Unlocking the genetic basis of monarch butterflies' use of medicinal plants.

If there was any doubt of the primary role that plant secondary metabolites play in host-parasite co-evolution, the "From the Cover" paper by Tan et al. (2019) featured in this issue of Molecular Ecology will lay these doubts to rest. The group's previous work on monarch butterflies (Danaus plexippus) infected with the protozoan pathogen Ophryocystis elektroscirrha (OE) demonstrated higher survival and lower spore load on high cardenolide-producing milkweed (Asclepias curassavica) (Figure 1a) compared with low cardenolide-producing milkweed (A. incarnata) (de Roode, Pedersen, Hunter, & Altizer, 2008) (Figure 1b). The mechanism of this protective effect is not directly clear, but a leading hypothesis is that the cardenolides confer protection through toxicity to the parasite. However, the role of the caterpillar immune system in managing this parasite is largely unknown. Novel insights into the influence of toxic plant metabolites on caterpillar immunity are explored in Tan et al. (2019). Using transcriptomics to probe this model system, the authors found that herbivore immune genes were down-regulated and detoxification genes were up-regulated when larvae were reared on the milkweed species with high cardenolide concentrations (A. curassavica). Surprisingly, immune genes were not significantly up- or down-regulated in response to protozoan infection alone. This tantalizing result suggests that sequestered plant metabolites, not immunity, is reining in protozoan infections in these larvae, and promoting survival. As the authors point out, the strategy to invest in sequestration may come at a cost, which is to the detriment of the immune response (Smilanich, Dyer, Chambers, & Bowers, 2009). However, the cost becomes worth the investment when chemical sequestration takes on an antipathogen role. The novelty of the Tan et al. (2019) paper is that they show the investment in sequestration leading to a possible divestment in immunity.