bel^neo30/bel⁶ mutant testes display a germ cell loss phenotype, with complete absence of vas-stained germline stem cells (GSCs) attached to the hub at the apical testis tip (but sometimes testes contain residual vas-stained germ cells that exhibit morphological defects or size abnormalities such as giant nuclei), compared to controls (bel/+ heterozygotes); half of mutant testes do not contain germ cells at all. bel^neo30/bel⁶ mutant testes do not contain wild type spectrosomes and fusomes, showing complete loss or disrupted fusome structure. Mutant bel^neo30/bel⁶ testes show severe depletion of germinal content (including GSCs, spermatogonial cells and spermatocytes). bel^neo30/bel⁶ mutants show expanded expression of hub markers and significantly more somatic cyst stem cells in the testes than controls. Older (6-7 days old) mutant male testes show an overpopulation of somatic cells and an absence of germ cells, compared to controls. bel^neo30/bel⁶ germline cells (but not somatic cyst cells) in the testis show increased rates of apoptosis compared to controls.

Overall distribution of germinal and somatic cells and hub structure in third instar larval bel^neo30/bel⁶ mutant testes is similar to controls, however mutants show significant testis size atrophy and a reduced number of GSCs; mutant spermatogonia often have abnormally large cellular and nuclear volumes, possibly due to delay in G2 growth phase.

bel^neo30/bel^EY08943 fly testes contain a pre-meiotic germ cell population that is reduced nearly two fold compared to heterozygote sibling controls, and maintain only a few GSCs. Mutant spermatogonial cells often have abnormally large cellular and nuclear volumes.

Adult bel⁶/bel^neo30 transheterozygotes have severely reduced testes compared to wild-type and have complete loss of germ cells prior to the spermatocyte growth stage.

Homozygous mutant males have testes that have a high frequency of defective onion stage early spermatid cysts: nucleus size often varies and abnormally large mitochondrial derivatives associated with several nuclei are common. Flagella elongate in mutants, nut spermatid bundles are disorganised. Spermatid nuclei also elongate but are scattered instead of clustered at the end of the flagella bundles. In bel^neo30/bel⁶ females oogenesis initially proceeds normally, but at around stage 9 the majority of egg chambers degenerate. A minority of egg chambers proceed beyond this stage to produce mature eggs. bel^neo30/bel⁶ and bel^neo30/bel² ovaries produce eggs that have very fragile chorions and very short dorsal appendages.

Hemizygous males show a more extreme phenotype than homozygotes and are poorly viable if raised at 18^oC. Hemizygous females are sterile and produce eggs with weak chorion. Homozygous females are fertile.