Abstract
The tardigrade brain has been the topic of several neuroanatomical studies, as it is key to understanding the evolution of the central nervous systems in Panarthropoda (Tardigrada + Onychophora + Arthropoda). The gross morphology of the brain seems to be well conserved across tardigrades despite often disparate morphologies of their heads and cephalic sensory structures. As such, the general shape of the brain and its major connections to the rest of the central nervous system have been mapped out already by early tardigradologists. Despite subsequent investigations primarily based on transmission electron microscopy or immunohistochemistry, characterization of the different regions of the tardigrade brain has progressed relatively slowly and open questions remain. In an attempt to improve our understanding of different brain regions, we reinvestigated the central nervous system of the heterotardigrade Echiniscus testudo using anti-synapsin and anti-acetylated α-tubulin immunohistochemistry in order to visualize the number and position of tracts, commissures, and neuropils. Our data revealed five major synapsin-immunoreactive domains along the body: a large unitary, horseshoe-shaped neuropil in the head and four neuropils in the trunk ganglia, supporting the hypothesis that the dorsal brain is serially homologous with the ventral trunk ganglia. At the same time, the pattern of anti-synapsin and anti-tubulin immunoreactivity differs between the ganglia, adding to the existing evidence that each of the four trunk ganglia is unique in its morphology. Anti-tubulin labeling further revealed two commissures within the central brain neuropil, one of which is forked, and additional sets of extracerebral cephalic commissures associated with the stomodeal nervous system and the ventral cell cluster. Furthermore, our results showing the innervation of each of the cephalic sensilla in E. testudo support the homology of subsets of these structures with the sensory fields of eutardigrades.
Highlights
The composition and segmental nature of the tardigrade brain has been a long-disputed and controversial topic
Two pairs of connectives link the brain and first trunk ganglion, neural processes are arranged superficially as a ring around the mouthparts giving the appearance of a circumpharyngeal brain, and the number of sensory structures in or on the head often varies between species (e.g., Mayer et al, 2013; Persson et al, 2014; Schulze & Schmidt-Rhaesa, 2013; Zantke et al, 2008)
The central brain neuropil is located within this synapsin-rich region, and, while it largely appears as a complex mesh of neuronal connections, we identify two prominent commissures associated with the central brain neuropil, numbered from anterior to posterior (Figures 1 and 3a)
Summary
The composition and segmental nature of the tardigrade brain has been a long-disputed and controversial topic (summarized in Smith & Goldstein, 2017). A consensus has begun to emerge that the brain represents a single neuromere that may be homologous with the protocerebrum of arthropods and the anteriormost brain region of onychophorans (Gross & Mayer, 2015; Mayer et al, 2013; Smith et al, 2016; Smith et al, 2018) This hypothesis is supported by innervation patterns (Gross & Mayer, 2015; Mayer et al, 2013) as well as expression data from Hox genes and nervous system patterning genes, which suggest more broadly that the entire head is derived from only one segment (Smith et al, 2016; Smith et al, 2018; but see Persson et al, 2012; Persson et al, 2014 for alternative view). We use data generated using high-resolution confocal laser scanning microscopy with an Airyscan module to build upon previous investigations and provide a detailed, updated model of the central nervous system and the neurites associated with the cephalic sensory structures in the heterotardigrade Echiniscus testudo
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