Methylation of histone H3 at Lys4 differs between paternal and maternal chromosomes in Sciara ocellaris germline development.

An outstanding example of programmed chromosome elimination and genomic imprinting is found in sciarid flies (Diptera, Sciaridae), where whole chromosomes of paternal origin are selectively discarded from the genome during development. In early germ cells a single paternal X chromosome is eliminated in embryos of both sexes and in male meiotic cells the whole paternal complement is discarded. In sciarids, differential acetylation of histones H3 and H4 occurs between chromosomes of different parental origin, both in early germ nuclei and in male meiotic cells (Goday and Ruiz, 2002). We here investigated histone methylation modifications between chromosomes in germline cells of Sciara ocellaris. In early germ nuclei, maternal chromosomes show high levels of di- and trimethylated histone H3 at Lys4, whereas this histone modification is not detected in paternal chromosomes. In male meiosis, only the eliminated paternal chromosomes exhibit high levels of di- and trimethylated histones H3 at Lys4 and dimethylated H4 at Lys20. In early germ nuclei, RNA polymerase II associates to maternally-derived chromosomes but lacks phosphorylation of the C-terminal domain on Ser2. We found that histone H3 methylation at Lys4 does not correlate with transcriptional activity in early Sciara germline nuclei. The results support the conclusion that specific covalent chromatin modifications are involved in the imprinted behaviour of germline chromosomes in Sciara.