Evolution of GSTD1 in Cactophilic Drosophila.

GSTD1 is an insect glutathione S-transferase that has received considerable attention because of its role in detoxification of xenobiotic compounds, specifically pesticides and plant allelochemicals involved in detoxification, or in the use of some substrates as a nutritional source. GSTD1 has been implicated in the adaptation to a new cactus host in Drosophila mojavensis and thus constitutes an interesting candidate to study ecological genetics of adaptation in Drosophila. We conducted population genetic and molecular evolution analyses of the GstD1 gene in the context of association with different cactus hosts (Opuntia sp. vs. Columnar) in nine Drosophila species from the repleta group. We observed strong evidence of selection in GstD1 from D. hexastigma. This species is associated with a diverse set of columnar cacti with very complex chemistries. GstD1 sequences from D. hexastigma show evidence of a recent selective sweep, and positive selection at one residue just outside of the active site of the enzyme. The substitution (Q116T) at the site under selection leads to a conformational change in the enzyme that could have important consequences for substrate binding efficiency. Our results suggest that GSTD1 from D. hexastigma may have evolved improved substrate binding in order to adapt to the diverse chemical environments that this species encounters in the wild.