How do different animals grow to be different sizes? The answer is programmed in their genes, encoding for the myriad of protein-based signaling pathways that regulate growth of cells, tissues, and the entire body. Whereas many developmental signaling pathways have been found to control cell-fate, some pathways have been discovered that specifically regulate growth. How exactly these signaling pathways control growth is still not fully understood and there are likely still undiscovered components. In addition, knowing the normal mechanisms cells use to coordinate growth can help us understand how it becomes misregulated in human diseases such as cancer. The goal of this thesis was to identify novel genes controlling tissue growth and characterize their function, using the imaginal discs in Drosophila melanogaster as a model tissue.

In collaboration with a colleague in the lab, I identified Crumbs as a regulator of cell competition, a phenomenon that affects the survival of cells in growing tissues. Crumbs is a transmembrane protein known to control apical cell polarity and binds homophilically on adjacent cells. Cells that express higher levels of Crumbs die when in contact with cells that express lower levels of Crumbs. Therefore, Crumbs binding on adjacent cells may be a general mechanism for how cells "sense" competitive ability. I contributed by sequencing alleles of crumbs and finding that mutant cells behave as supercompetitors.

I identified the Fbxl7 gene as a novel regulator of Fat and Hippo signaling. The atypical cadherin Fat localizes in a planar polarized manner in cells of the wing imaginal disc, which prevents the membrane localization of an atypical myosin named Dachs. It was not previously known how Fat keeps Dachs off of the membrane. I showed that Fbxl7 associates with the Fat intracellular domain and prevents Dachs from localizing to membrane. Fbxl7 is conserved in humans and may be a novel tumor suppressor.

I identified a role for autophagy members in controlling tissue growth of the eye imaginal disc. Autophagy allows the recycling of cytoplasmic components during starvation via double membrane vesicles called autophagosomes. Atg2 and Atg18 form a complex and are critical for autophagosome formation. Loss of these genes in the eye imaginal disc disrupts formation of autophagosomes in developing photoreceptors and increases growth in the adult eye. Surprisingly, mutations in other autophagy genes do not produce the same phenotype, implying Atg2 and Atg18 may regulate growth through autophagy-independent mechanisms.

I developed a unique genetic system called "CoinFLP" to facilitate the identification additional growth genes, as well as characterize cell-cell interactions occurring with tissues. CoinFLP uses stochastic recombination in a developmentally controlled fashion to produce patches of mutant and wild-type tissue in imaginal discs at predictable ratios. I used CoinFLP to conduct a large-scale screen and identified geminin as a novel regulator of cell competition. In addition, I used CoinFLP to manipulate gene expression in two separate populations and directly visualize cell-cell contacts between them.

In conclusion, I have described three novel components regulating growth in Drosophila imaginal discs, and established a new genetic tool to identify and characterize additional regulators of tissue growth.

The survival and growth of cells can be influenced by the properties of adjacent cells. This reflects the ability of cells to communicate with each other and can result in either cooperation or competition for limited resources. Cell competition is an example of a situation in which cells exert a non-autonomous influence over the survival of their neighbors. The phenomenon was first discovered over forty years ago during studies of genetically mosaic Drosophila melanogaster. Clones of slow-growing, though viable, "Minute" cells were found to be eliminated from tissues in which they were surrounded by wild-type cells. It was later discovered that signaling between the two cell types at their clone borders causes the apoptosis and elimination of the Minute cells. Conversely, faster growing cells known as "supercompetitors", such as cells overexpressing the transcription factor, dMyc, can induce the elimination of wild-type cells. Thus, cell competition is a short- range interaction between cells with different growth rates in the same tissue and leads to the elimination of the slower-growing cells. The nature of this interaction is still unknown. Specifically, there are two important, unanswered questions regarding the mechanism of cell competition: 1) How do cells at clonal boundaries compare their properties and designate a "winner" and "loser"? 2) What are the signals that then induce the death of the designated losers?

Understanding the mechanism of cell competition is important from several standpoints. First, cell competition has been proposed to play a role in regulating organ size and tissue composition during normal development. Thus, understanding the mechanism of cell competition may advance our understanding of basic developmental biology. Second, many of the genes involved in cell competition are also misregulated in certain cancers. Thus, cell competition would be expected to occur at the borders between such tumors and their host tissue. A competitive advantage could make the tumor more malignant, while a competitive disadvantage might make the tumor more manageable. Therefore, a better understanding of the mechanisms involved in cell competition could also advance our understanding of tumor progression and improve our ability to treat tumors more effectively. Finally, a thorough understanding of cell competition and the ability to manipulate it could eventually be useful in therapies. For example, it may become possible in regenerative medicine to give grafts the ability to replace injured or dysfunctional tissue.

