The Drosophila fusome, organelle biogenesis and germ cell differentiation: if you build it....

From stem cells to oocyte, Drosophila germ cells undergo a short, defined lineage. Molecular genetic analyses of a collection of female sterile mutations have indicated that a germ cell-specific organelle called the fusome has a central role at several steps in this lineage. The fusome grows from a prominent spherical organelle to an elongated and branched structure that connects all mitotic sisters in a germ cell syncytium. The organelle is assembled from proteins normally found in the membrane skeleton and, additionally, contains an extensive membranous reticulum, the probable product of differentiation-dependent vesicle trafficking. This review briefly summarizes a current view of the processes that drive germ cell differentiation particularly the various roles that the fusome might play in regulating the developmental events. Future efforts will consider to what extent an organelle assembly-dependent model for differentiation is heuristic and whether the Drosophila fusome represents a homolog of a similar organelle in vertebrate lymphocytes.