The path to "femmes fatales": the evolution of toxin resistance in predatory fireflies.

Many species of fireflies (Lampyridae) are defended from predation by toxic cardiotonic steroids (CTS), which inhibit the physiologically critical enzyme Na+,K+-ATPase (NKA). While most firefly genera defended by these toxins produce them autogenously, species of the genus Photuris obtain them from a surprising source: predation on other fireflies. Associated differences in the physiology of toxin metabolism of Photuris versus other firefly genera raise the possibility that they may have evolved distinct strategies to prevent self-intoxication. We show that both Photuris and their firefly prey have evolved highly resistant NKAs. Phylogenetic analysis of the target of CTS, the alpha-subunit of NKA (ATPa), suggests that the initial steps toward resistance were shared between Photuris and other firefly lineages. Despite these shared steps, the Photuris lineage subsequently underwent several rounds of duplication and neofunctionalization of increasingly CTS-resistant ATPα paralogs, whereas other firefly species retain a single copy. Our results implicate gene duplication as a facilitator in the transition of Photuris to its distinct ecological role as predator of toxic firefly prey. ### Competing Interest Statement The authors have declared no competing interest.