Lec 16: Origins and Evolution of Viruses Flashcards

1
Q

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

A

…fossils

…there is very little fossil record of viruses!

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2
Q

The discussion of the possible origins of viruses =

But…

A

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

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3
Q

All studies about viral origin is purely based on

A

sequence

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4
Q

Viruses are parasites of cells, so

A

there could be no viruses until cells had evolved.

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5
Q

which viruses came first, prokaryotic vs eukaryotic?

A

Prokaryotic viruses came first (before eukaryotic)

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6
Q

Where did viruses come from?

However, we speculate about…

A

Basically, we do not know!

…Possible ancestors of viruses

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7
Q

Possible ancestors of viruses:

A
  1. ) molecular precursors of cellular organisms
  2. ) Components of cells
  3. ) Intracellular micro-organisms
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8
Q

(Molecular precursors of cellular organisms)

Before cellular organisms had come into existence…

A

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

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9
Q

(Molecular precursors of cellular organisms)

Once cells had evolved perhaps some were…

A

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

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10
Q

possible 1st viruses =

A

RNA viruses (not DNA viruses).

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11
Q

(Components of cells)

Perhaps some cellular components evolved abilities…

A

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

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12
Q

(Components of cells)

Potential candidates: (3)

A
  1. ) mRNA molecules
  2. ) DNA molecules such as plasmids
  3. ) transposons
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13
Q

(Intracellular Micro-organisms)

There is strong evidence that…

A

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

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14
Q

(Intracellular Micro-organisms)

The parasite prokaryotic cells adopted…

A

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

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15
Q

(Intracellular Micro-organisms)

Perhaps the evolutionary process continued further, leading to…

A

…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.

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16
Q

(Intracellular Micro-organisms)

What indicates that viruses had multiple origins?

A

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

17
Q

(Intracellular Micro-organisms)

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

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

A

molecular precursors

cellular precursors.

18
Q

(Mechanisms of virus evolution)

The underlying processes that drive virus evolution are

A

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

19
Q

(Mechanisms of virus evolution)

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

A

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

20
Q

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

A

mutations
new gene arrangements
acquisition of cell genes

21
Q

DNA rep (DNA pol) = low error rate =

RNA rep (RNA pol) = high error rate =

Rev transc = high error rate =

A

= slow evolution

= fast evolution

= fast evolution

22
Q

Reassortment =

A

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

23
Q

If a cell is co-infected by:

A

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

24
Q

(Acquisition of cell genes)

The genomes of some retroviruses contain…

A

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

25
Q

(Acquisition of cell genes)

2 possible mechanisms:

A
  1. ) recombination.

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

26
Q

(Evolution of new viruses)

Many virus infections across species barriers =

But sometimes…

A

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

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

27
Q

(Co-evolution of viruses and their hosts)

Co-evolution is more common for those viruses…

A

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

28
Q

The potential for evolution is usually much greater for

A

a virus than it is for its host.

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

29
Q

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

A

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.