Transposable elements: insertion pattern and impact on gene expression evolution in hominids.

Transposable elements (TEs) can affect the regulation of nearby genes through several mechanisms. Here, we examine to what extent recent TE insertions have contributed to the evolution of gene expression in hominids. We compare expression levels of human and chimpanzee orthologs and detect a weak increase in expression divergence (ED) for genes with species-specific TE insertions compared with unaffected genes. However, we show that genes with TE insertions predating the human-chimpanzee split also exhibit a similar increase in ED and therefore conclude that the increase is not due to the transcriptional influence of the TEs. These results are further confirmed by lineage-specific analysis of ED, using rhesus macaque as an outgroup: Human-chimpanzee ortholog pairs, where one ortholog has suffered TE insertion but not the other, do not show increased ED along the lineage where the insertion occurred, relative to the other lineage. We also show that genes with recent TE insertions tend to produce more alternative transcripts but find no evidence that the TEs themselves promote transcript diversity. Finally, we observe that TEs are enriched upstream relative to downstream of genes and show that this is due to insertional bias, rather than selection, because this bias is only observed in genes expressed in the germ line. This provides an alternative neutral explanation for the accumulation of TEs in upstream sequences.