An Ancient Detoxification Mechanism Co-Opted in the Recent Transition to Mustard-Feeding by Drosophilid Flies: A Role for Glutathione S-Transferase

Abstract

Herbivorous insects comprise more than 25% of all known living species; however, knowledge of the evolutionary adaptive processes that enable insects to overcome toxic plant defense compounds is unknown. In this study, we investigated the possible evolutionary co-option of the generalized mercapturic acid detoxification pathway in mustard feeding Scaptomyza that recently derived from microbe feeding drosophilids. The first step of this pathway utilizes the ancient dimeric detoxification enzyme glutathione S-transferase (GST) to catalyze conjugation of toxic electrophilic substrates, such as mustard derived isothiocyantes (ITCs), to glutathione (GSH). Of all GSTs, the insect specific GstD1 was selected as a candidate gene. Our findings showed S. nigrita GSTD1 (SnigGSTD1A) has a specificity constant (k(cat)/K(m)) two-fold to five-fold higher against the exceptionally toxic benzyl isothiocyanate (BITC) when compared to non-mustard feeding drosophilids. X-ray crystallographic structure comparison suggests that mutations in and around the hydrophobic substrate binding pocket (H-site) may be responsible for this enhanced activity against ITCs.