Clues to future evolution of HIV come from African green monkeys

Monkey viruses related to HIV may have swept across Africa more recently than previously thought, according to new research from The University of Arizona in Tucson.

A new family tree for African green monkeys shows that an HIV-like virus, simian immunodeficiency virus, or SIV, first infected those monkeys after the lineage split into four species. The new research reveals the split happened about 3 million years ago.

Previously, scientists thought SIV infected an ancestor of green monkeys before the lineage split, much longer ago.

"Studying SIV helps us learn more about HIV," said the paper's first author Joel Wertheim, a doctoral candidate in the UA department of ecology and evolutionary biology. "This finding sheds light on the future direction of HIV evolution."

All SIVs and HIVs have a common ancestor, added senior author Michael Worobey, a UA assistant professor of ecology and evolutionary biology.

The new work suggests African green monkeys' SIVs, or SIVagm, may have lost their virulence more recently than the millions of years previously thought. Green monkeys almost never get sick from SIVagm. If SIVagm was once a monkey killer, the change in its virulence may shed light on the future course and timing of the evolution of HIV.

The new research also challenges the idea that one ancient SIV was transmitted vertically, down through time, and evolved into many SIVs as its original host diverged into many different species.

Wertheim and Worobey suggest various SIVs arose because SIVs were transmitted horizontally, between primate species, and evolved into a new host-specific form only after transmission.

HIV arose from chimpanzee SIV that was transmitted to humans, probably when people had contact with chimpanzee blood from hunting and butchering the animals, Worobey said.

The team's research article, "A Challenge to the Ancient Origin of SIVagm Based on African Green Monkey Mitochondrial Genomes," is in the July issue of PLoS Pathogens and can be found at http://www.plospathogens.org. The National Science Foundation and National Institutes of Health funded the research.

Previous research had sketched out the family trees, or phylogenies, of the four species of African green monkeys and their accompanying SIVagm, but Wertheim wanted to know more.

"I wasn't convinced by the evidence out there that these monkeys were infected before they speciated," Wertheim said. "So I set out to perform a rigorous test of that hypothesis."

He extensively sequenced the mitochondrial DNA genes of the four species of African green monkeys. Mitochondrial DNA is passed from mother to child.