Fig. 1. Schematic of the anterior part of a female ovariole and male testis in Drosophila melanogaster. The description of the developmental stages of the germ cells is to the left of each structure and the somatic cells are to the right. Anterior is up. In the female ovariole (A) and male testis (B), germ-line stem cells (GSCs) are located at the anterior tip of the gonad. Upon division, the posterior daughter cell differentiates to a cystoblast (Cb, females) or a gonialblast (males). The differentiation may be gradual, as depicted by increasingly stronger shades of green. The daughter cell divides further and forms a cyst. GSCs and their immediate daughter cells harbor a spherical fusome (here marked in black). This organelle grows and extends into every cell of the cyst (black marking within the cyst). In females, the niche includes terminal filament (TF) and cap cells, which are located most anteriorly. Inner sheath (IS) cells may also be part of the niche and may perform similar functions to those of early somatic cyst cells in males. Somatic stem cells (SSCs) are located `midway' down the germarium and they give rise to the follicle cells that envelope the cyst. In males, SSCs are attached to the hub, and their descendents (cyst cells) encapsulate the gonialblast. Color-coding is used to mark cells that have a similar function in males and females. Shades of red have been used for TF and cap cells as these two populations, although similar, are not identical in their gene expression profiles (Forbes et al., 1996a).