Retrotransposon mediated genomic fluidity in the human and chimpanzee lineages

Abstract

LINE-1s (Long interspersed elements or L1s) and Alus are highly successful non-long terminal repeat retrotransposons with copy numbers of ~520,000 and >1 million within the human genome, respectively. They are associated with human genetic variation and genomic rearrangement. Although they are abundant throughout primate genomes, their propagation strategy remains poorly understood. The recently released human and chimpanzee draft genome sequences provide the opportunity to compare the human genome with the chimpanzee genome. Thus, we were able to assess how these elements expanded in primate genomes and how they create genomic instability during their integration into the host genome as well as subsequent post-insertion recombination between elements. To understand the expansion of Alu elements, we first analyzed the evolutionary history of the AluYb lineage which is one of most active Alu lineages in the human genome. We suggest that the evolutionary success of Alu elements is driven at least in part by “stealth driver” elements that maintain low retrotransposition activity over extended periods of time and occasionally produce short-lived hyperactive copies responsible for the formation and remarkable expansion of Alu elements within the genome. Second, we conducted a detailed characterization of chimpanzee-specific L1 subfamily diversity. Our results showed that L1 elements have experienced different evolutionary fates in humans and chimpanzees lineages. These differential evolutionary paths may be the result of random variation or the product of competition between L1 subfamily lineages. Third, we report 50 deletion events in human and chimpanzee genomes directly linked to the insertion of L1 elements, resulting in the loss of ~18 kb of human genomic sequence and ~15 kb of chimpanzee genomic sequence. This study provides the basis for developing models of the mechanisms for small and large L1 insertion-mediated deletions. Fourth, we analyzed the magnitude of Alu recombination-mediated deletions in the human lineage subsequent to the human-chimpanzee divergence. We identified 492 human-specific deletions (for a total of ~400 kb of sequence) attributable to this process. The majority of the deletions coincide with known or predicted genes, which implicates this process in creating a substantial portion of the genomic differences between humans and chimpanzees.