Systemic signalling and local effectors in developmental stability, body symmetry, and size

Developmental instability, body mass, and reproduction predict immunological response in short-tailed bats

Developmental instability (DI) is a phenomenon whereby organisms are unable to buffer developmental disturbances, resulting in asymmetric variation of paired traits. Previous research has demonstrated a negative relationship between DI, measured as forearm asymmetry, and survival in the bat Carollia perspicillata. This study aims to test the hypothesis that individuals with higher DI exhibit a lower immune response. We measured a delayed-type hypersensitivity to the antigen phytohemagglutinin (PHA) on 74 males and 65 females of C. perspicillata before and after the breeding season (BS). Linear models were used to predict the immunological response based on body mass (BM), forearm asymmetry, sex, BS, and testicle length. The best-fitting model accounted for 29% of the variation in immune response and included asymmetry, BM, sex, and BS as predictors. The immune response was negatively associated with asymmetry and testicle length in males but positively related to asymmetry in females. Both sexes showed a reduced immune response in the late BS. Additionally, the association between immune response and BM changed direction seasonally, with heavier individuals showing weaker responses early in the BS and stronger responses later. Individual variation in male immunity was predicted by individual attributes, whereas variation in immune response in females was mostly seasonal. Our results support the link between DI, survival, and immune response in short-tailed bats, and suggest that the immunological component measured by the PHA response may be under finer selection in males due to its stronger correlation with individual traits.

A mathematical model of development shows that cell division, short-range signaling and self-activating gene networks increase developmental noise while long-range signaling and epithelial stiffness reduce it

The position of cells during development is constantly subject to noise, i.e. cell-level noise. We do not yet fully understand how cell-level noise coming from processes such as cell division or movement leads to morphological noise, i.e. morphological differences between genetically identical individuals developing in the same environment. To address this question we constructed a large ensemble of random genetic networks regulating cell behaviors (contraction, adhesion, etc.) and cell signaling. We simulated them with a general computational model of development, EmbryoMaker. We identified and studied the dynamics, under cell-level noise, of those networks that lead to the development of animal-like morphologies from simple blastula-like initial conditions. We found that growth by cell division is a major contributor to morphological noise. Self-activating gene network loops also amplified cell-level noise into morphological noise while long-range signaling and epithelial stiffness tended to reduce morphological noise.

Self-organization underlies developmental robustness in plants

Development is a self-organized process that builds on cells and their interactions. Cells are heterogeneous in gene expression, growth, and division; yet how development is robust despite such heterogeneity is a fascinating question. Here, we review recent progress on this topic, highlighting how developmental robustness is achieved through self-organization. We will first discuss sources of heterogeneity, including stochastic gene expression, heterogeneity in growth rate and direction, and heterogeneity in division rate and precision. We then discuss cellular mechanisms that buffer against such noise, including Paf1C- and miRNA-mediated denoising, spatiotemporal growth averaging and compensation, mechanisms to improve cell division precision, and coordination of growth rate and developmental timing between different parts of an organ. We also discuss cases where such heterogeneity is not buffered but utilized for development. Finally, we highlight potential directions for future studies of noise and developmental robustness.

Mother–Fetus Immune Cross-Talk Coordinates “Extrinsic”/“Intrinsic” Embryo Gene Expression Noise and Growth Stability

