Flies carrying Top3α^{M1L.T:Avic\GFP-EYFP} in a Top3α⁵⁴ background show fertility defects in both sexes. The females are completely sterile. They lay a normal number of eggs during a 12 day period, but the eggs appear to be smaller as a whole and 17% shorter in length than normal and are prone to collapse. The remaining intact embryos do not hatch. 0-1 hour embryos laid by the mutant females show a 20-fold decrease in mtDNA copy number and a 2 to 3-fold decrease in ATP content compared to wild type. 33% of newly eclosed males are completely sterile, with the remainder having a fertility less than 10% of their heterozygous siblings. The mutant males completely lose fertility after 6 days, with 31% of testes being completely devoid of germ-line cells. Spermatogenesis is disrupted in newly eclosed males, with individualisation complexes scattered along the testis. The testes of 3 day old males are 50% smaller in diameter than those of controls and half of these testes have no motile sperm in the seminal vesicles.