Population genetics of piRNAs in Drosophila

Transposable elements (TEs) selfish genetic elements that survive by replicating within host genomesare one of the most important kinds of parasitic elements. Most genomes are riddled with substantial numbers of TEs, e.g, the human genome is comprised of 44% TE-derived DNA, compared to only 1-2% protein coding DNA. Such a substantial evolutionary burden is likely to provoke a strong evolutionary response; in fact, parasites in general have been consistently shown to be among the primary driving forces underlying rapid adaptation. However, the evolutionary response to TEs is comparatively poorly understood. The reason is that the TE defense system has a important small RNA component, the piRNAs, and the evolution of such small RNA systems is relatively poorly studied. Here, we aim to study the evolution of piRNA loci, asking if these loci show the same sort of rapid evolutionary response as other immunity loci. Specifically, we will investigate the population genetics of piRNA loci, looking for rapid population differentiation in response to active TEs. To this end, we plan to survey Drosophila populations for the presence of protective piRNA loci. At the same time, we plan to survey these same populations for the transposable element content, to obtain a picture of both sides of the co-evolutionary equation.