Comparative aspects of peptidergic signaling pathways in the nervous systems of arthropods

Comparative aspects of arthropod peptidergic systems--in principle--can be studied on the level of precursor sequences (genes, preprohormones), peptide sequences (peptide families), and peptide expression patterns within the nervous system. The number of known arthropod neuropeptide precursor sequences is as yet far too small to provide a reasonably large basis for extended comparative studies. Comparative studies of peptide sequences have shown that many peptides belong to families with homologous members in both invertebrates and vertebrates. Comparative research on peptide expression has to find out whether phylogenetic necessities lead to "hard wired" neurochemical identities, i.e., a more or less fixed "Bauplan" that not only determines the lineage and morphology of a neuron but also its transmitter(s), or whether these necessities demand greater flexibility (plasticity), and hence cause great variability that would complicate comparative studies. As will be shown here, both possibilities appear to exist. On the one hand, peptidergic neurons may exist in comparable form in different groups of arthropods. On the other hand, the neurochemical identity of cells may vary in segmented organisms when comparing serially homologous sets of nerve cells in different segments. As a further complication, identical or similar peptides may serve different functions, even in closely related species. In view of these functional aspects in particular, it appears that peptidergic signalling pathways represent rapidly evolving systems. This conclusion, although very interesting in itself, reduces the use of such systems for general comparisons. However, arthropod nervous systems represent excellent model systems for the study of homology. At least for morphological and ontogenetic aspects arthropods provide numerous opportunities to study homology on the level of the individually identified peptidergic nerve cell.