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Investigating Perception Under Dynamic Auditory Conditions in the Acoustic Parasitoid Fly Ormia Ochracea

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Investigating Perception Under Dynamic Auditory Conditions in the Acoustic Parasitoid Fly Ormia Ochracea Investigating Perception Under Dynamic Auditory Conditions in the Acoustic Parasitoid Fly Ormia ochracea by Dean Koucoulas A thesis submitted in conformity with the requirements for the degree of Master of Science Cell and Systems Biology University of Toronto © Copyright by Dean Koucoulas 2013 Investigating Perception Under Dynamic Auditory Conditions in the Acoustic Parasitoid Fly Ormia ochracea Dean Koucoulas Master of Science Cell and Systems Biology University of Toronto 2013 Abstract Behavioural phonotaxis (oriented movement in response to sound) is an effective means to quantify auditory perception in acoustically communicating insects. Previous phonotaxis studies on the acoustic parasitoid fly Ormia ochracea (Diptera: Tachinidae) have described stereotyped, reflex-like responses towards auditory stimuli modeled after their preferred cricket hosts, yet their ability to demonstrate plasticity of responses in the context of dynamically changing auditory cues has not previously been described. Using a behavioural sensitization protocol, I compared phonotaxis towards behaviourally irrelevant (non-attractive) test stimuli presented alone, and when preceded with the natural, response-evoking cricket song (attractive). Results demonstrate the cricket song as a sensitizing stimulus mediating phonotaxis towards otherwise non-attractive sounds, and differential walking patterns depending on temporal delay between song offset and test stimulus onset. My findings suggest an ecological purpose of sensitization, allowing flies to maintain orientation towards a cricket host amidst conditions of signal disruption in the environment. ii Acknowledgments Throughout my academic career, I have had the immense privilege of being surrounded by an amazing support network of family, friends, peers, and colleagues. I would first like to thank my supervisor, Dr. Andrew C. Mason for being a continual source of support for me, and for giving me the opportunity to explore and incorporate a multitude of interests in the lab, especially re- igniting my passion for electronics. With Andrew’s guidance, I was able to grow both personally, and professionally and owe my sincerest gratitude for having first welcomed me into the lab as a summer student volunteer. I would like to thank my thesis advisory committee members, Drs. Mark J. Fitzpatrick, and Kenneth C. Welch for their continual feedback during the progress of my research, and for promoting my ability to develop as a critical thinker, and as an independent scientist. I am extremely grateful to Dr. Patrick O. McGowan for his continual support during the final stages of my thesis, and for always being open to hearing about the progress of my research. When it comes to my fellow lab members, I cannot thank them enough for their daily encouragement and motivation from hearing about my research ideas, to giving me company during long nights in the lab. I had the extreme privilege of overlapping my graduate studies with Dr. Norman Lee, Dr. Paul A. De Luca, Jenn Van Eindhoven, Sen Sivalighem, and the amazing Andrade lab, and I look forward to maintaining our collaborations well into the future. Thank you also to all the undergraduate research assistants and volunteers including Juli Rasanayagam, Steven Susanto, Alisha Patel, Kiran Beera, Paula Tactay, Olivia Murray, and Michelle Leung for their committed dedication to ensuring the well being of our fly population. Thank you also to the University of Toronto Scarborough, and the many Departmental staff for all your help throughout my time as both an undergraduate, and graduate student. I would like to thank my wonderful parents for their continued support, for always encouraging me to achieve my best, and for always believing in me. To my brothers, thank you for always being at my side, and for all the support and motivation you have provided me along the way. To my grandparents, thank you for all that you have taught me, and for giving me the opportunity to freely pursue the aspirations I am striving for now. It is because of you that I am able to say there are no limits to what I may achieve in my lifetime. iii Table of Contents Abstract……………………………………………………………...……………………ii Acknowledgements………………………………………………………………………iii Table of Contents……………………………………………………………………...…iv List of Tables……………………………………………………………………………..vi List of Figures…………………………………………………………………………...vii Chapter 1 General Introduction…………………………………………………….……..1 1.1 Hearing and the role of sound in insects…………………………………………1 1.2 Auditory challenges for insects………...