gio^Z3934 mutants are defective in spermatocyte cytokinesis and display large and incompletely constricted actomyosin rings.

Flies homozygous for vib^Z3934 show frequent multinucleate spermatids.

Larval brains of vib^Z3934 mutants exhibit frequent polyploid cells but have normal frequences of anaphases, consistent with a defect in neuroblast cytokinesis.

vib^Z3934 homozygotes and vib^Z3934/Df(3R)Dl-BX12 mutants exhibit defective telophase, with incompletely constricted contractile rings and central spindles that are either normal or less dense than their wild-type counterparts.

A significant proportion of gio^Z3934 mutant spermatids have one large nebenkern with two nuclei of normal size, indicating failure of cytokinesis in one of the meiotic divisions. Some spermatids have four nuclei and a large nebenkern, indicating a cytokinetic failure in both meiotic divisions. A small proportion of spermatids have three nuclei, which may be due to failure of cytokinesis in the first meiotic division, followed by budding off of only one daughter cell in the second meiotic division. The spermatids of gio^Z3934/Df(3R)Dl-BX12 mutants show the two and four nuclei phenotypes. During early telophase of gio^Z3934 spermatocytes, there is an actin ring of normal appearance and and a normal central spindle. However, as telophase progresses the actin ring often fails to fully constrict and in late telophases where there is a high degree of constriction, there is a greater amount of F-actin at the ring. In a small proportion of late telophase figures, the central spindle is also defective. gio^Z3934/Df(3R)Dl-BX12 mutants show the same phenotype.