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Dentified in mammals, all of which have four to five exons. The Drosophila genome encodes eight PANX/INX genes with three to eight exons [109]. Twenty-five PANX/INX genes are present in the C. elegans genome, with 3?1 exons [110]. In Aplysia californica, 18 PANX/INX genes have been identified, with 1?0 exons (figure 4). Non-bilaterian metazoans have the most unusual complements of PANX/INX genes. The A. queenslandica [30] and T. adhaerens [29] genomes and the publically available sponge transcriptome data [111] have no identified pannexins. Cnidarians appear to have both losses and expansions of the PANX/INX genes. The hydrozoan Hydra magnipapillata has at least 19 pannexins [112,113]. By contrast, the anthozoan Nematostella vectensis has only one recognized PANX/INX gene (figure 4 and electronic supplementary material, figure S1), which may be involved in the electrical coupling between blastomeres in embryos [114]. However, no PANX/INX genes have been detected in the genomes of three other anthozoans, Aiptasia [115], Acropora digitifera [116] and Stylophora pistillata. There are also no identified PANX/INX genes in the scyphozoan Cyanea capillata (based on our transcriptome profiling). It is still unknown whether the cubozoans have PANX/INX genes. Gap junction proteins (both innexins and connexions) tend to be present in multiple copies in almost all metazoans, with the exception of Nematostella. Interestingly, the predicted Nematostella PANX/INX protein is clustered at the base of the chordate clade (electronic supplementary material, figure S1; this has also been observed by Abascal Zardoya [106]). Given these observations, it has been suggested that there has been a horizontal transfer of a PANX/INX gene from an ancestral chordate to Nematostella [106]. In contrast to the cnidarians, all sequenced ctenophores have an enormous diversity of electrical synapses or gaprstb.royalsocietypublishing.org Phil. Trans. R. Soc. B 371:junctions [28], forming a distinct branch in tree topology (electronic supplementary material, figure S1). However, no connexins are encoded in the genomes of Pleurobrachia and Mnemiopsis, or in any of the other ctenophore transcriptomes analysed. In general, the ctenophore PANX/INX genes contain more exons than their orthologues in Hydrozoa and bilaterians (the number of exons varies from 1 to 14; figure 4). Although both P. bachei and M. leidyi have 12 PANX/INX genes in their genomes, only four of them form genealogical sister pairs between species (electronic supplementary material, figure S1), further suggesting the widespread lineage-specific radiation and parallel SB-431542 MedChemExpress evolution of this family. Functional analysis of the electrical synapses in ctenophores is in its infancy [70,117]. Interestingly, the PANX/ INX genes are one of the most highly expressed transcripts in the adult aboral organ of P. bachei (figure 5d; [28]), but they are also expressed in the combs and conductive tracts and in the neuron-like subepithelial cells. Gap junctions have previously been identified in the ciliated grooves by electron microscopy [95], which run from the aboral organ to the first comb plate of each comb row and through the endoderm of the meridional canals [117]. These data suggest that a significant fraction of the synaptic transmission in the conductive tracts and the aboral organ is electrical. In addition to conducting electrical synapses, gap junctions also mediate mechanoreception [118] and the direct exchange of sma.