Figure 2: Evolutionary consequences of L1-mediated 3′ transduction.

If exons (open arrows) of gene A are downstream of an active L1 element, they can be incorporated into a chimaeric (L1-A) pre-mRNA produced by L1 read-through transcription. This transcript terminates when it encounters gene A's strong polyadenylation signal. Processing of the pre-mRNA yields an mRNA consisting of L1 fused to the distal A exons. When the resulting L1-A cDNA is inserted into an intron of gene B, a long 3′ A exon including the strong polyadenylation signal is added to B. Even if inserted into the middle of gene B, this long A exon may become the terminal exon of a new hybrid B-A gene (this could help explain why 3′ terminal exons are often much longer than internal exons¹¹). The distal exons of gene B will be lost if cleavage/polyadenylation at the newly introduced gene A signal is 100% efficient (a). But inefficient cleavage/polyadenylation might produce a longer primary transcript that could be alternatively spliced (b). In this way, additional mRNAs can be formed through alternative splicing of the longer primary transcript, and the organism can sample a new gene variant while maintaining the original design. This provides a new hypothesis for the genesis of alternative splicing. (Vertical arrows point to the location of polyadenylation signals.)