The goal of my thesis research has been to identify and characterize novel regulators of cell competition in order to gain a better understanding of the underlying mechanism. Accordingly, I devised an assay that would allow me to readily identify mutations that make cells into supercompetitors. The "supercompetitor assay" described here takes advantage of established mosaic analysis techniques in Drosophila. Small, marked clones of cells that are heterozygous for a mutation are created in an eye primordium that is primarily composed of homozygous-mutant cells. If the mutant cells are supercompetitors, the marked heterozygous clones are eliminated and cannot be recovered in the adult eye. Using this method I was able to test candidates from a collection of mutations in tumor suppressor genes from several different pathway that regulate growth. While mutations in components of some growth-regulating pathways, notably the Hippo pathway, caused cells to become supercompetitors, mutations in other pathways did not. Thus, only a subset of the pathways that regulate growth appear to be involved in cell competition.

The supercompetitor assay was also used to conduct an unbiased genetic screen for novel mutations that make cells supercompetitors. Among the mutations found in the screen were several alleles of crumbs (crb), a gene that regulates apicobasal polarity. This gene had no previously defined role in cell competition or growth. I found that while loss of Crb causes cells to become supercompetitors, overexpression of Crb causes cells to be eliminated from wild-type epithelia. Furthermore, as expected in instances of cell competition, high levels of apoptosis were observed preferentially at boundaries between wild-type and crb mutant cells, as well as at boundaries between Crb-overexpressing and wild-type cells. Thus, cells that express higher levels of Crb appear to be eliminated through a mechanism that resembles cell competition when they are near cells that express lower levels of Crb.

It is still unclear how cells compare their Crb levels. Cells may compare the levels of a molecule that is downstream of Crb. One candidate is the Hippo pathway, which has been repeatedly linked to cell competition. I found that crb mutant cells upregulate some of the transcriptional targets of the Hippo pathway suggesting that Crb impinges upon the Hippo signaling pathway. Alternatively, cells may directly compare their levels of Crb, as it is a transmembrane protein with a large extracellular domain of unknown function. Interestingly, the extracellular domain of Crb appears to be required to elicit some of the heterotypic interactions that I observe. For example, cells that express the intracellular domain of Crb alone are not eliminated. Furthermore, I see evidence of interactions between Crb molecules on adjacent cells that could be the basis of a direct comparison mechanism. Future work aimed at testing such models may yield important insights into a mechanism of cell competition.

During normal development, organ growth is tightly regulated such that a high degree of fidelity to both size and shape is maintained. Often this constancy is achieved by a robust arrest of growth once final size has been reached. In response to injury, however, some organs are able to engage in additional growth, thereby regenerating the lost or damaged tissue. Regenerative capacity varies widely across the animal kingdom. Some organisms can replace lost appendages, such as limbs or tails, restore damaged internal organs, such as the heart or liver, or even regenerate their entire body plan from a small fragment. Other animals, notably humans, exhibit markedly reduced regenerative capacity. Damaged tissue is replaced by a scar and organ function is accordingly lost or reduced. Why some organs and organisms possess impressive regenerative abilities, while others lack this seemingly beneficial trait remains largely unresolved. Improving our understanding of how regeneration occurs in the former may inform therapeutic efforts to improve regeneration and repair in damaged human tissues.

Tissue damage elicits local and systemic responses that determine the efficacy of the ensuing repair and regeneration. Locally, damaged or dying cells are removed and additional tissue growth or remodeling must be undertaken to restore tissue integrity. On the systemic level, injury can lead to the recruitment of immune cells and the activation of an inflammatory response. Long-range signaling from the wound site may also broadly impact animal physiology by affecting processes such as metabolism. Reciprocally, regeneration may be influenced by organismal factors, such as age and nutrient availability. Very little is known about the cellular and molecular mechanisms underpinning these various aspects of regeneration.

