When sperm cells are formed, they eliminate the histone proteins that package the DNA in all our other cells, replacing them with protamines, special proteins found in the sperm nucleus. A recent study led by a team of the LBMC (CNRS/ENS de Lyon) and published in Science, shows for the first time the role of this mechanism, and demonstrates that this elimination of histones is very important in preserving the epigenetic identity of paternal chromosomes after fertilization.
The global replacement of histones with protamines in sperm chromatin is widespread in animals, including insects, but its actual function remains enigmatic. We show that in the Drosophila paternal effect mutant paternal loss , sperm chromatin retains germline histones H3 and H4 genome wide without impairing sperm viability. However, after fertilization, sperm chromosomes are targeted by the egg chromosomal passenger complex and engage into a catastrophic premature division in synchrony with female meiosis II. We show that encodes a rapidly evolving transition protein specifically required for the eviction of (H3-H4)2 tetramers from spermatid DNA after the removal of H2A-H2B dimers. Our study thus reveals an unsuspected role of histone eviction from insect sperm chromatin: safeguarding the integrity of the male pronucleus during female meiosis.
