Octopamine neurons mediate flight-induced modulation of visual processing in Drosophila melanogaster

Activity-dependent modulation of sensory systems has been documented in many organisms, and is likely to be essential for appropriate processing of information during different behavioral states. However, the mechanisms underlying these phenomena, and often their functional consequences, remain poorly characterized. I investigated the role of octopamine neurons in the flight-dependent modulation observed in visual interneurons in the fruit fly Drosophila melanogaster. The vertical system (VS) cells exhibit a boost in their response to visual motion during flight compared to quiescence. Pharmacological application of octopamine evokes responses in quiescent flies that mimic those observed during flight, and octopamine neurons that project to the optic lobes increase in activity during flight. Using genetic tools to manipulate the activity of octopamine neurons, I find that they are both necessary and sufficient for the flight-induced visual boost. This work provides the first evidence that endogenous release of octopamine is involved in state-dependent modulation of visual interneurons in flies. Further, I investigated the role of a single pair of octopamine neurons that project to the optic lobes, and found no evidence that chemical synaptic transmission via these neurons is necessary for the flight boost. However, I found some evidence that activation of these neurons may contribute to the flight boost. Wind stimuli alone are sufficient to generate transient increases in the VS cell response to motion vision, but result in no increase in baseline membrane potential. These results suggest that the flight boost originates not from a central command signal during flight, but from mechanosensory stimuli relayed via the octopamine system. Lastly, in an attempt to understand the functional consequences of the flight boost observed in visual interneurons, we measured the effect of inactivating octopamine neurons in freely flying flies. We found that flies whose octopamine neurons we silenced accelerate less than wild-type flies, consistent with the hypothesis that the flight boost we observe in VS cells is indicative of a gain control mechanism mediated by octopamine neurons. Together, this work serves as the basis for a mechanistic and functional understanding of octopaminergic modulation of vision in flying flies.

Neuronal mechanism of state control in Drosophila melanogaster

The changes in internal states, such as fear, hunger and sleep affect behavioral responses in animals. In most of the cases, these state-dependent influences are “pleiotropic”: one state affects multiple sensory modalities and behaviors; “scalable”: the strengths and choices of such modulations differ depending on the imminence of demands; and “persistent”: once the state is switched on the effects last even after the internal demands are off. These prominent features of state-control enable animals to adjust their behavioral responses depending on their internal demands. Here, we studied the neuronal mechanisms of state-controls by investigating energy-deprived state (hunger state) and social-deprived state of fruit flies, Drosophila melanogaster, as prototypic models. To approach these questions, we developed two novel methods: a genetically based method to map sites of neuromodulation in the brain and optogenetic tools in Drosophila.

These methods, and genetic perturbations, reveal that the effect of hunger to alter behavioral sensitivity to gustatory cues is mediate by two distinct neuromodulatory pathways. The neuropeptide F (NPF) – dopamine (DA) pathway increases sugar sensitivity under mild starvation, while the adipokinetic hormone (AKH)- short neuropeptide F (sNPF) pathway decreases bitter sensitivity under severe starvation. These two pathways are recruited under different levels of energy demands without any cross interaction. Effects of both of the pathways are mediated by modulation of the gustatory sensory neurons, which reinforce the concept that sensory neurons constitute an important locus for state-dependent control of behaviors. Our data suggests that multiple independent neuromodulatory pathways are underlying pleiotropic and scalable effects of the hunger state.

In addition, using optogenetic tool, we show that the neural control of male courtship song can be separated into probabilistic/biasing, and deterministic/command-like components. The former, but not the latter, neurons are subject to functional modulation by social experience, supporting the idea that they constitute a locus of state-dependent influence. Interestingly, moreover, brief activation of the former, but not the latter, neurons trigger persistent behavioral response for more than 10 min. Altogether, these findings and new tools described in this dissertation offer new entry points for future researchers to understand the neuronal mechanism of state control.

