Fig. 2

The hypothesized mechanism of intron gain assisted by target site duplication (TSD) formation during insertion of an introner element with terminal inverted repeats (TIRs). Introner (marked in green) integrates into the TCA target sequence. Site repair results in the duplication of this sequence on each side of the introner. These short, duplicated TSD sequences are highlighted in yellow; the TIRs are underlined. Acquired donor and acceptor splicing sites (bold) of the newly gained intron (lowercase) are either introduced by the introner (5’ gc or gt), or co-opted from TSD (3’ ag). The example sequence originates from M. pusilla introner family IE1 [6] and illustrates the potential sequence of events leading to the IE-derived intron gain [17, 28]. The actual mechanisms of introner insertion are not known