Investigation of Various Factors Shaping the Hearing Organ of Drosophila: The Johnston’s Organ

Abstract

This thesis deals with the investigation of the consequences of several internal and environmental factors on the Drosophila hearing organ, the Johnston’s organ (JO). JO is evolving as a potential model system to examine the hearing associated defects in vertebrates. Drosophila can be utilized as a model to investigate the effects of many parameters that may affect the hearing organ of the vertebrates. In the case of vertebrates, several studies suggest parameters like aminoglycosides, alteration of circadian rhythm (CR), excess noise, low pressure, and aging can affect hearing. The present study examines the oral toxicity of different types of aminoglycosides (gentamicin, kanamycin, neomycin) at a discrete concentration (50, 100,150, 200, 250 μg/ml) from the first instar larva onwards. The third instar larvae display defective sound avoidance behavior and crawling behavior (regulated by sensory motor neuron). The adult flies hatched after aminoglycoside treatment have lesser amounts of acetylcholine esterase, higher amounts of reactive oxygen species (ROS), and altered hearing-related behaviors. All the genes involved in the hearing associated behaviors such as nompC, inactive, nanchung, pyrexia, and serotonin were downregulated after aminoglycoside treatment. The effect of environmental factors like low pressure, noise, and altered circadian rhythm was investigated on Drosophila development from the embryonic stage onwards. In the environmentally challenged third instar larval antenna imaginal disc, cell death was detected. Flies hatched from the environmentally challenged larvae have increased amounts of antioxidant enzymes, reactive oxygen species, and fewer amounts of mitochondria. Altered hearing-related behaviors like climbing, aggression, and courtship suggest the antenna is defective. The effect of age was investigated on the JO of Drosophila. Perhaps with age, the climbing behavior declined and the antennae have less amount of catalase and contains more amounts of carbonyl. This suggests that there would be more accumulation of H2O2 and redox imbalance in the antennae. Superoxide dismutase (SOD) plays a key role in the redox balance within the body. Thus, SOD mutants were checked for hearing-related behaviors in different developmental stages. Adult flies have impaired climbing behavior. The third instar larva shows malformed startle, sound avoidance, and crawling behavior. Hearing or mechanosensory-related behaviors such as startle and sound avoidance, and larva crawling behavior were found to be affected in the SOD mutants. All these assays point that SOD flies have hearing defects. With the age, advanced glycosylated end products (AGE) get accumulated within the body. To induce aging, we fed the 1st instar larvae with age compounds. In AGE treated antennae, more autofluorescence was detected. AGE-treated flies have impaired climbing behavior. AGEcompound treated flies has more amount of carbonyl content, SOD level, and less catalase activity. This study signifies that age-related hearing defect in the Drosophila is due to redox imbalance and accumulation of AGE compound with age. This thesis finds that factors like aminoglycosides, noise, pressure, circadian rhythm (CR), and age can alter the behaviors associated with antennae in Drosophila.