Temporal and spacial control of RNA stability in the early embryo of Drosophila melanogaster

Early embryogenesis in metazoa is controlled by maternally synthesized products. Among these products, the mature egg is loaded with transcripts representing approximately two thirds of the genome. A subset of this maternal RNA pool is degraded prior to the transition to zygotic control of development. This transfer of control of development from maternal to zygotic products is referred to as the midblastula transition (or MBT). It is believed that the degradation of maternal transcripts is required to terminate maternal control of development and to allow zygotic control of development to begin. Until now this process of maternal transcript degradation and the subsequent timing of the MBT has been poorly understood. I have demonstrated that in the early embryo there are two independent RNA degradation pathways, either of which is sufficient for transcript elimination. However, only the concerted action of both pathways leads to elimination of transcripts with the correct timing, at the MBT. The first pathway is maternally encoded, is triggered by egg activation, and is targeted to specific classes of mRNAs through cis-acting elements in the 3' untranslated region (UTR}. The second pathway is activated 2 hr after fertilization and functions together with the maternal pathway to ensure that transcripts are degraded by the MBT. In addition, some transcripts fail to degrade at select subcellular locations adding an element of spatial control to RNA degradation. The spatial control of RNA degradation is achieved by protecting, or masking, transcripts from the degradation machinery. The RNA degradation and protection events are regulated by distinct cis-elements in the 3' untranslated region (UTR). These results provide the first systematic dissection of this highly conserved process in development and demonstrate that RNA degradation is a novel mechanism used for both temporal and spatial control of development.

Developmentally regulated transcription factors in Drosophila melanogaster

During early stages of Drosophila development the heat shock response cannot be induced. It is reasoned that the adverse effects on cell cycle and cell growth brought about by Hsp70 induction must outweigh the beneficial aspects of Hsp70 induction in the early embryo. Although the Drosophila heat shock transcription factor (dHSF) is abundant in the early embryo, it does not enter the nucleus in response to heat shock. In older embryos and in cultured cells the factor is localized within the nucleus in an apparent trimeric structure that binds DNA with high affinity. The domain responsible for nuclear localization upon stress resides between residues 390 and 420 of the dHSF. Using that domain as bait in a yeast two-hybrid system we now report the identification and cloning of a nuclear transport protein Drosophila karyopherin-α3(dKap- α3). Biochemical methods demonstrate that the dKap-α3 protein binds specifically to the dHSF's nuclear localization sequence (NLS). Furthermore, the dKap-α3 protein does not associate with NLSs that contain point mutations which are not transported in vivo. Nuclear docking studies also demonstrate specific nuclear targeting of the NLS substrate by dKap-α3.Consistant with previous studies demonstrating that early Drosophila embryos are refractory to heat shock as a result of dHSF nuclear exclusion, we demonstrate that the early embryo is deficient in dKap-α3 protein through cycle 12. From cycle 13 onward the transport factor is present and the dHSF is localized within the nucleus thus allowing the embryo to respond to heat shock.

The pair-rule gene fushi tarazu (ftz) is a well-studied zygotic segmentation gene that is necessary for the development of the even-numbered parasegments in Drosophila melanogastor. During early embryogenesis, ftz is expressed in a characteristic pattern of seven stripes, one in each of the even-numbered parasegments. With a view to understand how ftz is transcriptionally regulated, cDNAs that encode transcription factors that bind to the zebra element of the ftz promoter have been cloned. Chapter Ill reports the cloning and characterization of the eDNA encoding zeb-1 (zebra element binding protein), a novel steroid receptor-like molecule that specifically binds to a key regulatory element of the ftz promoter. In transient transfection assays employing Drosophila tissue culture cells, it has been shown that zeb-1 as well as a truncated zeb-1 polypeptide (zeb480) that lacks the putative ligand binding domain function as sequencespecific trans-activators of the ftz gene.

