The chromosomes supplied by sperm and egg undergo different chromatin reorganization events in the embryo. The paternal chromosomes transition from a highly compacted protamine-rich state to a mitosis-competent histone-rich state. This process involves several maternally-deposited proteins, but the paternal contribution to paternal genome remodelling is unclear. A study in Drosophila melanogaster now reveals that the testis-specific heterochromatin protein 1e (HP1e) is essential for priming the paternal genome to enter embryonic mitosis in synchrony with the maternal genome. Sperm from HP1e-depleted males fertilized eggs, but the post-fertilization condensation of paternal chromosomes (in particular of heterochromatin-rich sex chromosomes) in these embryos was delayed, leading to mitotic catastrophe and embryonic lethality. HP1e was not inherited: it was expressed late in spermatogenesis but disappeared in mature sperm.