Viruses Agents of Evolutionary Invention

When I reviewed Planet of Microbes, I remarked that microbes are everywhere. If you are willing to stretch the definition of life a bit further still, there is one entity that is even more numerous and omnipresent: the humble virus. We tend to think of viruses almost exclusively in the context of disease (see for example The Invisible Enemy). But, as virologist and pharmaceutical researcher Michael Cordingley shows here, they are so much more than mere pathogens and have a huge influence on evolutionary processes in all organisms. This book paints a remarkable portrait of these unusual life forms.

And let there be no mistake, viruses are unusual. They straddle the border between living and non-living matter. By themselves, they are inert collections of nucleic acids (either RNA or DNA), often, but not always, wrapped in a protein capsule. They don’t eat, they don’t breathe, they don’t move. For all intents and purposes, they are just another collection of macromolecules that make up the world. But give them a living host and they appear alive: invading, multiplying, and evolving. In the first few chapters, Cordingley highlights many peculiarities and traits that make viruses so important and unique in evolution, and I will highlight three of them here.

For one, they are numerous. Because viruses can infect all lifeforms, including microbes (themselves already very numerous) the virosphere contains an astronomical number of individuals at any given time. A conservative estimate of 10³¹ viruses is mentioned. This is a lot of genetic information for natural selection to work with. The viruses that infect microbes, bacteriophages, make up a large part of this and reproduce quickly due to the short generation time of bacteria.

Second, viruses literally inject their genetic material into cells, and some of it can and will be accidentally incorporated into the host’s genetic blueprint (its genome). This means genetic information is not only moving between generations – from parents to offspring during conventional reproduction – but also within generations in a sideways fashion, which is known as horizontal gene transfer. Quammen will further explore the importance of this in his forthcoming book The Tangled Tree: A Radical New History of Life, but consider for a moment that this mechanism is implicated in the speed with which bacteria can develop resistance to antibiotics, previously discussed in Superbugs. The incorporation of viral DNA in the host’s genome is directly causing mutations and generating variation on which natural selection can act.

The final example I want to highlight are RNA viruses, such as the poliovirus. Replication of RNA, the single-stranded relative of double-stranded DNA, is notoriously messy. Error rates are as high as one per 1000 to 10,000 bases, compared to one per ten million bases for DNA viruses (a base is the unit of genetic information, consisting of an organic molecule known as a nucleotide – in DNA these occur in pairs, hence base pairs, which form the familiar double helix when strung together). This means that the individuals in a population of RNA viruses are so genetically diverse you can’t really call them conventional species anymore. Cordingley introduces the concept of quasispecies: diffuse groups of closely related but genetically distinct individuals. This hypervariability makes viruses particularly adept at adapting. When environments change, the genetic variation present in a population of “conventional” organisms is probably not enough to immediately supply an individual that is optimally adapted, so further evolution will happen. As environments are constantly changing, in reality this means that most “conventional” species are constantly lagging behind, chasing the evolutionary optimum. The hyperdiverse members of an RNA virus population, meanwhile, are far more likely to already have well-adapted members present when environments change, which will immediately be favoured in reproduction. I had not heard of this concept before, but it blew my mind when considering it.

A large part of the book deals with human viruses and the diseases they cause: influenza, HIV-1 and AIDS, ebolavirus, cross-species infections and animal reservoirs of viral zoonoses (i.e. diseases that can be transmitted from animals to humans), and the unique world of retroviruses. Most of these topics have been the subject of dedicated books (e.g. Virus Hunt, Ebola: Profile of a Killer Virus, and Spillover), though not yet retroviruses. This group *must* nestle themselves into the host’s DNA before they can replicate, which means they are by definition heritable. In contrast, most other viruses will use their host for their own replication, but if the host survives and reproduces, the viral DNA is not passed on to the host’s offspring. Skalka will be writing more about these unique viruses in her forthcoming book Discovering Retroviruses.

In all these chapters, the focus is on the mechanics of these viruses and the particular evolutionary viral tricks they use. While introductions are light and terminology and concepts are explained, the reading quickly gets technical, sometimes to the point that I felt I was reading a journal paper rather than a popular science book. Although I found Viruses an incredibly fascinating book, I can’t say it is easily accessible. A basic understanding of microbiology is a must to really get the most out of this book. The lack of a glossary doesn’t help, but probably the biggest drawback is the complete absence of illustrations. The many complicated and often theoretical concepts could have been made much more accessible by illustrations. A Crack in Creation is a shining example of illustrating concepts in bacterial and viral genetics. Given the diversity of topics covered here, the number of required illustrations would perhaps have been prohibitive.

Speaking of A Crack in Creation, given what we now know about the incorporation of viral DNA in the host’s genome post-infection to be used in future immune responses, I was surprised to find no mention of CRISPR in this book whatsoever. Not even in the chapter on using viruses as human tools that covers such topics as the use of viruses in vaccines, as biological control agents, or as agents to combat cancer. It seems like an odd omission in an otherwise very thorough and wide-ranging book.

Though not a popular science book such as, say, A Planet of Viruses, Cordingley nevertheless provides an up-to-date and detailed account of the evolutionary power and prowess of viruses that should appeal to biologists with an interest in evolution or microbiology.