The Oct factors are members of the POU family of transcription factors that are shown to play important roles during development in mammals. Chapter IV reports the eDNA cloning and expression of a Drosophila Oct transcription factor. Whole mount in-situ hybridization experiments revealed that the spatial expression patterns of this gene during embryonic development have not yet been observed for any other gene. In early embryogenesis, its transcripts are transiently expressed as a wide uniform band from 20-40% of the egg length, very similar to that of gap genes. This pattern progressively resolves into a series of narrower stripes followed by expression in fourteen stripes. Subsequently, transcripts from this gene are expressed in the central nervous system and the brain. When expressed in the yeast Saccharomyces cerevisiae, this Drosophila factor functions as a strong, octamer-dependent activator of transcription. The data strongly suggest possible functions for the Oct factor in pattern formation in Drosophila that might transcend the boundaries of genetically defined segmentation genes.

Circadian clock mutants of Drosophila melanogaster

Three mutants of Drosophila melanogaster have been isolated in which the free-running period of the circadian eclosion rhythm and the adult locomotor activity rhythm is affected. One mutant is arrhythmic, another has a short period of 19 hours, and the third has a long period of 28 hours. The mutants retain their phenotypes over the temperature range 18° to 25° C. All three mutants map near the tip of the X chromosome (distal to the centromere). By deficiency mapping, the short-period mutation has been localized to the 3B1-2 region. Complementation tests show that all three mutations affect the same functional gene.

Analysis of activity rhythms of individual mosaic flies indicates that the site of action of the short-period mutation is probably located in the head of the fly. A few activity patterns of split-head and mixed-head mosaics appear to possess both mutant and heterozygous components, suggesting that the fly head may contain two complete clocks capable of maintaining their periodicities independently.

The short-period mutation affects both the duration of the light-insensitive part of the oscillation and the degree to which the clock can be reset during the light-sensitive part of the oscillation.

Both the short-period and long-period mutant eclosion rhythms can be entrained to a period of 24 hours by a 12:12 light-dark cycle having a light intensity at least two orders of magnitude greater than that required to entrain the normal rhythm. The arrhythmic mutant does not entrain under these conditions. In the presence of a temperature cycle, however, the arrhythmic mutant does entrain, but its rhythm damps out when the temperature cycle is removed.

Evidence is presented that Pittendrigh's two-oscillator model for the clock in D. pseudoobscura applies to D. melanogaster as well. The three clock mutations primarily affect the light- sensitive driving oscillator. The arrhythmic mutation appears to have eliminated the driving oscillator while leaving the temperature-sensitive driven oscillator relatively intact.

Molecular neurogenetics of eye development in Drosophila

The compound eye of Drosophila melanogaster begins to differentiate during the late third larval instar in the eye-antennal imaginal disc. A wave of morphogenesis crosses the disc from posterior to anterior, leaving behind precisely patterned clusters of photoreceptor cells and accessory cells that will constitute the adult ommatidia of the retina. By the analysis of genetically mosaic eyes, it appears that any cell in the eye disc can adopt the characteristics of any one of the different cell types found in the mature eye, including photoreceptor cells and non-neuronal accessory cells such as cone cells. Therefore, cells within the prospective retinal epithelium assume different fates presumably via information present in the environment. The sevenless^+ (sev^+) gene appears to play a role in the expression of one of the possible fates, since the mutant phenotype is the lack of one of the pattern elements, namely, photoreceptor cell R7. The sev^+ gene product had been shown to be required during development of the eye, and had also been shown in genetic mosaics to be autonomous to presumptive R7. As a means of better understanding the pathway instructing the differentiation R7, the gene and its protein product were characterized.

The sev+ gene was cloned by P-element transposon tagging, and was found to encode an 8.2 kb transcript expressed in developing eye discs and adult heads. By raising monoclonal antibodies (MAbs) against a sev^+- β-galactosidase fusion protein, the expression of the protein in the eye disc was localized by immuno-electronmicroscopy. The protein localizes to the apical cell membranes and microvilli of cells in the eye disc epithelium. It appears during development at a time coincident with the initial formation of clusters, and in all the developing photoreceptors and accessory cone cells at a time prior to the overt differentiation of R7. This result is consistent with the pluripotency of cells in the eye disc. Its localization in the membranes suggests that it may receive information directing the development of R7. Its localization in the apical membranes and microvilli is away from the bulk of the cell contacts, which have been cited as a likely regions for information presentation and processing. Biochemical characterization of the sev^+ protein will be necessary to describe further its role in development.

