Functional genomics of cardenolide sequestration in monarch butterflies (Danaus plexippus)

Many herbivorous insects sequester dietary toxins to defend themselves against predators and parasites. Monarch butterflies (Danaus plexippus) are a textbook example of this behavior, sequestering toxic cardiac glycosides (cardenolides) from their milkweed host plants. Despite decades of research into cardenolide sequestration in monarchs, major gaps remain in our understanding of the physiological and genetic basis of this behavior. Here, we describe two experimental approaches aimed at addressing these gaps that leverage variation among two monarch butterfly populations (eastern North America, Puerto Rico) in their ability to sequester and variation in milkweed host species (Asclepias curassavica, A. syriaca) in their cardenolide content. In one experiment, we factorially reared caterpillars from each population on each host and generated RNA-seq data from four caterpillar tissues: midgut, Malpighian tubules, cuticle, and ventral nerve cord. In the second experiment, we used genetic mapping of F2 hybrid butterflies reared on each host to find loci associated with wing cardenolide concentration. Preliminary results corroborate earlier findings of strong population-level differences in sequestration across hosts, and RNA-seq data suggest an important role for UDP-glucronosyltransferases (UGTs) and other transporters in the sequestration process. Our data further suggest that the ability to sequester cardenolides from A. syriaca and other temperate North American milkweed species may be a recently derived phenomenon in monarch butterflies, consistent with genetic evidence supporting post-glacial demographic expansion in both monarchs and milkweed.