show Abstracthide AbstractWe study the relatively unexplored evolutionary consequences of the epigenetic effects of transpoable elements (TEs) by providing the first genome-wide quantification of such effects in wild-derived D. melanogaster and D. simulans strains. Surprisingly, over half of euchromatic TEs show spread of repressive epigenetic marks to nearby DNA, resulting in differential epigenetic states of homologous genic alleles and, in return, selection against TEs. Interestingly, compared to D. melanogaster, the lower TE content in D. simulans is correlated with stronger epigenetic effects of TEs and higher levels of host genetic factors known to promote epigenetic silencing. We conclude that the epigenetic effects of euchromatic TEs, and host genetic factors modulating such effects, play a critical role in the population dynamics of TEs within and between species. Overall design: Examine the epigenetic effects of transposable elements