DNA and Histone Modifications Identify a Putative Controlling Element (CE) on the X Chromosome of Sciara coprophila

The term chromosomal imprinting was introduced to denote the parent-of-origin-dependent behavior of chromosomes in the fungus gnat originally named Sciara coprophila (current taxonomic name is Bradysia coprophila). Such behavior is observed in Sciara coprophila embryos, where paternal X chromosomes (X_p) are specifically eliminated during the 7th-8th cleavage divisions. Elimination is regulated by a controlling element (CE) that has been mapped to heterochromomere II (H2) within the sub-telomeric short arm of polytene X chromosomes. Here, using a combination of a new Sciara genome assembly, along with ChIP-Seq and MeDIP analyses, we show that a 1.2 Mb region within the CE locus has a repressive epigenetic signature that is characterised by enrichments of H3K9me3, H4K20me3 and 5'-methyl cytosine (5meC). These data provide evidence for a model where the H3K9me3/HP1/H4K20me3 pathway operates to assemble a heterochromatin-like complex at the CE that renders it silent on X_p chromosomes that are not eliminated. In this regard, our findings support the idea that the H3K9me3/HP1/H4K20me3 pathway represents the most evolutionarily conserved mechanism linked to chromosomal imprinting in animals.