Virus Evolution and Virulence: The Case of Myxoma Virus in Rabbits, Slides of Human Resource Management

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Exchange of gene&c informa&on

41

Selec&on: Is virulence a posi&ve or nega&ve trait?

• Idea: increased virulence reduces transmissibility because hosts die faster, reducing exposure to uninfected hosts
• ExpectaNon: all viruses evolve to be maximally infecNous and avirulent
• But this is not what is observed
• Persistent infecNons lie dormant for years, then kill host at end stage
• Virulent viruses for one species may be maintained as asymptomaNc infecNons in another
• For some diseases, increased virulence increases transmissibility and is selected for in natural infecNons 42

An experiment in virus evolu&on

• The myxoma leporipox virus was released in Australia in the 1950s in an aoempt to rid the conNnent of the rabbits
• The natural host of myxoma virus is the cooontail rabbit, the brush rabbit of California, and the tropical forest rabbit of Central and South America
• The virus is spread by mosquitoes; infected rabbits develop superficial warts on their ears
• European rabbits are a different species, infecNon is 90-­‐99% fatal 44

• In the first year, the released virus was efficient in

killing rabbits with a 99.8% mortality rate

• By the second year the mortality dropped to 25%

• In subsequent years, the rate of killing was lower than

the reproducNve rate of the rabbits, and hope for 100% eradicaNon was dashed

An experiment in virus evolu&on

45

What was learned?

• You always get what you select, but you o]en don’t

get what you want

• EliminaNon of rabbits with a virus was flawed idea

-­‐ Selec-ve forces that could not be controlled were at work

• In Australia, apparently nothing: experiments in

biological control of rabbits using a lethal calicivirus are under way 47

The origin of viruses

• Regressive theory : viruses are derived from

intracellular parasites that have lost all but essenNal genes

• Cellular origin theory : viruses arose from cells by

reducNve evoluNon

• Independent-­‐en&ty theory : viruses coevolved with

cells from the origin of life (‘pickpockets’)

• But there is no fossil record, and few viral stocks more

than 80 years old

• BioinformaNcs has provided insight

48

Very large viral DNA genomes

• Mimivirus (1.2 Mbp), Phycodnavirus (330 kbp) genomes have sequence coherence: are not mosaics
• Homogeneity of genomes within family, lack of homology among families, difficult to explain using model that they arose by the acquisiNon of exogenous genes by a primordial precursor viral genome
• Megavirus 50