Anke Van Wyk /
Those puppy kisses may not be so benign afer all, according to new research.
updated 5/25/2011 4:32:07 PM ET 2011-05-25T20:32:07

Humans and dogs may have exchanged genetic material over the millennia via viruses, scientists conjecture.

Retroviruses — the most infamous example of which is likely HIV, the virus that causes AIDS — have the ability to incorporate their genetic material into that of their hosts. In this manner, these hitchhikers can reproduce when their hosts do.

All mammals and most vertebrates, or creatures with backbones, apparently possess these "endogenous retroviruses" in their genomes. In fact, nearly 1 percent of the human genome consists of these unwelcome guests. Mice and opossums are even more greatly compromised, with these viruses making up about 2 percent of their genomes.

To get a broader picture of how deeply retroviruses have invaded genomes, scientists in Sweden analyzed the first sequenced carnivore genome, that of a female dog of the boxer breed.

The researchers discovered that endogenous retroviruses seem to make up only 0.15 percent of the dog genome, six times less than humans. Dogs may have better mechanisms to protect their genomes against retroviruses, or their genomes may house unknown types of retroviruses that current techniques can't yet detect, the researchers say.

Intriguingly, the scientists discovered a novel group of retroviral material in dogs that is highly similar to endogenous retroviruses seen in humans. They belong to a type of virus known as gammaretroviruses, the most frequent type found in mammals to date.

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This specific group of retroviruses seems to have invaded the dog genome relatively recently. This suggests that dogs and humans may have passed these germs to each other due to close interactions during our millennia of history together, a phenomenon known as "lateral transmission." It remains uncertain how such transmission might have occurred — perhaps from wet doggie kisses, for instance.

"We need to stress that we can only say 'potential for possible lateral transmission between dog and human,'" said researcher Göran Andersson, a molecular geneticist at the Swedish University of Agricultural Sciences.

To shed light on if, when and how much this happened, "DNA from more dogs will be analyzed," Andersson told LiveScience.

Such research might not only discover evidence for such lateral transmission, but also could reveal how dogs might be protecting themselves against retroviruses. Such knowledge might help lead to therapies against retroviruses, including perhaps HIV, Andersson said.

The scientists detailed their findings online May 12 in the journal PLoS ONE.

In addition to Andersson, the authors of "The First Sequenced Carnivore Genome Shows Complex Host-Endogenous Retrovirus Relationships" include Alvaro Martinez Barrio, Marie Ekerljung, Patric Jern, Farid Benachenhou, Goran O. Sperber, Erik Bongcan-Rudloff and Jonas Blomberg.

Follow LiveScience for the latest in science news and discoveries on Twitter @livescience and on Facebook.

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Explainer: The world’s 10 smartest animals

  • EBU

    We humans have the ability to learn, to reason and solve problems. We're self-aware, and we're also conscious of the presence, thoughts and feelings of others. We make tools and practice the art of deception. We're creative. We think abstractly. We have language and use it to express complex ideas. All of these are arguably signs of intelligence. Scientists may not agree on the best and fullest definition of intelligence — but they generally agree that humans are highly intelligent.

    Other members of the animal kingdom exhibit signs of intelligence as well, and some scientists might say the definition of animal vs. human intelligence is merely a matter of degree - a point that was brought home in 2005 when the London Zoo put "Homo sapiens" on display in the exhibit pictured here. Click ahead to learn about nine other species that stand out for their smarts.

  • Chimps are almost like us

    Tetsuro Matsuzawa  /  AP

    If we humans possess intelligence, chimpanzees must have some as well: Our genomes are at least 98 percent identical. Chimps make and use tools, hunt in organized groups and engage in acts of violence. Wild troops have distinct behaviors and customs. Field observations and lab experiments show chimps are capable of empathy, altruism and self-awareness. In the experiment pictured here, chimps performed better than humans on a number memory test.

