Author: Frank Ryan
Viruses are widely acknowledged as the most vicious microorganisms to afflict human beings, and their infamous history of maiming and killing stretches far back in time. It is however little known that viruses have played, and continue to play, an indispensable role in the survival of the human race. This symbiotic relationship is the subject of this book which explores how, over millennia, viruses repeatedly infiltrated and permanently entrenched themselves in the human genome, a process the author designated as symbiogenesis. In this way, viruses have acted as an ‘evolutionary force‘ in transforming both the structure and the function of the human genome (pages 62-70). The author’s key message is that human-viral co-evolution is a fundamental mechanism of natural selection, and to fully understand their impact on us, he advocated studying viruses in their natural hosts (pages 5 and 26).
The major theme of the book is the mechanism whereby retroviruses, over the ages, have repeatedly integrated their genes into human DNA, and in this way they have significantly restructured the human genome. Whilst the eventual outcome is a favourable remodelling of the human genome, the initial phase is nothing but destruction on an epic scale; this is the stage of aggressive symbiosis during which viruses mutilate and decimate humans in the course of fierce epidemics and pandemics. The author argues that these ‘exogenous retroviral colonisation events’ have been regular occurrences ‘over the hundred million or so years of our mammalian history’, and they eventually result in a ‘retroviral-induced genomic reconstitution‘ (page 141). Significantly, the author shows that these events are still taking place, supporting this assertion with the example of HIV which he says is ‘the latest wave’ of retroviral infections to have ‘evolutionary implications‘ in humans (pages 77, 97, and 133).
Perhaps the most striking feature of the book is its depiction of the process of viral colonisation of the human genome. The author describes this as a stereotyped routine which is triggered when a virus jumps into humans from a closely related species. A classical example the author cites is how HIV infection emerged when the simian version of the virus crossed over from chimpanzees and mangabey monkeys (page 86). Such transitions provoke a deadly phase of ‘plague culling‘ during which the virus kills ‘all those who cannot live with its presence’; the virus and the survivors of the outbreak subsequently settle into ‘long-term co-evolution‘ and ‘mutualistic partnership‘ (pages 90-93). The author cites other contemporary examples of this type of symbiogenesis, the most fascinating probably being the one between koala bears and the koala retrovirus (KoRV); this symbiosis has already gone through the aggressive culling phase, and the virus is now invading the germ lines of the surviving koala bears, an ‘endogenisation‘ process that will result in ‘genome fusion‘ and a ‘new holobiontic partnership‘ (pages 121 and 104-114).
To support his assertion that viruses have influenced the evolution of the human genome, the author points to the large amounts of viral material that is found in human DNA (page 99). Indeed he demonstrates that a staggering 43% of the human genome is made up of viral elements; the influential human endogenous retroviruses (HERVs), for example, make up 9% of the human genome, far exceeding the 1.5% of the genome that distinguishes us as human. The book refers to several other retroviral constituents of the human genome such as retrotransposons, and short and long interspersed nuclear elements (SINEs and LINEs). Because viral colonisation of the human genome is a continuing process, the most recent to be ‘endogenised‘ being HERV-K, the author makes the self-evident point that the viral proportion of the human genome is only likely to increase with time (page 102, 129, and 137).
The consequences of symbiogenesis on the host are predominantly beneficial, and the book illustrates this in several places. Notable examples of the invaluable roles of viruses in the human genome include the part endogenous retroviral genes play in regulating the production of keratin, parathyroid hormone, and amylase, and their pivotal role in governing the development of the placenta, a tissue which is crucial in nourishing the foetus, and in protecting it from the maternal immune system (pages 145-148). The book also points to the way viral components of our DNA regulate important metabolic functions in organs as diverse as the adrenals, the testes, and the brain (pages 150-152). Symbiogenesis unfortunately also has an enduring dark side, and the book portrays this with the way deletions and duplications in HERV genes result in disorders such as systemic lupus erythematosus (SLE), rheumatoid arthritis, multiple sclerosis (MS), neurofibromatosis, haemophilia, and cancer (chapters 8 and 9, and page 174).
It is quite revealing that symbiosis is a ubiquitous phenomenon in nature, and there are many examples of this throughout the book. One of the most remarkable is the symbiotic relationship of parasitic wasps with polydnaviruses; in this partnership, the wasp injects both its eggs and the virus into butterfly or moth caterpillars. Once inside the caterpillars, the virus aids the maturation and survival of the eggs by first disabling the caterpillar’s immune system, and then by ‘compelling the caterpillar to produce sugars to feed the larvae’. More intriguingly, the virus prevents the caterpillar from prematurely developing into a butterfly or moth (page 94). Another striking example is how a virus assists the sea slug, Elysia chlorotica, to attach its larvae to the algae Vaucheria litoria; the virus then assists the larvae to absorb chloroplasts from the algae, and to use this to develop its own photosynthetic mechanism (pages 9-12). The pervasiveness of symbiosis in nature is perhaps best reflected by the breath-taking scale of the symbiotic relationships that exist between fungi and plants, the author pointing out that almost every known plant has a partnership with a fungus (pages 61 and 82).
This book explores a truly groundbreaking concept, the far-reaching influence of viruses in human genomic evolution. With very compelling and illustrative examples, the author convincingly shows how endogenous retroviruses aggressively colonise the human genome, and how this eventually turns out to be a largely profitable venture for humans. The book graphically illustrates the implications of human-viral co-evolution on human health and disease, and it outlines the importance of further research in this field. The prose is a bit disjointed, sometimes tending to be unfocused and long-winded, but this did not detract from the fascinating and impactful content which is revealing and insightful.
Viruses play a significant role in health, and this book demonstrates the scale of this influence. It depicts how aggressive retroviral infections emerge, and how the viruses frequently endogenise and produce long term benefits in the human genome. The author urges a better understanding of the relationship of viruses and humans, particularly advocating research in their natural habitats. The lessons of the book are important for healthcare, and I recommend it to all doctors
Publisher, Place, Year: Collins, London, 2009
Number of chapters: 15
Number of pages: 390
Star rating: 4