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.
Highlights
The investigation of the anatomy of invertebrate nervous systems, and of euarthropod (Arthropoda sensu stricto, strictly the extant subphyla Chelicerata, Myriapoda, Crustacea, and Hexapoda) nervous systems in particular, has undergone a renaissance and received increasing attention in recent decades (e.g., [1, 2])
In the more basal euarthropod taxa Myriapoda and Chelicerata, we investigated the first representative of Diplopoda, Glomeris marginata, and added two arachnid taxa (Scorpiones, Pseudoscorpiones), in order to reach a broader taxon sampling in these rather neglected clades
In the present report, we provide a detailed analysis of the histaminergic system in the ventral ganglia of representatives of each major euarthropod taxon
Summary
The investigation of the anatomy of invertebrate nervous systems, and of euarthropod (Arthropoda sensu stricto, strictly the extant subphyla Chelicerata, Myriapoda, Crustacea, and Hexapoda) nervous systems in particular, has undergone a renaissance and received increasing attention in recent decades (e.g., [1, 2]). A close relationship of Crustacea and Hexapoda (building the Tetraconata or Pancrustacea) and the Myriapoda being the sister group to the (2019) 5:36. Alternative views on their relationship have been postulated, including concepts such as Tracheata/Atelocerata (Myriapoda + Hexapoda) and Myriochelata/Paradoxopoda (Chelicerata + Myriapoda) (e.g., [9, 10])). The internal phylogeny of the major taxa is far from clear. The internal phylogeny of Chelicerata and Myriapoda remains under debate [12,13,14,15]
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