Other mutations in Drosophila have eye phenotypes. These were analyzed to find which ones affected the initial patterning of cells in the eye disc, in order to identify other genes, like sev, whose gene products may be involved in generating the pattern. The adult eye phenotypes ranged from severe reduction of the eye, to variable numbers of photoreceptor cells per ommatidium, to sub de defects in the organization of the supporting cells. Developing eye discs from the different strains were screened using a panel of MAbs, which highlight various developmental stages. Two identified matrix elements in and anterior to the furrow, while others identified the developing ommatidia themselves, like the anti-sev MAb. Mutation phenotypes were shown to appear at many stages of development. Some mutations seem to affect the precursor cells, others, the setting up of the pattern, and still others, the maintenance of the pattern. Thus, additional genes have now been identified that may function to support the development of a complex pattern.

Studies on hymenopteran parasitism of Drosophila

Studies on Hymenopteran Parasitism of Drosophila

Flies of the genus Drosophila are subject to attack by a number of parasitic forms. Sturtevant (1921) has listed records of parasitism by protozoa (Leptomonas), fungi (Muiaria and Stigmatomyces), nematodes, mites and v~rious hymenoptera. According to Sturtevant, Perkins (1913) has bred at least five species of hymenoptera, belonging to the proctotrupoid, cynipoid and chalcidoid groups, upon Drosophiline flies. H.S. Smith has bred an unidentified proctotrupoid and a chalcidoid, Pachy crepoideus dubius Ashmead* from both Drosophila melanogaster ani D. hydei. Kieffer ( 1913) has described three species of hymenoptera from Africa collected by Silvestri and stated by him to be parasitic on Drosophila, species not given. They are Trichopria (Planopria) rhopalica (Diapriidae), Ashmeadopria drosophilae (Diapriidae), and the insect which forms the subject matter of the present investigation, Eucoila drosophilae (Figitidae).

There are in addition a number of predacious enemies among wasps, spiders, flies and beetles.

The present account is concerned with parasitism of various species of Drosophila by Eucoila drosophilae Kieff. The wasps were found b y Dr. w. P. Spencer who exposed traps in an effort to collect Drosophila at Long Lake, Ohio, in Sept. 1934 . Drosophila larvae from the trap gave a large number of pupae from which wasps emerged in considerable proportions. Since that time stock s have been maintained in culture on Drosophila melanogaster.

How resources control aggression in Drosophila

How animals use sensory information to weigh the risks vs. benefits of behavioral decisions remains poorly understood. Inter-male aggression is triggered when animals perceive both the presence of an appetitive resource, such as food or females, and of competing conspecific males. How such signals are detected and integrated to control the decision to fight is not clear. Here we use the vinegar fly, Drosophila melanogaster, to investigate the manner in which food and females promotes aggression.

In the first chapter, we explore how food controls aggression. As in many other species, food promotes aggression in flies, but it is not clear whether food increases aggression per se, or whether aggression is a secondary consequence of increased social interactions caused by aggregation of flies on food. Furthermore, nothing is known about how animals evaluate the quality and quantity of food in the context of competition. We show that food promotes aggression independently of any effect to increase the frequency of contact between males. Food increases aggression but not courtship between males, suggesting that the effect of food on aggression is specific. Next, we show that flies tune the level of aggression according to absolute amount of food rather than other parameters, such as area or concentration of food. Sucrose, a sugar molecule present in many fruits, is sufficient to promote aggression, and detection of sugar via gustatory receptor neurons is necessary for food-promoted aggression. Furthermore, we show that while food is necessary for aggression, too much food decreases aggression. Finally, we show that flies exhibit strategies consistent with a territorial strategy. These data suggest that flies use sweet-sensing gustatory information to guide their decision to fight over a limited quantity of a food resource.