Developmental instability (DI) is thought to be inversely related to a capacity of an organism to buffer its development against random genetic and environmental perturbations. DI is represented by a trait’s inter- and intra-individual variabilities. The inter-individual variability (inversely referred to as canalization) indicates the capability of organisms to reproduce a trait from individual to individual. The intra-individual variability reflects an organism’s capability to stabilize a trait internally under the same conditions, and, for symmetric traits, it is expressed as fluctuating asymmetry (FA). When representing a trait as a random variable conditioned on environmental fluctuations, it is clear that, in statistical terms, the DI partitions into “extrinsic” (canalization) and “intrinsic” (FA) components of a trait’s variance/noise. We established a simple statistical framework to dissect both parts of a symmetric trait variance/noise using a PCA (principal component analysis) projection of the left/right measurements on eigenvectors followed by GAMLSS (generalized additive models for location scale and shape) modeling of eigenvalues. The first eigenvalue represents “extrinsic” and the second—“intrinsic” DI components. We applied this framework to investigate the impact of mother–fetus major histocompatibility complex (MHC)-mediated immune cross-talk on gene expression noise and developmental stability. We showed that “intrinsic” gene noise for the entire transcriptional landscape could be estimated from a small subset of randomly selected genes. Using a diagnostic set of genes, we found that allogeneic MHC combinations tended to decrease “extrinsic” and “intrinsic” gene noise in C57BL/6J embryos developing in the surrogate NOD-SCID and BALB/c mothers. The “intrinsic” gene noise was negatively correlated with growth (embryonic mass) and the levels of placental growth factor (PLGF), but not vascular endothelial growth factor (VEGF). However, it was positively associated with phenotypic growth instability and noise in PLGF. In mammals, the mother–fetus MHC interaction plays a significant role in development, contributing to the fitness of the offspring. Our results demonstrate that a positive impact of distant MHC combinations on embryonic growth could be mediated by the reduction of “intrinsic” gene noise followed by the developmental stabilization of growth.

Evaluation of Middle Cerebral Artery Symmetry: A Pilot Study for Clinical Application in Mechanical Thrombectomy

BACKGROUND

Endovascular mechanical thrombectomy (MT) has now evolved to become the standard treatment for acute ischemic stroke due to large vessel occlusion. Arterial perforation is a potential complication of MT, and the risk of this event during blind crossing the occlusion site has been elucidated. The intracranial arterial system shows morphological structural symmetry, so we investigated the utility of the bilateral symmetry of the proximal middle cerebral artery (MCA) as a preprocedural evaluation to predict hidden running course distal to the thrombus.

METHODS

This study retrospectively analyzed 191 consecutive patients (mean age, 67.5 ± 15.5 years; 100 women) who underwent time-of-flight-magnetic resonance angiography in our institution. Four landmarks of the MCA were assessed: division pattern, early branching pattern, length, and course pattern. Each geometric property was compared between cerebral hemispheres. Frequencies of symmetry and symmetry breaking were assessed.

RESULTS

In 91% (bifurcation type, 87%; trifurcation type, 4%), branching patterns of the left and right M1 were symmetrical. Early frontal and/or temporal branches were observed in 31%, and the presence/absence of early branches was symmetrical in 70% cases. In 19%, M1 was classified as short M1, and classifications were identical between hemispheres in 74%. Running course of the M1 was symmetrical in 63%. Two or more parameters were symmetrical in 181 cases (95%).

CONCLUSIONS

The symmetry of bilateral M1-2 structures was demonstrated in most cases from the perspectives of 4 parameters. The MCA symmetry can predict the running course of the MCA before crossing the occlusion site and shows potential benefits for neurointerventionalists.

A Dilp8-dependent time window ensures tissue size adjustment in Drosophila

The control of organ size mainly relies on precise autonomous growth programs. However, organ development is subject to random variations, called developmental noise, best revealed by the fluctuating asymmetry observed between bilateral organs. The developmental mechanisms ensuring bilateral symmetry in organ size are mostly unknown. In Drosophila, null mutations for the relaxin-like hormone Dilp8 increase wing fluctuating asymmetry, suggesting that Dilp8 plays a role in buffering developmental noise. Here we show that size adjustment of the wing primordia involves a peak of dilp8 expression that takes place sharply at the end of juvenile growth. Wing size adjustment relies on a cross-organ communication involving the epidermis as the source of Dilp8. We identify ecdysone signaling as both the trigger for epidermal dilp8 expression and its downstream target in the wing primordia, thereby establishing reciprocal hormonal feedback as a systemic mechanism, which controls organ size and bilateral symmetry in a narrow developmental time window.

Mechanisms ensuring developmental precision are poorly understood. Here Blanco-Obregon et al. report reciprocal feedback between Dilp8 and Ecdysone, two hormones required during a precise time window of Drosophila development for organ size adjustment.

