title: Insights into the segmental identity of post-oral commissures and pharyngeal nerves in Onychophora based on retrograde fills.
abstract: BACKGROUND
  While the tripartite brain of arthropods is believed to have evolved by a fusion of initially separate ganglia, the evolutionary origin of the bipartite brain of onychophorans-one of the closest arthropod relatives-remains obscure. Clarifying the segmental identity of post-oral commissures and pharyngeal nerves might provide useful insights into the evolution of the onychophoran brain. We therefore performed retrograde fills of these commissures and nerves in the onychophoran Euperipatoides rowelli.
  
  
  RESULTS
  Our fills of the anterior and posterior pharyngeal nerves revealed groups of somata that are mainly associated with the deutocerebrum. This resembles the innervation pattern of other feeding structures in Onychophora, including the jaws and several lip papillae surrounding the mouth. Our fills of post-oral commissures in E. rowelli revealed a graded arrangement of anteriorly shifted somata associated with post-oral commissures #1 to #5. The number of deutocerebral somata associated with each commissure decreases posteriorly, i.e., commissure #1 shows the highest and commissure #5 the lowest numbers of associated somata, whereas none of the subsequent median commissures, beginning with commissure #6, shows somata located in the deutocerebrum.
  
  
  CONCLUSIONS
  Based on the graded and shifted arrangement of somata associated with the anteriormost post-oral commissures, we suggest that the onychophoran brain, which is a bipartite syncerebrum, might have evolved by a successive anterior/anterodorsal migration of neurons towards the protocerebrum in the last onychophoran ancestor. This implies that the composite brain of onychophorans and the compound brain of arthropods might have independent evolutionary origins, as in contrast to arthropods the onychophoran syncerebrum is unlikely to have evolved by a fusion of initially separate ganglia.
authors: Animal Evolution and Development, Institute of Biology, University of Leipzig, Talstraße 33, 04103, Leipzig, Germany. christine.martin@uni-leipzig.de.|Department of Zoology, Institute of Biology, University of Kassel, Heinrich-Plett-Str. 40, 34132, Kassel, Germany. christine.martin@uni-leipzig.de.
authors: Christine
authors: C
authors: Martin
authors: Animal Evolution and Development, Institute of Biology, University of Leipzig, Talstraße 33, 04103, Leipzig, Germany. gmayer@onychophora.com.|Department of Zoology, Institute of Biology, University of Kassel, Heinrich-Plett-Str. 40, 34132, Kassel, Germany. gmayer@onychophora.com.
authors: Georg
authors: G
authors: Mayer
mesh_terms: D000818:Animals; D007448:Invertebrates / Q000033:anatomy & histology*; D018613:Microscopy, Confocal; D008856:Microscopy, Fluorescence; D009055:Mouth / Q000033:anatomy & histology / Q000294:innervation; D009420:Nervous System / Q000033:anatomy & histology; D056347:Neuroanatomical Tract-Tracing Techniques; D056325:Neuronal Tract-Tracers; D009474:Neurons / Q000166:cytology
chemical_list: D056325:Neuronal Tract-Tracers