Development of the nervous system in <it>Phoronopsis harmeri</it> (Lophotrochozoa, Phoronida) reveals both deuterostome- and trochozoan-like features

• Main Authors: Temereva Elena, Wanninger Andreas
• Format: Article
• Language: English
• Published: BMC 2012-07-01
• Series: BMC Evolutionary Biology
• Subjects: Neurogenesis; Evolution; Phylogeny; Ventral nerve cord; Last common lophotrochozoan ancestor
• Online Access: http://www.biomedcentral.com/1471-2148/12/121

<p>Abstract</p> <p>Background</p> <p>Inferences concerning the evolution of invertebrate nervous systems are often hampered by the lack of a solid data base for little known but phylogenetically crucial taxa. In order to contribute to the discussion concerning the ancestral neural pattern of the Lophotrochozoa (a major clade that includes a number of phyla that exhibit a ciliated larva in their life cycle), we investigated neurogenesis in <it>Phoronopsis harmeri</it>, a member of the poorly studied Phoronida, by using antibody staining against serotonin and FMRFamide in combination with confocal microscopy and 3D reconstruction software.</p> <p>Results</p> <p>The larva of <it>Phoronopsis harmeri</it> exhibits a highly complex nervous system, including an apical organ that consists of four different neural cell types, such as numerous serotonin-like immunoreactive flask-shaped cells. In addition, serotonin- and FMRFamide-like immunoreactive bi- or multipolar perikarya that give rise to a tentacular neurite bundle which innervates the postoral ciliated band are found. The preoral ciliated band is innervated by marginal serotonin-like as well as FMRFamide-like immunoreactive neurite bundles. The telotroch is innervated by two neurite bundles. The oral field is the most densely innervated area and contains ventral and ventro-lateral neurite bundles as well as several groups of perikarya. The digestive system is innervated by both serotonin- and FMRFamide-like immunoreactive neurites and perikarya. Importantly, older larvae of P. harmeri show a paired ventral neurite bundle with serial commissures and perikarya.</p> <p>Conclusions</p> <p>Serotonin-like flask-shaped cells such as the ones described herein for <it>Phoronopsis harmeri</it> are found in the majority of lophotrochozoan larvae and therefore most likely belong to the ground pattern of the last common lophotrochozoan ancestor. The finding of a transitory paired ventral neurite bundle with serially repeated commissures that disappears during metamorphosis suggests that such a structure was part of the “ur-phoronid” nervous system, but was lost in the adult stage, probably due to its acquired sessile benthic lifestyle.</p>