Repeated adaptive evolution in a single protein.

Parallel evolution provides a portrait of the evolutionary dynamics of naturally occurring substitutions under a shared selection pressure. To study these dynamics, we have exploited the insect-herbivore community living on milkweed plants as a model system.  Milkweeds produce defensive steroid glycosides that deter herbivores with the exception of a group of specialists, many of which actively sequester these compounds for their own defense.  We have used de novo transcriptome assembly as a means to rapidly and efficiently survey the target of these inhibitors, the alpha subunit of Na+,K+-ATPase (ATPalpha), in a large number of insects feeding on milkweed plants and are integrating this information with the extensive functional and structural data for this particular protein-ligand interaction. ATPalpha is crucial for animal cellular processes including neural function, muscle contraction, and osmoregulation. We found a number of parallel amino-acid substitutions that are predicted to reduce the enzyme’s sensitivity to steroid glycosides. Despite the large number of potential targets for modulating steroid glycoside sensitivity suggested by functional studies, our data strongly suggest that some of these substitutions are strongly preferred over others by natural selection, likely due to negative pleiotropic effects of less preferred mutations. We also discovered three independent episodes of duplication and divergence expression of ATPalpha paralogs associated with specialization on milkweeds (e.g. see Figure 1), underscoring the importance of duplication in releasing an organism from the constraints of negative pleiotropic effects.

In NIH R01-funded research, we we are building on preliminary work (outlined above) by expanding the survey of Na+,K+-ATPase to include more insect orders (Orthoptera, Diptera, Hymenoptera) and predator-prey community analogs of the plant-herbivore system (i.e. fireflies, bufonid toads and their predators).  In addition, using state-of-the-art genome engineering approaches, we are developing a Drosophila melanogaster-based model system to study the specific biochemical, physiological and behavioral effects associated with specific putatively adaptive changes observed in steroid glycoside resistant taxa.  This work should greatly improve our understanding of adaptation and the origins of organismal complexity. 

Representative papers:

Zhen Y*, Aardema ML*, Medina EM, Schumer M*, Andolfatto P. 2012. Parallel molecular evolution in an herbivore community. Science. 337:1634-7.

Aardema ML*, Andolfatto P.  2016. Phylogenetic incongruence and the evolutionary origins of cardenolide-resistant forms of Na+,K+-ATPase in Danaus butterflies. Evolution, 70:1913-21.