ASSEMBLY OF RING CANALS IN THE MALE GERM-LINE FROM STRUCTURAL COMPONENTS OF THE CONTRACTILE RING

Stable intercellular bridges called ring canals form following incompl ete cytokinesis, and interconnect mitotically or meiotically related g erm cells. We show that ring canals in Drosophila melanogaster males a re surprisingly different from those previously described in females, Mature ring canal walls in males lack actin and appear to derive direc tly from structural proteins associated with the contractile ring. Rin g canal assembly in males, as in females, initiates during cytokinesis with the appearance of a ring of phosphotyrosine epitopes at the site of the contractile ring, Following constriction, actin and myosin II disappear. However, at least four proteins present at the contractile ring remain: the three septins (Pnut, Sep1 and Sep2) and anillin. In s harp contrast, in ovarian ring canals, septins have not been detected, anillin is lost from mature ring canals and filamentous actin is a ma jor component, In both males and females, a highly branched vesicular structure, termed the fusome, interconnects developing germ cells via the ring canals and is thought to coordinate mitotic germ cell divisio ns, We show that, in males, unlike females, the fusome persists and en larges following cessation of the mitotic divisions, developing additi onal branches during meiosis, During differentiation, the fusome and i ts associated ring canals localize to the distal tip of the elongating spermatids.