The final stage of spermatid terminal differentiation involves the removal of their bulk cytoplasm in a process known as spermatid individualization. Here we show that apoptotic proteins play an essential role during spermatid individualization in Drosophila melanogaster. Several aspects of sperm terminal differentiation, including the activation of caspases, are reminiscent of apoptosis. Notably, caspase inhibitors prevent the removal of bulk cytoplasm in spermatids and block sperm maturation in vivo, causing male sterility. We further identified loss-of-function mutations in one of the two Drosophila cyt-c genes, cyt-c-d, which block caspase activation and subsequent spermatid terminal differentiation. Finally, a giant ubiquitin-conjugating enzyme, dBruce, is required to protect the sperm nucleus against hypercondensation and degeneration. These observations suggest that an apoptosis-like mechanism is required for spermatid differentiation in Drosophila.
