Immunity, behavior, and circadian regulation are important ways that animals maintain homeostasis. Defects in these physiologies often lead to disease or even death, yet many questions remain about how these physiologies are related. I explored the interactions between innate immunity, behavior, and circadian regulation by using Drosophila melanogaster, a convenient, genetically tractable model organism with both functionally and molecularly conserved innate immune and circadian clock systems. In the first chapter, I show that feeding, a circadian-regulated behavior, increases immunity to a sepsis-like infection. In the second, I present evidence suggesting that aging-related changes in immunity may be linked to circadian defects. Finally, I use a novel automated method to demonstrate that reduced grooming is a conserved sickness behavior in Drosophila. The feeding project ultimately showed that mutating TORC2 components could increase the host’s ability to kill and clear a bacterial infection, as well as survive the pathogenic effects of infection. Therefore we have identified a possible drug target to create host-based therapies for sepsis patients. We also have established Drosophila as a model system for studying a conserved sickness behavior: reduced grooming. This experimental paradigm will allow researchers to isolate mutants that do not show reduced grooming, and investigate whether this sickness behavior is adaptive or not.

Color vision is widespread in the animal kingdom, and describes the ability to discriminate between objects purely based on the wavelengths that they reflect. Experiments across many species have isolated wavelength comparison in the brain as a computation underlying color vision. This comparison takes place in color opponent neurons, which respond with opposite polarity to wavelengths in different parts of the spectrum. In this work, I explore color opponency in the genetically tractable organism Drosophila melanogaster, where these circuits have only just begun to be described. Using two-photon calcium imaging, I measure the spectral tuning of photoreceptors in the fruit fly and identify circuit mechanisms that give rise to opponency. I find two pathways: an insect-specific pathway that compares wavelengths at each point in space, and a horizontal-cell-mediated pathway similar to that found in mammals. The horizontal-cell-mediated pathway enables additional spectral comparisons through lateral inhibition, expanding the range of chromatic encoding in the fly. Together, these two pathways enable efficient decorrelation and dimensionality reduction of photoreceptor signals while retaining maximal chromatic information. This dual mechanism combines motifs of both an insect-specific visual circuit and an evolutionarily convergent circuit architecture, endowing flies with the ability to extract chromatic information at distinct spatial resolutions.

Sox proteins all contain a single ~70 amino acid High Mobility Group (HMG) DNA-binding domain with strong homology to that of Sry, the mammalian testisdetermining factor. In Drosophila melanogaster, there are four closely related members of the B group, Dichaete (D), Sox Neuro (Sox N), Sox 21a, and Sox 21b that each exhibit ~90% sequence identity within the HMG domain.The previous study has shown that Dichaete plays a major role in embryonic nervous system development and is expressed in several clusters of neurons in the brain, including intermingled olfactory LNs and central-complex neurons strongly expressed in local interneuron of the olfactory system. However, little is known about the possible expression and functions of the related group B Sox genes in the larval and adult brain. In particular, it is unclear if Sox N may function along with Dichaete in controlling the development or physiology of the adult olfactory system. Our data suggests Sox N potential role in the elaboration of the olfactory circuit formation. / Includes bibliography. / Dissertation (Ph.D.)--Florida Atlantic University, 2017. / FAU Electronic Theses and Dissertations Collection

Models of sensory processing have historically abstracted underlying biological circuits, due to unknown connectivity and/or complexity. In contrast, the use of tractable and anatomically well-characterized model organisms such as the fruit fly Drosophila melanogaster allows us to utilize biological constraints in models of sensory processing to understand underlying circuit mechanisms and make more accurate predictions. This approach has been used to dissect motion vision circuits, but investigations into color vision - a salient visual feature for many animals - have been limited. Here, we investigate the circuit mechanisms of the early color circuit of the fruit fly and assess its information processing capabilities. Using in vivo two-photon calcium imaging and genetic manipulations, we measure the chromatic tuning properties of photoreceptor axons and their primary targets in the medulla neuropil. At the level of photoreceptor axons, we show that opponent processes are the result of a dual mechanism - a direct pathway specific to insect physiology and an indirect pathway found across the animal kingdom. Both pathways are necessary to decorrelate incoming signals and efficiently represent chromatic information. We built an anatomically constrained model that is able to quantitatively reproduce these color opponent responses without fitting synaptic weights. Instead, we used electron-microscopy-derived synaptic count, an anatomically defined measure, as a proxy for synaptic weight, thereby linking structure to function. Downstream of photoreceptors, we find that neurons shift their tuning and become highly selective for particular directions in color space - similar to “hue-selective” neurons in primate cortex. To achieve this selectivity, these neurons require input from all types of photoreceptors and an interneuron that determines the neuron's preferred chromatic direction. We extended our anatomically constrained model to incorporate these downstream neurons and are able to predict their responses, qualitatively and quantitatively.In summary, the detailed reconstruction of the fly circuit anatomy predicts the mechanisms of multiple stages of color information processing and allows us to infer functional roles for each part of the circuit. The circuit motifs, we uncover, are shared across species and hint at convergent mechanisms that underlie the transformation from an opponent neural code to a hue selective code.

The brain is incredibly complex, with billions of diverse cells performing a variety of necessary functions. It is fascinating then, that a small group of progenitor cells are capable of generating all of the neural cell types. During development, robust and stable expression of identity factors is necessary for diverse cell fate determination, but progenitor cells must also be flexible to quickly change expression programs in response to developmental cues. The metazoan genome is non-randomly organized, and this organization is thought to underlie cell type specific gene expression programs. However, the process by which genome organization is stabilized, and then reorganized, is not well-understood. A Drosophila neuroblast nuclear factor, Distal antenna (Dan), was previously identified as a key regulator of this process. Downregulation of Dan is necessary for a developmentally-timed genome reorganization in neural progenitors that terminates their competence to specify early-born cell types. Maintaining Dan expression prevents genome reorganization, extending the early competence window, and implicating Dan in the stabilization of the early competence state. The mechanisms through which Dan functions to stabilize the genome architecture is not known. In this work, we take advantage of the Drosophila embryonic ventral nerve cord model system to study Dan and its role in regulating neuroblast competence. We find that Dan, a DNA- binding protein that localizes throughout the nucleus in distinct puncta, coalesces into large, liquid condensates that relocalize to the nuclear periphery when DNA-binding is inhibited. The size of the droplets increases as impairment to the DNA-binding domain increases, suggesting that Da normally exists in a competitive tug-of-war between genome binding and protein condensation at the nuclear periphery. We further find that while Dan is a highly intrinsically disordered protein, formation of the large droplets requires a LARKS domain – a glycine-rich, structural motif that forms kinked beta-sheets associated with labile interactions that underlie phase-separation. In embryos, Dan’s ability to maintain neural progenitor early competence requires both its Pipsqueak motif DNA-binding domain and phase separation properties. Finally, we find that Dan interacts with proteins of the nuclear pore complex. In particular, we find that Elys, a core scaffold protein which has been shown to bind DNA and regulate nuclear architecture, is required for termination of the early competence window. Together, we propose a mechanism by which a single protein can exert opposing forces between DNA binding and self- association to organize progenitor genome architecture and regulate neuronal diversification.