Df(2L)ΔMst35B displays a dose dependent reduction in male fertility (i.e. homozygotes present a lower fertility than heterozygotes), as compared to wild-type controls.

Homozygous males are fully viable. A reduction in fertility is seen only when mutant males are allowed to mate with a large excess of virgin females (no defect is seen with a 1:1 sex ratio). Spermiogenesis is severely disorganised in the mutant males. Many elongating spermatids have abnormal nuclear morphology (nuclei appear bent, with the anterior tip sometimes folded into a hook-like structure) and a large proportion of mutant spermatid nuclei are scattered along the cysts instead of being tightly grouped in bundles of 64 nuclei. The mutant males have significantly less gametes in their seminal vesicles compared to wild type. Some of the gametes in the seminal vesicles appear morphologically abnormal.