Evidence for a Diverse Cys-Loop Ligand-Gated Ion Channel Superfamily in Early Bilateria

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Abstract
The genome sequences of Caenorhabditis elegans and Drosophila melanogaster reveal a diversity of cysteine-loop ligand-gated ion channels (Cys-loop LGICs) not found in vertebrates. To better understand the evolution of this gene superfamily, I compared all Cys-loop LGICs from rat, the primitive chordate Ciona intestinalis, Drosophila, and C. elegans. There are two clades of GABA receptor subunits that include both verterbate and invertebrate orthologues. In addition, I identified nine clades of anion channel subunits found only in invertebrates, including three that are specific to C. elegans and two found only in Drosophila. One well-defined clade of vertebrate cation channel subunits, the α7 nicotinic acetylcholine receptor subunits (nAChR), includes invertebrate orthologues. There are two clades of invertebrate nAChRs, one of α-type subunits and one of non-α subunits, that are most similar to the two clades of vertebrate neuronal and muscle α and non-α subunits. There is a large group of divergent C. elegans nAChR-like subunits partially resolved into clades but no orthologues of 5HT3-type serotonin receptors in the invertebrates. The topology of the trees suggests that most of the invertebrate-specific Cys-loop LGIC clades were present in the common ancestor of chordates and ecdysozoa. Many of these disappeared from the chordates. Subsequently, selected subunit genes expanded to form large subfamilies.