Structural and Functional Correlates of the Taste System of Drosophila Larvae

Abstract

The detection and discrimination among chemical cues is essential for living organisms to survive. Remarkable progress has been made in the knowledge of chemosensory perception in insects, largely based on the identification of chemo receptor genes in Drosophila melanogaster. Four receptor gene families underlie chemical detection of taste: the gustatory receptor(Gr), DEG/ENaC pickpocket (Ppk), the transient receptor potential (Trp) and the ionotropic receptor (Ir) gene families. Drosophila larvae sense and respond to a multitude of taste stimuli, though their taste system has a remarkably low number of neurons. Nevertheless, it triggers stereotypic behavioral responses associated with taste sensing, and the sensory coding of taste can be studied by genetic tools at the single cell level. These properties make Drosophila larvae particular suitable to study the mechanisms underlying sensation and processing of taste. The present thesis investigates anatomical and functional properties of sense organs in the taste system of Drosophila larvae. In particular, the morphology of larval sensilla was examined using a combination of volume electron microscopy and 3D image reconstruction. The use of genetically manipulated larvae in combination with anatomical, behavioral and physiological tests allowed to specify the expression and function of receptor genes and neurons in the larval taste system. The present thesis reveals that taste organs along the pharynx contribute to the perception of bitter compounds dependent on the behavioral context in which the bitter stimulus is presented to the larva. It is shown that a single pair of taste neurons in the dorsal pharyngeal sensilla sense the bitter compound caffeine. Signaling through this pair of neurons is required for the larva to learn the association between the taste of caffeine and an odor but dispensable to reduce feeding on a substrate containing caffeine. Three members of the Gr gene family are required for the reception of caffeine in the pharyngeal neuron pair: Gr93a,Gr33a, Gr66a. The major external taste organ of larvae is the terminal organ (TO) situated on the tip of the larval head. Present thesis provides a systematic anatomical and molecular analysis of the TO sensilla and sensory neurons. The use of focused ion beam scanning electron microscopy (FIB-SEM) enabled serial sectioning of the TO sensilla and subsequent 3D image analysis. Due to this detailed analysis precise classification of sensilla was possible. Further, the repertoire of receptor genes, belonging to the Ir, Gr, Ppk, and Trp gene families, for each sensillum was determined. A map of the TO was generated, in which the receptor genes are assigned to neurons of individual sensilla. This map of the TO provides the basis for further investigation of how taste is encoded by the signals of the taste neurons. By optimizing the preparation protocol for FIB-SEM for the investigation of insect sensilla it was possible to efficiently analyze the morphology of further larval sense organs on the head (dorsal, labial and ventral organ), thoracic and abdominal segments (hair, peg and spot sensilla). The present work provides a comprehensive analysis of these organs based on their external and internal morphology. It was possible to clarify the nomenclature and putative function of sensilla proposed in previous literature. The findings of the present thesis show, that, except for the olfactory dorsal organ and the gustatory TO, larval sensilla mostly display morphological features of mechanosensation. The principles underlying the reception of sensory stimuli can only be captured by deciphering both the anatomy and the functional properties of sense organs. Providing comprehensive anatomical analysis of sensory organs and their functional implications this work therefore contributes to the understanding of sensory reception in Drosophila larvae.