We have used the regenerative capacity of Drosophila larval imaginal discs to investigate the local proliferative and the systemic immune responses to tissue damage. Chapter 2 describes a system that we have developed to produce tissue damage and regenerative growth in imaginal discs. Using genetic mosaics we generate clones of cells that carry a temperature-sensitive cell-lethal mutation. We can thus selectively ablate these cells upon shifting to an elevated temperature and assess the regenerative capacity of the surviving sister clones. This system has allowed us to conduct a genetic screen for recessive mutations that selectively impair regenerative growth. Using traditional methods and whole-genome re-sequencing we have identified several of the mutations isolated in our screen. We have found that regenerative growth occurs, in part, by an acceleration of tissue growth and thus appears to sensitize cells to the loss of factors generally required for more rapid growth. In this way, regenerative growth may share features with some types of overgrowth. Indeed, two of our mutations are common targets for anti-cancer drugs.

In Chapter 3, we investigate the innate cellular immune response to tissue damage in Drosophila. Using ionizing radiation to generate widespread apoptosis in proliferating larval tissues, we find that broad changes in the blood cell population ensue. These include macrophage activation and the differentiation of lamellocytes, an otherwise cryptic cell fate. Using the power of Drosophila genetics, we consider a model in which the engulfment of apoptotic cells by activated macrophages allows these cells to stimulate a broader, inflammatory-like immune response.

A remarkable feat of animal development is the precision with which organs grow to a

consistent and characteristic size. The lengths of the right and left arm of a human, for

instance, match with an accuracy of about 0.2%, and the size of adult mouse brains varies

by only about 5%. How a developing organ|and the individual cells within|can know

when it is time to stop growing has long-fascinated biologists.

Reciprocal organ transplantation experiments from the 1920s were among the rst to

demonstrate that, for an individual organ, growth is subject to both extrinsic and intrinsic

programs. Working with two closely related but dierent-sized species of salamanders, the

faster-growing yet smaller-sized Ambystoma punctatum and the slower-growing yet largersized

Ambystoma tigrinum, Harrison found that the growth rate of a limb transplanted from

one species to the other matched that of the host, but the overall size of the transplanted

limb matched that of the donor. He concluded, therefore, that while some circulating factor,

perhaps a hormone, dictated the rate of limb growth, the organ's size overall was determined

by some intrinsic "growth potential."

Now, close to a century later, we know the identities of many evolutionarily-conserved

factors that are critical to achieving a properly-sized organ, but our understanding of what

ultimately determines an organ's size remains far from complete. Much of what we do know

about the mechanisms regulating tissue growth comes from studies using the model organism

Drosophila melanogaster, whose genetic tractability and high reproducibility of organ size

allow us to easily evaluate the eects of genetic perturbations on tissue growth.

In Chapter 1, I provide an overview of some of these growth-regulatory mechanisms

and describe how genetic studies using a well-characterized tissue model of growth|the

Drosophila wing imaginal disc|have helped to provide some clues into how these mechanisms

might operate. I will highlight two signaling pathways|the Hippo pathway and the JNK

pathway|that have very dierent functions but seem to both impact tissue growth in unique

ways. Activities of both of these pathways have been areas of interest for my dissertation

research.

The Hippo pathway regulates growth in most multicellular organisms and is altered

(either directly or indirectly) in many human cancers. Since the elucidation of the core

pathway approximately a decade ago, a key aim for researchers has been to identify the

upstream signals that link this pathway to external cues and understand how these signals

are sensed. An important upstream regulator of Hippo signaling is the transmembrane

protein Fat (Ft), though the mechanistic link between the core pathway and this upstream

factor is not fully clear. We identied the Drosophila F-box protein, Fbxl7, as a downstream

eector of Ft activity that is important for regulating the cellular distribution of Dachs, a

protein that mediates much of Ft's eect on tissue growth. This work, described in Chapter

2, provides a more complete understanding of the functional link between Ft signaling and

the Hippo pathway.

The JNK pathway is a well-conserved pathway involved in several morphogenetic processes

during development and commonly activated in response to stress. JNK signaling

promotes apoptosis, yet|quite paradoxically|is also important in promoting tissue proliferation

during regeneration and tumorigenesis. The role this pathway plays in regulating

growth during organ development is less established. We found that JNK signaling is active

at low levels in the developing wing imaginal disc and regulates an enhancer of the

gene bantam (ban), which encodes a microRNA that promotes growth. This ban enhancer

activity is opposed by the transcriptional co-repressor CtBP, which we characterize as a

negative growth-regulator in Drosophila. These ndings, described in Chapter 3, support a

role for JNK signaling in promoting tissue growth and suggest that CtBP may help to direct

this broadly-functioning signaling pathway towards specic eects on growth during normal

development.

