Molecular Evolution of Glutathione S-Transferases in the Genus Drosophila

Wai Yee Low^*^,, Hooi Ling Ng, Craig J. Morton, Michael W. Parker^,, Philip Batterham^*^, and Charles Robin^*^,^,1

^* Department of Genetics, University of Melbourne, Victoria 3010, Australia, Bio21 Molecular Science and Biotechnology Institute, University of Melbourne, Victoria 3010, Australia and Biota Structural Biology Laboratory and the ACRF Rational Drug Discovery Facility, St. Vincent's Institute of Medical Research, Melbourne, Victoria 3065, Australia

^¹ Corresponding author: Department of Genetics, University of Melbourne, Gate 12 Royal Parade, Parkville, Melbourne, VIC 3010, Australia.
E-mail: crobin{at}unimelb.edu.au

As classical phase II detoxification enzymes, glutathione S-transferases(GSTs) have been implicated in insecticide resistance and mayhave evolved in response to toxins in the niche-defining feedingsubstrates of Drosophila species. We have annotated the GSTgenes of the 12 Drosophila species with recently sequenced genomesand analyzed their molecular evolution. Gene copy number variationis attributable mainly to unequal crossing-over events in thelarge ${delta}$ and ${varepsilon}$ clusters. Within these gene clusters there are alsoGST genes with slowly diverging orthologs. This implies thatthey have their own unique functions or have spatial/temporalexpression patterns that impose significant selective constraints.Searches for positively selected sites within the GSTs identifiedG171K in GSTD1, a protein that has previously been shown tobe capable of metabolizing the insecticide DDT. We find thatthe same radical substitution (G171K) in the substrate-bindingdomain has occurred at least three times in the Drosophila radiation.Homology-modeling places site 171 distant from the active sitebut adjacent to an alternative DDT-binding site. We proposethat the parallel evolution observed at this site is an adaptiveresponse to an environmental toxin and that sequencing of historicalalleles suggests that this toxin was not a synthetic insecticide.