Histaminergic interneurons in the ventral nerve cord: assessment of their value for Euarthropod phylogeny

Abstract

Despite numerous approaches to the resolution of euarthropod phylogeny, mainly based on modern sequence information and traditional external morphology, the resulting hypotheses are often contradictory and leave many questions about euarthropod evolution unanswered. The comparison of developmental and structural aspects of the nervous system has shown to be a valuable contribution to the assessment of current phylogenetic hypotheses. One promising approach for the generation of new character sets is the morphology of transmitter systems and the discovery of individually identifiable neurons, which allow phylogenetic comparisons on the single cell level. In this context, the serotonin transmitter system has been investigated to a considerable degree. Studies to date have yielded important stimuli to our understanding of euarthropod relationships and the evolution of their nervous systems. However, data on other transmitter systems remain fragmented, and their value with respect to phylogenetic questions remains speculative. The biogenic amine histamine is a promising transmitter; a substantial amount of data has been reported in the literature and the homology of some histaminergic neurons has been suggested. Here, we present a comprehensive review of histaminergic neurons in the ventral nerve cord of Euarthropoda. Using immunocytochemical labeling of histamine combined with confocal laser-scanning microscopy, we investigated the transmitter system in phylogenetically relevant taxa, such as Zygentoma, Remipedia, Diplopoda, and Arachnida. By reconstructing ground patterns, we evaluated the significance of this specific character set for euarthropod phylogeny. With this approach, we identified a set of neurons, which can be considered homologous within the respective major taxon. In conclusion, the histaminergic system contains useful information for our understanding of euarthropod phylogeny, supporting the proposed clades Tetraconata and Mandibulata. Furthermore, this character set has considerable potential to help resolve relationships within the major clades at a deeper level of taxonomy, due to the considerable variability in neurite morphology.

Structure of the pecten neuropil pathway and its innervation by bimodal peg afferents in two scorpion species

“…Recently, this antibody has been applied as structural marker in the central nervous system of the scorpion Euscorpius italicus , resulting in the labelling of all neuropil structures, e.g. the arcuate body and pecten neuropils [ 101 ].…”

“…This antibody was raised against a Drosophila Glutathione S-Transferase (GST)-synapsin fusion protein. The antibody reacts with a highly conserved epitope, as it labels neuropilar structures in many euarthropod taxa [13,16,[96][97][98][99] including arachnids [100,101]. Recently, this antibody has been applied as structural marker in the central nervous system of the scorpion Euscorpius italicus, resulting in the labelling of all neuropil structures, e.g.…”

The visual pathway in sea spiders (Pycnogonida) displays a simple serial layout with similarities to the median eye pathway in horseshoe crabs

“…This conforms with an interpretation of the pycnogonid AB as a higher-order multimodal processing center that integrates (among others) visual cues, in correspondence to the euchelicerate AB [ 16 , 24 , 57 , 59 , 77 ]. With regard to neuroactive substances other than catecholamines, additional similarities between the pycnogonid and euchelicerate ABs include the absence of significant histamine immunoreactivity [ 61 , 62 , 65 , 81 ] and the expression of serotonin in some of its layers [ 58 , 65 , 82 , 83 ].…”

“…Alternatively, some specimens were pre-fixed in 4% EDAC/PBS overnight at 4 °C, followed by PFA/PBS post-fixation for 4 h at RT. These different fixation protocols have previously yielded positive results in various arthropod taxa, including chelicerates [ 61 , 65 , 81 , 126 ].…”

“…The monoclonal mouse anti-SYNORF1 antibody (deposited at the Developmental Studies Hybridoma Bank by E. Buchner, University Hospital Würzburg) was raised against a Drosophila melanogaster GST-synapsin fusion protein and has proved a useful marker of synaptic neuropil across many arthropod taxa, including various chelicerates [ 81 , 127 , 128 ].…”