Viruses and the Evolution of Life
Author: Luis P. Villarreal1Category: Viruses and Viral Pathogenesis
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Viral Studies conducted in the 1950s contributed tremendously to the scientific understanding of molecular mechanisms present in living organisms. Viruses and the Evolution of Life is an intriguing presentation of the virus-host relationship, as revealed through an examination of host evolution. This new volume avails the informed reader of a new perspective on the evolution of life while targeting the expert reader with discussion of specific scientific literature.
While it is generally accepted among evolutionary biologists that viruses are not self-sustaining and therefore cannot be alive, their impact on life is dramatic. Starting with prebiotic precursors of life, the book features an engaging discussion of various life forms and how each group has been affected by viruses. The author develops the concept of and distinction between acute and persisting viruses, an area of study that has heretofore largely been ignored. The relationship between the familiar disease-causing viruses and those viruses from which they emerge is also clarified.
Electronic Only, 395 pages, illustrations, index.
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Chapter 1 : Overall Issues of Virus and Host Evolution
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Abstract:
This chapter presents the evolution of viruses from the perspective of the evolution of their hosts. It provides an introduction to the book Viruses and the Evolution of Life that presents broad patterns for the evolution of life, evaluates the role of viruses in host evolution as well as the role of the host in virus evolution and also seeks to broadly consider and present the role of persistent viruses in evolution. Another way to think of virus evolution is to consider it from the perspective of host evolution. A chapter in this book that presents plants and insects has the unusual organization of considering the evolution of plants, insects, and their viruses all together. Another chapter is presented from an evolutionary perspective, initially addressing those animals that were first to evolve and diverge and also considering the viruses that infect them. Viruses are part of this world and have an evolutionary power that is immense and unmatched by any other living entity.
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Chapter 2 : Insights from Simulated Evolution
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Abstract:
All biological systems, including viruses, are essentially systems that store, copy, and express information. This chapter presents some of the insights of simulated evolution and attempts to evaluate the relevance of the simulations to extant biological systems and processes that we can now observe. The study of chemical replicators, attempts to create models of catalysis from which prebiotic characteristics can be determined. The parasites of parasitic replicators would correspond to the defective viruses that are observed for most types of viruses. Defective viruses are thus exactly the parasitic replicators of a functional virus, itself a parasitic replicator. These parasites of parasites are expected to have existed even under prebiotic conditions. In addition, computer-based modeling of replicator evolution also suggests a role for the parasitic replicators as well as the parasites of parasites. The temporal component for persistent virus selection may be even more extended than for lambda. Clearly, models must first develop more formal ways to define the issues of persistence before they can provide useful insights into the successful life strategies of a persistent virus.
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Chapter 3 : Viruses and Unicellular Organisms
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The viruses that infect unicellular organisms were among the very first viruses to be studied, and to this day, they remain the best understood of all viruses. The study of phage-phage gene function is especially well developed in the very large microbiological populations of the dairy fermentation industry. The terms temperate and lysogenic are often used interchangeably, although they can sometimes be differentiated. Other systems of virus restriction are also known, such as small interfering RNAs. These virus restriction systems are common to all bacteria and, among prokaryotes, are essentially invariant. Both temperate phages and lytic phages appear to become parasitized by either defective viral or subviral agents. This chapter discusses defective versions of lambda and P2 and the relationship of P2 to P1. The likely origins of pili are discussed, but their similarity to the capsid proteins of filamentous phages suggests that these sex plasmids are derived from persistent viral parasites. It has always been clear that a prophage can confer on its host bacterium the rather complex phenotypes associated with the acquisition of virulence factors. For the most part, these factors are phageborne toxin genes, such as those for diphtheria toxin, erythrogenic toxins, staphylokinase, enterotoxin A, Shiga-like toxin, and botulinum toxin. Pathogenicity islands (PAIs) constitute a well-studied, plasmid-mediated genetic system which has received much attention due to its obvious medical importance.
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Chapter 4 : The Dilemma of the Big Transition in Evolution: the Eukaryotes
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The evolution of the eukaryotic nucleus and the eukaryotic cell was the largest discontinuity in the evolution of all life. This chapter presents the possible role of viruses in acquisition of the nucleus, as well as other evolutionary discontinuities. As large DNA virus families have clear links to both prokaryotes and eukaryotes, they could be of central importance in the evolution of eukaryotes, although it is clear that extant viruses may also have developed and diverged after the nucleus was formed. The pore structures of the eukaryotic nucleus pose another significant dilemma for the possible prokaryotic origin of the nucleus. The author discusses the dilemma from the perspective of a putative viral origin. The viruses that are now commonly found in unicellular eukaryotes such as microalgae are of special interest in the evolution of eukaryotes because they may shed light on the relationships of large DNA viruses to the eukaryotic host and its evolution. Paramecium bursaria Chlorella virus 1 (PBCV-1) is the prototype for the phycodnavirus family, which includes chlorella viruses. In addition, photosynthetic organisms, such as cyanobacteria and Chlorella green algae, undergo photoinactivation of photosynthesis. Numerous other PBCV-1 genes also show a related pattern of similarity to eukaryotic genes and to viruses of eukaryotes and prokaryotes, but not to prokaryotic cellular genes. Overall, the viruses that infect algae appear to have most of the characteristics that would be needed to span the prokaryotic and eukaryotic kingdoms.
