Evolution__3rd_Edition

..
Myxomatosis reduced rabbit
populations
The myxoma virus decreased its
virulence
Rabbits evolved resistance
proportion of its host’s cells, it will be more likely to kill its host and therefore be more
virulent than one that consumes less host cells. The second topic we look at is whether
or not the phylogenies of parasites and their hosts have the same shape.
22.5.1 Evolution of parasitic virulence
CHAPTER 22 / Coevolution 625
Parasitic virulence and host resistance show evolutionary changes in Australian
myxoma virus and rabbits
The myxoma virus (which causes myxomatosis) in Australian rabbits provides the classic
illustration that the virulence of a parasite can change evolutionarily. The rabbits in
question belong to a species (Oryctolagus cuniculus) which is native to Europe but was
introduced to Australia, where it thrived and became a pest. The natural host of the
myxoma virus is another kind of rabbit, Sylvilagus brasiliensis, from South America, in
which the virus probably has low virulence. In 1950 the virus was deliberately introduced
into Australia in an attempt to control the pestiferous rabbits. It was, initially, a
deadly success. It spreads (in Australia, at least) from rabbit to rabbit by means of
mosquitoes and the large population of these biting insects enabled the myxoma virus
to sweep through the southeast Australian rabbit population, and round the south
coast as far as Perth in the west by 1953. Myxomatosis initially almost annihilated the
rabbit population: it declined by 99% in some hard hit areas. The virus was introduced
into France in 1952, and started to spread through Europe, and was surreptitiously
introduced into the UK in 1953.
The myxoma virus was highly virulent when it first hit the Australian (and
European) rabbit population; it killed 100% of infected hosts. Soon, however, the kill
rate declined. This decline could result from any combination of increasing host resistance
and decreasing viral virulence, and normally we should not know which was
operating. But in this case, a carefully controlled set of experiments allowed the two
factors to be teased apart.
The decline in virulence of the myxoma virus was demonstrated by infecting standard
rabbit strains in the laboratory with the viruses taken from the wild in successive
years. Because the rabbit strain was controlled and constant, any decline in the kill rate
must be due to a decline in virulence in the virus. Table 22.2 shows the results, in
Australia and Europe. In both places the virus started off maximally virulent (killing
100% of infected rabbits), but there was then a rapid increase in the less virulent strains
in the viral population a the less virulent strains kill a lower proportion of infected
rabbits and take longer to kill them when they do. Meanwhile, the rabbits were also
evolving resistance. This could be shown by challenging wild rabbits through a series
of times with standard strains of the virus; now the virus was held constant and any
decline in kill rate must be due to changes in the rabbits. Table 22.3 shows the results of
a series of such experiments through the 1960s and 1970s, in which resistance did
indeed manifestly increase.
Therefore, both parasitic virulence and host resistance can evolve. Natural selection
will clearly always favor increased resistance in hosts, but how will it operate on
virulence in parasites?