Sara Reynolds, Ph.D.
Assistant Professor, Fairleigh Dickinson University, Madison, New Jersey
It is widely accepted that the basic unit of all life is a cell, a single entity composed of a double-stranded DNA genome containing regions called genes. These genes encode the information necessary to make ribosomes, enzymes that synthesize proteins and act as the core of the cell’s metabolism, as well as other proteins and enzymes that respond to the cell’s environment. Surrounding the genome is a gelatinous cytoplasm where much of the cell’s metabolism takes place. All of this is bounded within an outer cell membrane.
Some organisms have evolved complexity above this basic cellular structure (check out some of the previous Taxon of the Month posts). For example, eukaryotes have extra membranes to organize and separate cellular components.
In contrast to cells, the basic unit of a virus is a genome composed of either single- or double-stranded DNA or RNA surrounded by a protein coat called a capsid. That’s it - no ribosomes or gelatinous cytoplasm and no metabolism of their own. The number of genes found on a virus is surprisingly small. For example, while the bacteria E. coli has about 500 (Lukjancenko, 2010) and you (a human) have about 19,000 -25,000 (Ezkurdia 2014), the ever burdensome HIV virus has only 9 genes (Li 2015) in its genome.
While viruses might lack morphological complexity, they certainly have evolved a wide range of behaviors. For example, some viruses wrap themselves in membranes derived from host cells (Figure 2; Buchmann 2015). Most viruses have a narrow host range, but some viruses have accumulated enough adaptations that they actually develop the ability to infect new host species or types of host cells, broadening their host range (Haller 2014).
As our understanding of the genomic material of humans and viruses has increased one very interesting aspect of evolution has emerged; viruses and their hosts sometimes exchange genetic material. Examples of viral genetic material has been found in the genomes of animals (Blanc 2015), plants (Maumus 2014) and more (Sharma 2014). As a matter of fact, the encounters that eukaryotes, including humans, have with viruses appears to provide an “evolutionary push” (Enard 2016; Moeling 2013; Yi 2004). Similarly, viruses can capture host genes and adapt them for use in the viral life cycle (Filée 2014). In poxviruses, many of these genes are altered to inhibit the host defense against the viral infection (Ouyang 2014; Martin 2012).
The discovery of larger, complex viruses has added new fuel to the debate regarding whether viruses are alive (like cells) and the implications that has regarding the origin of viruses. Recently discovered Klosneuviruses have genomes that are more cell-like than any previously identified virus with the largest known panel of enzymes critical for ribosome activity (Schulz 2017), but they still require host ribosomes. These viruses belong to the Nucleo-Cytoplasmic Large DNA Viruses (NCLDV), a monophyletic clade of large and complex viruses that contain up to 2500 genes (Koonin 2010). Members of the NCLDV family infect a wide range of eukaryotic cells and the adaptations that allow infection also demonstrate virus evolution, just like how living hosts have evolved to defeat them.
Another family found within NCLDVs are Poxviruses, some of which famously use humans as their host. They have a double-stranded DNA genome containing between 130 and 360 genes (Hughes, 2010). The family is divided into two subfamilies; one that infects insects (entomopoxvirinae), and another that infects a variety of vertebrates (chordopoxvirinae, Figure 3). Within the chordopox subfamily is the orthopoxvirus genus, which includes numerous species important to humans (Figure 1). Variola virus causes smallpox, one of the most devastating diseases in human history; and vaccinia virus, the weaker vaccine virus that eradicated smallpox from the natural world (Henderson 2009; Smithsom 2014). Closely related is monkeypox, an emerging zoonotic virus in Africa. In 2003, it made its way to the U.S. and infected at least 37 people (Reed 2004). While no one died, monkeypox has emerged as a potential bioterrorism threat with up to 10% mortality in some regions of Africa (McCollum 2014). Monkeypox, and related cowpox, both infect a wide range of hosts but cause a milder form of the disease than variola (Haller 2014; McCollum 2014). Cases of CPXV continue to appear in humans and animals in Europe and Asia (Vorou 2008), and even the smallpox vaccine virus has emerged as a pathogen in South American cattle (Franco-Luiz 2014). Molluscum contagiosum virus is a human-specific non-orthopoxvirus that, although it shows no mortality today (Mguyen 2014), could potentially evolve and adapt to better parasitize its human host.
