Evolution of animal cholinesterases

Cholinesterases emerged from a family of enzymes and proteins with adhesion properties. This family is absent in plants and expanded in multicellular animals [1,2]. True cholinesterases appeared in triploblastic animals together with the cholinergic system. Lineage specific duplications resulted in two acetylcholinesterases in most hexapods and in up to four genes in nematodes and arachnids. In the vertebrates, the duplication leading to acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) is now considered to be an ancient event which occurred before the split of the Ostheichthyes [3]. The product of one or the other of the paralogues is responsible for the physiological hydrolysis of acetylcholine, depending on the species lineage and tissue considered. The BChE gene seems to have been lost in some fish lineages [4]. The complete genome of amphioxus (Branchiostoma floridae: cephalochordate) or Saccoglossus kowalevskii (Acorn worm) contains a large number of duplicated genes or pseudogenes of cholinesterases [5]. Parasitic nematodes or ticks also show amplification of cholinesterase like genes. The mode of attachment through alternative C-terminal exons seems to have evolved independently from the catalytic part of the gene.