Evolution of vertebrate nicotinic acetylcholine receptors

Many physiological processes are influenced by nicotinic acetylcholine receptors (nAChR), ranging from neuromuscular and parasympathetic signaling to modulation of the reward system and long-term memory. Due to the complexity of the nAChR family and variable evolutionary rates among its members, their evolution in vertebrates has been difficult to resolve. In order to understand how and when the nAChR genes arose, we have used a broad approach of analyses combining sequence-based phylogeny, chromosomal synteny and intron positions.

Our analyses suggest that there were ten subunit genes present in the vertebrate predecessor. The two basal vertebrate tetraploidizations (1R and 2R) then expanded this set to 19 genes. Three of these have been lost in mammals, resulting in 16 members today. None of the ten ancestral genes have kept all four copies after 2R. Following 2R, two of the ancestral genes became triplicates, five of them became pairs, and three seem to have remained single genes. One triplet consists of CHRNA7, CHRNA8 and the previously undescribed CHRNA11, of which the two latter have been lost in mammals but are still present in lizards and ray-finned fishes. The other triplet consists of CHRNB2, CHRNB4 and CHRNB5, the latter of which has also been lost in mammals. In ray-finned fish the neuromuscular subunit gene CHRNB1 underwent a local gene duplication generating CHRNB1.2. The third tetraploidization in the predecessor of teleosts (3R) expanded the repertoire to a total of 31 genes, of which 27 remain in zebrafish. These evolutionary relationships are supported by the exon-intron organization of the genes.

The tetraploidizations explain all gene duplication events in vertebrates except two. This indicates that the genome doublings have had a substantial impact on the complexity of this gene family leading to a very large number of members that have existed for hundreds of millions of years.