Figure 1

Mechanisms of intron gain in retrogenes. In the parental gene, rectangles represent exons, ‘H’-like tags represent introns, the retroposed regions are indicated in purple, and other regions are indicated in blue. In the retrogene, the retroposed region is indicated in purple and the newly evolved intronic regions are indicated in yellow. Semi-rectangle lines with arrows indicate the direction of transcription. (A) The retrogene RPS3AP5 gained an intron by insertion of an external sequence; (B) the retrogene XXyac-R12DG2.2 evolved a new intron after transcription in the opposite orientation compared to the parent; (C) in retrogene HSP90AA4P three new introns were generated by intronization. There is no mutation at the splice sites in the two introns near the 5′ terminus, whereas one transition from ‘A’ to ‘G’ (indicated in red) at the splice sites occurred in the intron near the 3′ terminus.