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Chapter 5 : Microscopic Aquatic Organisms and Their Viruses
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In spite of common immersion in pools of mainly diverse types of DNA-containing viruses, the orders of Protista are not typically associated with infections by large DNA viruses but are instead more often associated with RNA virus infections. This chapter collectively deals with the nonalgal aquatic microscopic species that make up a rather diverse set of eukaryotic organisms, which includes the protists, ciliated protozoa, dinoflagellates, and the lower and higher Fungi. It examines the best-studied examples of virus and host to consider virus-host interactions. The main types of viruses to be considered are dsRNA viruses, single-stranded RNA (ssRNA) viruses of Fungi, and linear dsDNA viruses of Fungi. The majority of mycoviruses viruses have dsRNA genomes and, like most fungal viruses, lack an extracellular transmission phase. The various aquatic microorganisms, their general characteristics, the types of viral agents they support, and the relationship of virus and host presented in the chapter are summarized in a table. In conclusion, it is worth considering why all distinct characteristics of fungal virus-host interactions are mostly peculiar to the fungal orders of organisms and yet are generally absent from the plants and animals that are descendants of Fungi, as well as the algae or prokaryote predecessors.
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Chapter 6 : A Virus Odyssey from Worms to Fish: Viruses of Early Animals and Aquatic Animals
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This chapter explores the virology and host evolution of animals which have evolved from Fungi or depended on Fungi for their evolution. It focuses on the emergence of early animals and their viruses. Furthermore, some of the characteristic virus-tissue associations known for mammals are seen for aquatic animals and their viruses. However, it often appears that there are some specific examples in which specific viruses are able to persist and inapparently infect younger forms of specific hosts. In these cases, the persistently infected hosts often function as reservoirs for viruses that cause disease in other, sometimes related species. The evolutionary reentry of some viruses (reoviruses, parvoviruses, and birnaviruses) which were present in ancestral lineages (such as Fungi) but had been absent from less distant basal metazoan predecessors (worms and basal deuterostomes) are discussed. The currently accepted view concerning the function of the adaptive immune response is that a surprisingly complex system has evolved to control parasitic invasion, especially by viruses.
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Chapter 7 : Viruses, Land Plants, and Insects: a Trinity of Virus, Host, and Vector
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This chapter examines the deep issues concerning the origin and evolution of land plants, insects, and their viruses together. Human activity, especially agricultural activity in the last 10,000 years, has generated large, closely spaced, and genetically homogeneous plant populations, which have frequently been introduced into new habitats. The chapter examines the relationship of algae to the evolution of higher green plants and considers the oceanic crustaceans along with their viruses and the evolution of terrestrial insects. The role that viruses have played in the evolution of their hosts has seldom been addressed in the context of either plant or insect evolution. The chapter presents the overall patterns of host plant evolution and then addresses the evolution of host insects. It is now accepted that a progenitor of land plants was the green microalgae, based on phylogenetic analysis of chloroplastic and mitochondrial DNA. Interestingly, in filamentous brown Fungi and other higher Fungi, zoospore formation is frequently associated with reactivation of species-specific persistent virus replication. Due to their enormous numbers, hexapods will be the main focus of this discussion of viruses and the origin of insects. The major groups of viruses of insects are clearly distinct from those found in both plants and animals. Most plant viruses need an insect vector for transmission, but in almost all cases the virus does not replicate in that insect. The most common vectors for plant viruses are homopterans, with their piercing and sucking mouthparts.
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Chapter 8 : Evolution of Terrestrial Animals and Their Viruses
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As vertebrates emerged from the oceans to become land-dwelling animals, numerous basic changes in their physiology and organ structures were necessary. This chapter examines the roles that persistent viruses and especially endogenous retroviruses (ERVs) have had in the evolution of terrestrial vertebrates. The mammalian paramyxovirus lineage may trace its evolutionary origins from reptiles, through avians, to mammals, and during the course of this adaptation the reptilian and avian viruses appear to have gained the ability to establish species-specific persistent infections. Although monotreme-like mammals are evolutionarily old (predating the dinosaurs), little is known about the viruses or genomes of these early mammalian predecessors. The majority of small DNA viruses (adenoviruses, papillo-maviruses, polyomaviruses, and TT viruses) have persistent life strategies, are host specific, and are phylogenetically congruent with the evolution of their hosts. The epidemiological concept of a reservoir species has been used for many decades to help explain the recurrence of viral disease after it has apparently been eliminated from a population or habitat. With the completion of the sequencing of the human genome, one can now consider what types of genetic changes were associated with this recent stage in human evolution. The successful genomes that have colonized host cells might be expected to persist in the ecosystem and contribute this vast genetic creativity to the tree of life.
There are no separately available contributors for this publication.
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