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

Animals frequently defend themselves against predators using diet-derived toxins. Monarch butterflies are a preeminent example of toxin sequestration, gaining protection via cardenolides in their milkweed hosts. Few studies have considered genetic variation in sequestration ability, in monarchs or other species. Here, we use two approaches to study natural selection on cardenolide sequestration in monarchs. First, we conducted a reciprocal rearing experiment with six monarch populations and six associated host species from around the world to determine whether sequestration is higher in monarchs reared on sympatric host species. Second, we compared sequestered cardenolides in wild-caught monarchs from Guam—an island where bird predators have been functionally extirpated for >40 years—to a nearby island with intact birds. We found substantial genetic variation in sequestration ability, though no consistent sequestration advantage in sympatric combinations. One monarch population from Puerto Rico showed greatly reduced sequestration from Asclepias syriaca , likely reflecting a lack of evolutionary association with this host. Monarchs from Guam showed reduced sequestration from A. curassavica , both in a cross-island comparison and when reared under controlled conditions. 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.