title: Nervous and muscle system development in Phascolion strombus (Sipuncula).
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.
authors.affiliation: Department of Cell Biology and Comparative Zoology, Institute of Biology, University of Copenhagen, Universitetsparken 15, 2100, Copenhagen Ø, Denmark. awanninger@bi.ku.dk
authors.forename: Andreas
authors.initials: A
authors.lastname: Wanninger
authors.forename: Demian
authors.initials: D
authors.lastname: Koop
authors.forename: Lindell
authors.initials: L
authors.lastname: Bromham
authors.forename: Erin
authors.initials: E
authors.lastname: Noonan
authors.forename: Bernard M
authors.initials: BM
authors.lastname: Degnan
mesh_terms: D000818:Animals; D019521:Body Patterning / Q000502:physiology*; D004625:Embryo, Nonmammalian / Q000196:embryology; D007150:Immunohistochemistry; D007814:Larva / Q000254:growth & development; D018613:Microscopy, Confocal; D008855:Microscopy, Electron, Scanning; D024510:Muscle Development / Q000502:physiology; D009132:Muscles / Q000196:embryology*; D009348:Nematoda / Q000196:embryology* / Q000254:growth & development / Q000648:ultrastructure; D009420:Nervous System / Q000196:embryology* / Q000254:growth & development; D011793:Queensland; D013045:Species Specificity; D013548:Sweden