Enrichment or depletion of active and repressive histone modifications in retrotransposons. Histone modifications were classified as active or repressive based on expression enrichment (Additional file 1, Figure S2). The log2 normalized ratios of the number of tags of active or repressive modifications located within each family of retrotransposons over the total number of tags taken as the genomic background is shown. Retrotransposon families are arranged according to their relative ages. Spearman rank correlations (ρ) between active and repressive transposable element (TE)-modification enrichments (depletions) and the relative ages of TE families are shown.