Dokument

Summary:
Chapter 3: Adult neurogenesis in the mushroom bodies The ground pattern of the central nervous system is genetically encoded. However, following development, when the first sensory contact with environmental stimuli occurs, adaptation to actual conditions and stimuli is essential for survival. Therefore, the genetically predetermined neuronal wiring scheme must be adaptable. This adaptability is also called plasticity and is typically limited to specific time windows of increased receptivity to sensory stimuli, called critical periods or sensitive phases. Primarily, plasticity is achieved by changes in the synaptic connections of existing neurons, biochemical changes at synapses, and the addition and removal of neurons. Research on critical periods has long focused on vertebrates, where such occur during the postnatal development of the visual system or during song learning in birds. However, several studies have shown the existence of critical phases in insects as well - primarily in the olfactory system. The sense of smell is used by insects for a variety of tasks, such as finding food sources, reproductive partners, or suitable habitats. In holometabolous insects, like Tribolium castaneum, the first contact of the adult olfactory system with the environment occurs following the pupal phase at the time of adult eclosion. Based on the already known existence of adult neurogenesis in the mushroom bodies of T. castaneum, this subproject investigates to what extent this depends on olfactory input during the first days as an imago and whether there is a time limit. Reliable labeling of the newborn daughter cells of the mushroom body neuroblasts in the adult animal provided the basis for the study and allowed the determination of the number of newborn Kenyon cells under different olfactory conditions. Within the first week after adult eclosion, the emergence of new Kenyon cells can be divided into at least two phases. In the first three days, the genesis of Kenyon cells is not dependent on olfactory stimuli and is therefore probably due to genetic programming and represents a continuation of the developmental processes from metamorphosis (nature). Directly afterward, however, the genesis of Kenyon cells probably mainly depends on the olfactory environment (nurture). Given the role of the mushroom bodies as centers for learning and memory, neurogenesis is most likely an important component in remodeling the neuronal network, ultimately leading to environmental adaptation and behavioral optimization. Chapter 4: Metamorphic development of the olfactory system For insects, the olfactory sense is essential for survival. Olfactory signals are processed and ultimately translated into behavior by a rather complex system. The nervous system - and thus the olfactory system - of holometabolous insects, like Tribolium castaneum, is typically subject to major remodeling during metamorphosis. These changes are necessary because the lifestyle of imagines of holometabolous insects is typically very different from that of their larvae. Larvae of holometabolous insects already possess olfactory processing centers and sensory appendages, although their complexity differs among species. In T. castaneum, as well as ist close relative Tenebrio molitor, already the larvae possess an elaborate antenna consisting of three distinguishable segments (scape, pedicel, flagellum). The larval antenna of the tobacco hawkmoth Manduca sexta has a similar structure, while flies have only functionally equivalent dorsal organs. The current scientific picture sees the larval and adult antennae/olfactory sensory appendages as independent structures. The primary processing centers are already glomerularly organized in the mealworm beetle T. molitor and in Drosophila melanogaster, while this is not the case, for example, in the tobacco hawkmoth M. sexta or the western honeybee Apis mellifera. Previous studies suggest that olfactory sensory neurons are necessary for the correct formation of glomeruli in adult antennal lobes. De-antennation in M. sexta leads to nonglomerular antennal lobes and in the ant Ooceraea biroi to a reduction in the number of glomeruli. In the ant, the same effect could also be achieved by knocking out the co-receptor Orco, which forms a functional unit together with a specific odorant receptor. However, it should be noted that the authors of the study attributed this to the resulting absence of olfactory sensory neurons. Furthermore, studies in D. melanogaster could detect Orco only after the formation of the glomeruli. Thus, in the fly, Orco cannot affect glomeruli formation. In this subproject, the metamorphic development of the olfactory system of the beetle T. castaneum was studied. It could be shown that for the adult antenna, structures of the larval antennae are reused. This contradicts the previous scientific picture that they are independent structures. The same is true for the larval antennal lobes as primary olfactory processing centers, which are transformed into their adult counterparts during metamorphosis. This is supported by the fact that olfactory sensory neurons can be detected in the antennal structures and the antennal lobes throughout metamorphosis. Furthermore, it could be shown that the co-receptor Orco is not necessary for the initial formation of the glomerular structure of the antennal lobes - but is necessary for the later differentiation of the glomeruli in the earlyimagines. The formation of the glomeruli of the antennal lobes occurs about midway through metamorphosis, as in all holometabolous insects studied to date. The same is true for the glomeruli of the gnathal olfactory center first described in T. castaneum. Chapter 5: The palpal olfactory pathway In the current scientific picture of the olfactory system of holometabolous insects, the paired antennal lobes are the sole primary processing centers. Accordingly, these receive input from the sensory neurons of the antennae and mouthparts. In hemimetabolous insects, however, the olfactory signals, which are received by the sensory neurons of the mouthparts, are processed separately. This is also true for the holometabolous beetle Tribolium castaneum, where the signals from the mouthparts are processed in the glomerular lobes (LG), previously known only from hemimetabolous insects, and the gnathal olfactory center (GOC), first described in the beetle. This subproject focuses on a detailed description of the GOC, LGs, and their sensory inputs. Using scanning electron microscopy and comparison with confocal microscopy images of the mouthparts in a transgenic line labeling the olfactory sensory neurons, the olfactory sensilla on the mouthparts containing the dendrites of the olfactory sensory neurons innervating the GOC and LGs were identified. This revealed that at least two of the three sensilla types have an olfactory identity, which contrasts previous findings from Drosophila melanogaster, where only one sensilla type on the mouthparts is olfactory. Using 3D reconstructions based on various histological stains, the anatomical description of the GOC was made more precise. The GOC consists of approximately 30 glomeruli. Thus, the number of glomeruli is at the lower end of the previously suspected range. This number of glomeruli also matches the 28 specific odorant receptors that are significantly enriched in the mouthparts compared to the body, as determined by RNA sequencing. For the first time, the neuromediators repertoire of the GOC and LGs is investigated. For this purpose, ones that are also found in the antennal lobes were exemplarily investigated. Underlining the role of the GOC and LGs as primary olfactory processing centers, all investigated neuromediators were also found in the GOC and LGs, albeit with differences in the pattern of innervation. Therefore, it might be concluded that the neuromediator repertoire is rather not correlated with the complexity of the processing centers, but rather a necessity to ensure fine-tuning and initial evaluation of incoming signals.

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