Trade-off between thermal tolerance and insecticide resistance in Plutella xylostella.

Fitness costs associated with resistance to insecticides have been well documented, usually at normal temperature conditions, in many insect species. In this study, using chlorpyrifos-resistant homozygote (RR) and chlorpyrifos-susceptible homozygote (SS) of resistance ace1 allele of Plutella xylostella (DBM), we confirmed firstly that high temperature experience in pupal stage influenced phenotype of wing venation in insecticide-resistant and insecticide-susceptible Plutella xylostella, and SS DBM showed significantly higher thermal tolerance and lower damages of wing veins under heat stress than RR DBM. As compared to SS DBM, RR DBM displayed significantly lower AChE sensitivity to chlorpyrifos, higher basal GSTs activity and P450 production at 25°C, but higher inhibitions on the enzyme activities and P450 production as well as reduced resistance to chlorpyrifos under heat stress. Furthermore, RR DBM displayed significantly higher basal expressions of hsp69s, hsp72s, hsp20,hsp90,Apaf-1, and caspase-7 at 25°C, but lower induced expressions of hsps and higher induced expressions of Apaf-1,caspase-9, and caspase-7 under heat stress. These results suggest that fitness costs of chlorpyrifos resistance in DBM may partly attribute to excess consumption of energy caused by over production of detoxification enzymes and hsps when the proteins are less demanded at conducive environments but reduced expressions when they are highly demanded by the insects to combat environmental stresses, or to excess expressions of apoptotic genes under heat stress, which results in higher apoptosis. The evolutionary and ecological implications of these findings at global warming are discussed.

Identification and characterization of ace2-type acetylcholinesterase in insecticide-resistant and -susceptible parasitoid wasp Oomyzus sokolowskii (Hymenoptera: Eulophidae).

A full-length acetylcholinesterase (AChE) cDNA sequence (Os-ace2.s) from insecticide-susceptible (S) parasitoid Oomyzus sokolowskii (Hymenoptera: Eulophidae) and a partial cDNA sequence (Os-ace2.r) from insecticide- resistant (R) O. sokolowskii were identified firstly. Both Os-ace2.s (encoding a protein of 639 amino acid residues) and Os-ace2.r (encoding a protein of 530 amino acid residues) contained the typical conserved motifs, including FGESAGdomains, catalytic triad, acyl pocket, three oxy-anino hole, choline binding site, peripheral anionic site, omega loop and conserved aromatic residues. The multiple alignment and Blast results indicated that Os-ace2.s were ace2 member of AChE gene. There were three replacements of the amino acid residues (Glu 115 Leu, Phe 394 Leu, and Lys 424 Arg) between Os-ace2.s and Os-ace2.r. The ace2 of O. sokolowskii was the AChE gene firstly isolated from hymenopteran parasitoid so far. R O. sokolowskii displayed about 15-20-folds resistance ratios to methamidophos and avermectin. The bimolecular rate constant (k i) value in S O. sokolowskii was 3.8-folds for methamidophos and 12.3 for dichlorvos, respectively higher than those in R O. sokolowskii. The results indicated that the insensitive AChE and replacements of the amino acid residues in Os-ace2 might be involved in the resistance to methamidophos in R O. sokolowskii.

Identification and expression of caspase-1 gene under heat stress in insecticide-susceptible and -resistant Plutella xylostella (Lepidoptera: Plutellidae).

A caspase gene in Plutella xylostella (DBM) was identified firstly and named Px-caspase-1. It had a full-length of 1172 bp and contained 900 bp open reading frame that encoded 300 amino acids with 33.6 kDa. The deduced amino acid of Px-caspase-1 had two domain profile including caspase_p20 (position 61-184) and caspase_p10 (position 203-298) (i.e. the big and small catalytic domains), and the highly conserved pentapeptide QACQG in caspase_p20 domain (the recognized catalytic site of caspases). Being highly homologous to effector caspase genes in other insect and mammalian species, Px-caspase-1 was thought to be an effector caspase gene. Heat stress could result in significant mortality increase on adult DBM. Px-caspase-1 mRNA expression and caspase-3 enzyme activity (a effector caspase) were elevated with age and heat treatment. And, heat stress facilitated the procession of Px-caspase-1 expression. Significantly higher mRNA transcription levels were found in a chlorpyrifos-resistant DBM strain, as compared to those in insecticide-susceptible DBM. The results indicated that high temperature could significantly promote apoptosis process resulting in an the increased DBM mortality rate, and that insecticide-susceptible DBM had a significantly higher physiological fitness at high temperatures than insecticide-resistant DBM.