Читать книгу Principles of Virology - Jane Flint, S. Jane Flint - Страница 56

The study of viruses has increased our understanding of the importance and ubiquitous existence of these diverse agents and, in many cases, yielded new and unexpected insight into the molecular biology of host cells and organisms. Indeed, because viruses are obligate molecular parasites, every tactical solution encountered in their reproduction and propagation must of necessity tell us something about the host as well as the virus. Some of the important landmarks and achievements in the field of virology are summarized in Fig. 1.14. It is apparent that much has been discovered about the biology of viruses and about host defenses against them. Yet the more we learn, the more we realize that much is still unknown.

In the first edition of this textbook (published in 2000), we noted that the most recent (1995) report of the ICTV listed 71 different virus families, which covered most new isolates. We speculated therefore that: “As few new virus families had been identified in recent years, it seems likely that a significant fraction of all existing virus families are now known.” In the intervening years, this prediction has been shattered, not only by the discovery of new families of viruses, including giant viruses with genome sizes that surpass those of some bacteria, but also by results from metagenomic analyses. For example, the fact that a high percentage (93%) of protein-coding sequences in the genomes of the giant Pandoraviruses have no homologs in the current databases was totally unexpected. The unusual morphological features and atypical reproduction process of these viruses were also surprising. In addition, it is mind-boggling to contemplate that of almost 900,000 viral sequences identified in samples of only one type of ecosystem (raw sewage), more than 66% bore no relationship to any viral family in the current database. From these analyses, and similar studies of other ecosystems (i.e., oceans and soil), it has been estimated that only a minor percentage of extant viral diversity has been explored to date. Clearly, the viral universe is far more vast and diverse than suspected only a decade ago, and there is much fertile ground for gaining a deeper understanding of the biology of viruses and their host cells and organisms.

Figure 1.13 Viral families sorted according to the nature of the viral genomes. A wide variety of sizes and shapes are illustrated for the families of viruses that infect vertebrates. Families are identified by Latinized names and organized in seven distinct classes, based on the nature of their genomes. Genome replication cycles are illustrated in the column at the left. Similar diversity exists for the families of viruses that infect other life forms, but the chart lists only the approximate number found to date in each class. As noted in the 9th and 10th ICTV Reports, in some cases there are as yet no examples. Data from King AMQ et al. 2012. Virus Taxonomy: The Classification and Nomenclature of Viruses (https://talk.ictvonline.org/ictv-reports/), with assistance from Dr. Elliot J. Lefkowitz, Department of Microbiology, Director of Informatics, UAB Center for Clinical and Translational Science, Birmingham, AL (http://www.uab.edu/bioinformatics/).

Figure 1.14 Landmarks in the study of viruses. Key discoveries and technical advances are listed for each time interval. The pace of discovery has increased exponentially over time. Abbreviations: AAV, adenovirus-associated virus; EU, European Union; EMA, European Medical Association; FDA, U.S. Food and Drug Administration; FMDV, foot-and-mouth disease virus; HAART, highly active antiretroviral therapy; HCV, hepatitis C virus; HHV-8, human herpesvirus 8; HIV-1, human immunodeficiency virus type 1; HPV, human papillomavirus; MHC, major histocompatibility complex; RSV, Rous sarcoma virus; SV40, simian virus 40; TBSV, tomato bushy stunt virus; TMV, tobacco mosaic virus; WHO, World Health Organization.