Comparison of acetylcholinesterase genes from cattle ticks

We describe the isolation and characterisation of two putatively new acetyl cholinesterase genes from the African cattle ticks Boophilus decoloratus an d Rhipicephalus appendiculatus. The nucleotide sequences of these genes had 93% homology to each other and 95% and 91% identity, respectively, to the acetylcholinesterase gene from an Australian strain of another cattle tick, Boophilus microplus. Translation of the nucleotide sequences revealed puta tive amino acids that are essential for acetylcholinesterase activity: the active site serine, and the histidine and glutamate residues that associate with this serine to form the catalytic triad. All known acetylcholinestera ses have three sets of cysteines that form disulfide bonds; however, the ac etylcholinesterase genes of these three species of ticks encode only two se ts of cysteines. Acetylcholinesterases of B. microplus from South Africa, Z imbabwe, Kenya and Mexico had 98-99% identity with acetylcholinesterase fro m B. microplus from Australia, whereas acetylcholinesterase from B. micropl us from Indonesia was identical to that from Australia. Preliminary phyloge netic analyses surprisingly indicate that the acetylcholinesterases of tick s are closer phylogenetically to acetylcholinesterases of vertebrates than they are to those of other arthropods. (C) 1999 Australian Society for Para sitology Inc. Published by Elsevier Science Ltd. All rights reserved.