Phylogenetic Position of Ctenophores and the Origin(s) of Nervous Systems

Abstract

Ctenophores' phylogenetic position is debated, with some studies suggesting they are the sister group to all other metazoans. This implies either two independent evolutions of the nervous system or a single origin followed by loss in sponges and placozoans. Key characters like neuropeptide signaling and ciliary photoreceptors suggest a shared ancestry of nervous systems. Further studies are needed.

Introduction

Ctenophores (comb jellies) are a non-bilaterian lineage crucial for understanding early animal evolution. They possess comb rows of cilia for swimming and tentacles with colloblasts for prey capture. Unlike sponges, ctenophores have a gut, complex nervous system, and muscles.

Ctenophore Nervous System

The ctenophore nervous system includes an epithelial and mesogleal nerve net with sensory cells. It controls cilia, bioluminescence, and muscular contractions. Some species have giant axons for escape responses. Ctenophores were classified as eumetazoans but recent studies suggest they are the sister group to all other metazoans due to the absence of Hox genes and microRNAs. This placement implies either independent evolution or loss of nerves and muscles in sponges and placozoans. Technical issues may affect phylogenetic reconstruction.

Uncertain Phylogenetic Position of Ctenophora

Phylogenomic analyses support ctenophores as the sister lineage to all other animals. Long-branch attraction (LBA) is a concern in phylogenetic tree reconstruction. Using distant out-groups can increase LBA effects. Exclusion of problematic taxa and use of closer out-groups tend to support the 'sponges first' view. The age of the last common ancestor (LCA) of ctenophores may contribute to placement problems due to a LCA-LBA effect. Alternative phylogenetic markers could be explored. Gene-content analysis can be distorted by gene loss.

Comparisons of Ctenophore and Eumetazoan Nervous Systems

Comparing molecular and cellular characteristics can inform us about nervous system origins. The absence of neurotransmitter receptors in Pleurobrachia should be considered cautiously as some genes are bilaterian-specific or may be present in other ctenophores. The presence of neurotransmitter pathways doesn't necessarily indicate a nervous system, as sponges and placozoans also contain such genes. Ctenophores have neuropeptide precursors and G-protein-coupled receptors. A family of putative peptide-gated ion channels in ctenophores presents an interesting avenue of future research. The presence of pre-synapse components suggests chemical synapses were present in the last common ancestor of ctenophores and eumetazoans. Ctenophores also express a diversity of innexins, proteins that form gap junctions. Ctenophore sensory systems may show homologies to the eumetazoan nervous system at the level of neuron types. Ciliary photoreceptors may represent a sensory neuron type shared by ctenophore and eumetazoan nervous systems. Overall, similarities are compatible with a single origin of ctenophore and eumetazoan nervous systems, but the independent origin of complex nervous systems from a simple precursor cannot be ruled out.

Influence of Phylogeny on Origin Scenarios

Clarifying the deep branching order of metazoans will help resolve the problems surrounding the origin of nervous systems. If ctenophores belong to eumetazoans, the homology of nervous systems gains phylogenetic support. If ctenophores are sister to the rest of metazoans, this implies that sponges and placozoans have lost the nervous system, or that nervous systems evolved independently. Complete loss of a complex, highly integrated nervous system in sponges and placozoans is highly improbable, given their active ciliary locomotion and sensory systems. If the metazoan common ancestor had only a few protoneuron types, a scenario of simplification in sponges and placozoans and independent complexification in ctenophores and eumetazoans becomes tenable.

Conclusions

The phylogenetic position of Ctenophora is still uncertain. Methodological problems with the placement of ctenophores, arguing that the available data do not strongly challenge the traditional view of sponges occupying a position as a sister group of the remaining eumetazoans. If future studies support the 'ctenophores first' phylogeny, we favor the view of an independent complexification of nervous systems from a simple precursor state.