Retrotransposons and the mammalian germline

Retrotransposons are an abundant class of mobile genetic elements in mammalian genomes that contribute to genetic instability and variation in the population by integrating at new sites in the genome. Retrotransposons need to be active in the germline so that new retrotransposon integrations can accumulate in the genome during evolution and therefore retrotransposons contain sequences to drive their expression in cells in the germline. While mammals appear to have evolved mechanisms in the germline to limit retrotransposon activity and the resulting mutational load, retrotransposons also appear to be contributing to the regulation of gene expression and driving evolution of transcriptional networks in germline cells. This review will discuss the interplay between retrotransposons and their host cells in the mammalian germline, the genome defence systems that germ cells and pluripotent cells use to limit the mutagenic activity of retrotransposons, and the impact that retrotransposons are having on the biology of mammalian germ cells and pluripotent cells.