Origin of viruses and virus strains

Explanation on theories on Origin of viruses and virus strains

1. Origin of viruses and
virus strains
N. H. SHANKAR REDDY
Ph.D., Plant Pathology
Annamalai University

2. •Isolate
When a virus is derived from a field or experimentally from infected
host by a single isolation is known as isolate
•Variant
Isolateof the same virus that is differ in some property
(host range, transmission, serology or nucleotide sequence)
•Strain
A variant of virus within a species that can be recognized by some
phenotypic characteristic is known a “strain”

3. There are three hypothesis
1. The virus first hypothesis (Independent entities theory)
2. The reduction hypothesis (The second virus theory/ Degenerate hypothesis/
Regressive evolution theory)
3. The escape hypothesis (The third virus hypothesis)

4. 1. The virus first hypothesis (Independent
entities theory)
Viruseshaveevolvedfromaself-replicatingmaterialthatexistedintheprebiotic
mRNAworld.
Theyaretheremnantsofthepre-cellularlifeforms.RNAanditsorigin,
perceivedasthefirstmoleculesoflife,wasabletoactasanucleicacidcarrying
informationandcapableofself-replicationhenceanentityofindependent
existence.
Inthistheory,RNAvirusesarethoughttohavebeendescendantsoftheRNA
worldandtheDNAvirusesevolvedlaterfromRNA.

5. 2.Thereductionhypothesis(Thesecondvirus
theory/Degeneratehypothesis/Regressive
evolutiontheory)
Thisstatesthatvirusesoriginatedasaresultofreductionofunicellularorganisms
viaparasitic-drivenevolution.
Thismeansthatcellsofunicellularorganismslosttheirmembranes,cellwalls
andotherstructuresaswellastheircytoplasm,henceretainingonlythenucleic
acidandsomevitalproteinsinordertoliveinothercellsasparasites.

6. 3.Theescapehypothesis(Thethirdvirus
hypothesis)
Virusesarethoughttohaveoriginatedfromfragmentsofgeneticmaterialthat
escapedfromthecontrolofthecellandbecameparasitic.
Plasmidsandmobilegeneticelementsareextracellulargeneticmaterialscapable
ofexitandentryintothegenomesofothercells.Thisphenomenonprovidesa
glimpseofthepossibilitiesofescapebythegeneticelementsprovidedbythis
theory.

7. Variations in viruses
1.Mutation-Asinglenucleotidechangeeitherinsertionordeletioneitherinthe
noncodingregion;Thechangeincodingregion-resultinginchangeinamino
acid.
(Itmayoccurduringreplication;purineorpyrimidinisreplacedbypurineor
pyramidinrespectivelyitiscalledtransversion).
2.Hybridization/Recombination-exchangeofsegmentsofgenome-bothin
RNAandDNAviruses.
3.Pseudorecombination/reassortment-Inviruseswithdividedgenome
shufflingofgenomicsegments–geminiviruses.

8. Types of evolution:
•Microevolution–mutationsorsmallinsertionordeletionchangingsomeregionsof
thevirusbutnotaffectingthemajorfunctions.
•Macroevolution–byrecombination,acquiringnewgenesleadtogenerationofnew
species/genera/family

9. Muller’s Ratchet
•Mutationsthatleadtolossofcriticalfunctionswouldnotbepropagatedina
populationunlesstheywerecomplementedbyothermembersofpopulation.
•Ifmutationonlycauseslightdeclineinfitnesstheywillsurvive.
•PlantvirusescanovercomeconstraintofMullersRachetforeg.Nematode
transmissionofTobaccorattlevirus(TRV).
•DefectiveRNAofRNA2hasmodifiedcoatprotein,whichwillinterferewith
replication.Insteadofremovalofthisvariantvirushasovercomethisproblemby
encapsidationofbothdefectiveRNAandRNA2together.Whichare
nonfunctionalandfunctionalintransmission.

10. Video available at you tube
Channel –Geeky Researcher