Lec 16: Origins and Evolution of Viruses

Question 1

The origins and evolution of many cellular organisms can be inferred from….
But…

Answer 1

…fossils

…there is very little fossil record of viruses!

Question 2

The discussion of the possible origins of viruses =

But…

Answer 2

highly speculative
…firm ground as sequencing of virus genomes enables the construction of phylogenetic trees and the monitoring of virus evolution as it occurs

Question 3

All studies about viral origin is purely based on

Answer 3

sequence

Question 4

Viruses are parasites of cells, so

Answer 4

there could be no viruses until cells had evolved.

Question 5

which viruses came first, prokaryotic vs eukaryotic?

Answer 5

Prokaryotic viruses came first (before eukaryotic)

Question 6

Where did viruses come from?

However, we speculate about…

Answer 6

Basically, we do not know!

…Possible ancestors of viruses

Question 7

Possible ancestors of viruses:

Answer 7

1. ) molecular precursors of cellular organisms
2. ) Components of cells
3. ) Intracellular micro-organisms

Question 8

(Molecular precursors of cellular organisms)

Before cellular organisms had come into existence…

Answer 8

…RNA molecules evolved and developed enzyme activities (ribozymes) and the ability to replicate themselves

Question 9

(Molecular precursors of cellular organisms)

Once cells had evolved perhaps some were…

Answer 9

…parasitized by some of these RNA molecules, which somehow acquired capsid protein genes —- possible 1st viruses = RNA viruses (not DNA viruses).

Question 10

possible 1st viruses =

Answer 10

RNA viruses (not DNA viruses).

Question 11

(Components of cells)

Perhaps some cellular components evolved abilities…

Answer 11

…to replicate themselves, independent of host cell control, and thus became parasites of those cells.

Question 12

(Components of cells)

Potential candidates: (3)

Answer 12

1. ) mRNA molecules
2. ) DNA molecules such as plasmids
3. ) transposons

Question 13

(Intracellular Micro-organisms)

There is strong evidence that…

Answer 13

…the mitochondria and chloroplasts of eukaryotic cells are derived from prokaryotic cells.

Question 14

(Intracellular Micro-organisms)

The parasite prokaryotic cells adopted…

Answer 14

…new modes of life within the cells of host organisms.

Question 15

(Intracellular Micro-organisms)

Perhaps the evolutionary process continued further, leading to…

Answer 15

…greater degeneracy and loss of functions such as protein synthesis, until the intracellular intruder was no longer a cell or an organelle, but had become a virus.

Question 16

(Intracellular Micro-organisms)

What indicates that viruses had multiple origins?

Answer 16

The great variety of virion structures, virus genome types, and replication strategies

Question 17

(Intracellular Micro-organisms)

Small simple viruses (ex: parvoviruses, picornaviruses)
perhaps evolved from:

Some large complex viruses (ex: mimivirus)
perhaps evolved from:

Answer 17

molecular precursors

cellular precursors.

Question 18

(Mechanisms of virus evolution)

The underlying processes that drive virus evolution are

Answer 18

the same as those that drive the evolution of cellular organisms.

Question 19

(Mechanisms of virus evolution)

The vast majority of the variants generated during the evolution processes are

Answer 19

deleterious and do not survive, but a few provide an advantage in a particular niche

Question 20

Virus genome variants arise as a result of: (3)

Answer 20

mutations
new gene arrangements
acquisition of cell genes

Question 21

DNA rep (DNA pol) = low error rate =

RNA rep (RNA pol) = high error rate =

Rev transc = high error rate =

Answer 21

= slow evolution

= fast evolution

= fast evolution

Question 22

Reassortment =

Answer 22

a category of recombination that may occur with segmented genome viruses.

Question 23

If a cell is co-infected by:

Answer 23

2 strains of one of these viruses the progeny virions may contain mixtures of genome segments from the two parental strains.

Question 24

(Acquisition of cell genes)

The genomes of some retroviruses contain…

Answer 24

…oncogenes that they probably acquired from their host cells (taken from host), which contain similar genes (proto-oncogenes).

Question 25

(Acquisition of cell genes)

2 possible mechanisms:

Answer 25

1. ) recombination.

2. ) synthesis of a DNA copy of a cell mRNA, followed by insertion of the DNA into the virus genome.

Question 26

(Evolution of new viruses)

Many virus infections across species barriers =

But sometimes…

Answer 26

dead ends for the viruses as no transmissions to further individuals occur.

…a virus adapts to enable transmission from host to host.

Question 27

(Co-evolution of viruses and their hosts)

Co-evolution is more common for those viruses…

Answer 27

…whose host suffers little or no harm (ex: reoviruses, where ‘o’ means ‘orphan’ (not associated with any disease))

Question 28

The potential for evolution is usually much greater for

Answer 28

a virus than it is for its host.

the reproductive capacity of HIV (1010–1012 new virions per day).

Question 29

A good example of co-evolution of a virus–host relationship is

Answer 29

Myxoma virus in the European rabbit.

(In the natural host of the virus, a rabbit species in South America, infection results in the development of skin tumors, which eventually heal)

• However, when myxoma virus infects the European rabbit it causes myxomatosis, and is almost always fatal.
• For this reason, the virus was introduced into Australia as a biological control agent against the European rabbit.
• Within a year of its introduction, there was evidence of virus evolution.
• Myxoma virus strains with attenuated virulence had evolved.
• Subsequently, selection for rabbits with resistance to myxomatosis occurred
• Both the virus and its new host have continued to co-evolve.