Convergent adaptive evolution - how insects master the challenge of cardiac glycoside-containing host plants

Cardiac glycosides are a prime example of highly toxic plant secondary compounds, which block an essential transmembrane carrier in animals, the Na, K-ATPase. Nevertheless, over 100 insect species from diverse orders are known to feed on plants containing these compounds and in many cases these toxins are additionally sequestered without ill effect. We investigated whether the insects' adaptations for handling cardiac glycosides are based on a single physiological mechanism or whether various strategies have evolved across groups. We analyzed gene sequences of the Na, K-ATPase a-subunit from cardiac glycoside-adapted insects and screened for amino-acid substitutions which could alter the affinity of the enzyme toward cardiac glycosides. In representatives from five insect orders, separated by over 300 million years of evolutionary divergence, we uncovered amino-acid substitutions at identical positions. Especially striking is the convergent substitution of a histidine for the conserved asparagine at position 122, which we report here for the first time in a sawfly, Monophadnus latus Costa (Hymenoptera: Tenthredinidae), and which was previously observed in the orders Lepidoptera, Coleoptera, Hemiptera, and Diptera. Prior in vitro expression and enzyme assays indicated that this substitution as well as combined substitutions with other residues result in a strongly increased cardenolide resistance of the Na, K-ATPase. The substitutions to threonine 111 and histidine 122 observed in M. latus are highly effective and were previously known only in lygaeid bugs. However, not all insects dealing with dietary cardenolides rely on target-site insensitivity as a mechanism of resistance. An impermeable gut or the exclusion of cardenolides from the nervous tissue with the greatest expression of Na, K-ATPase by the perineurium, the insect blood brain barrier, apparently represent alternative strategies. Immuno-histochemical data presented here support the existence of P-glycoprotein-like efflux transporters in insect gut membranes that might prevent the uptake of allelochemicals like cardenolides.