The shape of modern genomes reflects evolutionary forces that have operated on them for millions of years. According to the exon-shuffling hypothesis, which was first suggested in these pages over 20 years ago, new genes are assembled from chunks of old ones. But the molecular mechanisms by which these exons (protein-coding sequences) could be glued together remained speculative. A report in Science by Moran and colleagues now suggests an unexpected and efficient source of such genomic reshufflings. The authors looked at the abundant L1 (LINE-1) retrotransposons — transposable elements that can replicate within the mammalian genome using reverse transcriptase, which copies the retrotransposon RNA into DNA. L1 usually moves only its own sequence from one genomic location to another (Fig. 1a). But Moran et al. show that it can, with surprising efficiency, co-mobilize a 3′ flanking segment of non-L1 DNA to new genomic locations. In this way, two previously unlinked host genomic segments are juxtaposed in a process referred to as 3′ transduction (Fig. 1b).
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