Massive expansion and diversity of nicotinic acetylcholine receptors in lophotrochozoans

Nicotinic acetylcholine receptors (nAChRs) are among the oldest and most conserved transmembrane receptors involved in signal transduction. Despite the prevalence and significance of cholinergic signaling, the diversity and evolution of nAChRs are not fully understood. By comparative genomic analysis, we found massive expansions of nAChR genes in molluscs and some other lophotrochozoans. The expansion is particularly pronounced in stationary bivalve molluscs with simple nervous systems, with the number of nAChR genes ranging from 99 to 217 in five bivalves, compared with 10 to 29 in five ecdysozoans and vertebrates. The expanded molluscan nAChR genes tend to be intronless and in tandem arrays due to retroposition followed by tandem duplication. Phylogenetic analysis revealed diverse nAChR families in the common ancestor of bilaterians, which subsequently experienced lineage-specific expansions or contractions. The expanded molluscan nAChR genes are highly diverse in sequence, domain structure, temporal and spatial expression profiles, implying diversified functions. Some molluscan nAChR genes are expressed in early development before the development of the nervous system, while others are involved in immune and stress responses. The massive expansion and diversification of nAChR genes in bivalve molluscs may be a compensation for reduced nervous systems as part of adaptation to stationary life under dynamic environments, while in vertebrates a subset of specialized nAChRs are retained to work with advanced nervous systems. The unprecedented diversity identified in molluscs broadens our view on the evolution and function of nAChRs that are critical to animal physiology and human health.