Models of spliceosomal intron gain. (a) Intron transposition. An intron from one gene is spliced out of an mRNA transcript. That intronic RNA sequence then reinserts into a previously intronless site of a transcript of the same or a different gene. That structure is then retroposed to give a DNA copy of the gene containing an intron at a new site. The retroposed copy then undergoes gene conversion with a genomic copy (not shown). (b) Transposon insertion. A transposon inserts into a contiguous coding region and is transformed into an intron. (c) Tandem genomic duplication. A region including part or all of an exon with an internal AGGT is duplicated. The two homologous AGGTs are then used as 5' and 3' splicing boundaries for a new intron comprising the 3' end of the upstream copy and the 5' end of the downstream copy. (d) Intron transfer. A gene undergoes a gene conversion or simple double recombination with an intron-containing paralog. (e) A self-splicing type II intron, presumably from an organelle of the same organism, inserts into a contiguous region of coding sequence of a nuclear genome and is then converted to a spliceosomal intron.