The chemical architecture of the nervous system as a character complex of arthropod phylogeny

The phylogenetic relationships within the Arthropoda are a matter of debate for more than a century. Although the close relationship of Crustacea and Hexapoda, termed Tetraconata, is now widely accepted, their internal affinities remain unresolved. One of the most controversial issues is the crustacean sister group to the Hexapoda. Because of numerous often conflicting hypotheses independent data are crucial to contribute new insights to tetraconate and arthropod phylogeny. The investigation of the structure and development of nervous systems with an evolutionary emphasis has gained increasing attention and an impressive amount of neurophylogenetic studies has emerged over the last decades. However, our knowledge of the nervous system in certain key taxa like the crustacean Remipedia and basal insects is still limited. In this thesis, I focused on two aspects of the tetraconate nervous system as phylogenetic characters. In the first part, I explored the neuroanatomy of the remipede brain and in particular the olfactory pathway. Neurite connectivity and neuropil outlines were investigated using antisera against acetylated alpha-tubulin and synapsin. I applied immunolabeling of the catalytic subunit of the cAMP-dependent protein kinase (DC0) and the glutamic acid decarboxylase (GAD), as well as DiI labeling. The results were supplemented by a similar study in two species of marine Isopoda. In the second part, I described the distribution of individually identifiable neurons and their neurite morphology in the ventral nerve cord of the basal insect taxa Archaeognatha and Zygentoma with an antibody against serotonin. This standard protocol was complemented by immunolabeling of tryptophan hydroxylase and preincubation experiments with the serotonin precursor 5-hydroxy-L-tryptophan and serotonin. The preincubation experiments allowed the distinction between neuronal serotonin uptake and enzymatic serotonin synthesis. The brain anatomy of Remipedia, especially in the olfactory pathway, resembles that of Malacostraca and Hexapoda. This comprises the connectivity of the two unpaired medial midline neuropils (protocerebral bridge and central body) by four tracts termed W-, X-, Y-, Z-tracts. A conspicuous chiasm of the olfactory globular tracts, which house the axons of olfactory projection neurons, is a synapomorphy of Remipedia and Malacostraca. The presence of a GABAergic interneuronal feedback loop in the lateral protocerebrum unites Remipedia and Hexapoda. However, most of the structural similarities in the olfactory pathway have to be interpreted as plesiomorphic features of Tetraconata or even Mandibulata requiring verification by denser taxon sampling. The pattern of serotonin containing neurons in basal insects, in particular the presence of medially positioned individually identifiable cells, suggests a close relationship of Remipedia, Cephalocarida, and Hexapoda. However, data on developmental origins of these cells and tetraconate outgroup taxa are rather sparse, hindering detailed phylogenetic conclusions. The neuroanatomical data on adult animals of this thesis provide novel characters for evolutionary analyses and support a growing corpus of studies proposing Remipedia as a derived crustacean taxon with a potential phylogenetic affinity to Hexapoda.