A large proportion of fws^Z0161 mutant spermatids have one large nebenkern with four nuclei of normal size, indicating a cytokinetic failure in both meiotic divisions. A smaller proportion have two nuclei and a large nebenkern, indicating failure of only one of the meiotic divisions. Occasionally spermatids are seen (0.6% of cases) with 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. Additionally, some spermatids have more than four nuclei, which may result from defects in the mitotic divisions preceding mitosis. fws^Z0161/Df(2L)qua¹³⁷⁴ mutant spermatids show the same defects but in different proportions; over a quarter have more than four nuclei, while ~10% have two nuclei, a further ~10% have four nuclei and <2% have three nuclei. During early telophase of fws^Z0161 spermatocytes, there is an actin ring of normal appearance and and a normal central spindle. However, as telophase progresses the actin ring fails to fully constrict and the central spindle either becomes less dense or completely disorganized. The cleavage furrow initiates ~10 minutes after anaphase onset as in wild type, but shows a limited ingression and then regresses leading to a failure of cytokinesis. The same phenotype occurs in fws^Z0161/Df(2L)qua¹³⁷⁴ mutants.

Spermatids from fws^z-0161 mutants contain a single abnormally large mitochondrial derivative (Nebenkern derivative) and multiple nuclei of normal size. The numbers of nuclei per spermatid varies dramatically between cysts in the same testis. Spermatocytes from fws^z-0161 homozygotes and fws^z-0161/Df(2L)qua¹³⁷⁴ adult males look normal in early telophase, but the majority exhibit failure of constriction of their contractile ring during mid to late telophase. The contractile ring appears broken in many of these mid to late telophase spermatocytes and the central spindle appears less dense than wild-type. These defects in constriction of the contractile ring are accompanied by failure of cleavage furrow ingression. In fws^z-0161/Df(2L)qua¹³⁷⁴ testes, cysts remain ovoid at stages when wild-type cysts have undergone dramatic elongation. This is despite the fact that substantial elongation of individual axonemes and Nebenkern derivatives does occur, although they form a tangled mess rather than the ordered bundles seen in wild-type. As a consequence of this tangling, it is impossible to tell whether elongation is complete. At the beginning of elongation, the acroblasts in these mutant spermatids have abnormal morphology. By the time the mitochondrial derivatives have slightly elongated, the acroblast is absent.