  • Dolphins get creative

    Janet Mann  /  Georgetown University

    This dolphin in Australia uses a sponge to protect her snout when foraging on the seafloor, a tool use behavior that is passed on from mother to daughter. Scientists say that's just one sign of dolphin smarts. Other signs include distinct whistles and clicks that may serve as dolphin names, perhaps used in a type of language. A famous 1960s experiment found that a pair of dolphins entered a tizzy of creativity once they figured out their novel behaviors were rewarded with fish. Frustrated human test subjects just let out a sigh of relief when they caught on to the idea.

  • Elephants exhibit self-awareness


    The sheer size of their brains suggests that elephants must know a thing or two about the ways of the world. They have been seen consoling family members, helping other species in times of need, playing in water and communicating with one another via vibrations sensed in their feet. A crowning achievement, some researchers say, was when this female Asian elephant named Happy recognized herself in the mirror. The complex behavior is shared only with humans, great apes and dolphins.

  • Cephalopods have big brains

    Binyamin Hochner

    Are octopi, squids and cuttlefish smart? That's a matter of scientific intrigue, but such cephalopods are certainly among the brainiest invertebrates in the sea. The cephalopod brain surrounds the esophagus, but shares with the human brain features of complexity such as folded lobes and distinct regions for processing visual and tactile information. The how-smart debate swirls around deciphering observations that the creatures have a seemingly irrepressible curiosity, a disdain for boredom, an ability to learn and the capacity to use tools. The octopus pictured here exerts precise muscle control to eat.

  • Crows get crafty

    Alex Kacelnik et al.  /  University of Oxford

    Crows are crafty critters: They fashion tools from twigs, feathers and other bits of debris to snare food from hard-to-reach places. A crow named Betty, pictured here, uses a straight wire she bent into a hook to retrieve food from a tube. The birds are born with a tool-making ethic, but they hone their craft by watching their elders, a sign of higher intelligence. Ravens, a type of crow, have even been shown to manipulate the outcomes of their social interactions for added protection and more food.

  • Squirrels can be deceptive

    Gabriel Bouys  /  AFP - Getty Images file

    Is the squirrel pictured here plotting deception? Perhaps. Researchers recently reported that the rodents put on elaborate shows of deceptive caching to thwart would-be thieves. The behavior increased in a lab experiment after squirrels observed humans stealing their peanuts. The researchers called the finding a sign that squirrels can interpret intentions of others, though it could just be a case of learned behavior. Other studies have shown the critters make three-dimensional maps to recall where they cache their nuts. And squirrels in California will cover their fur in the scent of rattlesnakes to mask their own scent from predators.

  • Man's best friend

    University of Vienna

    Are dogs intelligent or just really good at basic obedience? They can learn to sit, lie down and fetch, for example, but can they read their owner's intentions? Research suggests they can at least find food in response to non-verbal cues, a type of understanding that scientists think may be akin to the human ability to understand someone else's point of view. The dog in the experiment pictured here accurately discriminated between photos of dogs and photos of landscapes — an indication the dog was able to form the concept of "dog."

  • Cats are adaptable

    Bob Pennell  /  AP

    Like dog owners, some cat owners have trained their pets to sit down, roll over and jump through hoops. Cats learn the tricks by observation and imitation, egged on with positive reinforcement. But training cats is harder than dogs. Does that mean they are less intelligent? Not necessarily. Cat experts say felines are just different. They are solitary animals, motivated by the need to survive. This has allowed them to adapt to a variety of domestic environments for at least 9,500 years - even the hoods of cars.

  • Pigs are wise ... and clean

    Paulo Whitaker  /  Reuters

    Here's the dirt on pigs: They are perhaps the smartest, cleanest domestic animals known - more so than cats and dogs, according to some experts. But pigs don't have sweat glands, so they roll around in the mud to stay cool. A sign of their cleverness came from experiments in the 1990s. Pigs were trained to move a cursor on a video screen with their snouts and used the cursor to distinguish between scribbles they knew and those they were seeing for the first time. They learned the task as quickly as chimpanzees.


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