In the genome of the endangered gray mouse lemur, scientists have found DNA fossils of an ancient version of HIV.

Its exact origins are a mystery, but the clues could help solve a more modern and pressing unknown: how humans can beat AIDS.

Remnants of the ancient retrovirus were found by virologists studying the squirrel-sized primate on its island home of Madagascar. The DNA was deposited when viruses managed to infect sperm or egg cells, writing permanent and heritable copies of themselves into the genetic code.

Virtually every animal's genome contains fragments left by retroviruses over millions of years. (Incredibly, retrovirus genes may outnumber human genes in our own genome.) To a virologist, every genome is like a Grand Canyon cross-section or piece of Burgess shale.

But deciphering genetic strata is more difficult than dating sedimentary deposits: It's hard to tell exactly when a virus showed up. It's possible that this particular virus — technically known as pSIVgml, or prosimian immunodeficiency virus of the gray mouse lemur — was present when Madagascar split from Africa.

That would make pSIVgml at least 65 million years old — the oldest ancestor of modern HIV yet found. But it's also possible, write Stanford University virologists Robert Gifford and Robert Shafer in the latest Proceedings of the National Academy of Sciences, that pSIVgml arrived on the island at some later date, jumping from another species just as HIV jumped from primates to people.

But while HIV destroys human immune systems, leaving bodies vulnerable to ultimately lethal diseases, pSIVgml doesn't affect the lemurs. They may have evolved an immunity to it.

In an accompanying commentary, Harvard Medical School microbiologist

Welkin Johnson suggests that scientists compare the ancient virus with its descendant, resurrecting ancient genes and comparing them to modern counterparts. Perhaps we can discover the lemurs' secret.

"The lemurs may present a novel opportunity to glimpse ... a distant mirror of the AIDS epidemic," wrote Johnson.

Unfortunately, there's a catch: Like so many other animals on Madagascar, the gray mouse lemur is in grave danger of going extinct. The few specimens gathered won't be enough.

"One or a few DNA sequences does nothing to capture the diversity represented by an entire species or population," he said in an interview. "It would be virtually impossible to deduce social or behavioral characteristics, or even complex physical characteristics, by glancing at a single DNA sequence or by testing a biological sample."

In the case of some endangered species, conservationists must appeal to basic sympathies for other living species. Such arguments don't always work: Snail darters and spotted owls are wonderful creatures, but won't be universally missed.

Helping the gray mouse lemur, however, means helping ourselves.

They're a great opportunity: "Hopefully," writes Johnson, "an opportunity gained, not lost."

A proviral puzzle with a prosimian twist [PNAS] (not yet online)*

A transitional endogeneous lentivirus from a basal primate and implications for lentivirus evolution [PNAS]

Image: flickr/Arthur Chapman

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