It's been known for a long time that only 2% - 3% of human DNA codes for proteins. Much of the rest of our genomes - often referred to as junk DNA - consists of retroelements, some of which can occasionally replicate and move to a new location in the genome.
If a retrovirus invades the germline (sperm or egg cells) it can be passed on to offspring. And when such an endogenous retrovirus inserts into the genome near a gene, it can alter gene function and can influence the evolution of its host. Over 8% of our genome is made of these infectious remnants - infections that scientists believe occurred before Old World and New World monkeys diverged (25 - 35 million years ago).
In a new study published in the premier open-access online journal PLoS Biology, Evan Eichler and colleagues scanned the chimpanzee genome sequence for endogenous retroviruses and found one that does not occur in humans. Searching the genomes of various apes and monkeys revealed that the retrovirus had integrated into the germline of African great apes and Old World monkeys - but was not present in humans and Asian apes (orangutan, siamang, and gibbon).
The authors also mapped the locations of more than 100 copies of the retroviruses. The results all point to the conclusion that the retroviruses have not been conserved from a common ancestor, but reflect independent infections. Specifically, Eichler and colleagues estimate that gorillas and chimps were infected about 3 - 4 million years ago, and baboon and macaque about 1.5 million years ago.
As for how this retroviral infection bypassed orangutans and humans, the authors offer a number of possible scenarios. It could be that African apes evolved a susceptibility to infection, for example, or that humans and Asian apes evolved resistance. A better understanding of the evolutionary history and population genetics of great apes will help identify the most likely scenarios. And knowing how these retroviral elements infiltrated some apes while sparing others could provide valuable insights into the process of evolution itself.
Source: Eurekalert & othersLast reviewed: By John M. Grohol, Psy.D. on 21 Feb 2009
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