Theories, laws, and models in evo-devo

Evolutionary developmental biology (evo-devo) is the study of the evolution of developmental mechanisms. Here, I review some of the theories, models, and laws in evo-devo, past and present. Nineteenth-century evo-devo was dominated by recapitulation theory and archetypes. It also gave us germ layer theory, the vertebral theory of the skull, floral organs as modified leaves, and the "inverted invertebrate" theory, among others. Newer theories and models include the frameshift theory, the genetic toolkit for development, the ABC model of flower development, the developmental hourglass, the zootype, Urbilateria, and the hox code. Some of these new theories show the influence of archetypes and recapitulation. Interestingly, recent studies support the old "primordial leaf," "inverted invertebrate," and "segmented head" theories. Furthermore, von Baer's first three laws may now need to be rehabilitated, and the hourglass model modified, in view of what Abzhanov has pointed out about the maternal-zygotic transition. There are many supposed "laws" of evo-devo but I argue that these are merely generalizations about trends in particular lineages. I argue that the "body plan" is an archetype, and is often used in such a way that it lacks any scientific meaning. Looking to the future, one challenge for evo-devo will be to develop new theories and models to accommodate the wealth of new data from high-throughput sequencing, including single-cell sequencing. One step in this direction is the use of sophisticated in silico analyses, as in the "transcriptomic hourglass" models.

The steroid-hormone ecdysone coordinates parallel pupariation neuromotor and morphogenetic subprograms via epidermis-to-neuron Dilp8-Lgr3 signal induction

Innate behaviors consist of a succession of genetically-hardwired motor and physiological subprograms that can be coupled to drastic morphogenetic changes. How these integrative responses are orchestrated is not completely understood. Here, we provide insight into these mechanisms by studying pupariation, a multi-step innate behavior of Drosophila larvae that is critical for survival during metamorphosis. We find that the steroid-hormone ecdysone triggers parallel pupariation neuromotor and morphogenetic subprograms, which include the induction of the relaxin-peptide hormone, Dilp8, in the epidermis. Dilp8 acts on six Lgr3-positive thoracic interneurons to couple both subprograms in time and to instruct neuromotor subprogram switching during behavior. Our work reveals that interorgan feedback gates progression between subunits of an innate behavior and points to an ancestral neuromodulatory function of relaxin signaling.

What determines organ size during development and regeneration?

The sizes of living organisms span over 20 orders of magnitude or so. This daunting observation could intimidate researchers aiming to understand the general mechanisms controlling growth. However, recent progress suggests the existence of principles common to organisms as diverse as fruit flies, mice and humans. As we review here, these studies have provided insights into both autonomous and non-autonomous mechanisms controlling organ growth as well as some of the principles underlying growth coordination between organs and across bilaterally symmetrical organisms. This research tackles several aspects of developmental biology and integrates inputs from physics, mathematical modelling and evolutionary biology. Although many open questions remain, this work also helps to shed light on medically related conditions such as tissue and limb regeneration, as well as metabolic homeostasis and cancer.

Regulation of Body Size and Growth Control

The control of body and organ growth is essential for the development of adults with proper size and proportions, which is important for survival and reproduction. In animals, adult body size is determined by the rate and duration of juvenile growth, which are influenced by the environment. In nutrient-scarce environments in which more time is needed for growth, the juvenile growth period can be extended by delaying maturation, whereas juvenile development is rapidly completed in nutrient-rich conditions. This flexibility requires the integration of environmental cues with developmental signals that govern internal checkpoints to ensure that maturation does not begin until sufficient tissue growth has occurred to reach a proper adult size. The Target of Rapamycin (TOR) pathway is the primary cell-autonomous nutrient sensor, while circulating hormones such as steroids and insulin-like growth factors are the main systemic regulators of growth and maturation in animals. We discuss recent findings in Drosophila melanogaster showing that cell-autonomous environment and growth-sensing mechanisms, involving TOR and other growth-regulatory pathways, that converge on insulin and steroid relay centers are responsible for adjusting systemic growth, and development, in response to external and internal conditions. In addition to this, proper organ growth is also monitored and coordinated with whole-body growth and the timing of maturation through modulation of steroid signaling. This coordination involves interorgan communication mediated by Drosophila insulin-like peptide 8 in response to tissue growth status. Together, these multiple nutritional and developmental cues feed into neuroendocrine hubs controlling insulin and steroid signaling, serving as checkpoints at which developmental progression toward maturation can be delayed. This review focuses on these mechanisms by which external and internal conditions can modulate developmental growth and ensure proper adult body size, and highlights the conserved architecture of this system, which has made Drosophila a prime model for understanding the coordination of growth and maturation in animals.

