View Program - 2022 Population, Evolutionary, and Quantitative Genetics Conference

Transposable elements (TEs) are widespread genetic parasites that copy and insert themselves across host genomes. The replicative nature and generally deleterious effects of TEs raise an outstanding question about how TE copy number is stably contained in host populations. Classic theoretical analyses predicted that, when the decline in fitness due to each additional TE insertion is greater than linear, or when there is synergistic epistasis, selection against TEs can result in a stable equilibrium of TE copy number. While several mechanisms have been predicted to yield synergistic deleterious effects of TEs, empirical studies of the presence of such epistatic interactions are still lacking. To investigate the presence and prevalence of synergistic epistasis among TEs, we leveraged the fact that purifying selection with synergistic epistasis generates repulsion linkage between deleterious alleles. We investigated this population genetic signal in the likely ancestral Drosophila melanogaster population and found evidence supporting the presence of synergistic epistasis among TE insertions, especially TEs expected to exert large fitness impacts. Even though synergistic epistasis of TEs has been predicted to arise through ectopic recombination and TE-mediated epigenetic silencing mechanisms, we only found mixed support for the associated predictions. We observed signals of synergistic epistasis for a large number of TE families, which is consistent with the expectation that such epistatic interaction mainly happens among copies of the same family. Curiously, significant repulsion linkage was also found among TE insertions from different families, suggesting the possibility that synergism of TEs’ deleterious fitness effects could arise above the family level and through mechanisms similar to those of simple mutations. Our findings set the stage for investigating the prevalence and importance of epistatic interactions in the evolutionary dynamics of TEs.

Synergistic epistasis of the deleterious effects of transposable elements