Intracellular battlegrounds: conflict and cooperation between transposable elements

Transposable elements (TEs) are genomic parasites that amplify their own representation on hosts' chromosomes by inserting into new positions. It is traditionally thought that their copy number is regulated by purifying selection that eliminates hosts with higher than average TE abundance. Here, we stress that selection due to beneficial or harmful interactions between TEs introduces a whole new dimension, with implications for TE evolutionary trajectories and TE loads on hosts. This framework poses new questions requiring conceptual and experimental advances. Considering primarily Drosophila data, we make a case for within host selection on TEs by thinking expansively about the lifecycle of several TE families.