Nervous and muscle system development in <Emphasis Type="Italic">Phascolion strombus</Emphasis> (Sipuncula) |
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Authors: | Email author" target="_blank">Andreas?WanningerEmail author Demian?Koop Lindell?Bromham Erin?Noonan Bernard?M?Degnan |
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Institution: | 1.Department of Cell Biology and Comparative Zoology, Institute of Biology,University of Copenhagen,Copenhagen ?,Denmark;2.School of Integrative Biology,University of Queensland,Brisbane,Australia;3.Centre for the Study of Evolution, School of Life Sciences,University of Sussex,Brighton,UK |
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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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Keywords: | Evolution Development Segmentation Confocal microscopy Phylogeny |
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