Evidence for a diverse Cys-loop ligand-gated ion channel superfamily in early bilateria.
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 vertebrate 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 alpha 7 nicotinic acetylcholine receptor subunits (nAChR), includes invertebrate orthologues. There are two clades of invertebrate nAChRs, one of alpha-type subunits and one of non-alpha subunits, that are most similar to the two clades of vertebrate neuronal and muscle alpha and non-alpha 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.
Dent
Joseph A
JA
Department of Biology, McGill University, 1205 Dr. Penfield Avenue, Montreal, Quebec H3A 1B1, Canada. joseph.dent@mcgill.ca
D000595:Amino Acid Sequence; D000818:Animals; D002938:Ciona intestinalis / Q000235:genetics; D003545:Cysteine / Q000737:chemistry / Q000378:metabolism*; D019143:Evolution, Molecular; D007448:Invertebrates / Q000235:genetics; D015640:Ion Channel Gating; D007473:Ion Channels / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D008024:Ligands; D008322:Mammals / Q000235:genetics; D008958:Models, Molecular; D008969:Molecular Sequence Data; D005810:Multigene Family / Q000235:genetics*; D010802:Phylogeny; D018079:Receptors, GABA / Q000737:chemistry; D011978:Receptors, Nicotinic / Q000235:genetics; D014714:Vertebrates / Q000235:genetics
D007473:Ion Channels; D008024:Ligands; D018079:Receptors, GABA; D011978:Receptors, Nicotinic; D003545:Cysteine