A quantitative analysis of growth control in the Drosophila eye disc

Avalanches during epithelial tissue growth; Uniform Growth and a drosophila eye disc model

Epithelial tissues constitute an exotic type of active matter with non-linear properties reminiscent of amorphous materials. In the context of a proliferating epithelium, modeled by the quasistatic vertex model, we identify novel discrete tissue scale rearrangements, i.e. cellular rearrangement avalanches, which are a form of collective cell movement. During the avalanches, the vast majority of cells retain their neighbors, and the resulting cellular trajectories are radial in the periphery, a vortex in the core. After the onset of these avalanches, the epithelial area grows discontinuously. The avalanches are found to be stochastic, and their strength is correlated with the density of cells in the tissue. Overall, avalanches redistribute accumulated local spatial pressure along the tissue. Furthermore, the distribution of avalanche magnitudes is found to obey a power law, with an exponent consistent with sheer induced avalanches in amorphous materials. To understand the role of avalanches in organ development, we simulate epithelial growth of the Drosophila eye disc during the third instar using a computational model, which includes both chemical and mechanistic signaling. During the third instar, the morphogenetic furrow (MF), a ~10 cell wide wave of apical area constriction propagates through the epithelium. These simulations are used to understand the details of the growth process, the effect of the MF on the growth dynamics on the tissue scale, and to make predictions for experimental observations. The avalanches are found to depend on the strength of the apical constriction of cells in the MF, with a stronger apical constriction leading to less frequent and more pronounced avalanches. The results herein highlight the dependence of simulated tissue growth dynamics on relaxation timescales, and serve as a guide for in vitro experiments.

Quantitative Relationships Between Growth, Differentiation, and Shape That Control Drosophila Eye Development and Its Variation

The size of organs is critical for their function and often a defining trait of a species. Still, how organs reach a species-specific size or how this size varies during evolution are problems not yet solved. Here, we have investigated the conditions that ensure growth termination, variation of final size and the stability of the process for developmental systems that grow and differentiate simultaneously. Specifically, we present a theoretical model for the development of the Drosophila eye, a system where a wave of differentiation sweeps across a growing primordium. This model, which describes the system in a simplified form, predicts universal relationships linking final eye size and developmental time to a single parameter which integrates genetically-controlled variables, the rates of cell proliferation and differentiation, with geometrical factors. We find that the predictions of the theoretical model show good agreement with previously published experimental results. We also develop a new computational model that recapitulates the process more realistically and find concordance between this model and theory as well, but only when the primordium is circular. However, when the primordium is elliptical both models show discrepancies. We explain this difference by the mechanical interactions between cells, an aspect that is not included in the theoretical model. Globally, our work defines the quantitative relationships between rates of growth and differentiation and organ primordium size that ensure growth termination (and, thereby, specify final eye size) and determine the duration of the process; identifies geometrical dependencies of both size and developmental time; and uncovers potential instabilities of the system which might constraint developmental strategies to evolve eyes of different size.

The Drosophila Larval Locomotor Circuit Provides a Model to Understand Neural Circuit Development and Function

It is difficult to answer important questions in neuroscience, such as: "how do neural circuits generate behaviour?," because research is limited by the complexity and inaccessibility of the mammalian nervous system. Invertebrate model organisms offer simpler networks that are easier to manipulate. As a result, much of what we know about the development of neural circuits is derived from work in crustaceans, nematode worms and arguably most of all, the fruit fly, Drosophila melanogaster. This review aims to demonstrate the utility of the Drosophila larval locomotor network as a model circuit, to those who do not usually use the fly in their work. This utility is explored first by discussion of the relatively complete connectome associated with one identified interneuron of the locomotor circuit, A27h, and relating it to similar circuits in mammals. Next, it is developed by examining its application to study two important areas of neuroscience research: critical periods of development and interindividual variability in neural circuits. In summary, this article highlights the potential to use the larval locomotor network as a "generic" model circuit, to provide insight into mammalian circuit development and function.

