|General similarities of the olfactory system from insects to mammals make it tempting to extrapolate from one species to the other. However, the olfactory system is remarkably adaptive and diversified among animal lineages and species occupying different ecological niches. The amphibians are a particularly interesting group of animals, since they are the descendants of the first land living vertebrates and still partially depend on water. In my thesis, I examined adaptive features of the olfactory system of anurans across different distantly related species and in different developmental stages.
• Anuran tadpoles have radiated into many different aquatic microhabitats and have adopted several different lifestyles and feeding habits. Nevertheless, I found that the organization of glomerular clusters in the MOB is remarkably conserved in anuran larvae, constituting a morphological and possibly functional blueprint for tadpole olfaction.
• Before the onset of metamorphosis, the main olfactory system of Xenopus consists of a sensory epithelium in the PC, which connects to the glomerular clusters in the vMOB. In this thesis, I showed that during metamorphosis, projections to the vMOB are gradually replaced by cells from the newly formed MC (adult ‘water nose’) until the metamorphotic climax. Despite the complete rewiring, the vMOB retains its coarse functional organization and odorant-mediated behavior to waterborne stimuli is still present. In the terrestrial Dendrobates tinctorius, the glomerular clusters of the vMOB show signs of degeneration, possibly as a sign of less reliance on aquatic olfaction.
• During metamorphosis, cells in the larval PC are completely replaced to form the adult ‘air nose’. The new PC neurons project their axons towards a new glomerular projection field in the dMOB. I found that in the dMOB, RN projections from the left and right PC show a substantial amount of overlap, and individual glomeruli around the midline receive bilateral input. Postsynaptic projection neurons in the dMOB extend multiple primary dendrites to multiple glomeruli, putatively integrating bilateral sensory input. It remains to be shown, whether this particular pattern might be behaviorally relevant, e.g. in olfactory-guided spatial orientation.
• Single RN axons in vertebrates are generally believed to be unbranched and only innervate a single glomerulus in the MOB. In my thesis, I show that multi-glomerular projections of individual RNs are conserved among ecologically diverse anurans and are also present in the axolotl. In contrast, the sea lamprey and the zebrafish almost exclusively showed a uni-glomerular projection pattern. Axonal bifurcations seem to be an ancestral feature in amphibians, and are possibly more common among vertebrates, forming an alternative odorant processing channel.