Chapter 8 Flashcards
What is evolution in the context of a virus?
A change in the genetic composition of a DNA or RNA virus population over time, irrespective of the time frame involved
What is positive selection?
a change that confers an increase in fitness
What is negative selection?
a change that confers a decrease in fitness
What is neutral selection?
a change that has no positive or negative effect on fitness
What does selection act on?
the population, not the individual genotype
What are viruses selected for?
the ability to spread within an infected host and among individuals in the population
What do viruses exert selective pressure on?
host populations
When does a large number of viral mutations arise?
when viral genomes replicate
How often do RNA virus mutations occur?
1 in 10,000 to 100,000
Error rate for RNA viruses are a ______ fold greater tan rate of host genome.
million
Error rate for RNA viruses are equivalent to _ mutation for every _ -_ genomes.
1, 1-10
How often do DNA virus mutations occur?
3 for every 1,000,000 - 10,000,000
What DNA viruses are more error prone?
small single-stranded viruses
What is a quasispecies?
a virus population
Ocassionally, even the most ___ genotype in a population may become the most predominant after a single selective event.
rare
What are the 3 forces of selection in vivo?
- Eliminate ill-adapted genotype and favors genotypes with replication fitness
- Is of the moment, not based on the prospect of building something better for the future
- Can result in antigenic drift or antigenic shift
What do high rates of replication over short periods result in?
the accumulation of large quantities of infectious virus particles
Why do virus mutation lead to great diversity?
every genome has a mutation, virus evolution is relentless, new mutants will always arise, and the possibilities are unimaginable
What are the sources of diversity in virus populatinos?
Genetic drift (mutations and hypermutation) and Genetic shift (recombination and reassortment)
What is a synonymous mutation?
a mutation that does not result in a change in amino acid
What is a non-synonymous mutation?
a mutation that results in change in an amino acid
Non-synonymous mutations may confer a ______ advantage.
selective
What is recombination?
The formation of new, covalently linked combinations of genetic material from two parental genomes
What does recombination result in?
re-arrangement of genetic sequences within a linear genome, or between two linear genomes
What does recombination provide?
a one-step mechanism for creation og new combinations of many mutations - may be essential for survival and may rapidly expand tissue tropism/host range
What is reassortment?
genetic recombination between viruses with segmented genomes, whereby a progeny virus acquires genome segments from each parental virus
What can reassortment produce?
new genome constellations conferring new phenotypic traits
Does reassortment or mutation occur more?
mutations
What is antigenic drift?
small, gradual changes that occur through point mutations
What is antigenic shift?
an abrupt, major change
What are the 5 factors that contribute to viral emergence?
- Population density and health
- Changing host populations and environment
- Changing climate and animal populations
- Expansion of host range by mutation or recombination
- Accidental or deliberate infection of new species
How does high population density impact virus spread?
close contact and large supply of susceptible hosts and can result in maintenance and continued evolution of the virus
What are some ways to reduce the effective population density?
physical separation of small groups, quarantine, immunizing the group such that the large majority are immune
What parameters increase virus spread within a population?
age (old and young), poor nutrition, immunosuppression
Provide an example of how climate can influence viral disease.
Spring precipitation in the Southwest US increases food resources for rodents and lead to increased rodent population sizes, these rodents are carriers for Hantavirus
List 4 examples of expansion of host range following mutation or recombination.
- Influenza viruses - transmission from avian reservoir
- HIV-1 and HIV-2 in humans from non-human primates
- SARS virus in humans from civet
- Canine parvovirus in dogs from cats
