Doctor of Philosophy in Zoology (PhD)
Taste processing in Drosophila melanogaster
Most projects centre on the neural circuits underlying taste processing and feeding in the fly brain. Potential projects include looking at circuit mechanisms underlying taste integration, hunger, feeding, taste memories, chemosensory integration, and taste circuit development.
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Dissertations completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest dissertations.
The full abstract for this thesis is available in the body of the thesis, and will be available when the embargo expires.
To survive animals must find and consume nutritive foods. The chemical composition of food sources is evaluated using gustation. Because of its importance to survival, feeding behaviors are tightly regulated. Changes in feeding occur in response to the external environment and the animal’s internal physiological state. Expendable behaviours can be suppressed during starvation to prioritize feeding. This thesis examines the role of two factors in feeding decisions: the location of taste input and starvation. Pharyngeal sense organs in Drosophila are the last evaluation point before food is ingested. It was previously unclear whether they served a unique function in feeding. To investigate this, we focused on appetitive gustatory pharyngeal neurons and showed they express nine gustatory receptors which respond to sweet compounds. Mutants lacking peripheral taste have functional pharyngeal sense organs. In the absence of peripheral taste cues, the pharyngeal sense organs can drive the choice and ingestion of sweet compounds by sustaining consumption. Knocking down pharyngeal neurons in these mutants allowed us to examine a sweet-blind fly in a short term feeding assay. Putatively sweet-blind flies do not show a preference for sweet compounds in a short-term feeding assay, suggesting that nutrient sensing is not operating in this context. Starved flies have an increased tolerance for bitter foods, which is mediated by sensitization and desensitization of sweet and bitter taste, respectively. Mechanisms that cause the sensitization of sweet taste have been studied, but those that underlie desensitization of bitter taste were unknown. We identify a pair of octopaminergic/tyraminergic modulatory neurons called the ventrolateral cluster of octopaminergic neurons (OA-VLs). Because OA-VLs exist in close physical proximity to bitter neuron axon terminals but are not postsynaptic to them, we examined their function as modulators of bitter neuron output. Tonic firing rate of OA-VLs decreases as a function of starvation. Octopamine and tyramine are sufficient to potentiate bitter neuron response in starved flies, suggesting that reduction in OA-VL activity during starvation depotentiates bitter neuron output. Silencing OA-VLs causes a reduction in bitter neuron output and increased acceptance of bitter compounds. OA-VLs may act directly on bitter sensory neurons through the Oct-Tyr receptor.
Theses completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest theses.
Female Aedes aegypti mosquitoes are anthropophilic. They locate human hosts by using multimodal cues like visual cues, carbon-dioxide, body heat, odor, and taste. Upon landing, female mosquitoes assess taste cues with their tarsi and proboscis to evaluate the surface before probing in to draw blood. Lactic acid is an important component of human body secretions, which plays a role in attracting mosquitoes to the human host. However, not much has been uncovered about the role of mosquito gustation in bite-initiation behavior. Does lactic acid taste on human skin elicit an appetitive response in the gravid female mosquitoes, initiating blood-feeding? While blood-feeding behavior is specific to female Aedes aegypti, both males and females engage in sugar feeding to acquire the nutrients for life processes. Is lactic acid taste an attractive gustatory cue for the sugar-feeding program? Does sexual dimorphism play a role in their gustatory response towards lactic acid? We have developed novel mosquito feeding and engorgement assays to answer these questions. We found that lactic acid is an attractive contact cue for the mosquitoes both during sugar-feeding and engorgement behavior.How acid taste is encoded, evaluated, processed, and modulates behavior are not well understood. To understand the neural coding of lactic acid taste, we used Drosophila melanogaster (flies). Flies show an appetitive response to lactic acid taste at low concentration, however as the concentration increases, they start demonstrating aversive behavior. We showed that attraction towards lactic acid taste is mediated by sweet Gustatory Receptor Neurons (GRNs), expressing Gr64f, which is a gustatory receptor similar to the G-protein-coupled receptor family. Further delving into the molecular mechanism, we found that lactic acid taste attraction in fruit flies is mediated by receptors belonging to two different families - the Gustatory Receptor (GR) family and the Ionotropic Receptor (IR) family. To our best knowledge, it is the first time the involvement of both an IR and a GR has been demonstrated in mediating taste response to a single modality.
