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.

Keywords: Ciliary photoreceptor; Cnidarian; Ctenophore; DEG/ENaC channels; Metazoan phylogeny; MicroRNA; Nervous system evolution; Neuropeptide; Sponge; Trichoplax.

Figure 1

Ctenophore diversity. A, Pleurobrachia bachei (D Kent); B, Mnemiopsis leidyi (K Brandt); C, Beroe gracilis with Pleurobrachia pileus in the stomach (C Marneff); D, Thalassocalyce inconstans (L Madin); E, Coeloplana astericola (yellow-brown with thin, branched tentacles) on the red sea-star Echinaster luzonicus (D Fugitt). Reproduced with permission from D Kent, K Brandt, C Marneff, L Madin and D Fugitt.

Figure 2

Four scenarios for the origins of nervous systems. Four scenarios for the origins of nervous systems in the Animal Kingdom depending on the homology of their components and the phylogenetic position of ctenophores. If nervous systems are homologous across metazoans, and if ctenophores are the earliest-diverging animals, then nervous systems were lost in sponges and placozoans. In contrast, if nervous systems are not homologous across animals then they arose more than once, a result that is not made more or less likely by any of the possible placements for ctenophores. Vignettes from phylopic.org and C Nielsen.