How and when organisms edit their own genomes.

Mutations are often thought of as untargeted and non-adaptive, but in rare cases, organisms perform programmed, targeted and adaptive rearrangements of their own DNA sequences. Notable examples include the somatic diversification of immunoglobulin genes, which is the foundation of the vertebrate immune system, and natural CRISPR spacer arrays in bacteria, which recognize and cleave foreign DNA. These systems, along with a dozen known analogs scattered across the tree of life, often underlie critical biological functions, particularly in host-pathogen conflicts. In this Review, we compare the mechanisms by which organisms edit their own genomes. We show that superficially dissimilar editing systems often rely on surprisingly similar genetic mechanisms, regardless of function or taxon. Finally, we argue that the recurrence of editing in host-pathogen conflicts and the bias to a handful of well-studied organisms strongly suggest that new editing systems will be found in understudied pathogens and their hosts.