Fusome fragmentation during spermatogenesis and loss of maximal branching.

A) Starting with n = 62 reconstructed fusomes, a range of cyst sizes can be inferred by identifying the largest connected portions of fusome within each sample. These fusome portions, and their corresponding ring canals, can be used to identify the number of cells connected by each fusome fragment (fusome-connected cells). In approximately half of these, the number of fusome-connected cells was a power of 2 (n = 31). B) Two examples of fusomes connecting 16 and 15 cells, respectively, whose topology cannot be embedded onto that of a maximally branched network. C) Projections of 3D images of dividing cysts with labeled fusome (Adducin, gray), ring canals (Pav, red), and mitotic chromosomes (Phosphohistone 3, cyan), showing examples of a 2-cell (left), 4-cell (middle), and 8-cell cyst (right) during mitosis. In at least one location in the 8-cell cyst, cells are dividing without forming a new branch; as such, the resulting 16-cell tree will not possess a maximally-branch topology. In all examples, yellow dotted lines denote the likely plane of division for actively dividing cells, while yellow arrows depict clear bundles of aggregated or migrating chromosomes. In addition, nodes representing the locations of existing cells, as well as the likely positions of newly-formed cell based on the plane of division are shown. Scale bars = 10 μm.