LINEs and SINEs of Primate Evolution

The primate order is a monophyletic group thought to have diverged from the Euarchonta more than 65 mya.(1) Recent paleontological and molecular evolution studies place the last common ancestor of primates even earlier (>= 85 mya).(2) More than 300 extant primate species are recognized today,(3,4) clearly emphasizing their diversity and success. Our understanding of the evolution of primates and the composition of their genomes has been revolutionized within the last decade through the increasing availability and analyses of sequenced genomes. However, several aspects of primate evolution have yet to be resolved. DNA sequencing of a wider array of primate species now underway will provide an opportunity to investigate and expand on these questions in great detail. One of the most surprising findings of the human (Homo sapiens) genome project was the high content of repetitive sequences, in particular of mobile DNA.(5) This finding has been replicated in all available and analyzed primate draft genome sequences analyzed to date.(5-7) In fact, transposable elements (TEs) contribute about 50% of the genome size of humans,(5) chimpanzees (Pan troglodytes),(6) and rhesus macaques (Macacca mulatta).(7) The proportion of TEs among the overall genome content is likely even higher due to the decay of older mobile elements beyond recognition, rearrangements of genomes over the course of evolution, and the challenge of sequencing and assembling repeat-rich regions of the genome.(8,9) Retrotransposons, in particular L1, long interspersed element 1 (LINE1), and Alu, a short interspersed element (SINE), are prominent in primate genomes, and have played a major role in genome evolution and architecture. The evolution and success of the primate-specific LINE and SINE subfamilies (L1 and Alu in particular), their application in phylogenetic studies, and their impact on the architecture of primate genomes will be the focus of this review. In addition, we will briefly cover the emergence and impact of SVA (SINE-R/VNTR/Alu), a composite retrotransposon of relatively recent origin, and of other SINEs that are not common to all primates.