Evolutionary specialization of neuronal core circuits in arthropod olfactory systems: structure and function of convergence/divergence in crustaceans versus insects

Steffen Harzsch – Universität Greifswald
Martin Nawrot – Universität zu Köln
Jürgen Rybak – MPI for Chemical Ecology, Jena

According to all recent molecular phylogenies, insects must be regarded as a highly successful terrestrial animal group that originated from marine crustaceans. The main chemosensory organ of both taxa for distance olfaction is the most anterior pair of head appendages, the deutocerebral antennae. Olfactory sensory neurons associated with sensilla on the antennae project their axons into the brain’s primary olfactory centres, the bilaterally paired antennal lobes (insects)/olfactory lobes (crustaceans). There, sensory olfactory afferents synapse with two classes of olfactory interneurons (OSNs), the local olfactory interneurons (LNs) and the olfactory projection neurons (PNs), within specialized neuropil compartments, the olfactory glomeruli. Although the principle wiring pattern within the glomeruli of crustaceans and insects share many similarities, nevertheless there also exist pronounced difference of these olfactory core circuits. Therefore, this system can be seen as an ideal playground for those interested in analysing evolutionary diversification of neuronal core circuits. For example, our results from phase 1 of the PP strongly suggest that what is currently described as „canonical olfactory system organization” e.g. in the benchmark system Drosophila melanogaster likely is a derived character state that evolved from a distributed representation / combinatorial coding, and more and more examples from crustaceans and from within the insects emerge as deviations from this pattern. By comparing insects and crustaceans we can study the end point of past evolutionary optimization processes. This project brings together two experts on the functional and evolutionary morphology of crustacean (HAR) and insect (RYB) olfactory system and a computational neuroscientist with a strong background in arthropod olfactory systems (NAW). In the prolongation proposal, we want to continue our successful collaboration from phase 1 of the PP and gain more detailed insights into the neuroanatomy of the central olfactory pathway in non-model insects and crustaceans beyond the well studied model system D. melanogaster. By quantifying the numbers of the involved neuronal elements (olfactory sensory neurons, local olfactory interneurons, projection neurons, olfactory glomeruli) and by analysing projectomic characteristics of the OSN to PN to output pathway (mushroom bodies) we will gain additional insights into divergence versus convergence and evolutionary specialization of the central olfactory pathways in arthropod lineages. Furthermore, by obtaining ultrastructural data we will continue to understand the evolutionary optimization of the connectivity of OSNs to PNs. Finally, by further evolving a mathematical model of information processing within olfactory glomeruli of different arthropod lineages as developed in phase 1, we want to further understand the existing differences of olfactory coding mechanisms in insects versus crustaceans.