The nervous system of Neodasys chaetonotoideus (Gastrotricha: Neodasys) revealed by combining confocal laserscanning and transmission electron microscopy: evolutionary comparison of neuroanatomy within the Gastrotricha and basal Protostomia

Abstract

We present a reconstruction of the nervous system of Neodasys chaetonotoideus Remane, 1927 (Gastrotricha, Chaetonotida) based on different microscopical methods: (1) immunohistochemistry (anti-acetylated a-and b-tubulin-, anti-5-HT-and anti-FMRFamide labelling) and (2) histochemistry (labelling of musculature and nuclei) by the means of confocal laser scanning microscopy (cLSM) and (iii) ultrastructure by means of transmission electron microscopy (TEM). All parts of the nervous system contain structures with an immunoreaction against the used immunohistochemical markers and labelling of histochemical markers. Results of both techniques (cLSM, TEM) reveal that the nervous system of N. chaetonotoideus is composed of a ''dumb-bell-shaped'' brain and one pair of posterior longitudinal neurite bundles. The brain is made up of a pair of laterally located clusters of neuronal somata, a large dorsal interconnecting dorsal commissure and two tiny ventral commissures in the region of the lateral clusters. From this, it follows that the brain is circumpharyngeal in position. The innervation of the head region is conducted by three pairs of anterior-directed neurite bundles. We describe here the gross anatomy of the nervous system and give additional details of the ultrastructure and the 5-HT and RFamide-like IR components of the nervous system. We compare our newly obtained data with already published data on the nervous system of gastrotrichs to reconstruct the hypothetical ground pattern of the nervous system in Gastrotricha, respectively, in Macrodasyida.

1. Gastrotricha

“…1.16 A-C and 1.17 A). All somata stained with any neuronal marker are positioned lateral of the dorsal commissure, a result that gets support by ultrastructural data (e.g., Rothe et al 2011a). IR against other neuronal components may show only subsets, as is shown exemplary in the IR against serotonin, histamine, and FMRFamides in Dactylopodola species, which all stain only some fibers within the dorsal commissure (Rothe & Schmidt-Rhaesa 2009).…”

“…The brain has been reconstructed in different ways. Both investigations found a different distribution of somata, which cover a broader area, either more or less homogeneous over the brain (Wiedermann 1995) or separated in an anterior and a posterior part (Teuchert 1977a (Hochberg & Atherton 2011), and Neodasys chaetonotoideus (Rothe et al 2011a) confirmed the description by Zelinka and Remane. According to these authors, the brain consists of a bridge of neurons (the dorsal commissure) dorsal of the pharynx and cells (=somata) on both sides of this "bridge".…”

“…There is a broad commissure composed only of neurites dorsal of the pharynx (Figs. 1.16 A-C), a count of anti-FMRF amide IR cells in the brain of Nesodasys chaetonotoideus and Xenodasys riedli yielded a mean number of 24 cells per hemisphere of the brain (Hochberg 2007, Rothe et al 2011a. This commissure is well shown with immunoreactivity (IR) against tubulin.…”

1. Gastrotricha, Cycloneuralia and Gnathifera: General History and Phylogeny

“…Their sequences constantly cluster with spiralian sequences, in particular with those from flatworms (e.g. This architecture is in contrast to the cycloneuralian brain (Rothe & Schmidt-Rhaesa 2009, Rothe et al 2011. In molecular analyses, gastrotrichs are never associated with Ecdysozoa or Cycloneuralia.…”

An Integrated Morphological and Molecular Approach to the Description and Systematisation of a Novel Genus and Species of Macrodasyida (Gastrotricha)

“…One of the problems that makes revision difficult is that the anatomical ground patterns of taxa putatively belonging to the different evolutionary lines are still not well-known (e.g., [1, 29–32]). Since the process of re-systematisation benefits from additional surveys of insufficiently known taxa [33], the discovery of new species with novel characteristics could help to identify plesiomorphy in these morphologically diverse animals, thus providing a more solid ground for their natural grouping [1, 21].…”