Interplay between sex determination cascade and major signaling pathways during Drosophila eye development: Perspectives for future research

Understanding molecular mechanisms of sexually dimorphic organ growth is a fundamental problem of developmental biology. Recent quantitative studies showed that the Drosophila compound eye is a convenient model to study the determination of the final organ size. In Drosophila, females have larger eyes than males and this is evident even after correction for the larger body size. Moreover, female eyes include more ommatidia (photosensitive units) than male eyes and this difference is specified at the third larval instar in the eye primordia called eye imaginal discs. This may result in different visual capabilities between the two sexes and have behavioral consequences. Despite growing evidence on the genetic bases of eye size variation between different Drosophila species and strains, mechanisms responsible for within-species sexual dimorphism still remain elusive. Here, we discuss a presumptive crosstalk between the sex determination cascade and major signaling pathways during dimorphic eye development. Male- and female-specific isoforms of Doublesex (Dsx) protein are known to control sex-specific differentiation in the somatic tissues. However, no data on Dsx function during eye disc growth and patterning are currently available. Remarkably, Sex lethal (Sxl), the sex determination switch protein, was shown to directly affect Hedgehog (Hh) and Notch (N) signaling in the Drosophila wing disc. The similarity of signaling pathways involved in the wing and eye disc growth suggests that Sxl might be integrated into regulation of eye development. Dsx role in the eye disc requires further investigation. We discuss currently available data on sex-biased gene expression in the Drosophila eye and highlight perspectives for future studies.

Short communication: Context matters: Adult size is contingent on embryonic temperature in Drosophila melanogaster

Temperature is a critical factor in shaping ectothermic development. Developmental temperature may constrain, alter, or redirect phenotypes expressed later in life. Recent studies have begun to analyze the consequences of mismatches between developmental and adult environments. Few studies analyze the consequences environmental mismatches during development yield on adult phenotypes. The aim of this study was to determine how mismatched temperatures during development affect adult size in Drosophila melanogaster. We employed a full factorial design in which eggs were incubated for 24 h in one of two temperature treatments (18 °C or 28 °C) with half of the flies subsequently being switched to the opposite temperature treatment for the remainder of development. We measured body size shortly after eclosure. We found that variation in size after eclosure was contingent upon the temperature during the embryo stage. Flies reared initially in 18 °C eclosed larger regardless of the subsequent temperature until eclsoure. Flies reared initially in 28 °C, however, eclosed smaller only if they remained in 28 °C until eclosure. The degree of plasticity in size was therefore contingent upon temperature during the embryo stage. We discuss the implications of employing full factorial approaches to consider the full context of phenotypic outcomes in light of changing developmental environments.

The evolution and development of eye size in flies

The compound eyes of flies exhibit striking variation in size, which has contributed to the adaptation of these animals to different habitats and their evolution of specialist behaviors. These differences in size are caused by differences in the number and/or size of ommatidia, which are specified during the development of the retinal field in the eye imaginal disc. While the genes and developmental mechanisms that regulate the formation of compound eyes are understood in great detail in the fruit fly Drosophila melanogaster, we know very little about the genetic changes and mechanistic alterations that lead to natural variation in ommatidia number and/or size, and thus overall eye size, within and between fly species. Understanding the genetic and developmental bases for this natural variation in eye size not only has great potential to help us understand adaptations in fly vision but also determine how eye size and organ size more generally are regulated. Here we explore the genetic and developmental mechanisms that could underlie natural differences in compound eye size within and among fly species based on our knowledge of eye development in D. melanogaster and the few cases where the causative genes and mechanisms have already been identified. We suggest that the fly eye provides an evolutionary and developmental framework to better understand the regulation and diversification of this crucial sensory organ globally at a systems level as well as the gene regulatory networks and mechanisms acting at the tissue, cellular and molecular levels. This article is categorized under: Establishment of Spatial and Temporal Patterns > Regulation of Size, Proportion, and Timing Invertebrate Organogenesis > Flies Comparative Development and Evolution > Regulation of Organ Diversity.

A neurodevelopmental origin of behavioral individuality in the Drosophila visual system

The genome versus experience dichotomy has dominated understanding of behavioral individuality. By contrast, the role of nonheritable noise during brain development in behavioral variation is understudied. Using Drosophila melanogaster, we demonstrate a link between stochastic variation in brain wiring and behavioral individuality. A visual system circuit called the dorsal cluster neurons (DCN) shows nonheritable, interindividual variation in right/left wiring asymmetry and controls object orientation in freely walking flies. We show that DCN wiring asymmetry instructs an individual’s object responses: The greater the asymmetry, the better the individual orients toward a visual object. Silencing DCNs abolishes correlations between anatomy and behavior, whereas inducing DCN asymmetry suffices to improve object responses.

Genetic interactions between Protein Kinase D and Lobe mutants during eye development of Drosophila melanogaster

Background

In Drosophila, the development of the fly eye involves the activity of several, interconnected pathways that first define the presumptive eye field within the eye anlagen, followed by establishment of the dorso-ventral boundary, and the regulation of growth and apoptosis. In Lobe (L) mutant flies, parts of the eye or even the complete eye are absent because the eye field has not been properly defined. Manifold genetic interactions indicate that L influences the activity of several signalling pathways, resulting in a conversion of eye tissue into epidermis, and in the induction of apoptosis. As information on the molecular nature of the L mutation is lacking, the underlying molecular mechanisms are still an enigma.

