Origin, evolution and emergence Flashcards
Where do viruses come from?
Controversial topic 3 Theories proposed: “Virus first” theory Reduction theory Escape theory
Unlike animals, no fossils to study therefore evidence scanty
Virus first theory:
- Remnants of pre-cellular forms
- Primordial “RNA world”
- Existed as self-replicating units which became more organized and more complex over time – developed enzymes for membrane synthesis
- Present RNA viruses could be relics of RNA world
- Retroviruses: relics of RNA/DNA transition
- Theory rejected because viruses are obligate intracellular parasites
Notes:
Viruses are obligate intracellular parasites.
Theory of evolution
Primordial=existing from the beginning of time.
Reduction theory:
- Descendants of unicellular organisms that over time, adapted a parasitic replication strategy
- Symbiotic relationship between organisms but over time one organism became dependent on the other, eventually losing it’s essential genes.
- Poxviruses – large complex genome, depends less on host cell for replication
- Theory rejected because no known intermediate form between cells and viruses. Also, if viruses derived from cells, would retain cellular characteristics.
Notes on the reduction theory:
*May be descendants of previously free-living organisms that adapted a parasitic replication strategy
*Autologous organisms developed a symbiotic
relationship
*Overtime the relationship became parasitic with one organism depending on the other and lost essential genes
*Poxviruses illustrate this hypothesis
large genome size with greater complex
depend less on host cell for replication
Escape theory:
- Viruses came from fragments of cellular genetic material capable of moving within a genome.
- Gained ability to exit one cell and enter another
- Escaped from the control of the cell and became parasitic
- Theory favoured because present-day viruses can integrate cellular genes into their own genomes.
- Drawbacks: Doesn’t specify how nucleic acid acquired a capsid. Also viral proteins have no cellular homologues
Notes on the escape theory:
- Viruses originated through progressive process
- Views viruses as elements of cell genome
- Viruses originated from fragments of mobile genetic material capable of moving within a genome
- Gained ability to exit one cell and enter another
- Escaped from the control of the cell and became parasitic – plasmids and mobile elements are considered precursors
Virus evolution
Viruses have a greater genetic diversity than any
other group of organisms
Diversity produced by natural selection acting on genomes that are continuously changing as a result of:
mutation
recombination
reassortment
Viral replication:
Viral replication →millions of progeny virions
Errors are inevitable→ mutations
lethal
disadvantageous
neutral
selective advantage
Def: localized inheritable alteration of the nucleotide sequence of a nucleic acid
Mechanisms - mutagenesis
Types
Spontaneous
Template miscopying by the viral RNA/DNA polymerases –
chance errors during replication
Activity of cellular enzymes i.e. deaminases
Induced
chemical damage to the viral nucleic acid eg. nitrosoguanidine
physical agents – UV light, irradiation
use of base analogues – mutagenic when viruses are grown in their presence
site-specific mutagenesis – genetic engineering
Notes on RNA viruses:
RNA viruses have higher mutation rates than DNA viruses
RNA polymerases – one mistake per 103 to 105 nucleotides incorporated
Error-rate of RNA replication»_space;> DNA; lack of proofreading
mechanism
Rate of base substitution is ~106 times higher than Euk DNA
Most substitutions are deleterious
Non-lethal mutations in the genome accumulate very rapidly
Mutations continue to accumulate in the viral genome during passage in cultured cells, just as they do in natural replication in the host
Notes on DNA viruses:
DNA viruses:
Replicate in the nucleus ~ similar to error-rate of DNA in eukaryotic cells
Proofreading exonuclease error correction
Error-rate: one mistake per 107 to 109 nucleotides incorporated
Emerging & Re - emerging viruses:
Interface between humans, animals, & environments source of diseases impacting public health
Zoonoses = infections transmitted from wild or domestic animals to man
Emerging Viruses = Viral infections that have newly appeared or that have appeared previously, but are expanding in incidence & geographic distribution or threaten to increase in near future”John R Su, Clin Lab Med 24 (2004) 773-795
Emerging = Newly discovered
Re-emerging or Resurgent = Re-appearing
Notes on emerging & Re - emerging viruses:
Most human viral infections maintained in nature within animals
Many viruses efficiently transmit between different species
Majority of emerging viral infections are zoonotic - due to microbial traffic - transfer of existing agents to new host populations
Introduction of viruses into humans often due to human activities
Viral zoonoses occur in variety of ecological settings
Very limited ecological & geographical foci
Viruses cause little or no overt disease in animal host
Some zoonotic viruses very limited host ranges
Others may infect a wide range of vertebrates
Human infection - Asymptomatic or mild to severe or life threatening depending on specific virus & immune response
Potential for epidemic spread or pandemics
Examples of emerging & Re - emerging viruses:
Animal RNA viruses common source of emerging viruses
RNA viruses lack proofreading high error rate during replication high mutation rate
Most RNA viruses zoonoses transmitted initially to humans from mammals or avian hosts e.g. influenza, Nipah, SARS, MERS viruses
Some viruses commonly transmitted exclusively between humans - HIV & HCV - likely animal origins
The infection of emerging & Re - emerging viruses is acquired by:
Direct contact with animal Ingestion of meat or animal products Contact with animal urine or faeces Aerosol inhalation Arthropod vector Inoculation of saliva in a bite wound
Global emerging infections since 1973:
1973 Rotavirus Enteritis/Diarrhea 1976 Cryptosporidium Enteritis/Diarrhea 1977 Ebola virus VHF 1977 Legionella Legionnaire’s Dx 1977 Hantaan virus VHF HRS 1977 Campylobacter Enteritis/Diarrhea 1980 HTLV-1 Lymphoma 1981 Toxin prod. S.aureus Toxic Shock Synd. 1982 E.coli 0157:H7 HUS 1982 HTLV-II Leukemia 1982 Borrelia burgdorferi Lyme disease
Global Emerging Infections since 1983:
1983 HIV AIDS 1983 Helicobacter pylori Peptic Ulcer Dx 1988 Hepatitis E Hepatitis 1989 Hepatitis C Hepatitis 1990 Guanarito virus VHF 1991 Encephalitozoon Disseminated Dx 1992 Vibrio cholerae O139 Cholera 1992 Bartonella henselae Cat Scratch Dx 1993 Sin Nombre Virus HPS 1994 Sabia virus VHF 1994 Hendra virus Respiratory Dx 1995 Hepatitis G Hepatitis
