In hostparasite associations, there is often a geneforgene mechanism of infection. What th
In host-parasite associations, there is often a gene-for-gene mechanism
of infection. What this means is that certain parasite genes must
"match" certain host genes for the parasite to be able to infect:
Gene 1 Gene 2
parasite A B
host A b
^---match ^---No match. No infection
parasite a B
host A b
^-No match ^-No match. No infection
parasite A b
host A b
^---match ^---match. Infection
Essentially, the fittest parasites in a population are those which can
match the genotype which is at the highest frequency in the population.
The fittest hosts, however, are those which are matched by the lowest-
frequency parasite genotype. Since parasites generally have shorter
generation times than their hosts, parasite genotype frequencies tend
to "track" host genotype frequencies; they are able to evolve so that
their most common genotype matches the host's most common genotype.
This has the seemingly-paradoxical result that THE HOST GENOTYPE WITH
THE HIGHEST FITNESS IS AT THE LOWEST FREQUENCY IN THE POPULATION. Of
course, that genotype therefore increases in frequency until it is at
a higher frequency. However, since the parasites are evolving faster,
almost as soon as that host genotype has reached high frequency, THAT
GENOTYPE BECOMES THE LOWEST-FITNESS GENOTYPE. This leads to really
neat genotypic cycling, where host genotypes bounce from common/low
fitness to rare/high fitness, and parasite genotypes follow them.
This cycling is the basis for the Red Queen theory for the maintenance
of sexual reproduction.
E-Mail Fredric L. Rice / The Skeptic Tank