Distinct functions of Mst77F and protamines in nuclear shaping and chromatin condensation during Drosophila spermiogenesis.

Chromatin reorganisation is a major event towards the end of mammalian and Drosophila spermatogenesis. In Drosophila, we previously identified protamine A, protamine B and Mst77F as major chromatin components of the mature sperm. Here, an antibody against Mst77F reveals a dual expression pattern of Mst77F as a chromatin component and in association with microtubules during nuclear shaping. Spermatids of ms(3)nc3 (Mst77F(1)) mutants show disturbed nuclear shaping, instability of perinuclear microtubules but no obvious chromatin condensation defects. Furthermore, we generated a deletion including both protamine genes (prot Delta) and observed that in Drosophila, protamine genes are not haploinsufficient in contrast to those of mice and humans. Moreover, we show that in prot Delta mutants, histone degradation, distribution of DNA breaks and Tpl(94D)-eGFP and Mst77F expression proceed as in wild-type males. Surprisingly, in homozygous prot Delta mutants, males are fertile and sperm are motile, while about 20% of sperm show abnormally shaped nuclei. The latter phenotype can be rescued by supplying protamine-eGFP but not by supplying Mst77F-eGFP. Finally, we demonstrate a 21% increase in X-ray-induced mutation rate of prot Delta sperm. These data support the long-standing hypothesis that the switch from a histone- to protamine-based chromatin protects the paternal genome from mutagens.