Comparative analysis of two acetylcholinesterase genes of Bombyx mandarina and Bombyx mori
Acetylcholinesterase (AChE), which contains two subfamilies, ace1 and ace2 in insects, was identified to be the target of organophosphorous and carbamate insecticides. Resistance to insecticides is apparently different between Bombyx mori and Bombyx mandarina. To compare the sequences and tissue expressions of the two aces between the two species, cDNAs encoding two ace genes were cloned and designated as Bmm-ace1 and Bmm-ace2 from the larvae of the B. mandarina. The amino acid sequence of Bmm-ace1 shared 99.71% homology with its homolog, Bm-ace1, in B. mori, with two mutations (G664S and S307P) and the amino acid sequence of Bmm-ace2 shared 99.37% homology with Bm-ace2, in Bombyx mori, with four mutations (M18I, N233S, I310V and G621S). Analysis of tissue expression showed that ace1 genes of the two species were highly expressed only in brain tissues and fat bodies, while ace2 genes were expressed in all tissues tested; the expression level of Bmm-ace and Bm-ace1 in brain tissue was almost the same, while the expression level of Bmm-ace2 was 4.17 fold as high as that of Bmm-ace2 in brain tissue. The results indicated that ace gene mutations and the difference in the expression level of ace2 were speculated to be the molecular basis for the difference in sensitivity to organophosphate insecticides between B. mori and B. mandarina. This was the first experimental report in which the ace2 gene was closely related to insecticide resistance in silkworm.
Key words: Bombyx mori, Bombyx mandarina, acetylcholinesterase gene, expression, insect, resistance.