Aggregation of symbionts on hosts depends on interaction type and host traits

Symbionts tend to be aggregated on their hosts, such that few hosts harbor
the majority of symbionts. This ubiquitous pattern can result from
stochastic processes, but aggregation patterns may also depend on the type
of host-symbiont interaction, plus traits that affect host exposure and
susceptibility to symbionts. Untangling how aggregation patterns both
within and among populations depend on stochastic processes, interaction
type and host traits remains an outstanding challenge. Here, we address
this challenge by using null models to compare aggregation patterns in a
neutral system of Balanomorpha barnacles attached to patellid limpets and
a host-parasite system of Trinidadian guppies (Poecilia reticulata) and
their Gyrodactylus spp. monogeneans. We first used a model to predict
patterns of symbiont-host aggregation due to random partitioning of
symbionts to hosts. This null model accurately predicted the aggregation
of barnacles on limpets, but the degree of aggregation varied across 303
quadrats. Quadrats with larger limpets had less aggregated barnacles,
whereas aggregation increased with variation in limpet size. Across 84
guppy populations, Gyrodactylus spp. parasites were significantly less
aggregated than predicted by the null model. As in the neutral
limpet-barnacle system, aggregation decreased with mean host size.
Parasites were also significantly less aggregated on males than females
because male guppies tended to have higher prevalence and lower parasite
burdens than predicted by the null model. Together, these results suggest
stochastic processes can explain aggregation patterns in neutral but not
parasitic systems, though in both systems host traits affect aggregation
patterns. Because the distribution of symbionts on hosts can affect
symbiont evolution via intraspecific interactions, and reciprocally host
behavior and evolution via host-symbiont interactions, identifying the
drivers of aggregation enriches our understanding of host-symbiont
interactions.