Following up on the findings of the first chapter, we asked how the presence of a conspecific female resource promotes male-male aggression. In the absence of food, group-housed male flies, who normally do not fight even in the presence of food, fight in the presence of females. Unlike food, the presence of females strongly influences proximity between flies. Nevertheless, as group-housed flies do not fight even when they are in small chambers, it is unlikely that the presence of female indirectly increases aggression by first increasing proximity. Unlike food, the presence of females also leads to large increases in locomotion and in male-female courtship behaviors, suggesting that females may influence aggression as well as general arousal. Female cuticular hydrocarbons are required for this effect, as females that do not produce CH pheromones are unable to promote male-male aggression. In particular, 7,11-HD––a female-specific cuticular hydrocarbon pheromone critical for male-female courtship––is sufficient to mediate this effect when it is perfumed onto pheromone-deficient females or males. Recent studies showed that ppk23⁺ GRNs label two population of GRNs, one of which detects male cuticular hydrocarbons and another labeled by ppk23 and ppk25, which detects female cuticular hydrocarbons. I show that in particular, both of these GRNs control aggression, presumably via detection of female or male pheromones. To further investigate the ways in which these two classes of GRNs control aggression, I developed new genetic tools to independently test the male- and female-sensing GRNs. I show that ppk25-LexA and ppk25-GAL80 faithfully recapitulate the expression pattern of ppk25-GAL4 and label a subset of ppk23⁺ GRNs. These tools can be used in future studies to dissect the respective functions of male-sensing and female-sensing GRNs in male social behaviors.

Finally, in the last chapter, I discuss quantitative approaches to describe how varying quantities of food and females could control the level of aggression. Flies show an inverse-U shaped aggressive response to varying quantities of food and a flat aggressive response to varying quantities of females. I show how two simple game theoretic models, “prisoner’s dilemma” and “coordination game” could be used to describe the level of aggression we observe. These results suggest that flies may use strategic decision-making, using simple comparisons of costs and benefits.

In conclusion, male-male aggression in Drosophila is controlled by simple gustatory cues from food and females, which are detected by gustatory receptor neurons. Different quantities of resource cues lead to different levels of aggression, and flies show putative territorial behavior, suggesting that fly aggression is a highly strategic adaptive behavior. How these resource cues are integrated with male pheromone cues and give rise to this complex behavior is an interesting subject, which should keep researchers busy in the coming years.

Engineered underdominance as a method of insect population replacement and reproductive isolation

Insect vector-borne diseases, such as malaria and dengue fever (both spread by mosquito vectors), continue to significantly impact health worldwide, despite the efforts put forth to eradicate them. Suppression strategies utilizing genetically modified disease-refractory insects have surfaced as an attractive means of disease control, and progress has been made on engineering disease-resistant insect vectors. However, laboratory-engineered disease refractory genes would probably not spread in the wild, and would most likely need to be linked to a gene drive system in order to proliferate in native insect populations. Underdominant systems like translocations and engineered underdominance have been proposed as potential mechanisms for spreading disease refractory genes. Not only do these threshold-dependent systems have certain advantages over other potential gene drive mechanisms, such as localization of gene drive and removability, extreme engineered underdominance can also be used to bring about reproductive isolation, which may be of interest in controlling the spread of GMO crops. Proof-of-principle establishment of such drive mechanisms in a well-understood and studied insect, such as Drosophila melanogaster, is essential before more applied systems can be developed for the less characterized vector species of interest, such as mosquitoes. This work details the development of several distinct types of engineered underdominance and of translocations in Drosophila, including ones capable of bringing about reproductive isolation and population replacement, as a proof of concept study that can inform efforts to construct such systems in insect disease vectors.

