Digestibility of defense proteins in latex of milkweeds by digestive proteases of Monarch butterflies, Danaus plexippus L.: A potential determinant of plant–herbivore interactions

This study describes the digestive protease activity extracted from the gut of fifth-instar Monarch butterfly larvae, and its proteolytic activity on latex proteins of their host plant, Calotropis procera (the milkweed) and related non-host species from the milkweed family. Gut extracts digested azocasein, BANA and BApNA. Cysteine protease inhibitors such as E-64 and iodoacetamide inhibited proteolytic activity on azocasein; however, the serine protease inhibitors PMSF and leupeptin were more effective. Gut extracts promptly digested LP and were not affected by endogenous latex proteases. Gut extracts, however, did not digest LP from Cryptostegia grandiflora and only slightly digested LP from Plumeria rubra, two plant species that are not consumed by Monarch larvae. The protein profiles of latex proteins extracted from healthy and attacked plants were different. A protein identified as glycoside hydrolase was detected in increased concentrations in latex from damaged plants. Larvae fed on artificial diets containing 1% or 5% latex proteins were not adversely affected and gained weight faster than control larvae. These results provide new information on the resistance of Monarch larvae fed on C. procera and suggest that the ability of Monarch proteolytic enzymes to promptly digest LP can explain (at least in part) how these insects overcome the defensive proteins found in C. procera latex.

Research highlights▶ Gut extracts of Monarch promptly digested laticifer proteins of Calotropis procera and were not affected by endogenous latex proteases. ▶ The protein profiles of latex proteins extracted from healthy and attacked plants were different. ▶ A protein identified as glycoside hydrolase was detected in increased concentrations in latex from damaged plants.