Molecular and temporal characteristics of human retropseudogenes

One of the primary forces driving genome evolution is retrotranscription. In addition to creating new genetic material from which new genes with new functions arise, retrotranscription leaves traces of its action in the form of retropseudogenes. These loci, which are intronless, retrotransposed copies of mature mRNAs from functional antecedent genes, are layered throughout genomes as a molecular fossil record of genome evolution. A survey of 138 functional source genes in the human genome has revealed more than three hundred retropseudogenes. Analysis of the characteristics of the source genes shows that, on average, their size, G/C content, and expression patterns fit the canonical features of source genes reported elsewhere. Details of insertion site duplications for these loci are consistent with a model of retropseudogene formation involving endogenous retrotranscription and enzymatic mobilization and retroposition. Retrotranscription event age estimates reveal a pattern in which the highest densities appear after major phylogenetic events in primate history and then decline. This temporal pattern suggests that the processes forging genome evolution are most active during periods of speciation and adaptive radiation and then steadily diminish until the next burst of activity.