Chapter 8

Question 1

What is evolution in the context of a virus?

Answer 1

A change in the genetic composition of a DNA or RNA virus population over time, irrespective of the time frame involved

Question 2

What is positive selection?

Answer 2

a change that confers an increase in fitness

Question 3

What is negative selection?

Answer 3

a change that confers a decrease in fitness

Question 4

What is neutral selection?

Answer 4

a change that has no positive or negative effect on fitness

Question 5

What does selection act on?

Answer 5

the population, not the individual genotype

Question 6

What are viruses selected for?

Answer 6

the ability to spread within an infected host and among individuals in the population

Question 7

What do viruses exert selective pressure on?

Answer 7

host populations

Question 8

When does a large number of viral mutations arise?

Answer 8

when viral genomes replicate

Question 9

How often do RNA virus mutations occur?

Answer 9

1 in 10,000 to 100,000

Question 10

Error rate for RNA viruses are a ______ fold greater tan rate of host genome.

Answer 10

million

Question 11

Error rate for RNA viruses are equivalent to _ mutation for every _ -_ genomes.

Answer 11

1, 1-10

Question 12

How often do DNA virus mutations occur?

Answer 12

3 for every 1,000,000 - 10,000,000

Question 13

What DNA viruses are more error prone?

Answer 13

small single-stranded viruses

Question 14

What is a quasispecies?

Answer 14

a virus population

Question 15

Ocassionally, even the most ___ genotype in a population may become the most predominant after a single selective event.

Answer 15

rare

Question 16

What are the 3 forces of selection in vivo?

Answer 16

1. Eliminate ill-adapted genotype and favors genotypes with replication fitness
2. Is of the moment, not based on the prospect of building something better for the future
3. Can result in antigenic drift or antigenic shift

Question 17

What do high rates of replication over short periods result in?

Answer 17

the accumulation of large quantities of infectious virus particles

Question 18

Why do virus mutation lead to great diversity?

Answer 18

every genome has a mutation, virus evolution is relentless, new mutants will always arise, and the possibilities are unimaginable

Question 19

What are the sources of diversity in virus populatinos?

Answer 19

Genetic drift (mutations and hypermutation) and Genetic shift (recombination and reassortment)

Question 20

What is a synonymous mutation?

Answer 20

a mutation that does not result in a change in amino acid

Question 21

What is a non-synonymous mutation?

Answer 21

a mutation that results in change in an amino acid

Question 22

Non-synonymous mutations may confer a ______ advantage.

Answer 22

selective

Question 23

What is recombination?

Answer 23

The formation of new, covalently linked combinations of genetic material from two parental genomes

Question 24

What does recombination result in?

Answer 24

re-arrangement of genetic sequences within a linear genome, or between two linear genomes

Question 25

What does recombination provide?

Answer 25

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

Question 26

What is reassortment?

Answer 26

genetic recombination between viruses with segmented genomes, whereby a progeny virus acquires genome segments from each parental virus

Question 27

What can reassortment produce?

Answer 27

new genome constellations conferring new phenotypic traits

Question 28

Does reassortment or mutation occur more?

Answer 28

mutations

Question 29

What is antigenic drift?

Answer 29

small, gradual changes that occur through point mutations

Question 30

What is antigenic shift?

Answer 30

an abrupt, major change

Question 31

What are the 5 factors that contribute to viral emergence?

Answer 31

1. Population density and health 2. Changing host populations and environment 3. Changing climate and animal populations 4. Expansion of host range by mutation or recombination 5. Accidental or deliberate infection of new species

Question 32

How does high population density impact virus spread?

Answer 32

close contact and large supply of susceptible hosts and can result in maintenance and continued evolution of the virus

Question 33

What are some ways to reduce the effective population density?

Answer 33

physical separation of small groups, quarantine, immunizing the group such that the large majority are immune

Question 34

What parameters increase virus spread within a population?

Answer 34

age (old and young), poor nutrition, immunosuppression

Question 35

Provide an example of how climate can influence viral disease.

Answer 35

Spring precipitation in the Southwest US increases food resources for rodents and lead to increased rodent population sizes, these rodents are carriers for Hantavirus

Question 36

List 4 examples of expansion of host range following mutation or recombination.

Answer 36

1. Influenza viruses - transmission from avian reservoir 2. HIV-1 and HIV-2 in humans from non-human primates 3. SARS virus in humans from civet 4. Canine parvovirus in dogs from cats