During the course of these studies, I made the surprising observation that a common

genetic technique used in Drosophila, namely shRNA-mediated gene knockdown using the

Gal4/UAS system, can lead to an unexpected result: clonal expression of shRNAs causes

knockdown in cells that do not express Gal4. Chapter 4 describes this phenomenon, which we

term "shadow RNAi," and shows how this eect can lead to erroneous conclusions regarding

cell-autonomous vs. non-autonomous genetic functions. We outline how shadow RNAi can

be mitigated, as well as how it can be exploited as an eective lineage-tracing tool.

Taken together, my work oers important clues for solving the mysteries of organ size

control and explores new applications of standard genetic techniques, while potentially providing

useful insights for the development of novel tools in cancer therapy and regenerative

medicine.

The size of an animal is determined by the growth, proliferation, and survival of every cell in its body. Individual cells must respond to local cues and properly adjust their growth behavior to collectively generate tissues and organs of the proper size. How cells regulate their growth, and how they coordinate to regulate the size of the organs, are two fundamental questions which have long captivated developmental biologists, and which remain incompletely understood.

Much of what is already known about growth regulation has been learned from the study of mutants. Mutations affecting multiple signaling pathways can lead to overgrowth or undergrowth. However, the study of genetic mosaics has shown that outcome of growth-disrupting mutations depends not only on the properties of affected cells, but also on the interactions those cells have with their neighbors. For example, cells heterozygous for a class of mutations called Minute are slow growing but viable when all cells carry the mutation, but are eliminated in the presence of wild type neighbors. This phenomenon, termed cell competition, highlights how cells can non-autonomously influence growth and survival within a mosaic tissue.

I conducted a genetic screen in Drosophila to identify new genes which regulate growth in mosaics, focusing specifically on cell adhesion genes. The screen identified 9 genes which, when knocked down in clones in the wing imaginal disc, alter the size, shape, or number of clones which are recovered later in development. Of particular interest was the gene echinoid (ed): clones of cells lacking ed are small, round, and underrepresented, but ed mutations affecting large populations of cells can cause tissue overgrowth. My dissertation work is aimed at understanding the mechanistic basis of these seemingly contradictory phenotypes and to clarify the role of ed in regulating cellular and tissue growth. I demonstrate that cells lacking ed die by apoptosis at increased rates, at least in part because they express lower levels of the anti-apoptotic protein Diap1. This contributes to the elimination of ed-deficient clones in mosaic tissues. I also confirm that organs which are mostly or entirely deficient of ed overgrow because of a failure to terminate growth when the organ has reached its appropriate final size.

Ed has been previously shown to have a function in restricting growth via its interactions with the Hippo pathway, which regulates the expression of pro-growth and anti-apoptotic genes downstream of the transcription factor Yorkie (Yki). I show that this prevailing model cannot account for many of the phenotypes observed in ed-depleted tissues. Contrary to other reports, I found that many—but not all—Yki target genes are expressed at lower levels in ed-depleted cells. In mosaics, many of these same Yki targets are expressed at higher levels in the wild-type neighbors of ed clones. These observations are consistent with clonal elimination but inconsistent ed having a simple, growth-inhibitory effect via the Hippo pathway.

To understand why ed mutant organs overgrow, I screened for dominant modifiers of the overgrowth phenotype caused by Gal4-driven knockdown of ed in the wing. The top hit in this screen was upd2Δ, which suppressed overgrowth and enhanced cell death. While characterizing these modification phenotypes, I determined that they were ed-independent artifacts of a UAS construct present in the upd2Δ allele which interacts with Gal4. Although ultimately uninformative to the overgrowth phenotype associated with ed, this line of inquiry uncovered important information about the properties of the upd2Δ allele which may be of interest to researchers who use this allele, or other alleles generated in a similar manner.

Overall, this work advances our understanding of how growth is regulated by echinoid, highlights the context-dependence of ed’s effects on growth, and paints a more complicated picture of how ed interacts with the Hippo pathway than previously appreciated.

Epithelial layers of cells are a fundamental building block of organs and organisms. More than simple flat sheets, epithelia form the branches of our lungs, the villi of our guts, and the lining of every vein and artery in our bodies. Epithelia most often exist as part of complex, three-dimensional organs that include multiple epithelial layers or other tissue types. To properly function, epithelia must coordinate their growth and development with neighboring tissues.

The relative growth rates of multiple tissue layers can determine the shape of an organ, from the curve of the retina to the folds of the cerebral cortex. Understanding these important developmental processes requires understanding how epithelial growth regulates, and is regulated by, other tissues. This growth can occur not only by cell proliferation, but also by changes in cell shape or cross-sectional area – an important, but relatively understudied, contributor to epithelial growth.