Without traditional fossil records, viral evolution is investigated primarily through genomics. Genomic data indicate that the parasitic lifecycle of viruses also makes their evolution highly dependent upon host evolution and availability (Hughes 2010). Indeed, the very thing that made variola virus so capable of devastating human populations, its strict human host specificity (6), also made smallpox the first disease ever eradicated. Variola virus only infected humans and lacked any animal reservoirs, which meant that the implementation of a vaccine in humans was able to break the chain of transmission from person-to-person and thus ending all new infections (Henderson 2009; Smithson 2014).
The culmination of competitive and host-derived pressures on viruses have shaped how viruses evolve, yet describing the exact nature of that evolution can be challenging. Understanding how the deadly smallpox virus (VARV) evolved to be the human-specific killer that plagued civilization for centuries is an example of that challenge. Yet, understanding the nature of how VARV evolved from ancestral poxviruses could help researchers to understand how other deadly pathogens could arise in the future. Here we will explore just a few hypotheses.
First, other poxviruses such as ectromelia virus (ECTV), an orthopoxvirus which infects mice, and myxoma virus (MYXV), a leporipoxvirus which infects rabbits, can have high mortality (death) rates in non-natural host species. Some have suggested that a host switch under specific circumstances of an ancestral, highly lethal poxvirus could explain the evolutionary origin of smallpox (Smithson 2014).
A similar, second hypothesis is derived from the belief that cowpox virus (CPXV) is the ancestral orthopoxvirus, containing all the genes found in any other orthopoxvirus. It is possible that the gradual restriction of host range on CPXV could lead to gene loss and evolution that created highly-specific and highly-lethal pathogens such as VARV (Smithson 2014; Haller 2014).
In contrast to these first two hypotheses, the phylogenetic analysis of poxviral genomes indicate that VARV is significantly more similar to camelpox (CMLV) and taterapox virus (TATV), than to other members of the orthopoxvirus family. However the presence of genetic recombination, or the swapping of gene material, likely played a significant role in the subsequent evolution of VARV which complicates the ability to examine phylogenetic relationships based on genomes sequence (Hughes 2010; Smithson 2014).
Researchers may never know who came first, the virus or the cell, but a better understanding of how viruses infect hosts and the influences that drive viral evolution will better prepare us to predict and understand the epidemics that emerge in the future. The parasitic nature of virus-host interactions makes it particularly challenging to study evolution. Viruses must adapt to defeat the host, but also to the adaptations that the host is making to defeat the virus. Strategies such as host gene capture attest to these adaptations, yet complicate our interpretations of viral relationships or the timelines of speciation. Regardless of the evolutionary path that has led to the diversity of poxviruses today, one fact is abundantly clear; poxviruses are not done evolving!
Interested in learning more about viruses and their impact on humans? I highly recommend the following books:
D. A. Henderson, M.D. 2014. Smallpox: the death of a disease.
C. J. Peters, M.D. 1997. Virus Hunter: thirty years of battling hot viruses around the world.
About the Author
Sara Reynolds received her Ph.D. in 2016 from the University of Maryland and in cooperation with the National Institutes of Health (NIH). Her dissertation research focused on a single large gene in cowpox, monkeypox and ectromelia poxviruses. Interestingly, this gene reduced the severity of disease during infections. Dr. Reynolds is currently an Assistant Professor at Fairleigh Dickinson University in Madison, New Jersey. Her present work focuses on using PCR technology to detect the presence of viral and bacterial species in the waters of Northern New Jersey. Correspondence can be directed to reysa@FDU.edu
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