The Drosophila Maternal Gene Sésame is Required for Sperm Chromatin Remodeling at Fertilization

The spermatozoon features an extremely condensed and inactive nucleus. The unique sperm chromatin organization is acquired during the late stages of spermatid differentiation by the replacement of somatic histones with sperm-specific chromosomal proteins. At fertilization, the inactive sperm nucleus must be rapidly transformed into a DNA replication competent male pronucleus before the formation of the zygote. The sequential events of this crucial process are well conserved among animals and are controlled by molecules present in the egg. We have previously identified a Drosophila maternal effect mutation called sésame, which specifically arrests male pronucleus formation at a late stage of chromatin decondensation. In this study, we show that sésame affects maternal histone incorporation in the male pronucleus, a situation that is expected to prevent nucleosomal organization of the paternal chromatin. As an apparent consequence, the male pronucleus is arrested before the first S-phase and does not condense mitotic chromosomes. However, centromeric heterochromatin is present on paternal centromeres, which occasionally interact with microtubules. The abnormal chromatin organization of the male pronucleus does not prevent the formation of a male pronuclear envelope, which breaks down and reassembles in synchrony with maternally derived nuclei present in the same cytoplasm.