Viruses evolve rapidly through mutation and recombination. Many viruses have high mutation rates due to error-prone replication and lack of repair mechanisms. There are three hypotheses for the origins of viruses: escape from host genes, evolution before cells, and reduction of small cells into parasites. Evolution occurs through random mutation, recombination, reassortment, and antigenic drift/shift. Reassortment of influenza virus segments leads to pandemics, while antigenic drift accumulates mutations to evade antibodies. Viral evolution poses challenges for vaccines and drugs as resistant mutations often emerge.

1. Evolution
and
mutation of viruses
Dr. Harinatha Reddy M.sc, Ph.D.
biohari14@gmail.com
Department of Microbiology
Sri Krishnadevaraya University
Anantapur, A.p. India

2. Introduction:
 Many viruses, in particular RNA viruses, have short
generation times and relatively high mutation rates.
 Viral evolution is an important aspect in the epidemiology of
viral diseases such as influenza virus, HIV, and Hepatitis C
viruses.
 The viral mutation also causes problems in the development
of successful vaccines and antiviral drugs, as resistant
mutations often appear within weeks or months after the
beginning of the treatment.

3.  The major hallmark of mutations in RNA genomes is the
error-prone replication and retro-transcription.
 The other reason is the absence of proofreading/repair
mechanisms that normally operate during replication of
cellular DNA.

4. There are three classical hypotheses on the origins of viruses:
 Escape hypothesis: Some viruses may have evolved from bits of
DNA or RNA that "escaped" from the genes of a larger organism.
 Virus-first hypothesis: Viruses could have evolved from complex
molecules of protein and nucleic acid before cells first appeared on
earth.
 Reduction hypothesis: Viruses may have once been small cells that
parasites larger cells.

5. Evolution occurs by several mechanisms:
Random mutation
Recombination
Reassortment/ Antigenic Shift
Antigenic drift

6. Random mutation:
 Virus genomes display a
higher mutation rate than
cellular organisms .
 Viral polymerases have a
higher mutation rate than
cellular polymerases, in
particular RNA dependent
RNA polymerases.
 Moreover virus replication
process often do not support
cellular repair mechanisms.

7. Self recombination:
 Self recombination occurs when two viral genomes
recombine by homologous crossing over.
 This self recombination is very common in retroviruses and
DNA viruses.
 These recombination events can be of evolutionary advantage
for the virus when it helps to escape from host immune
defenses, for example by changing surface protein
antigenicity.

8.  Recombination with host genome: occurs when a viral
genome recombine to acquire sequences from another
organism.
 Natural selection can retain acquired sequence if it gives an
evolutionary advantage to the virus and mutations can modify
its original functions.
 This event is common in large dsDNA viruses, some
eukaryotic viruses.

9. Re-assortment/Antigenic Shift:
 Reassortment occurs when two different viruses that are
infecting the same cell exchange genetic material.
 In particular, reassortment occurs among influenza viruses,
whose genomes consist of eight distinct segments of RNA.
These segments act like mini-chromosomes.
 It is particularly studied for Influenza virus, since
reassortment is the major event giving rise to new flu
pandemics.

10.  Best example for reassortment is exchange of RNA
segments between human flu and Bird flu viruses that give
rise to pandemic influenza i.e H1N1.

11. Influenza A viruses sub types:
 H2N2, which caused Asian Flu in 1957.
 H3N2, which caused Hong Kong Flu in 1968.
 H5N1, which caused Bird Flu in 2004.
 H1N1, which caused Spanish Flu in 1918, and Swine Flu in 2009.
 What next Flu?

12. Antigenic drift:
 Antigenic drift is a mechanism for variation in viruses that
involves the accumulation of mutations within the genes that
code for antibody-binding sites.
 This results in a new strain of virus particles which cannot be
inhibited as effectively by the antibodies that were originally
targeted against previous strains, making it easier for the virus
to spread throughout a partially immune population.
 Antigenic drift occurs in both influenza A and influenza B
viruses.

13. THANK YOU