Pinning down the phylogenetic position of Chaetognatha: architecture of their nervous system in comparison to potential outgroups

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We analyse the nervous system of Chaetognatha against an evolutionary background in order to provide new insights into the phylogenetic position of this taxon (Neurophylogeny). In this application, we thoroughly report our work progress during Phase 2 of SPP 1174 „ Deep Metazoan Phylogeny“ including joint activities of all Neurophylogeny groups in the SPP such as establishing a glossary of neuroanatomical terms in invertebrates. The major goal of our application for Phase 3 is to finally pin down the phylogenetic position of Chaetognatha. To that end, we suggest dedicating circa 60% of the Phase 3 work efforts to obtaining additional neuroanatomical data to fill gaps of knowledge that became obvious during Phase 1 and 2. Circa 40% of the Phase 3 work efforts will be directed towards the analysis of the data together with the other neurophylogeny groups in a joint data matrix and a combined analysis of morphological data. More precisely: 1. Because so far we have analyzed only two chaetognath species, we suggest extending the taxon sampling for a broad comparative analysis of brain structure and arrangement of individually identifiable neurons in the ventral nerve centre in order to work out the representative nervous system ground pattern of Chaetognatha as a solid basis for the following phylogenetic analysis with other protostomian taxa and for the correct coding of nervous system characters in the matrix. Furthermore, this approach will contribute characters to resolve chaetognath internal phylogeny. 2. We propose to use the full set of markers that we have already successfully used for the Chaetognatha to study the nervous system of Gnathostomulida, a potential outgroup of Chaetognatha. The aim is to obtain a stable reconstruction of the nervous system in the ground pattern of this taxon. This approach will be vital to fill the existing gaps in the neuronatomical data set for the matrix and will allow for an immediate test of the Gnathifera hypothesis as suggested by Nielsen (2001) in which the Chaetognatha are suggested to be relatives of Gnathostomulida, Rotatoria and other gnathiferan taxa. 3. One of the ultimate goals of the priority program is a combined analysis of all the morphological data compiled by the DMP collaborators, and in a second step the combination of morphological and molecular data. The analysis of the Neurophylogeny team will focus on structures which, primarily, we have studied in person. Other aspects of our innovative approach of data analysis include the development of a strict terminology and the critical conceptualization of characters, including a test of primary homology for these conceptualized characters. We have already development such a terminology by establishing a glossary of neuroanatomical terms for invertebrates in Phase 2. The morphological database of DMP, MorphDBase takes on a central function in the interlocking of the different morphological projects within the priority program and managing and facilitating the generation of a combined morphological character matrix. For establishing the character matrixs we will also interact with participants of the “Protostome Tree of Life” AtoL group.