Heliconius melpomene midgut
Analysis of xenobiotic metabolising enzymes in Heliconius melpomene
Insects, like all eukaryotes, depend mainly on three superfamilies of enzymes for detoxification and metabolism of xenobiotic compounds. These are glutathione-S-transferases (GSTs), cytochrome P450s (P450s) and carboxylesterases. The general mechanism underlying insecticide resistance is the amplification of the genes belonging to these three superfamilies. However, surprising diversity can be seen in the representation of these gene families in the insect genomes published to date. The Glutathione-S-transferase superfamily consists of diverse multifunctional enzymes found ubiquitously in most aerobic eukaryotes and prokaryotes. They play a key role in the detoxification of the xenobiotic compounds including insecticides and are also involved in biosynthesis of hormones, protection against oxidative stress and intracellular transport.        In insects cytochrome P450''s are involved in metabolism of xenobiotics, synthesis and degradation of juvenile hormones and ecdysteroids.    Similar to P450s, CCEs can function broadly in xenobiotic detoxification. CCE based resistance has been reported in several veterinary, medical and agriculture insect pests. In insect species, CCEs contribute to insecticide degradation and are involved in insecticide resistance.        Large scale gene duplication events have been assumed to correspond to major evolutionary changes. Acquisition of new biological functions has been associated with proliferation of gene families followed by functional diversification of paralogues. Comparative studies of gene sequences among different insects can point to species specific genes.  Comparing gene families responsible for xenobiotic metabolism in different insect pests can be important as many of the genes in these superfamilies are potential candidates for targets of selective insecticides.         Transcriptome data can be used as starting point for deducing coding sequences and function of genes that have not been previously isolated or sequenced.