LINES between species: Evolutionary dynamics of LINE-1 retrotransposons across the eukaryotic tree of life

LINE-1 (L1) retrotransposons are dynamic elements. They have the potential to cause great genomic change due to their ability to jump around the genome and amplify themselves, resulting in the duplication and rearrangement of regulatory DNA. Active L1, in particular, are often thought of as tightly constrained, homologous and ubiquitous elements with well-characterised domain organisation. For the past 30 years, model organisms have been used to define L1s as 6-8kb sequences containing a 5-UTR, two open reading frames working harmoniously in cis, and a 3-UTR with a polyA tail.\n\nIn this study, we demonstrate the remarkable and overlooked diversity of L1s via a comprehensive phylogenetic analysis of over 500 species from widely divergent branches of the tree of life. The rapid and recent growth of L1 elements in mammalian species is juxtaposed against their decline in plant species and complete extinction in most reptiles and insects. In fact, some of these previously unexplored mammalian species (e.g. snub-nosed monkey, minke whale) exhibit L1 retrotranspositional hyperactivity far surpassing that of human or mouse. In contrast, non-mammalian L1s have become so varied that the current classification system seems to inadequately capture their structural characteristics. Our findings illustrate how both long-term inherited evolutionary patterns and random bursts of activity in individual species can significantly alter genomes, highlighting the importance of L1 dynamics in eukaryotes.