A prominent feature of germ cell differentiation is the formation of the cytoplasmic bridges connecting the cystocytes. In oogenesis, germ line stem cells contain a spectrin -rich body known as the spectrosome. As the cystoblast divides, the spectrosome gives rise to an elongated structure known as the fusome, which extends through the ring canals that connect cystocytes to form branching connections between daughter cystocytes. A comparison was made between the structure of wild-type germaria and the germaria of flies mutant for either strong or weak alleles of otu . The otuPdelta1 allele constitutes a deletion of the otu coding region.
Mutant flies are female-sterile and produce either agametic ovarioles or tumorous egg chambers.
The otuPdelta1 mutation causes enlargement of germarial region I, resulting from an increased number of germ cells arrested at stages prior to when they would normally interact with follicle cells. Severe hypomorphic alleles of otu form primarily tumorous egg chambers that superficailly resemble the tumors produced by the otuPdelta1 null mutations. In normal ovaries, no actin, as revealed by phalloidin staining is apparent in the fusome. However, in contrast to wild type, substantial levels of actin filaments were detected in all mutant fusomes examined. The cytoplasmic dumping that occurs during stage 10 is believed to require subcortical actin for nurse cell contraction and a complex array of cytoplasmic actin filaments, which anchor the nurse cell nucleus to the plasma membrane.