Genomic organization of a voltage-gated Na+ channel in a hydrozoan jellyfish: Insights into the evolution of voltage-gated Na+ channel genes

Voltage-gated Na+ channels are responsible for fast propagating action pote ntials. The structurally simplest animals known to contain rapid, transient , voltage-gated currents carried exclusively by Na+ ions are the Cnidaria. The Cnidaria are thought to be close to the origin of the metazoan radiatio n and thus are pivotal organisms for studying the evolution of the Na+ chan nel gene. Here we describe the genomic organization of the Na+ channel a su bunit, PpSCN1, from the hydrozoan jellyfish, Polyorchis penicillatus. We sh ow that most of the 20 intron sites in this diploblast are conserved in mam malian Na+ channel genes, with some even shared by Ca2+ channels. One of th ese conserved introns is spliced by a rare U12-type spliceosome. Such conse rvation places the origin of the primary exon arrangement of Na+ channels a nd different intron splicing mechanisms to at least the common ancestors of diploblasts and triploblasts, similar to 600 million-1 billion years ago.