Vrp1^KO mutant flies are male sterile. The testis contain mature cysts that do not undergo appreciable coiling, cysts do not occupy the base of the testis tube and the seminal vesicle is completely empty of released sperm.

The microfilaments and nuclei of Vrp1^KO/Vrp1^D30 mutant late-stage spermatid cysts show significant structural alterations. There is a consistent failure to achieve the intertwined and tightly packed organisation of head cyst cell F-actin arrays and associated spermatid nuclei. The microfilament arrays appear sparse and misshapen, and the normally tight and uniformly oriented bundles of nuclei are loosely packed and partially split. The robust, transient network of head cyst cell microfilaments seen in wild type is far less frequently observed in Vrp1^KO/Vrp1^D30 mutants (5% of testis compared to 25% in wild type). However, actin-based projections remain properly associated with individual spermatids nuclei at late stages of cyst maturation.