. The pre-existing fusome (p-f) is spherical and resides at the anterior tip of the stem cell. (B) The stem cell (SC) and cystoblast (CB) are interconnected through an Orbit/Mast containing ring canal (rc) that divides the fusome into two unequal pieces. (C) Metaphase cyst, showing the Orbit/Mast on the spindles and pre-existing fusome (arrowhead). Only two out of four spindles in this cyst are shown. (D) Cyst featuring Orbit/Mast on spindle remnants with fusome plugs (arrow). The pre-existing fusome (arrowhead) is situated in the middle of the cyst. (E,F) Orbit/Mast accumulates in the ring canals surrounding the newly formed fusome plugs. Along the pre-existing fusome, Orbit/Mast shows a more diffuse distribution in addition to its presence in ring like structures. Some fusome plugs have already fused with the pre-existing fusome, while others are still 2-3 µm distant. Scale bar: 5 µm. (G) The dynamic changes in the subcellular localisation of Orbit/Mast and HtsF during a division cycle of a germline cyst. Two-cell cyst at metaphase: the asymmetric nature of this division is determined by the fusome-spindle interaction, as only one of the poles of each spindle is fusome anchored. Orbit/Mast accumulates at the spindle poles and decorates the spindle MTs. It is also present in the fusome and the fusome associated ring canal. Note that one of the cells contains a bigger part of the fusome. Two-cell cyst at telophase: central spindle and contractile ring assembly marks the onset of cytokinesis. Cleavage furrow arrest takes place upon contact with spindle remnants in a four-cell cyst. Orbit/Mast associates with central region of spindle remnants. Concomitantly, the fusome plug formation is initiated as inferred from the accumulation of HtsF between the spindle remnant and contractile ring. Ring canal migration: Orbit/Mast accumulates in the ring canals and to a less extent associates with the fusome plugs. The spindle remnants reorient towards the fusome and the fusome plugs together with their ring canals move towards the pre-existing fusome, changing the cyst geometry from a linear into a rosette configuration. Eventually, the pre-existing fusome fuses with fusome plugs and the spindle remnants disassemble. Once again the bigger fusome part will be retained by the cell that has inherited it during the previous division cycle.