Results

We have identified Protein Kinase D (PKD) as a strong modifier of the L mutant phenotype. PKD belongs to the PKC/CAMK class of Ser/Thr kinases that have been involved in diverse cellular processes including stress resistance and growth. Despite the many roles of PKD, Drosophila PKD null mutants are without apparent phenotype apart from sensitivity to oxidative stress. Here we report an involvement of PKD in eye development in the sensitized genetic background of Lobe. Absence of PKD strongly enhanced the dominant eye defects of heterozygous L² flies, and decreased their viability. Moreover, eye-specific overexpression of an activated isoform of PKD considerably ameliorated the dominant L² phenotype. This genetic interaction was not allele specific but similarly seen with three additional, weaker L alleles (L¹, L⁵, L^G), demonstrating its specificity.

Conclusions

We propose that PKD-mediated phosphorylation is involved in underlying processes causing the L phenotype, i.e. in the regulation of growth, the epidermal transformation of eye tissue and apoptosis, respectively.

Control of size, fate and time by the Hh morphogen in the eyes of flies

Molecules of the hedgehog (hh) family are involved in the specification and patterning of eyes in vertebrates and invertebrates. These organs, though, are of very different sizes, raising the question of how Hh molecules operate at such different scales. In this paper we discuss the strategies used by Hh to control the development of the two eye types in Drosophila: the large compound eye and the small ocellus. We first describe the distinct ways in which these two eyes develop and the evidence for the key role played by Hh in both; then we consider the potential for variation in the range of action of a "typical" morphogen and measure this range ("characteristic length") for Hh in different organs, including the compound eye and the ocellus. Finally, we describe how different feedback mechanisms are used to extend the Hh range of action to pattern the large and even the small eye. In the ocellus, the basic Hh signaling pathway adds to its dynamics the attenuation of its receptor as cell differentiate. This sole regulatory change can result in the decoding of the Hh gradient by receiving cells as a wave of constant speed. Therefore, in the fly ocellus, the Hh morphogen adds to its spatial patterning role a novel one: patterning along a time axis.

Dynamic Hh signalling can generate temporal information during tissue patterning

The differentiation of tissues and organs requires that cells exchange information in space and time. Spatial information is often conveyed by morphogens: molecules that disperse across receiving cells to generate signalling gradients. Cells translate such concentration gradients into space-dependent patterns of gene expression and cellular behaviour. But could morphogen gradients also convey developmental time? Here, by investigating the developmental role of Hh on a component of the Drosophila visual system, the ocellar retina, we have discovered that ocellar cells use the non-linear gradient of Hh as a temporal cue, collectively performing the biological equivalent of a mathematical logarithmic transformation. In this way, a morphogen diffusing from a non-moving source is decoded as a wave of differentiating photoreceptors that travels at constant speed throughout the retinal epithelium.

Growth and size control during development

The size and shape of organs are characteristic for each species. Even when organisms develop to different sizes due to varying environmental conditions, such as nutrition, organ size follows species-specific rules of proportionality to the rest of the body, a phenomenon referred to as allometry. Therefore, for a given environment, organs stop growth at a predictable size set by the species's genotype. How do organs stop growth? How can related species give rise to organs of strikingly different size? No definitive answer has been given to date. One of the major models for the studies of growth termination is the vinegar fly Drosophila melanogaster. Therefore, this review will focus mostly on work carried out in Drosophila to try to tease apart potential mechanisms and identify routes for further investigation. One general rule, found across the animal kingdom, is that the rate of growth declines with developmental time. Therefore, answers to the problem of growth termination should explain this seemingly universal fact. In addition, growth termination is intimately related to the problems of robustness (i.e. precision) and plasticity in organ size, symmetric and asymmetric organ development, and of how the ‘target’ size depends on extrinsic, environmental factors.

Computer modeling in developmental biology: growing today, essential tomorrow

D'Arcy Thompson was a true pioneer, applying mathematical concepts and analyses to the question of morphogenesis over 100 years ago. The centenary of his famous book, On Growth and Form, is therefore a great occasion on which to review the types of computer modeling now being pursued to understand the development of organs and organisms. Here, I present some of the latest modeling projects in the field, covering a wide range of developmental biology concepts, from molecular patterning to tissue morphogenesis. Rather than classifying them according to scientific question, or scale of problem, I focus instead on the different ways that modeling contributes to the scientific process and discuss the likely future of modeling in developmental biology.

