Comparative genetics of Na+/K+-ATPase in monarch butterfly populations with varying host plant toxicity

Abstract

Herbivores have evolved numerous behavioural and physiological adaptations to host plants; however, molecular adaptations are still poorly understood. One well-studied case comprises the specialist insects that feed on cardenolide-containing plants. Here, convergent molecular evolution in the Na+/K+-ATPase results in a reduced sensitivity to cardenolides across four insect orders. Because different plant species and genotypes differ in toxicity, Na+/K+-ATPase may be under differential selection from geographically varying host plants. We examined the α subunit of Na+/K+-ATPase in monarch butterflies (Danaus plexippus) from six worldwide populations to test whether differences in their host plant chemistry result in local adaptation at the molecular level. Although our study revealed multiple synonymous changes, we did not find these to be population-specific, nor did we identify nonsynonymous changes. Additionally, we compared the amino acid sequence of this subunit across 19 species. We identified two novel changes at sites 836 (K836N) and 840 (E840R) in the αM7-αM8 regions in the genus Danaus. Although previous studies focused on the first two trans-membrane domains, C-terminal domains may also interact with cardenolides. These results reveal a lack of molecular evolution of Na+/K+-ATPase at the population level, and call for additional attention regarding the C-terminal regions of this important enzyme.