Data from: Stepwise evolution of resistance to toxic cardenolides via genetic substitutions in the Na+/K+-ATPase of milkweed butterflies (Lepidoptera: Danaini)

Despite the monarch butterfly (Danaus plexippus) being famous for its
adaptations to the defensive traits of its milkweed host plants, little is
known about the macroevolution of these traits. Unlike most other animal
species, monarchs are largely insensitive to cardenolides, since their
target site, the sodium pump (Na+/K+-ATPase), has evolved amino acid
substitutions which reduce cardenolide binding (so-called target site
insensitivity, TSI). Since many, but not all, species of milkweed
butterflies (Danaini) are associated with cardenolide-containing host
plants, we analyzed 17 species, representing all phylogenetic lineages of
milkweed butterflies, for the occurrence of TSI by sequence analyses of
the Na+/K+-ATPase gene and by enzymatic assays with extracted
Na+/K+-ATPase. Here we report that sensitivity to cardenolides was reduced
in a stepwise manner during the macroevolution of milkweed butterflies.
Strikingly, not all Danaini typically consuming cardenolides showed TSI,
but rather TSI was more strongly associated with sequestration of toxic
cardenolides. Thus, the interplay between bottom-up selection by plant
compounds and top-down selection by natural enemies can explain the
evolutionary sequence of adaptations to these toxins.