The spermatid mitochondria in knon¹⁴⁰⁰;fzo¹ double mutants successfully elongate alongside the axoneme, although the the elongation appears to be slightly delayed.

fzo¹/Df(3R)P2O onion stage spermatids have a nebenkern composed of many small mitochondria, such that it appears irregular. At the leaf blade stage, numerous small mitochondria are seen adjacent to a single nucleus in the mutant spermatids, in contrast to the two mitochondrial derivatives seen in wild-type controls.

fzo¹ mutant testicles exhibit irregularly-shaped nebenkerns.

fzo¹/fzo² spermatids arrest development at mid-elongation with undifferentiated nuclei. However, caspase activation, only seen in wild-type cysts at the individualisation stage, still occurs.

Mitochondria in early spermatids of homozygous, fzo²/Df(3R)P2O or fzo¹/fzo² males aggregate and form misshapen Nebenkerns. The mitochondria fail to fuse into two giant mitochondria as in wild-type males, instead many smaller mitochondria appear to wrap around each other at the onion stage. The mitochondria do unfurl and elongate in early elongation stage spermatids, although the mitochondria appear fragmented compared to wild-type. At later stages, each spermatid has many elongating mitochondria, rather than the normal two elongating mitochondrial derivatives. Fertility and sperm motility is not restored in homozygous males carrying one or two copies of fzo^K99T or fzo^R249L. Some mitochondrial fusion occurs in homozygous males carrying fzo^R249L.