New ways to acquire resistance: imperfect convergence in insect adaptations to a potent plant toxin

Abstract

Evolution of insensitivity to the toxic effects of cardiac glycosides has become a model in the study of convergent evolution, as five taxonomic orders of insects use the same few similar amino acid substitutions in the otherwise highly conserved Na,K-ATPase α. We show here that insensitivity in pyrgomorphid grasshoppers evolved along a slightly divergent path. As in other lineages, duplication of the Na,K-ATPase α gene paved the way for subfunctionalization: one copy maintains the ancestral, sensitive state, while the other copy is resistant. Nonetheless, in contrast with all other investigated insects, the grasshoppers' resistant copy shows length variation by two amino acids in the first extracellular loop, the main part of the cardiac glycoside-binding pocket. RT-qPCR analyses confirmed that this copy is predominantly expressed in tissues exposed to the toxins, while the ancestral copy predominates in the nervous tissue. Functional tests with genetically engineered Drosophila Na,K-ATPases bearing the first extracellular loop of the pyrgomorphid genes showed the derived form to be highly resistant, while the ancestral state is sensitive. Thus, we report convergence in gene duplication and in the gene targets for toxin insensitivity; however, the means to the phenotypic end have been novel in pyrgomorphid grasshoppers.

Dietary cardenolides enhance growth and change the direction of the fecundity‐longevity trade‐off in milkweed bugs (Heteroptera: Lygaeinae)

“…In O . fasciatus , the three gene copies (α1A, α1B, and α1C) encoding α‐ subunit of cardenolide‐resistant Na + /K + ‐ATPases have diverse functions (Lohr et al, 2017 ) and duplicated α1 gene copies do not only vary in number and identity but also show specific expression patterns in different body tissues (Dobler et al, 2019 ; Yang et al, 2019 ; Zhen et al, 2012 ) allowing for complex regulation.…”

Wing musculature reconstruction in extinct flightless auks (PinguinusandMancalla) reveals incomplete convergence with penguins (Spheniscidae) due to differing ancestral states

“…Zanno 2020). The prevalence of incomplete convergence is now widely appreciated in many different systems, although the detailed underpinnings of incompleteness largely remain to be 1598 explored (e.g., Losos 2010Losos , 2011Kaeuffer et al 2012;Collar et al 2014;Moen et al 2016;Dobler et al 2019). Also unexplored is the extent to which the retention and/or modification of ancestral conditions have influenced evolutionary trajectories in these examples.…”

Constraints on the evolution of toxin-resistant Na,K-ATPases have limited dependence on sequence divergence

“…Similarly, the known CTS resistance substitution C104Y is observed among many natricid snakes (Supplementary Dataset 2) and CTS-adapted milkweed weevils (5). Chinchilla ( Chinchilla lanigera ) and yellow-throated sandgrouse ( Pterocles gutturalis ) show distinct single-amino acid insertions in the H1-H2 extracellular loop, a characteristic that has been previously associated with CTS resistance in pyrgomorphid grasshoppers (30, 35). Further, in lieu of variation at site 122, ATP1A3 of tetrapods harbors frequent parallel substitutions at site 120 (G120R, see also (7)).…”