Previous studies on the mechanism of action of pyrethroids have confirmed that voltage-gated sodium channels (VGSC) in the axon membrane are the major target site of these compounds. The use of pyrethroids to control maize weevils, Sitophilus zeamais, a major pest of stored maize in Brazil, has led to the occurrence of resistance. The work described here seeks to establish whether changes in VGSC of S.zeamais can explain pyrethroid resistance. The S. zeamais homologue of the Drosophila para VGSC was identified using degenerate primers and sequenced. Resistance mutations were examined by sequencing the IIS4-IIS6 region of the gene from laboratory strains of susceptible and resistant insects, revealing one amino acid replacement (T929I). The T929I mutation has been identified in other insects but always associated with a second mutation together producing a highly resistant phenotype. The occurrence of T929I in isolation is rare. DNA-based diagnostic assays were designed to screen weevils for the T929I mutation and analyse Brazilian field populations revealing a low frequency of heterozygous individuals carrying the mutation. The effect of the T929I mutation on VGSC function was investigated using whole cell patch clamping on cultured neurons isolated from thoracic ganglia of wild-type and resistant weevils. Inward currents were recorded by depolarizing the neuron to test potentials in the range -70mV to +70mV in 10mV increments for 25ms from a holding potential of -80mV. Current amplitudes were similar in cells from resistant weevils however other changes were apparent, notably a significant depolarizing shift in the voltage-dependence of activation of sodium currents in the resistant animals (P<0.05). Mutant neurons are also less sensitive to deltamethrin than the wild types.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:529081 |
| Date | January 2010 |
| Creators | Araújo, Rúbia Aparecida de |
| Publisher | University of Nottingham |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://eprints.nottingham.ac.uk/11604/ |
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