The Study Of Octopamine And GABA_A Receptors In Insect Adaptation To Insecticides And Plant Toxic Terpenoids

Insects have evolved various mechanisms against insecticides and plant defensive secondary metabolites.Toxins are toxins,whether insecticides or phytochemicals,suggesting that insects may employ similar adaptive mechanisms under similar selective pressure.Many insecticides and secondary compounds act on conserved receptors and ion channels in insects.Therefore,identifying their molecular targets and then comparing the genetic mechanisms of adaptation to them could provide new insights into long-term sustainable pest control.In this study,we mainly focused on the role of octopamine and GABA_A receptor in insect adaptation to insecticides and plant chemicals,since they are the targets of commercial insecticides and potential targets of phytochemicals.1.An octopamine receptor confers selective toxicity of amitraz on honeybees and Varroa mitesThe Varroa destructor mite is a devastating parasite of Apis mellifera honeybees.They can cause colonies to collapse by spreading viruses and feeding on the fat reserves of adults and larvae.Amitraz is used to control mites due to its low toxicity to bees;however,the mechanism of bee resistance to amitraz remains unknown.In this study,we found that amitraz and its major metabolite potently activated all four mite octopamine receptors.Behavioral assays using Drosophila null mutants of octopamine receptors identified one receptor subtype Octβ2R as the sole target of amitraz in vivo.We found that thermogenetic activation of octβ2R-expressing neurons mimics amitraz poisoning symptoms in target pests.We next confirmed that the mite Octβ2R was more sensitive to amitraz and its metabolite than the bee Octβ2R in pharmacological assays and transgenic flies.Furthermore,replacement of three bee-specific residues with the counterparts in the mite receptor increased amitraz sensitivity of the bee Octβ2R,indicating that the relative insensitivity of their receptor is the major mechanism for honeybees to resist amitraz.Our findings have important implications for resistance management and the design of safer insecticides that selectively target pests while maintaining low toxicity to non-target pollinators.2.A point mutation in the parasitoid wasp GABA_A receptor subunit RDL confers resistance to cyclodiene and phenylpyrazole insecticidesInsecticides have become the primary selective force in many insect species;however,whether beneficial insects developed resistance remains unknown.We analyzed the sequences of hymenopteran GABA_A receptor subunit gene Rdl(resistance to dieldrin),which encodes the target of cyclodiene and phenylpyrazole insecticides.The resistance-conferring A2?S mutations were found in seven parasitoid wasps and similar amino acid replacements at homologous sites have been identified in four of their resistant hosts.Our findings indicate how parallel molecular evolution at a single amino acid site confers adaptation in parasitoid wasps.3.Evolution of GABA_A receptor subunit RDL promotes insect adaptation to plant defensive terpenoidsPlants have evolved chemical defenses against insect herbivores;however,the genetic basis for insect adaptation is largely unknown.Plants produce diverse neurotoxic terpenoids that target the GABA_A receptor,a ligand-gated ion channel in the central nervous system.We surveyed the insect GABA_A receptor subunit gene Rdl and found eight independent duplications in four herbivore and one predator lineages spanning three orders associated with their host shifts.Additionally,parallel amino acid substitutions that may impair ligand binding were found in all duplicated copies.Using genome engineering in Drosophila melanogaster,we confirmed that duplications of Rdl increase terpenoid resistance and partially release constraints imposed by substitutions.Our results reveal how the evolution of novel traits drove the adaptive radiations of organisms at macroevolutionary scales.