Hamiltosporidium tvaerminnensis infection route diet quality

Linking characteristics of parasites to host diet-induced shifts in
host-parasite interactions is a critical step in predicting the abundance
of specific parasites within an ecosystem. A parasite’s infection route
reflects a suite of parasite characteristics that could mediate the
effects of host diet quality on host-parasite interactions, but the effect
of infection route on the response of host-parasite interactions to host
diet quality has not been quantified. We quantified these effects for the
first time by experimentally manipulating a parasite capable of
transmitting itself via multiple pathways utilized by a wide variety of
parasites. We altered the diet phosphorus (P) content of Daphnia infected
by the mixed mode transmitter Hamiltosporidian tvaerminnensis via one of
two infection routes: horizontal, via the ingestion of spores from dead
hosts, or vertical, via the infection of host offspring. Then, we
quantified the effect of infection route on host diet-induced shifts in
parasite load and virulence (i.e. host fecundity and survival). Overall,
we found that horizontal infections were more sensitive to host diet than
vertical infections. Low P diets decreased the parasite load and fecundity
of all hosts, but had stronger negative effects on the loads of
horizontally infected individuals than vertically infected ones. Likewise,
horizontal infection reduced the survival of hosts fed low P (but not high
P) diets while vertical infection did not affect survival. This increased
sensitivity of horizontal infections to diet quality empirically supports
the intuitive hypothesis that host diet quality exerts stronger effects on
parasites that transmit via ingestion into a host’s gut than on parasites
that transmit via routes less directly related to host nutrient uptake.
Through these effects, host diet quality may shape the ecology and
evolution of parasite communities.