Abstract
Recent interpretations of developmental gene expression patterns propose that the last common metazoan ancestor was segmented, although most animal phyla show no obvious signs of segmentation. Developmental studies of non-model system trochozoan taxa may shed light on this hypothesis by assessing possible cryptic segmentation patterns. In this paper, we present the first immunocytochemical data on the ontogeny of the nervous system and the musculature in the sipunculan Phascolion strombus. Myogenesis of the first anlagen of the body wall ring muscles occurs synchronously and not subsequently from anterior to posterior as in segmented spiralian taxa (i.e. annelids). The number of ring muscles remains constant during the initial stages of body axis elongation. In the anterior-posteriorly elongated larva, newly formed ring muscles originate along the entire body axis between existing myocytes, indicating that repeated muscle bands do not form from a posterior growth zone. During neurogenesis, the Phascolion larva expresses a non-metameric, paired, ventral nerve cord that fuses in the mid-body region in the late-stage elongated larva. Contrary to other trochozoans, Phascolion lacks any larval serotonergic structures. However, two to three FMRFamide-positive cells are found in the apical organ. In addition, late larvae show commissure-like neurones interconnecting the two ventral nerve cords, while early juveniles exhibit a third, medially placed FMRFamidergic ventral nerve. Although we did not find any indications for cryptic segmentation, certain neuro-developmental traits in Phascolion resemble the conditions found in polychaetes (including echiurans) and myzostomids and support a close relationship of Sipuncula and Annelida.
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Acknowledgements
The authors are indebted to Michael Thorndyke and the staff of the Kristineberg Marine Research Station (Sweden) and the Heron Island Research Station (Australia) for their hospitality and support throughout the years. We thank Olga Alexandrova and Charles N. David (Munich) for making confocal microscopy available. We are particularly grateful to Mark Q. Martindale (Hawaii) and three anonymous reviewers whose detailed criticisms greatly improved the manuscript. The financial support of the German Science Foundation (DFG) (grant WA 1580/1–2 to AW), the European Commission (ARI-programme, 5th framework, to AW), and the Australian Research Council (to BD) are gratefully acknowledged.
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Wanninger, A., Koop, D., Bromham, L. et al. Nervous and muscle system development in Phascolion strombus (Sipuncula). Dev Genes Evol 215, 509–518 (2005). https://doi.org/10.1007/s00427-005-0012-0
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DOI: https://doi.org/10.1007/s00427-005-0012-0