Multiple Creatures Share Cardiac Poison Protection

Foxglove, lily of the valley, milkweed butterflies, and bufonid toads are among the plant and animal producers of cardiac glycosides, a large family of defense poisons that can cause heart attacks. Cardiac glycosides are now used in a variety of applications, from rat poison to heart-resuscitating drugs. Given the prevalence of these poisons in nature, many insects, amphibians, reptiles, and mammals have developed resistance to the heart toxins. A team led by Nicholas R. Casewell of the Liverpool School of Tropical Medicine, in England, and Thomas Madsen of the University of Wollongong, in Australia, compared the resistance strategies of these diverse creatures and found that they all converge on a single 12-amino acid section of the ion-transport protein Na⁺/K⁺-ATPase (Proc. Natl. Acad. Sci. USA 2015, DOI: 10.1073/pnas.1511706112). This region of Na⁺/K⁺-ATPase is where cardiac glycoside poisons bind and thereby interfere with proper heart function. The researchers found that in all of the animal taxa the replacement of two amino acids in this stretch of protein conferred protection from the poison. They note that this is an extreme example of convergent evolution, where different species independently evolve a molecularly similar characteristic.