Pinning down the phylogenetic position of Chaetognatha: architecture of their nervous system in comparison to potential outgroups
Final Report Abstract
The phylogenetic position of the chaetognaths within the Bilateria is seriously debated but this taxon so far has resisted all serious attempts to pin down its phylogenetic affinities. Despite the ever increasing number of molecular phylogenetic studies and an emerging consensus for protostome affinities, we must conclude that still the position of the Chaetognatha is among the most enigmatic issues of metazoan phylogeny. Here, we review the current molecular phylogenetic studies and discuss morphological features that play important roles in the discussion of chaetognath affinities including the integument, musculature and neuromuscular innervation, sensory systems, the nervous system and aspects of embryology. The chaetognath genome is likely the product of a unique evolutionary history and witnesses a long isolation of this group. Furthermore, morphological characters provide evidence for the long evolutionary distance that separates the Chaetognatha from its closest (unknown) metazoan relative and suggest that this taxon in many aspects seems to have explored its own evolutionary pathways in generating tissue and organ diversity. Both the genome and morphological characters include many autapomorphies of this group in addition to a character mix of protostome and deuterostome features. However, despite intense analyses of their genome and tissue ultrastructure, so far we do not know of any convincing synapomorphies that clearly would suggest a sister-group relationship to another metazoan taxon. These facts are most parsimoniously interpreted such that Chaetognatha are likely an early offshoot of the protostome lineage but so far it is not possible to say precisely whether they are basal lophotrochozoans, basal ecdysozoans or sister-group to all others protostomes. All together, we currently tend to suggest a new basal rooting of the Bilateria, which considers the Chaetognatha as the sister-group to the Lophotrochozoa and Ecdysozoa (soft polytomy). This assemblage may be called Hyponeuria to indicate the ventralization of the nervous system as one of the most important steps in metazoan evolution.
