Primate and rodent specific intron gains and the origin of retrogenes with splice variants.

Retroposition, a leading mechanism for gene duplication, is an important process shaping the evolution of genomes. Retrogenes are also involved in the gene structure evolution as a major player in the process of intron deletion. Here, we demonstrate the role of retrogenes in intron gain in mammals. We identified one case of "intronization," the transformation of exonic sequences into an intron, in the primate specific retrogene RNF113B and two independent "intronization" events in the retrogene DCAF12L2, one in the common ancestor of primates and rodents and another one in the rodent lineage. Intron gain resulted from the origin of new splice variants, and both genes have two transcript forms, one with retained intron and one with the intron spliced out. Evolution of these genes, especially RNF113B, has been very dynamic and has been accompanied by several additional events including parental gene loss, secondary retroposition, and exaptation of transposable elements.