In Chapter 1, I introduce the system in which I study epithelial growth regulation: the Drosophila wing imaginal disc (wing disc), which develops into the adult wing and part of the thorax. The wing disc is composed of two epithelial sheets which grow in synchrony, the disc proper and the peripodial epithelium. The disc proper grows primarily through cell proliferation; the peripodial epithelium grows primarily through cell shape changes. I briefly review the existing knowledge on how these two layers interact, and introduce signaling pathways that are especially salient to my studies of growth regulation.

I found that the two layers of the wing disc grow in synchrony through a “leader/follower” mechanism, in which growth of the disc proper sets the pace for growth of the peripodial epithelium. I determined that several signaling pathways that are critical for growth of the disc proper are surprisingly dispensable for growth of the peripodial epithelium. In contrast, the adaptive growth of the peripodial epithelium absolutely requires the Hippo pathway, a signaling pathway that can respond to physical forces like tissue stretching. I propose that the Hippo pathway can sense mechanical forces caused by growth of the disc proper, which allows the peripodial epithelium to grow in response. This work is described in Chapter 2.

The disc proper is extraordinarily well-studied as a model of growth; however, much remains unknown about how patterned gene expression within the peripodial epithelium drives its development and morphogenesis. In Chapter 3, I delve into an existing single-cell RNA sequencing dataset generated by previous members of the laboratory, Melanie Worley and Nicholas Everetts, to identify patterns of differential gene expression and candidate regulators of cell shape changes within the peripodial epithelium. I tested whether a number of these candidate genes were required for cell shape changes, but the key “shape factor” remains unknown. Also in Chapter 3, I investigate the extent of physical contacts between the layers, as these could be a critical avenue for inter-layer communication.

Overall, my work presents a paradigm for synchronizing growth between tissue layers, determines a role for the Hippo pathway in growth regulation during normal development, and contributes to foundational knowledge on the biology of an important model system.

Why some tissues can regenerate, while other cannot is a fundamental question for regenerative medicine. The ability to regenerate at least some tissues is widespread across diverse animals. Some, such as Hydra and planaria can regenerate the majority of their body, while urodele amphibians such as salamanders can regenerate the spinal cord in their tails and also a complete limb. Humans can regenerate tissues such as the liver, muscle and skin. There are diverse mechanisms of regenerations, from stem cell based replacement to the local dedifferentiation of tissues adjacent to the site of injury. A better understanding of the genetic regulation of regenerative growth could potentially impact the design of therapeutic strategies to promote regeneration.

The genetically tractable model organism Drosophila melanogaster has the ability to regenerate imaginal disc tissues during larval development and is therefore a powerful system to study the molecular mechanisms that regulate regeneration. We have developed a system to study imaginal-disc regeneration on large scale. The new system is an improvement over methods used in the past in Drosophila, because it is non-surgical and therefore more suited to large-scale screens. Tissue damage is induced in the larval primordium of the adult wing (the wing imaginal disc), in a spatially and temporally controlled manner. Drosophila imaginal discs are capable of regenerating lost tissue. Following damage caused by inducing cell death in the central portion of the wing pouch in the wing imaginal disc, cells surrounding the damaged area undergo proliferation to replace the lost cells, a type of regenerative growth referred to as epimorphic regeneration. Salamander limb regeneration is also a form of epimorphic regeneration. The region of proliferating cells exhibits changed expression patterns of genes that are involved in patterning and growth regulation. In addition, the cells that are undergoing regenerative growth cease to express some markers associated with committed cell fates. The morphogen and Wnt-homolog, Wingless, along with the transcription-factor Myc are expressed at higher levels in the regenerating tissue. Wnt proteins have been found to play a role in vertebrate regeneration and Myc protein is involved in cell proliferation. Increasing the amount of Myc leads to improved regeneration in the wing imaginal disc, while increasing levels of other growth-promoting genes had no effect. This suggests that Myc levels may be specifically important for regenerative growth. In addition, the extent of regeneration of the imaginal discs differs based on the age of the larva. As the animal matures, less regeneration is observed. Using this decrease in regenerative capacity, we have screened for dominant enhancers of regenerative growth. However, the screen was unsuccessful in clearly identifying a negative regulator of regenerative growth, but we did uncover mutants that slow down developmental timing and suppressor of Eiger-induced cell death.

