Towards a ground pattern reconstruction of bivalve nervous systems: neurogenesis in the zebra mussel Dreissena polymorph
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ORIGINAL ARTICLE
Towards a ground pattern reconstruction of bivalve nervous systems: neurogenesis in the zebra mussel Dreissena polymorpha Anna Pavlicek 1 & Thomas Schwaha 1 & Andreas Wanninger 1 Received: 20 July 2017 / Accepted: 28 December 2017 / Published online: 18 January 2018 # The Author(s) 2018. This article is an open access publication
Abstract Bivalvia is a taxon of aquatic mollusks that includes clams, oysters, mussels, and scallops. Within heterodont bivalves, Dreissena polymorpha is a small, mytiliform, freshwater mussel that develops indirectly via a planktotrophic veliger larva. Currently, only a few studies on bivalve neurogenesis are available, impeding the reconstruction of a ground pattern in Bivalvia. In order to inject novel data into this discussion, we describe herein the development of the serotonin-like and α-tubulin-like immunoreactive (lir) neuronal components of D. polymorpha from the early trochophore to the late veliger stage. Neurogenesis starts in the early trochophore stage at the apical pole with the appearance of one flask-shaped serotonin-lir cell. When larvae reach the veliger stage, four flask-shaped serotonin-lir cells are present in the apical organ. At the same time, the anlagen of the cerebral ganglia start to form at the base of the apical organ. From the apical organ, one pair of cerebro-visceral connectives projects posteriorly and connects to a posterior larval sensory organ that contains serotonin- and α-tubulin-like flask-shaped cells. Additional, paired serotonin-lir neurites originate from the apical organ and project into the velum. One unpaired stomatogastric serotonin-lir cell develops ventrally to the stomach at the veliger stage. The low number of serotonin-lir cells in the apical organ of bivalve veligers is shared with larvae of basally branching gastropods and scaphopods and is thus considered a feature of the last common ancestor of Conchifera, while the overall simplicity of the larval neural architecture appears to be a specific trait of Bivalvia. Keywords Mollusca . Bivalvia . Evolution . Neurogenesis . Evodevo . Veliger . Trochophore
Introduction Mollusca is a highly diverse metazoan phylum with its earliest members most likely dating back to the Precambrian (Parkhaev 2008). The remarkable plasticity of bodyplans among molluscan clades, exemplified by worm-shaped forms such as the neomeniomorphs (solenogasters) and chaetodermomorphs (caudofoveates), polyplacophorans, as well as the conchiferan taxa Monoplacophora, Bivalvia, Scaphopoda, Gastropoda and Cephalopoda, renders this taxon particularly well-suited for developmental and evolutionary studies (e.g., Wanninger et al. 2008). Within Mollusca, Bivalvia is the second largest class-level taxon with the majority of species being marine but several linages such as the unionids or dreisseniids have colonized freshwater
* Andreas Wanninger [email protected] 1
Department of Integrative Zoology, University of Vienna, Althanstrasse 14, 1090 Vienna, Austria
environments. Bivalves are typicall
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