Extensive structural renovation of retrogenes in the evolution of the Populus genome

Retroposition, as an important copy mechanism for generating new genes, was believed to play a negligible role in plants. As a representative dicot, the genomic sequences of Populus (poplar; Populus trichocarpa) provide an opportunity to investigate this issue. We identified 106 retrogenes and found the majority (89%) of them are associated with functional signatures in sequence evolution, transcription, and (or) translation. Remarkably, examination of gene structures revealed extensive structural renovation of these retrogenes: we identified 18 (17%) of them undergoing either chimerization to form new chimerical genes and (or) intronization (transformation into intron sequences of previously exonic sequences) to generate new intron-containing genes. Such a change might occur at a high speed, considering eight out of 18 such cases occurred recently after divergence between Arabidopsis (Arabidopsis thaliana) and Populus. This pattern also exists in Arabidopsis, with 15 intronized retrogenes occurring after the divergence between Arabidopsis and papaya (Carica papaya). Thus, the frequency of intronization in dicots revealed its importance as a mechanism in the evolution of exon-intron structure. In addition, we also examined the potential impact of the Populus nascent sex determination system on the chromosomal distribution of retrogenes and did not observe any significant effects of the extremely young sex chromosomes.