A biochemical and structural characterization of Drosophila neuroglian

A number of cell-cell interactions in the nervous system are mediated by immunoglobulin gene superfamily members. For example, neuroglian, a homophilic neural cell adhesion molecule in Drosophila, has an extracellular portion comprising six C- 2 type immunoglobulin-like domains followed by five fibronectin type III (FnIII) repeats. Neuroglian shares this domain organization and significant sequence identity with Ll, a murine neural adhesion molecule that could be a functional homologue. Here I report the crystal structure of a proteolytic fragment containing the first two FnIII repeats of neuroglian (NgFn 1,2) at 2.0Å. The interpretation of photomicrographs of rotary shadowed Ng, the entire extracellular portion of neuroglian, and NgFnl-5, the five neuroglian Fn III domains, is also discussed.

The structure of NgFn 1,2 consists of two roughly cylindrical β-barrel structural motifs arranged in a head-to-tail fashion with the domains meeting at an angle of ~120, as defined by the cylinder axes. The folding topology of each domain is identical to that previously observed for single FnIII domains from tenascin and fibronectin. The domains of NgFn1,2 are related by an approximate two fold screw axis that is nearly parallel to the longest dimension of the fragment. Assuming this relative orientation is a general property of tandem FnIII repeats, the multiple tandem FnIII domains in neuroglian and other proteins are modeled as thin straight rods with two domain zig-zag repeats. When combined with the dimensions of pairs of tandem immunoglobulin-like domains from CD4 and CD2, this model suggests that neuroglian is a long narrow molecule (20 - 30 Å in diameter) that extends up to 370Å from the cell surface.

In photomicrographs, rotary shadowed Ng and NgFn1-5 appear to be highly flexible rod-like molecules. NgFn 1-5 is observed to bend in at least two positions and has a mean total length consistent with models generated from the NgFn1,2 structure. Ng molecules have up to four bends and a mean total length of 392 Å, consistent with a head-to-tail packing of neuroglian's C2-type domains.

Molecular genetics of the Drosophila eyes absent gene

The Drosophila compound eye has provided a genetic approach to understanding the specification of cell fates during differentiation. The eye is made up of some 750 repeated units or ommatidia, arranged in a lattice. The cellular composition of each ommatidium is identical. The arrangement of the lattice and the specification of cell fates in each ommatidium are thought to occur in development through cellular interactions with the local environment. Many mutations have been studied that disrupt the proper patterning and cell fating in the eye. The eyes absent (eya) mutation, the subject of this thesis, was chosen because of its eyeless phenotype. In eya mutants, eye progenitor cells undergo programmed cell death before the onset of patterning has occurred. The molecular genetic analysis of the gene is presented.

The eye arises from the larval eye-antennal imaginal disc. During the third larval instar, a wave of differentiation progresses across the disc, marked by a furrow. Anterior to the furrow, proliferating cells are found in apparent disarray. Posterior to the furrow, clusters of differentiating cells can be discerned, that correspond to the ommatidia of the adult eye. Analysis of an allelic series of eya mutants in comparison to wild type revealed the presence of a selection point: a wave of programmed cell death that normally precedes the furrow. In eya mutants, an excessive number of eye progenitor cells die at this selection point, suggesting the eya gene influences the distribution of cells between fates of death and differentiation.

In addition to its role in the eye, the eya gene has an embryonic function. The eye function is autonomous to the eye progenitor cells. Molecular maps of the eye and embryonic phenotypes are different. Therefore, the function of eya in the eye can be treated independently of the embryonic function. Cloning of the gene reveals two cDNA's that are identical except for the use of an alternatively-spliced 5' exon. The predicted protein products differ only at the N-termini. Sequence analysis shows these two proteins to be the first of their kind to be isolated. Trangenic studies using the two cDNA's show that either gene product is able to rescue the eye phenotype of eya mutants.

