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Virus evolution
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- 1Virus EvolutionRapid:
-Short generation time
-Large virus burst sizes (large number of progeny viruses)
-High mutation rate - 2Evolution forces-Mutation
-Recombination/Reassortment
-Selection
-Transmission between hosts - 3Virus ''Fitness''Definition: The capacity to replicate in a specific environment.
- 4Mutation: Error rates during genome replication -RNA Virus -DNA VirusRNA virus: per base ~10^-3/4
per genome: 1-3
DNA virus: per base 10^-8 to 10^-12
per genome: 10^-3 to 10^-4 range - 5What is a quasispecies?-A quasispecies is a pop of similar, not identical genomes
-A viral quasispecies was originally defined as the population of virus progeny not yet exposed to selection processes.
-term coined by manfred Eigen - 6Recombination or reassortment between viral genomes-Exchange of genome segments between viruses with segmented genomes
Example: Orthomyxoviridae (7-8 Segments ssRNA)
Reoviridae (10-12 Segments dsRNA)
-Use of different templates for synthesis of new genomes Ex: Retroviridae
-Breakage and reassmbly of nucleic acid fragments Ex: ds DNA viruses - 7Examples of Recombination between the host and virus nucleic acids-Cytopathogenic variants of the Pestivirus Bovine Viral Diarrhea Virus (Flaviviridae) can be generated by recombination of viral RNA and host RNAs
-Oncogenic Retroviruses: contain transforming sequences picked up from the host
-Poxviruses and herpesviruses: contain cell-derived sequences that encode modulators of the immune system - 8Genetic drift/shiftGenetic drift = Mutations in the genome + selection (may lead to antigenic drift)
-Genetic shift = Recombination or re-assortment of genomes or genome segments + selection (may lead to antigenic shift) - 9Genetic bottleneckEvent that reduce the genetic diversity of a population by eliminating a substantial proportion of the population
- 10Example for genetic bottleneck:Outgrowth of drug resistant viruses during antiviral treatment
- 11Events that may represent a genetic bottleneck for virus populationsTransmission to a new host
-animal-human (zoonosis)
-human human (horizontal)
-mother to child (vertical)
or
antiviral Therapy - 12Evolution within the host (Ex: HIV)-Primary infection of the host: R5 Virus: Macroophage tropic Virus; uses CCR5 chemokine receptor as co receptor
-Late stages of the disease
Mainly X4 Virus; T-cell tropic virus; uses CXCR4 as coreceptor
-Conclusion: The virus pop that replicates best in the host is different from the transmitted founder virus pop in a new host.
--> Transmission represents a genetic bottleneck for HIV. - 13Error thresholdThe point at which the mutation rate impairs viral fitness
-If the mutation rate is too high, essential virus information may be lost (''error catastrophe''
-If the mutation rate is too low, host defenses overcome the virus - 14Increasing the mutation rate of virusesmay be an antiviral strategy:
A ~30 fold increase in mutation rate was calculated to eliminate HIV-1 replication - 15Theories of virus origins1. Regressive Evolution: Viruses originate from previously independent life forms that have lost may functions and maintained only those required for the survival as parasites; Model Virus: Poxvirus
2. Cellular origins: Viral genomes originate from cellular nucleic acids that acquired the capacity to move from cell to cell. Models: Human endogenous retroviral sequences
3. Independent entities:
Evolution from primitive self replicating molecules; Evolution parallel to cellular organisms; Models: Viroids - 16Human endogeneous retroviral elements
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- Karteikarten am Virus evolution, erstellt von janina_198904 am 02/07/2015.
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