Mobile genetic elements are big players in mammalian genomes. Some of my previous Snippets have visited wildly selfish elements and our cells’ efforts to fight them, so I think it’s time to look at some of their positive (ish) contributions to mammalian evolution. In this case, “they” are the abundant LINE-1 elements that alone make up 1/6 of the human genome.
LINE-1 retrotransposons are so-called transposable elements (TEs), capable of hopping around inside of a genome using their own machinery. Their haphazard relocations can cause damage, but sometimes they create new genomic features in a single bound. For example, they can fuse with another sequence (often an RNA sequence) and give rise to a chimeric element, now newly inserted into a genome. These chimeric insertions are known to occur, but their evolutionary histories and genomic extent were unknown until now.
In a new paper in Cell Genomics, Cheuk-Ting Law and Kathleen Burns present a computational pipeline (called TiMEstamp) and analysis that finds many hundreds of chimeric LINE-1 elements along with their evolutionary birthdates. Have a look at Figure 2 (above), which is an illustration of the evolutionary history of subfamilies of the LINE-1 clan, a history the authors used to infer when chimeras formed in the past. The authors explored the nature of the “partners” in these chimeras and found some new characters. One very interesting new finding pertains to the “rescue” of a seemingly dead LINE-1 element: they describe how one element of a type that is typically “dead on arrival” (their words) co-opted a nearby genomic element to restore its ability to keep hopping around in the genome.
The authors end by suggesting that chimeric LINE-1 insertions, and the specific mechanisms that make them possible, explain the remarkable success of these ubiquitous genomic parasites. Now please enjoy this sample of their excellent writing from the Discussion:
Over millions of years, TEs have shaped our genome, though precisely how they have contributed to speciation and species-specific characteristics is only partially understood. Many primate-specific regulatory regions with cell-type-specific and developmental activities are derived from TEs. Timing TE insertions relative to events in speciation could narrow down those involved in primate evolution and guide functional investigations.
Comparative genomics reveals LINE-1 recombination with diverse RNAs
In Cell Genomics, 13 May 2026
From the group of Kathleen Burns at the Dana-Farber Cancer Institute
Snippet by Stephen Matheson
Image credit: Figure 2 from Law and Burns cited above (CC BY)