………………………………………...4 1.3 Insect solutions to complex auditory scenes……………………………………..5 1.4 Directional hearing in Ormia ochracea………………………………………….6 Chapter 2 Behavioural plasticity under dynamic auditory conditions in the acoustic parasitoid fly, Ormia ochracea……………………………………………………………………..10 2.1 Abstract…………………………………………………………………………10 2.2 Introduction……………………………………………………………………..11 2.3 Materials and Methods………………………………………………………….15 2.3.1 Animals…………………………………………………………………15 2.3.2 Acoustic Stimuli………………………………………………………...15 2.3.3 Experimental Apparatus………………………………………………...17 2.3.4 Protocol…………………………………………………………………18 2.3.5 Data Analysis……………………………………………..……………...19 iv 2.4 Results……………………………………………………………………………….21 2.4.1 Responses to Noise………………………………………………………21 2.4.2 Responses to Pulse Trains……………………………………………….24 2.5 Discussion…………………………………………………………………………....27 Chapter 3 General Discussion…………………………………………………………...56 References……………………………………………………………………………….59 v List of Tables Table 1. Noise following chirp…………………………………………………………..51 Table 2. Pulse-trains following chirp – long and short IPI…………………………...….52 Table 3. Pulse-trains following chirp – intermediate IPI………………………………...54 vi List of Figures Figure 1. Common acoustic definitions in cricket song structure………………………...9 Figure 2. Outline of auditory stimuli used in noise experiment………………...….……36 Figure 3. Outline of auditory stimuli used in pulse train experiment………………...…37 Figure 4. Experimental apparatus used to assess phonotaxis ………………………..…38 Figure 5. Velocity measurements used to quantify phonotaxis…………………………39 Figure 6. Time indeces for velocity calculations in pulse train experiment………….…40 Figure 7. Steering and forward velocities in noise experiment…………………………41 Figure 8. Delta steering and forward velocity in noise experiment……………………..42 Figure 9. 2-D walking paths and lateral deviation in noise experiment………………....43 Figure 10. Steering and forward velocities in pulse train experiment……………...……44 Figure 11. Closer look at steering and forward velocity in pulse train experiment……..45 Figure 12. Regression lines for steering velocity in pulse train experiment.……….…..46 Figure 13. Regression lines for forward velocity in pulse train experiment…………….47 Figure 14. Average velocity per pulse compared to naïve fly responses.…………….....48 Figure 15. 2-D walking paths and lateral deviation in pulse train experiment……….....49 Figure 16. Full 10 s long cricket chirp effects………………………….….……………50 vii Chapter 1 General Introduction Organisms that make use of auditory communication are exposed to a diversity of acoustic signals that together, comprise what is known as their auditory scene (Hulse, 2002). Auditory scene analysis is the process by which the complex mixture of individual sound sources entering the auditory system is segregated and identified as meaningful representations of the surrounding environment (Bregman, 1990; Fay, 2007). Much of the early work in auditory scene analysis focused on understanding the role of sound in human hearing and speech communication (Bregman, 1990; Bee and Micheyl, 2008). However, the challenge of extracting relevant sources of information amidst a complex auditory backdrop is common amongst all acoustically communicating animals, including insects. Despite receiving less attention than their vertebrate counterparts, investigation of auditory processing mechanisms in insect systems offers a unique opportunity to understand the ubiquitous nature of auditory scene analysis across taxa, and is what defines the topic of my thesis. 1.1 Hearing and the role of sound in insects Among certain insects, maintaining conspecific communication, avoiding predation, and ensuring reproductive success is largely dependent on the auditory system processing of acoustic stimuli (Hedwig, 2006; Virant-Doberlet and Čokl, 2004). The ability to maintain such critical processes is dependent on the effective recognition of relevant auditory cues in the environment, and localization of their sources. Recognition of conspecific signals is predominantly determined by assessing the spectral (varying across frequency), and temporal (varying across time) features of the incoming sound field (Pollack, 1998). The importance of spectral characteristics in insects is best realized by considering that the auditory receptors of many insect ears exhibit specialized tuning towards a specific range of sound frequencies, such that spectral characteristics of the surrounding acoustic space are represented by the differential activation of individual receptors (Pollack, 1998; Mason and Faure, 2004). Depending on the pattern of receptor activation, different 1 behavioural responses may be elicited that are matched to the appropriate context. As an example, female crickets of the species Gryllus bimaculatus actively seek mates based on the dominant 4.5 kHz calling song of their male suitors, but will only exhibit the mounting response for reproduction upon hearing the 13.5 kHz courtship song (Pollack,
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