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Today's selection -- from Evolving Ourselves by Juan Enriquez and Steve Gullans. Trillions of viruses inhabit each of our bodies, far outnumbering both our cells and the bacterial cells our bodies host. Collectively, these viruses are referred to as our "virome," and they impact the way our genome expresses itself and evolves. Whereas in the past, scientists thought of our genes (referred to collectively as our genome) as being the primary determinant of our development, scientists now believe that there are three additional layers that have a profound impact on how our genes express themselves and how our bodies develop and function -- the epigenome (the complex array of matter surrounding our genes), the microbiome (the trillions of bacterial cells hosted by our bodies), and the virome:
"We live in a world filled with viruses; they are everywhere that host species exist. Viruses infect and live among and within all species from bacteria to blue whales and redwoods. While there are an estimated 10 billion bacteria in a liter of seawater, there are also 100 billion viruses joyfully playing with them. (Think about that the next time you swallow a mouthful.) Even more may live inside the soil and dirt that cover your hands. Viruses continuously shuttle in and out of your body, and there are at least ten times more of them inside you, every day, than there are bacteria.
"The human virome includes trillions of viruses that live in and on our cells, plus even more that inhabit the bacteria in our microbiome. The virome is poorly understood and could be considered the 'dark matter of nature' and humanity; we know it is there but have a very hard time describing it or knowing what it is doing. The human virome is essentially our fourth genome; it interacts directly and indirectly with our other three genomes. Moreover, like your genome, epigenome, and microbiome, your virome is absolutely unique. Viruses live in our intestines, mouths, lungs, skin, and even in our blood, the latter being only discovered recently. But fret not; given that people are generally healthy day-to-day, the virome overall must be benign, and given the millennia of mutual coexistence, our viromes must provide benefits that we don't yet appreciate.
"Viruses are champions of DNA mutation. A 2013 study of the human gut virome tracked the identities, abundance, and mutations of native viruses in one person over two and a half years. There were 478 relatively abundant viruses, most of which had not been previously identified. A majority of the viruses were bacteriophage, the type that infects bacteria. Eighty percent of the viruses persisted for the entire 2.5 years, but they all mutated, some slowly, some very quickly. In some cases they mutated so fast that the virus would be deemed a new species within the 2.5 years. What came out of the body after symbiosis was very different from what went in.
"So viruses, our ubiquitous interlopers, are an important part of our rapid evolution; they carry, exchange, and modify the DNA between cells or from one species to another. They drive evolution at all scales, in bacteria, plants, animals, and humans. The best example of this is the spread of antibacterial-resistance genes from one bacterial organism to its fellow species, and then to other bacteria of all types, in all geographies. Once a beneficial mutation arises in a microbe, viruses help spread it quickly throughout the microbiome and beyond.
"A single sneeze propels 40,000 droplets, each containing up to 200 million individual viruses, across the room at speeds exceeding 200 miles per hour. (Amazing how viruses created a way to make us sneeze so they could infect new people.) After you breathe in this viral code, it enters your cells, reproduces, releases trillions of copies of itself, and proceeds to take over other cells. Sometimes viral DNA simply embeds itself in your own human DNA, where it can lie dormant or sometimes come back to life when you least want it, as occurs with recurring cold sores, shingles from a long-past chicken pox, and even some cancers -- particularly when our immune systems become weak. This is exactly what occurs with Kaposi's sarcoma in immunodeficient HIV patients. On some occasions, viral code can end up in the DNA in your sperm and eggs, which then gets passed on to future generations.
"Over long periods three things can happen as viruses invade organisms. One, they kill the organisms (for example: Ebola, or the 1914 flu). Viral epidemics kill millions of humans, animals, insects, trees, and bacteria, but in doing so the virus is left with fewer and fewer hosts to infect and eventually tends to disappear. A second option is the body kills the virus and the virus stops propagating. A third option is the virus and the host species (for example, humans) coevolve; the virus is ever less nasty and the body stops bothering to attack it. It is this third option, the symbiosis between humans and viruses, that we know the least about. This is largely because investigators tend to focus their energies on the evil viruses that are contagious killers. (And it is hard to get grants to study the nice viruses.)"
Juan Enriquez and Steve Gullans
Evolving Ourselves: How Unnatural Selection and Nonrandom Mutation are Changing Life on Earth
Copyright 2015 by Juan Enriquez and Steven Gullans