MSc Student
Advisor: Georg Brenneis
Unit for Integrative Zoology, Department of Evolutionary Biology
University of Vienna
Abstract
The evolutionary history of the Chelicerata (spiders, scorpions, ticks, mites and kin) is heavily debated due to considerable incongruities between phylogenetic hypotheses based on (mostly external) morphological characters and those generated from molecular datasets. In the past decades, comparative studies taking a closer look at the arthropod central nervous system (CNS) and its associated structures have produced comprehensive neuroanatomical datasets that can be analysed against the background of competing phylogenetic hypotheses.
In this context, one promising avenue is the study of the spatial expression of neuroactive substances (e.g. neuropeptides), in particular those that enable to reliably identify individual neurons for cross-taxa comparison. SIFamide is a highly conserved neuropeptide in arthropods, and first data for ticks and pycnogonids (sea spiders) indicate that characteristic SIFamide-immunoreactive neuron patterns are present in the chelicerate CNS.
Extending these preliminary findings, this study employed fluorescent immunolabeling coupled with confocal laser scan microscopy to describe and compare the SIFamide-immunoreactive neurons in the CNS of five additional chelicerate groups. Among taxon-specific patterns, several neuron types can be identified that display very similar soma positions and projections across taxa and are therefore proposed to be homologous. Given their presence in most of the groups studied, however, these neurons do not contribute to the current debate about euchelicerate relationships. On the other hand, one chelicerate brain neuron type with soma in the anterior protocerebrum and contralaterally descending main neurite shows striking similarities to SIFamide-immunoreactive neurons in all insects studied so far and potentially also in crustaceans. This indicates their homology across arthropod subphyla and thus potentially reveals a shared feature that dates back to the last common ancestor of arthropods.
