Fusome formation, structure, and patterns of division.

A) Schematic of cyst formation during oogenesis and spermatogenesis in Drosophila melanogaster. A founder cell gives rise to a germline cyst whose 16 cells are connected through ICBs or ring canals. During oogenesis, one cell becomes the oocyte while the remaining cells serve a supporting role. During spermatogenesis, two subsequent meiotic divisions with incomplete cytokinesis generate 64 haploid spermatids. In both sexes, germline cysts contain a fusome, which permeates the cysts through the ring canals. B) Schematic of the four rounds of synchronous cell divisions that give rise to the female germline cyst, comprising 16 cells (nodes) connected by 15 ring canals (edges). The fusome (cyan) derives from the spectrosome and grows with each cell division; association of the fusome with one of the two mitotic centrosomes orients the planes of cell division, ensuring the stereotypical and maximally branched pattern in female cysts. C) Projection of a 3D image showing the fusome (α-Spectrin, Spec) permeating the cyst through the ring canals (Pavarotti, Pav) in cysts of various sizes (2-, 4-,8- and 16-cell cysts, and a Stage 1 egg chamber (EC)). Scale bar = 5 μm. D) Schematic for the ‘predetermination’ model for oocyte selection, which posits that the initial bias in fusome volume determines which cell becomes the oocyte; as such, oocyte identity is determined at the 1-cell stage. E) Projection of a 3D image of developing cysts during spermatogenesis, where the fusome (Adducin) and ring canals (Pav) are shown. Scale bar = 20 μm. F) Schematic showing that cell divisions can occur in one of two ways based on the orientation that the internal cells (green) divide. If all internal cells divide to form new branches, the cyst will remain maximally branched; however, internal cells can also divide without forming new branches, thus creating in-line divisions and paving the way for topologies that deviate from the maximally branched pattern.