The fly eye: Through the looking glass

The developing eye-antennal disc of Drosophila melanogaster has been studied for more than a century, and it has been used as a model system to study diverse processes, such as tissue specification, organ growth, programmed cell death, compartment boundaries, pattern formation, cell fate specification, and planar cell polarity. The findings that have come out of these studies have informed our understanding of basic developmental processes as well as human disease. For example, the isolation of a white-eyed fly ultimately led to a greater appreciation of the role that sex chromosomes play in development, sex determination, and sex linked genetic disorders. Similarly, the discovery of the Sevenless receptor tyrosine kinase pathway not only revealed how the fate of the R7 photoreceptor is selected but it also helped our understanding of how disruptions in similar biochemical pathways result in tumorigenesis and cancer onset. In this article, I will discuss some underappreciated areas of fly eye development that are fertile for investigation and are ripe for producing exciting new breakthroughs. The topics covered here include organ shape, growth control, inductive signaling, and right-left symmetry. Developmental Dynamics 247:111-123, 2018. © 2017 Wiley Periodicals, Inc.

Growth control in the Drosophila eye disc by the cytokine Unpaired

A fundamental question in developmental biology is how organ size is controlled. We have previously shown that the area growth rate in the Drosophila eye primordium declines inversely proportionally to the increase in its area. How the observed reduction in the growth rate is achieved is unknown. Here, we explore the dilution of the cytokine Unpaired (Upd) as a possible candidate mechanism. In the developing eye, upd expression is transient, ceasing at the time when the morphogenetic furrow first emerges. We confirm experimentally that the diffusion and stability of the JAK/STAT ligand Upd are sufficient to control eye disc growth via a dilution mechanism. We further show that sequestration of Upd by ectopic expression of an inactive form of the receptor Domeless (Dome) results in a substantially lower growth rate, but the area growth rate still declines inversely proportionally to the area increase. This growth rate-to-area relationship is no longer observed when Upd dilution is prevented by the continuous, ectopic expression of Upd. We conclude that a mechanism based on the dilution of the growth modulator Upd can explain how growth termination is controlled in the eye disc.

An Unbiased Analysis of Candidate Mechanisms for the Regulation of Drosophila Wing Disc Growth

The control of organ size presents a fundamental open problem in biology. A declining growth rate is observed in all studied higher animals, and the growth limiting mechanism may therefore be evolutionary conserved. Most studies of organ growth control have been carried out in Drosophila imaginal discs. We have previously shown that the area growth rate in the Drosophila eye primordium declines inversely proportional to the increase in its area, which is consistent with a dilution mechanism for growth control. Here, we show that a dilution mechanism cannot explain growth control in the Drosophila wing disc. We computationally evaluate a range of alternative candidate mechanisms and show that the experimental data can be best explained by a biphasic growth law. However, also logistic growth and an exponentially declining growth rate fit the data very well. The three growth laws correspond to fundamentally different growth mechanisms that we discuss. Since, as we show, a fit to the available experimental growth kinetics is insufficient to define the underlying mechanism of growth control, future experimental studies must focus on the molecular mechanisms to define the mechanism of growth control.

A Model of the Spatio-temporal Dynamics of Drosophila Eye Disc Development

Patterning and growth are linked during early development and have to be tightly controlled to result in a functional tissue or organ. During the development of the Drosophila eye, this linkage is particularly clear: the growth of the eye primordium mainly results from proliferating cells ahead of the morphogenetic furrow (MF), a moving signaling wave that sweeps across the tissue from the posterior to the anterior side, that induces proliferating cells anterior to it to differentiate and become cell cycle quiescent in its wake. Therefore, final eye disc size depends on the proliferation rate of undifferentiated cells and on the speed with which the MF sweeps across the eye disc. We developed a spatio-temporal model of the growing eye disc based on the regulatory interactions controlled by the signals Decapentaplegic (Dpp), Hedgehog (Hh) and the transcription factor Homothorax (Hth) and explored how the signaling patterns affect the movement of the MF and impact on eye disc growth. We used published and new quantitative data to parameterize the model. In particular, two crucial parameter values, the degradation rate of Hth and the diffusion coefficient of Hh, were measured. The model is able to reproduce the linear movement of the MF and the termination of growth of the primordium. We further show that the model can explain several mutant phenotypes, but fails to reproduce the previously observed scaling of the Dpp gradient in the anterior compartment.