The Viruses That Make Us: A Role For Endogenous Retrovirus In The Evolution Of Placental Species

Chromosome evolution, higher order and parasitic elements. With the accumulation of genomic sequence data, certain unexplained patterns of genome evolution have begun to emerge. One striking observation is the general tendency of genomes of higher organisms to evolve an ever decreasing gene density with higher order. For example, E. Coli has a gene density of about 2 Kb per gene, Drosophila 4 Kb per gene and mammalian about 30 Kb per gene. Much of the decreased density is due to the increase in the accumulation of non-coding or 'parasitic DNA' elements, such as type one and two transposons. Current evolutionary theory does not adequately account for this observation (81). In addition mammals appear to have retained the presence of at least some copies of non-defective 'genomic retroviruses', such as intercysternal A-type particles (IAP's) or endogenous retroviruses (ERVs), (51,85). It is currently difficult to account for the selective pressure that retains these genomic viruses, since they often lack similarity to existing free autonomous retroviruses. It is widely accepted that viral agents act a negative selecting force on their host. However, viral agents have very high mutation and adaption rates. This character led Salvador Luria to speculate early on that perhaps viruses contribute to host evolution (52).