In a different type of genetic screen for modification of regeneration, I identified a mutant that increases the frequency of transdetermination following ablation. Transdetermination is a phenomenon where cells that have been determined to generate one type of structure swich fates to adopt a different determined state. This mutant, which we have named Chameleon, dominantly increases the frequency of notum-to-wing transdetermination following Eiger-induced ablation. We have characterized the formation of the ectopic wing pouch and the data suggest that multiple cells are adopting new fates, possibly switching compartment identity and that local proliferation generates the new structure. The molecular lesion that is responsible for the Chameleon mutant phenotype remains unknown, however I have candidate mutations identified using whole genome sequencing.

In order to better understand different changes in fate that might occur during the processes of regeneration and transdetermination, we have developed a new method, TIE-DYE, which allows simultaneous tracing of multiple cell lineages as well as the genetic manipulation of a subset of these populations. The method creates seven uniquely marked categories of cells and we have used the method to estimate the number of founder cells that give rise to the wing imaginal disc during normal development and following compensatory growth caused by X-ray irradiation of the founder cells. We also demonstrate the utility of this system in studying the consequences of alterations in growth, patterning and cell-cell affinity.

How multicellular organisms establish and maintain a diversity of incredibly complex body plans is a fundamental question in development. Complex body plans require patterning of limbs, organs, and tissues along axes, such as the segmentation seen along the anteroposterior axis in insects and the vertebrate central nervous system. This diversity of forms is regulated by an intricate network of genetic interactions, and allows for a diversity of functions in the living world. To reproducibly and correctly create complex systems, the genes that control patterning, known as selector genes, must be tightly regulated so that they are only expressed in their correct locations and at the correct time. Normally these patterns of gene expression are stable and invariant. How this regulation occurs continues to be a major area of study in developmental biology.

Key genes that confer patterning were first discovered in the fruit fly Drosophila melanogaster, and the utility of these selector genes was later found to be conserved throughout metazoans. Flies remain an incredibly valuable tool to study the principles of how patterns form. In Chapter 1, I discuss some basic principles of the genetics of pattern formation by selector genes and how this is regulated at the epigenetic level, with a focus on patterning in Drosophila. I also cover how cancer coopts these processes and what the relationship between tumorous growth and developmental patterning means for the treatment of cancer. I focus on two pathways, the Hippo pathway and the Hedgehog (Hh) pathway, which have been extensively studied for their conserved roles in growth and patterning, and which are the focus of my dissertation research.

The Hippo pathway has been implicated as a driver of tissue growth in multiple different types of cancer. Over the last fifteen years, the core pathway as well as several targets of the Hippo pathway have been identified, but it remains unclear exactly how the pathway acts through these targets to control growth, as well as what other effects the pathway has on processes including patterning, cell-cell communication, and regulation of chromatin modification factors. Some aspects of Hippo pathway interactions with signaling pathways that control patterning have been studied, including between the Hippo pathway and Decapentaplegic (Dpp), which is the ortholog of vertebrate BMP2/4. However, interactions with other pathways aren’t fully understood. There has been some work done on the relationship between the Hippo pathway and the Hh pathway, but this mostly relates to Hh’s role upstream of the Hippo pathway and not on whether the Hippo pathway works through the Hh pathway. The Hh pathway has also been implicated in various types of cancer, but it is still not fully clear what regulates the inappropriate activation of the Hh pathway in cancer, nor what its role might be in tumor progression. My work identifies a role for Yorkie (Yki), the transcriptional co-activator of the Hippo pathway, in activating inappropriate Hh pathway signaling in tumorous tissue, which leads to downstream effects within the tumor and in surrounding cells.

Chapter 2 describes my work investigating the relationship between these two pathways and how it relates to growth and heterotypic interactions between tumor tissue and surrounding cells. I characterized the intriguing phenomenon of Yki inappropriately activating selector gene expression in tumor tissue, and found that this phenomenon relies on Yki’s activity through two targets, the microRNA bantam and the chromatin regulator taranis. I identified evidence that Yki-mediated activation of these targets leads to changes in chromatin modifiers that facilitate inappropriate patterning gene expression, and I also found evidence that inappropriate developmental signaling from Yki-expressing tumors has an effect on the growth of surrounding wild type cells. Chapter 3 discusses the significance of this work in the budding study of how tumor cells control and communicate with their environment, as well as how they cause epigenetic changes that broadly alter gene expression.

My work provides key information about how oncogenes can hijack normal developmental and genetic processes to facilitate the formation of cancer in otherwise healthy tissue. It contributes to our knowledge of how cancer cells behave, which can be used in future study on the biology of tumor tissue, as well as how to treat, categorize, and screen for it.