Data accompanying: Population-specific patterns of toxin sequestration in monarch butterflies from around the world

Animals frequently defend themselves against predators and parasites using toxins obtained from their diets. Monarch butterflies are a preeminent example of toxin sequestration and gain protection from cardenolides in their milkweed host plants. Although sequestration behavior is well-studied in monarchs, relatively little research has studied genetic variation in sequestration ability. In this study, we use the monarch's global range expansion to test hypotheses about how cardenolide sequestration has evolved over recent evolutionary history. First, using a reciprocal rearing experiment involving six monarch populations and six associated milkweed host species, we test for whether natural selection has increased cardenolide sequestration in monarch populations reared on their sympatric hosts. Second, we test for whether contemporary species interactions affect sequestration by measuring cardenolides in monarchs from Guam, an oceanic island where bird predators have been functionally absent for approximately 40 years. We find evidence for substantial genetic variation in sequestration ability, although no consistent pattern of enhanced sequestration in sympatric monarch/milkweed combinations. One monarch population (from Puerto Rico) shows strong support for cross-hosts tradeoffs in sequestration ability, with elevated sequestration from two tropical milkweed species (Asclepias curassavica, Gomphocarpus physocarpus) but greatly reduced sequestration from two temperate species (A. syriaca, A. speciosa). Monarchs from Guam show some evidence for reduced cardenolide sequestration in both a cross-island comparison of wild-caught butterflies as well as a population-level comparisons of greenhouse-reared butterflies. Our results suggest that processes involved in toxin sequestration are subject to natural selection and may evolve in response to contemporary changes in species interactions.