Similar but different: Comparative neuroanatomy of the chemo- and mechanosensory pathways in scorpion pectines

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The chemical sense is an integral modality for virtually all forms of life. Regarding Arthropoda, the most diverse animal phylum, many representatives possess primary chemosensory organs – the antennae I - associated with the second head segment. Olfactory pathways of the Mandibulata (Hexapoda, Crustacea, and Myriapoda) are markedly similar, and their primary olfactory centers even resemble vertebrate olfactory bulbs in several respects. The remaining euarthropod taxon, the Chelicerata, do not possess dedicated chemosensory appendages associated with the second head segment. A particularly fascinating solution to compensate the lack of antennae is observed in scorpions. These animals possess so-called pectines as dedicated pairs of chemosensory appendages that also fulfil a mechanosensory function, much like the antenna I in Mandibulata. Intriguingly, the pectines are located on the ventral side of the body, just behind the walking legs and the genital operculum. They are comb-shaped organs studded with many thousands of mechano- and chemosensory sensilla that are arranged on the ventral sides of each tooth, or peg. Pectines have been suggested to play an important role in scanning the substrate as contact chemoreceptive organs. The sensory afferents project into numerous (lobular to glomerular) compartments of the central nervous pectine neuropil. These compartments are reminiscent of the glomerular organization of the olfactory centers in Mandibulata. Initial backfill experiments revealed a somatotopic projection of peg sensilla, which is quite uncommon for chemoreceptive systems, though typically observed in mechanosensory neuropils. This raises the question of how chemo- and mechanosensory inputs are integrated in the pectine neuropils.The present project sets out to investigate the neuroanatomical organization of the scorpion pectine neuropils and their projections in detail. We will focus on all components of this pathway: Projections of chemosensory neurons from the periphery into the CNS, structure and anatomy of putative glomeruli in the central nervous system, characteristics of local interneurons and their neurotransmitters, and connectivity to higher brain centers by projection neurons. We will compare the resulting data set to the mechanosensory pectine projections and thus characterize the mode of integration of the two sensory modalities. Developmental aspects will be addressed in all project parts, focusing on sensory structures and their primary projections. This will expand our understanding of potential functional necessities in the organization of the chemo- and mechanosensory neuropils and their interlacing.