Parallel Evolution of Insensitivity to Cardenolides in an Herbivore Community: A Molecular Basis

            Similar to foxglove and oleander, plants from the family Apocynaceae such as milkweed produce toxic steroids called cardenolides as a host-plant resistance mechanism to defend against predatory herbivory. Intriguingly, the larvae and adults of certain insect species are able to feed on and obtain nutrition from Apocynaceae without suffering from adverse effects that plague most herbivores. Moreover, some of these insects can sequester cardenolides as a defense against their own predation. For example, the larvae of the monarch butterfly feed on milkweed plants and sequester large amounts of cardenolide which renders them unpalatable to predators. Recently, scientists from Princeton University have disclosed studies on the genetic origins of this phenomenon (For more information about the study and some really great insect photos, see this).

            The Princeton team, led by Peter Andolfatto, surveyed the protein sequence of the alpha subunit of the sodium pump (ATPalpha) in 14 herbivorous insect species that feed on cardenolide-producing plants. Cardenolides bind to ATPalpha and inhibit essential physiological processes that depend on cation transport such as muscle contraction. It was shown by the Princeton team that amino acid substitutions associated with host-plant specialization are highly clustered, with many parallel substitutions. The mutations block ingested cardenolides from binding to ATPalpha.

            To examine the effects of the amino acid substitutions that were observed in the insect lineages on cardenolide binding affinity, molecular docking simulations were studied using an ATPalpha crystal structure bound to ouabain. Ouabain docking onto the wild-type (pig) protein was compared with a protein modified to incorporate the individual amino acid substitutions. A single parallel substitution, N122H, the replacement of an asparagine residue with histidine, occurs in five distinct insect lineages and is known from mutagenesis studies to confer resistance to ouabain in cells. This mutation appears to act by sterically blocking ouabain from entering the cardenolide-binding pocket of ATPalpha (depicted above). Stabilizing hydrogen bonds seem to occur between three polar amino acids in the ATPalpha binding pocket and hydroxyl groups located at positions C14 and C19 on the convex face of ouabain’s steroidal framework. The study provides a fascinating example of parallel evolution by diverse species in the context of a poison protection mechanism and enhances our understanding of the molecular basis for the observed physiological effects associated with cardenolide-type steroids.