An experimental test of parasite adaptation to common vs. rare host genotypes

A core hypothesis in coevolutionary theory proposes that parasites adapt
to specifically infect common host genotypes.  Under this
hypothesis, parasites function as agents of negative frequency-dependent
selection, favoring rare host genotypes.  This parasite-mediated
advantage of rarity is key to the idea that parasites maintain genetic
variation and select for outcrossing in host populations.  Here,
we report the results of an experimental test of parasite adaptation to
common vs. rare host genotypes.  We selected the bacterial
parasite Serratia marcescens to kill C. elegans hosts in uneven mixtures
of host genotypes.  To examine the effect of commonness itself,
independent of host identity, each of four host genotypes was represented
as common or rare in experimental host mixtures.  After
experimental selection, we evaluated a parasite line’s change in
virulence, the selected fitness trait, on its rare and common host
genotypes.  Our results were consistent with a slight advantage
for rare host genotypes: on average, parasites lost virulence against rare
genotypes but not against common genotypes.  The
response varied substantially, however, with distinct patterns across host
genotype mixtures.  These findings support the potential for
parasites to impose negative frequency-dependent selection, while
emphasizing that the cost of being common may vary with host genotype.