The eya gene exhibits interallelic complementation. This interaction is an example of an "allelic position effect": an interaction that depends on the relative position in the genome of the two alleles, which is thought to be mediated by chromosomal pairing. The interaction at eya is essentially identical to a phenomenon known as transvection, which is an allelic position effect that is sensitive to certain kinds of chromosomal rearrangements. A current model for the mechanism of transvection is the trans action of gene regulatory regions. The eya locus is particularly well suited for the study of transvection because the mutant phenotypes can be quantified by scoring the size of the eye.

The molecular genetic analysis of eya provides a system for uncovering mechanisms underlying differentiation, developmentally regulated programmed cell death, and gene regulation.

Flight dynamics in Drosophila through a dynamically-scaled robotic approach

Flies are particularly adept at balancing the competing demands of delay tolerance, performance, and robustness during flight, which invites thoughtful examination of their multimodal feedback architecture. This dissertation examines stabilization requirements for inner-loop feedback strategies in the flapping flight of Drosophila, the fruit fly, against the backdrop of sensorimotor transformations present in the animal. Flies have evolved multiple specializations to reduce sensorimotor latency, but sensory delay during flight is still significant on the timescale of body dynamics. I explored the effect of sensor delay on flight stability and performance for yaw turns using a dynamically-scaled robot equipped with a real-time feedback system that performed active turns in response to measured yaw torque. The results show a fundamental tradeoff between sensor delay and permissible feedback gain, and suggest that fast mechanosensory feedback provides a source of active damping that compliments that contributed by passive effects. Presented in the context of these findings, a control architecture whereby a haltere-mediated inner-loop proportional controller provides damping for slower visually-mediated feedback is consistent with tethered-flight measurements, free-flight observations, and engineering design principles. Additionally, I investigated how flies adjust stroke features to regulate and stabilize level forward flight. The results suggest that few changes to hovering kinematics are actually required to meet steady-state lift and thrust requirements at different flight speeds, and the primary driver of equilibrium velocity is the aerodynamic pitch moment. This finding is consistent with prior hypotheses and observations regarding the relationship between body pitch and flight speed in fruit flies. The results also show that the dynamics may be stabilized with additional pitch damping, but the magnitude of required damping increases with flight speed. I posit that differences in stroke deviation between the upstroke and downstroke might play a critical role in this stabilization. Fast mechanosensory feedback of the pitch rate could enable active damping, which would inherently exhibit gain scheduling with flight speed if pitch torque is regulated by adjusting stroke deviation. Such a control scheme would provide an elegant solution for flight stabilization across a wide range of flight speeds.

Complex behavior and perception in Drosophila emerges from iterative feedback-regulated reflexes

For a hungry fruit fly, locating and landing on a fermenting fruit where it can feed, find mates, and lay eggs, is an essential and difficult task requiring the integration of both olfactory and visual cues. Understanding how flies accomplish this will help provide a comprehensive ethological context for the expanding knowledge of their neural circuits involved in processing olfaction and vision, as well as inspire novel engineering solutions for control and estimation in computationally limited robotic applications. In this thesis, I use novel high throughput methods to develop a detailed overview of how flies track odor plumes, land, and regulate flight speed. Finally, I provide an example of how these insights can be applied to robotic applications to simplify complicated estimation problems. To localize an odor source, flies exhibit three iterative, reflex-driven behaviors. Upon encountering an attractive plume, flies increase their flight speed and turn upwind using visual cues. After losing the plume, flies begin zigzagging crosswind, again using visual cues to control their heading. After sensing an attractive odor, flies become more attracted to small visual features, which increases their chances of finding the plume source. Their changes in heading are largely controlled by open-loop maneuvers called saccades, which they direct towards and away from visual features. If a fly decides to land on an object, it begins to decelerate so as to maintain a stereotypical ratio of expansion to retinal size. Once they reach a stereotypical distance from the target, flies extend their legs in preparation for touchdown. Although it is unclear what cues they use to trigger this behavior, previous studies have indicated that it is likely under visual control. In Chapter 3, I use a nonlinear control theoretic analysis and robotic testbed to propose a novel and putative mechanism for how a fly might visually estimate distance by actively decelerating according to a visual control law. Throughout these behaviors, a common theme is the visual control of flight speed. Using genetic tools I show that the neuromodulator octopamine plays an important role in regulating flight speed, and propose a neural circuit for how this controller might be implemented in the flies brain. Two general biological and engineering principles are evident across my experiments: (1) complex behaviors, such as foraging, can emerge from the interactions of simple independent sensory-motor modules; (2) flies control their behavior in such a way that simplifies complex estimation problems.

