Evolution in an Ancient Detoxification Pathway Is Coupled with a Transition to Herbivory in the Drosophilidae

Abstract

Chemically defended plant tissues present formidable barriers to herbivores. Although mechanisms to resist plant defenses have been identified in ancient herbivorous lineages, adaptations to overcome plant defenses during transitions to herbivory remain relatively unexplored. The fly genus Scaptomyza is nested within the genus Drosophila and includes species that feed on the living tissue of mustard plants (Brassicaceae), yet this lineage is derived from microbe-feeding ancestors. We found that mustard-feeding Scaptomyza species and microbe-feeding Drosophila melanogaster detoxify mustard oils, the primary chemical defenses in the Brassicaceae, using the widely conserved mercapturic acid pathway. This detoxification strategy differs from other specialist herbivores of mustard plants, which possess derived mechanisms to obviate mustard oil formation. To investigate whether mustard feeding is coupled with evolution in the mercapturic acid pathway, we profiled functional and molecular evolutionary changes in the enzyme glutathione S-transferase D1 (GSTD1), which catalyzes the first step of the mercapturic acid pathway and is induced by mustard defense products in Scaptomyza. GSTD1 acquired elevated activity against mustard oils in one mustard-feeding Scaptomyza species in which GstD1 was duplicated. Structural analysis and mutagenesis revealed that substitutions at conserved residues within and near the substrate-binding cleft account for most of this increase in activity against mustard oils. Functional evolution of GSTD1 was coupled with signatures of episodic positive selection in GstD1 after the evolution of herbivory. Overall, we found that preexisting functions of generalized detoxification systems, and their refinement by natural selection, could play a central role in the evolution of herbivory.

Pseudomonas syringae enhances herbivory by suppressing the reactive oxygen burst in Arabidopsis

“…We previously identified two candidate xenobiotic metabolism genes, Glutathione S-tranferase D1 ( GstD1 ) and Peritrophin A. GstD1 is an important detoxification enzyme in Drosophila species, involved in xenobiotic and pesticide resistance, including resistance to ROS and downstream lipid peroxidation products (Landis et al, 2012; Mittapalli et al, 2007b; Sawicki et al, 2003), and to ITCs (Gloss et al, 2014). Peritrophin A is integral to the peritrophic membrane, which lines the midgut of most insects, and serves as a physical antioxidant in herbivorous insects protecting the epithelium from ROS (Summers, Felton, 1998).…”

“…After the eggs hatch, the larvae develop inside the leaves as leafminers, consuming the mesophyll tissue. To some extent adult females and larvae detoxify the lineage-specific defensive chemicals in the Brassicales, the glucosinolate-derived isothiocyanates (ITCs), using the mercapturic acid pathway (Gloss et al, 2014). One key enzyme in this pathway that has been characterized in Scaptomyza spp.…”

“…One key enzyme in this pathway that has been characterized in Scaptomyza spp. is Glutathione S -transferase D1 ( GstD1 ) (Gloss et al, 2014). Less is known about how Scaptomyza spp.…”

Recurrent specialization on a toxic fruit in an island Drosophila population

“…Although the exact function of these transmembrane proteins remains unknown, they are differentially expressed upon exposure to Arabidopsis glucosinolates in the herbivorous drosophilid S. flava (38) and they have expanded in herbivorous silkworm and pea aphid (50). Other candidates for convergence with herbivorous Scaptomyza include the above-mentioned Tweedle family (38) and cluster D GST genes (51). It is therefore possible that aspects of the parallel evolution between D. yakuba and D. sechellia may extend to other distantly related drosophilids and perhaps beyond.…”

Metabolism of Glucosinolates and Their Hydrolysis Products in Insect Herbivores

“…Interestingly, products of this pathway were also detected in species of Scaptomyza (Diptera), whose larvae are leaf-mining specialists on glucosinolate-containing plants. Unlike many glucosinolate specialists, these larvae do not employ a specifi c extracellular biochemical mechanism to prevent isothiocyanate formation, and instead metabolize these compounds intracellularly after exposure [ 143 ]. However, biochemical evidence suggests that particular GST enzymes in these species may have been modifi ed by evolution to better utilize isothiocyanates as substrates, in comparison to orthologous enzymes in related non-herbivorous dipterans.…”