Purifying Selection Shapes the Dynamics of P-element Invasion in Drosophila Populations

Background Transposable elements (TEs) are DNA sequences that can move within a host genome. Many new TE insertions have deleterious ebects on their host and are therefore removed by purifying selection. The genomic distribution of TEs thus reflects a balance between new insertions and purifying selection. However, the inference of purifying selection against deleterious TE insertions from the patterns observed in natural populations is challenged by the confounding ebects of demographic events, such as population bottlenecks and migration. Results We used Experimental Evolution to study the role of purifying selection during the invasion of the P-element, a highly invasive TE, in replicated Drosophila simulans populations under controlled laboratory conditions. Because the change in P-element copy number over time provides information about the transposition rate and the ebect of purifying selection, we repeatedly sequenced the experimental populations to study the P-element invasion dynamics. Based on these empirical data we used Gaussian Process surrogate models to infer parameter values characterizing the observed P-element invasion trajectory. We found that 73% of P-element copies are under purifying selection with a mean selection coebicient of -0.056, highlighting the central role of selection in shaping P-element invasion dynamics. Conclusion This study underscores the power of Experimental Evolution as a tool for studying transposable element invasions, and highlights the pivotal role of purifying selection in regulating P-element dynamics.