Species-specific chromatin landscape determines how transposable elements shape genome evolution

Transposable elements (TEs) are selfish genomic parasites that increase their copy number at the expense of host fitness. The "success," or genome-wide abundance, of TEs differs widely between species. Deciphering the causes for this large variety in TE abundance has remained a central question in evolutionary genomics. We previously proposed that species-specific TE abundance could be driven by the inadvertent consequences of host-direct epigenetic silencing of TEs--the spreading of repressive epigenetic marks from silenced TEs into adjacent sequences. Here, we compared this TE-mediated "epigenetic effect" in six species in the Drosophila melanogaster subgroup to dissect step-by-step the role of such effect in determining genomic TE abundance. We found that TE-mediated spreading of repressive marks is prevalent and substantially varies across and even within species. While this TE-mediated effect alters the epigenetic states of adjacent genes, we surprisingly discovered that the transcription of neighboring genes could reciprocally impact this spreading. Importantly, our multi- species analysis provides the power and appropriate phylogenetic resolution to connect species-specific host chromatin regulation, TE-mediated epigenetic effects, the strength of natural selection against TEs, and genomic TE abundance unique to individual species. Our findings point towards the importance of host chromatin landscapes in shaping genome evolution through the epigenetic effects of a selfish genetic parasite.