The ability to detect various tastes greatly enhances survivorship. The detection of nutritious or toxic foods leads to a promotion or inhibition of feeding. Although the characterization of taste-detection at the periphery is extensive, their connection to second-order taste neurons is only beginning to be elucidated. In Drosophila melanogaster, we can harness powerful genetic tools to help map out the neuronal circuitry that translates the sensory inputs of taste to the motor commands of feeding. Our objective is to identify higher-order neurons that bridge this sensorimotor circuitry. Understanding how taste information is processed in fruit flies may shed light on how its mammalian counterparts do so, as well. To accomplish this, we exploit the Sip-Triggered Optogenetic Behaviour Enclosure (STROBE) for its high-efficiency and biological relevance to screen for potential taste neurons. In total, 123 driver lines were selected and screened through optogenetic neuronal activation. One line, in particular, R70C07-GAL4, was chosen for the further characterization of its role in feeding inhibition. It predominantly labels two clusters of cell bodies that are bilateral to the brain’s primary taste center, the subesophageal zone (SEZ). It is predicted to be responsible for aversive feeding behaviour. To test this, we created split-GAL4 lines to narrow down this population before GRASP and additional optogenetic activation experiments were performed to confirm the identity of neurons that are responsible for altering feeding behaviour. Upon activation, one subset of the lateral SEZ population was revealed to induce significantly aversive feeding behaviour, while another induced appetitive feeding. Surprisingly, both subsets show a positive GRASP signal with both sweet and bitter gustatory receptor neurons (GRNs). The evidence presented here demonstrates that clear feeding preferences can be made even by activating a population of neurons that communicates with GRNs of opposing valence. This raises the question of whether feeding decisions are instigated by the relative activity between pathways of opposing taste valences, and if this ratio of activity is already encoded at the level of second-order taste neurons, possibly challenging the extension of the labelled-lines theory into higher-order neurons.
Salt is an important component of most of the food we ingest daily, yet its detection in food and the control of its consumption remain poorly understood. The gustatory system of an animal underlies the processing of salt taste. With the ability to taste on multiple parts of the body, Drosophila melanogaster makes critical decisions about beneficial tastants to consume, and potentially harmful tastants to avoid. Most chemicals sensed by the gustatory system drive either attraction or repulsion, but salt is unique in that it is appetitive at low concentrations and repulsive at high concentrations. How salt is encoded, processed, modulated, and drives behavior is still not well defined. Through immunolabeling, calcium imaging, and a variety of behavioral assays, we are working on understanding these questions. I have constructed a taste sensory neuron map by co-labeling different gustatory receptors and protein markers for the neurotransmitters glutamate and acetylcholine. Interestingly, while most sensory neurons are cholinergic, I have found that the ENaC family member ppk23 labels a population of glutamateric taste neurons in the fly labellum (the analog of the mammalian tongue). Using calcium imaging, I have characterized the responses of this and other major populations of taste sensory neurons, and found that most, if not all, taste sensory neurons respond to salt in some way. We look at the effect of salt deprivation on sugar and salt sensing labellar GRNs’ physiology to further understand the neural circuitry devoted to this salt sensing pathway. This work challenges the current paradigm of labelled lines coding in the gustatory system of flies, and instead presents evidence of combinatorial coding in salt. Understanding the basic neural circuitry in the fly gustatory system and how that drives behavior in the face of different internal states across modalities, will give insight into how taste information is translated into appropriate feeding responses.
Although feeding behavior is a matter of life and death for animals, the genetic factors that control it remain poorly understood. We have identified a novel regulator of hunger-induced behavior through comparison of transcriptomic changes in the fruit fly Drosophila melanogaster and the yellow fever mosquito Aedes aegypti. Head mRNA from each insect was sequenced at a roughly equivalent level of starvation. Using data gleaned from the protein orthology database OrthoDB, we looked for gene pairs in which both A. aegypti and D. melanogaster orthologs were significantly regulated by starvation. This identified Juvenile Hormone esterase (Jhe) as a possible modulator of hunger-induced behavior. Pan-neuronal knockdown of Jhe resulted in increased food consumption and caused enhancement of starvation-induced sleep suppression in Drosophila. These behavioral phenotypes were not caused by a developmental or metabolic defects, and were reproduced by feeding adult Drosophila methoprene, a synthetic Juvenile Hormone analog. Application of precocene I, an inhibitor of Juvenile Hormone biosynthesis, reversed the phenotype. Our analysis suggests that Jhe (and Juvenile Hormone by extension) is a novel and biologically relevant regulator of hunger-induced behavior.
The sense of taste plays a critical role in animal behaviour. The ability to taste prior to ingestion allows an animal to differentiate between beneficial substances, such as calorie-rich foods, from those that may be toxic and which often taste bitter. For animals to avoid harmful substances, exposure to bitter substances must not only activate bitter-sensing neurons, but also suppress the response of acceptance-mediating neurons. The ability to choose appropriately between consumption and avoidance requires functional neuronal circuits in the brain that start with the processing of taste input from the environment and end with a behavioural output. γ-aminobutyric acid (GABA) is the primary inhibitory neurotransmitter in the insect and mammalian brain. We show evidence of functional connections between GABAergic interneurons (those that release GABA) and taste sensory neurons in the fly brain. Neurons that detect palatable substances (i.e. sugars) express high levels of a metabotropic GABAB receptor (GABABR), whereas those that sense unpalatable substances (i.e. bitter compounds) express little to no GABABR. Using a behavioural assay (proboscis extension reflex) and calcium imaging, I investigated how GABAergic inhibition shapes sweet neuron output and contributes to the integration of competitive taste stimuli. When GABABR is knocked down in sugar neurons, the behavioural response to sugars is elevated. Pharmacological assays show that GABAergic activation suppresses the sweet neuron response while GABAergic blockade potentiates the response. In flies expressing GABABR knockdown, suppression of sugar neuron activity by bitter exposure is relieved both behaviourally, by increased acceptance of bitter mixtures, and cellularly, by increased calcium response. Our model proposes that GABA acts via GABABR in the fly taste circuit to produce ecologically relevant responses to both attractive and repellent energy sources.