Current theories assume that adaptive parasite evolution explains
variation in the level of virulence and parasite success.
In particular, mixed-genotype infections by parasites should generally
be
more virulent, and faster multiplying strains
more successful, either because fixed strategies have evolved or because
parasites facultatively alter virulence in response
to co-infecting competitors. We compared several measures of parasite
success and virulence between single-clone and
mixed-clone infections of 2 strains of the trypanosome Crithidia bombi
in its bumblebee host, Bombus terrestris. Contrary
to expectation, we could not find differences between single-clone and
mixed-clone infections in parasite prevalence,
infection success, duration and clearance rate. However, a clearly
significant effect of colony on infection intensity was
present, and the colony effect emerged in virtually all other measures.
We thus conclude that host environment as defined
by the family (colony) genotype and thus host heterogeneity are more
important in determining parasite virulence than
the parasite characteristics. This does not invalidate modern theories
of parasite evolution but suggests that variation in
both hosts and parasites must be taken into account in more detail.