Mechanisms of Transposable Element Repression by Piwi Proteins in the piRNA Pathway of Drosophila Germ Cells

The ability to reproduce is a defining characteristic of all living organisms. During reproduction, the integrity of genetic material transferred from one generation to the next is of utmost importance. Organisms have diverse strategies to ensure the fidelity of genomic information inherited between generations of individuals. In sexually reproducing animals, the piRNA pathway is an RNA-interference (RNAi) mechanism that protects the genomes of germ cells from the replication of ‘selfish’ genetic sequences called transposable elements (TE). When left unabated, the replication of TE sequences can cause gene disruption, double-stranded DNA breaks, and germ cell death that results in sterility of the organism. In Drosophila, the piRNA pathway is divided into a cytoplasmic and nuclear branch that involves the functions of three Piwi-clade Argonaute proteins—Piwi, Aubergine (Aub) and Argonaute-3 (Ago3)—which bind piwi-interacting RNA (piRNA) to form the effector complexes that represses deleterious TE sequences.

The work presented in this thesis examines the function and regulation of Piwi proteins in Drosophila germ cells. Chapter 1 presents an introduction to piRNA biogenesis and to the essential roles occupied by each Piwi protein in the repression of TE. We discuss the architecture and function of germ granules as the cellular compartments where much of the piRNA pathway operates. In Chapter 2, we present how Piwi in the nucleus co-transcriptionally targets genomic loci expressing TE sequences to direct the deposition of repressive chromatin marks. Chapter 3 examines the cytoplasmic function of the piRNA pathway, where we find that the protein Krimper coordinates Aub and Ago3 in the piRNA ping-pong pathway to adaptively target and destroy TE transcripts. Chapter 4 explores how interactions of Piwis with associated proteins are modulated by arginine methylation modifications. Lastly, in Chapter 5 I present evidence that the cytoplasmic branch of the piRNA pathway can potentially ‘cross-talk’ with the nuclear branch to transfer sequence information to better target and co-transcriptionally silence the genomic loci coding active TE sequences. Overall, the work presented in this thesis constitutes a part of the first steps in understanding the molecular mechanisms that protect germ cells from invasion by TE sequences.

I. Studies on the activation of Drosophila phenol oxidase. II. Studies on serum insulin in normal subjects and diabetics

Part I

Phenol oxidase is the enzyme responsible for hardening and pigmentation of the insect cuticle. In Drosophila, phenol oxidase is a latent enzyme. Enzyme activity is produced by the interaction of a number of protein components. A minimal activation scheme consisting of six protein components, designated Pre S, S activator, S, P. P' and Ʌ₁ is described. Quantitative assays have been developed for the S activator, S, P and P' proteins and these components have been partially purified. Experiments describing the interactions of the six components have been conducted and a model for the activation of phenol oxidase in a minimal system is proposed. Possible mechanisms of the reactions between the constituents of the activating system and potential regulatory mechanisms involved in phenol oxidase production and function are discussed.

Part II

A method has been developed for the partial purification of insulin from human serum. A procedure for the determination of the electrophoretic mobility of serum insulin on polyacrylamide gels is described. An electrophoretic analysis of insulin isolated from a normal subject is reported and in addition to a major band, the existence of a number of minor bands of immunoreactive insulin is described. A comparison of the electrophoretic patterns of insulin isolated from normal and diabetic subjects was carried out and indications that differences between them may occur are reported.