Resistance and tolerance to imperfectly specialized parasites: Milkweed butterflies and their protozoan parasites

Understanding host specificity and cross-species transmission of parasites
is crucial for predicting the risk and consequences of parasite spillover.
We experimentally examined these dynamics in two closely related,
sympatric, milkweed butterfly hosts: monarchs (Danaus plexippus) and
queens (D. gilippus). The debilitating protozoan Ophryocystis
elektroscirrha (OE) infects wild monarchs throughout their range, and
similar neogregarine parasites have been reported from queens. We compared
host resistance and tolerance to infection between hosts exposed to
parasites of conspecific and heterospecific origin and examined whether
differences in immune investment reflected variation in infection
outcomes. Results showed that monarchs were highly susceptible to both
conspecific and heterospecific parasites. In contrast, queens were
susceptible almost exclusively to conspecific parasites. Queens showed
greater tolerance to infection and greater immune defense in the form of
melanization activity and concentrations of encapsulating hemocytes.
Additionally, monarch parasites caused higher pre-adult mortality and more
wing deformities than queen parasites. Given that OE can reduce monarch
abundance and migratory performance, and that monarchs overlap with
butterflies that host similar parasites, quantifying cross-infection
outcomes is important for conservation management of these two butterfly
species. The greater susceptibility and costs of infection in monarchs
suggest potential fitness trade-offs against resistance and tolerance to
infection in migratory hosts and underscores the need to identify factors
that limit hosts’ adaptation to parasites.