Whenever evolution in response to a particular toxic chemical is revealed, one of the first questions that comes to mind is, how pervasive is resistance? Would a population of fish, for example, exposed to PCBs anywhere likely develop resistance? How much depends on the species? The conditions? And if resistance were to evolve would it likely happen in the same way or in very different ways? A recent study by Susan Dobler and colleagues, published in PNAS addresses this last question – and their findings are fascinating. That insect species develop resistance to defensive chemicals in plants (as well as to pesticides) is well known. In many cases it is clear that resistance evolved through different routes, leading to different mechanisms of resistance. Reduced binding at a receptor site. Increased metabolism of the toxicant. Increased production of a particular enzyme. Yet in a smaller number of cases (so far) not only is there convergence of the evolutionary ends (resistance in this case) but the molecular means as well.
Dobler’s group focused on cardenolides, toxicants we are most familiar with through the relationship of monarch butterfly caterpillars with milkweed. Cardenolides like those produced by milkweeds, interfere with the ubiquitous and essential enzyme Na,K-APTase, or the sodium pump. A key feature in all animal cell membranes, the sodium pump is highly conserved, and proper function is critical. But that doesn’t mean there isn’t the potential for an alteration here and there. And, given the long history of warfare between insects and plants and the list of plants producing cadenolides, it’s not surprising that monarchs aren’t the only resistant species. In monarch’s the swapping out at least one amino acid of the sodium pump for another is enough to enable the caterpillars to take advantage of a plant few other species can use. What is surprising, are the results of the group’s genetic analysis. Apparently monarchs aren’t the only ones to have stumbled across this path of resistance. In the majority of insect species evaluated (18 different species, spanning four orders of insects) resistance was conferred by the same altered amino acids. Write the authors, “Thus, across roughly 300 Myr of divergence….we find identical amino acid replacements in all four insect orders feeding on cardenolide-containing plants.” Amazing.
For more see: Community-wide convergent evolution in insect adaptation to toxic cardenolides by substitutions in the Na,K-ATPase, PNAS, July 24, 2012. See also an earlier post, Combating tetrodotoxin.