Resistance and Tolerance to Imperfectly Specialized Parasites: Milkweed Butterflies and Their Protozoan Parasites

Abstract

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 concentration 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, 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 underscore the need to identify factors that limit hosts' adaptation to parasites.

Keywords: Danaus gilippus; Danaus plexippus; Ophryocystis elektroscirrha; cross‐infection; host specificity; monarch butterfly; neogregarine; queen butterfly.

FIGURE 1

(A) The range of monarchs (light tan, Danaus plexippus ) and queens (orange, D. gilippus ) in the Americas. The overlapping range of monarchs and queens is shown in brown. Range maps were redrawn from (Jetz et al. 2012), and use data from (Balmer ; Brock and Kaufman ; Klimaitis et al. 2018). Inset shows the state of Florida and butterfly and parasite sources. W = Wakulla County, A = Alachua County, B = Broward County, D = Dixie County, La = Lake County, Le = Levy County. (B) Milkweed species used in this study; Sw = low‐cardenolide swamp milkweed ( Asclepias incarnata ) and Tr = high‐cardenolide tropical milkweed ( A. curassavica ). (C) Experimental design. Within each host species, larvae were chosen from either three (monarch) or twelve (queen) genetic lineages; for each parasite group, larvae were assigned to one of three isolates. Sample size was 50 larvae per treatment combination (N = 600 larvae). Butterfly and milkweed photographs by Icosahedron (2017), Flannery (2015), Mathias (2015), and Ramsey (2007). Images not to scale.

FIGURE 2

The outcome of exposure to conspecific and heterospecific parasites by source and host species. (A) Pre‐adult mortality (larva or pupae stage). (B.) Percentage of infected adults. (C) Average spore load (log₁₀) in infected adults. Error bars show interquartile ranges. C = control, M = monarch parasites, Q = queen parasites.

FIGURE 3

Immune defense metrics of fifth‐instar larvae by host species. (A) Median concentration of hemocytes (cells/μL) by milkweed species (larval diet). (B) Median plasmatocyte and spheroid cell concentration. (C) Average final absorbance (A₄₉₀) of the melanization reaction by parasite source. Error bars show interquartile ranges for (A) and (B) and standard errors for (C). Milkweed species: Sw = low‐cardenolide swamp milkweed, Tr = high‐cardenolide tropical milkweed. Hemocyte type: P = plasmatocytes, S = spheroid cells. Parasite source: C = control, M = monarch parasites, Q = queen parasites.

FIGURE 4

Fitness and tolerance of adult butterflies by host species and parasite source. (A) Median adult lifespan (days). Error bars show interquartile ranges. (B) Tolerance, as measured by the relationship between spore loads and lifespan. Parasite source: C/U = control/uninfected, M = monarch parasites, Q = queen parasites.

FIGURE A1

Kinetic curve of the melanization reaction (average absorbance per read ± standard errors) by host species and parasite source. The final absorbance value and the maximum slope of the melanization kinetic curve were highly correlated (r_s = 0.9). Parasite source: C = control, M = monarch parasites, Q = queen parasites.

FIGURE A2

Mean wing area by host species and milkweed host plant. Error bars show standard errors. Milkweed host plant species: Sw = low‐cardenolide swamp milkweed, Tr = high‐cardenolide tropical milkweed.