Shaping mammalian genomes: The regulation and co-option of transposable elements in early development and...

Transposable elements (TEs), or mobile genetic elements, are fragments of DNA capable of moving from one genomic location to another. Traditionally, transposons have been considered selfish genetic elements because they increase their copy number by jumping into new genomic locations and causing genomic instability. In mammals, TEs make up a substantial portion of the genome, far exceeding the fraction occupied by protein-coding genes or regulatory elements such as promoters or enhancers, but only a small fraction is still able to mobilize. Additionally, in most adult cells, TE expression is largely silenced by epigenetic mechanisms and other pathways that protect the genome from the deleterious consequences of TE expression. However, during gametogenesis and early development, epigenetic reprogramming creates a window in which TEs can be expressed. Throughout evolution, host genomes have repurposed transposon-derived sequences to exert a function in a process known as co-option. Here, we summarize how TE expression is regulated and can be harnessed for the host's benefit, with a particular focus on the co-option of TEs during gametogenesis and early development, and present current evidence for their extensive regulatory roles.