Oocyte determination relies on polarized microtubule-dependent intercellular transport from the nurse cells into the oocyte through ring canals. The basis for this polarity appears to be established as early as in the cystoblast. Since the fusome is associated with microtubules, it has been suggested that the fusome helps establish and maintain polarity during cystoblast divisions and the amplifying of germ cell divisions. In bam mutant ovaries, polarity cannot be established, and the cystoblast cell fate cannot be assigned, resulting in an overproliferation of cells with stem cell characteristics. Little can be gleaned from the protein sequence since Bam is a novel protein.
Some information is provided by the phenotype especially with regard to fusome appearance.
In the female, each round of cystocyte mitosis is accompanied by the growth of a germ cell-specific organelle, the fusome. Stem cells and cystocytes contain a large sphere of fusomal material, termed the spectrosome. During the four cystocyte mitoses, one pole of each spindle associates with the fusome, and following each mitosis, as the spindles disaggregate, additional fusomal material accumulates. Thus, by the fourth division, the fusome forms one large branched structure that extends though the ring canals into all the cells in a cyst. Bam is a component of the fusome.