title: The phylogenetic position of ctenophores and the origin(s) of nervous systems.
abstract: Ctenophores have traditionally been treated as eumetazoans, but some recent whole genome studies have revived the idea that they are, rather, the sister group to all other metazoans. This deep branching position implies either that nervous systems have evolved twice, in Ctenophora and in Eumetazoa, or that an ancestral metazoan nervous system has been lost in sponges and placozoans. We caution, however, that phylogenetic-tree construction artifacts may have placed ctenophores too deep in the metazoan tree. We discuss nervous system origins under these alternative phylogenies and in light of comparative data of ctenophore and eumetazoan nervous systems. We argue that characters like neuropeptide signaling, ciliary photoreceptors, gap junctions and presynaptic molecules are consistent with a shared ancestry of nervous systems. However, if ctenophores are the sister group to all other metazoans, this ancestral nervous system was likely very simple. Further studies are needed to resolve the deep phylogeny of metazoans and to have a better understanding of the early steps of nervous system evolution.
authors.affiliation: Max Planck Institute for Developmental Biology, Spemannstrasse 35, 72076 Tübingen, Germany.
authors.forename: Gáspár
authors.initials: G
authors.lastname: Jékely
authors.affiliation: Department of Zoology, University of Oxford, South Parks Rd, Oxford, OX13PS UK.
authors.forename: Jordi
authors.initials: J
authors.lastname: Paps
authors.affiliation: The Natural History Museum of Denmark, University of Copenhagen, Universitetsparken 15, DK-2100 Copenhagen, Denmark.
authors.forename: Claus
authors.initials: C
authors.lastname: Nielsen
keywords: Ciliary photoreceptor; Cnidarian; Ctenophore; DEG/ENaC channels; Metazoan phylogeny; MicroRNA; Nervous system evolution; Neuropeptide; Sponge; Trichoplax