Drosophila Insulin-Like Peptide 8 (DILP8) in Ovarian Follicle Cells Regulates Ovulation and Metabolism

In Drosophila melanogaster eight insulin-like peptides (DILP1-8) are encoded on separate genes. These DILPs are characterized by unique spatial and temporal expression patterns during the lifecycle. Whereas, functions of several of the DILPs have been extensively investigated at different developmental stages, the role of DILP8 signaling is primarily known from larvae and pupae where it couples organ growth and developmental transitions. In adult female flies, a study showed that a specific set of neurons that express the DILP8 receptor, Lgr3, is involved in regulation of reproductive behavior. Here, we further investigated the expression of dilp8/DILP8 and Lgr3 in adult female flies and the functional role of DILP8 signaling. The only site where we found both dilp8 expression and DILP8 immunolabeling was in follicle cells around mature eggs. Lgr3 expression was detected in numerous neurons in the brain and ventral nerve cord, a small set of peripheral neurons innervating the abdominal heart, as well as in a set of follicle cells close to the oviduct. Ovulation was affected in dilp8 mutants as well as after dilp8-RNAi using dilp8 and follicle cell Gal4 drivers. More eggs were retained in the ovaries and fewer were laid, indicating that DILP8 is important for ovulation. Our data suggest that DILP8 signals locally to Lgr3 expressing follicle cells as well as systemically to Lgr3 expressing efferent neurons in abdominal ganglia that innervate oviduct muscle. Thus, DILP8 may act at two targets to regulate ovulation: follicle cell rupture and oviduct contractions. Furthermore, we could show that manipulations of dilp8 expression affect starvation resistance suggesting effects on metabolism. Possibly this reflects a feedback signaling between ovaries and the CNS that ensures nutrients for ovary development. In summary, it seems that DILP8 signaling in regulation of reproduction is an ancient function, conserved in relaxin signaling in mammals.

Poikilosis - pervasive biological variation

Biological systems are dynamic and display heterogeneity at all levels. Ubiquitous heterogeneity, here called for poikilosis, is an integral and important property of organisms and in molecules, systems and processes within them. Traditionally, heterogeneity in biology and experiments has been considered as unwanted noise, here poikilosis is shown to be the normal state. Acceptable variation ranges are called as lagom. Non-lagom, variations that are too extensive, have negative effects, which influence interconnected levels and once the variation is large enough cause a disease and can lead even to death. Poikilosis has numerous applications and consequences e.g. for how to design, analyze and report experiments, how to develop and apply prediction and modelling methods, and in diagnosis and treatment of diseases. Poikilosis-aware new and practical definitions are provided for life, death, senescence, disease, and lagom. Poikilosis is the first new unifying theory in biology since evolution and should be considered in every scientific study.

Poikilosis - pervasive biological variation

Biological systems are dynamic and display heterogeneity at all levels. Ubiquitous heterogeneity, here called for poikilosis, is an integral and important property of organisms and in molecules, systems and processes within them. Traditionally, heterogeneity in biology and experiments has been considered as unwanted noise, here poikilosis is shown to be the normal state. Acceptable variation ranges are called as lagom. Non-lagom, variations that are too extensive, have negative effects, which influence interconnected levels and once the variation is large enough cause a disease and can lead even to death. Poikilosis has numerous applications and consequences e.g. for how to design, analyze and report experiments, how to develop and apply prediction and modelling methods, and in diagnosis and treatment of diseases. Poikilosis-aware new and practical definitions are provided for life, death, senescence, disease, and lagom. Poikilosis is the first new unifying theory in biology since evolution and should be considered in every scientific study.

Role of Serotonin Transporter in Eye Development of Drosophila melanogaster

Serotonin transporter (SerT) in the brain is an important neurotransmitter transporter involved in mental health. However, its role in peripheral organs is poorly understood. In this study, we investigated the function of SerT in the development of the compound eye in Drosophila melanogaster. We found that SerT knockdown led to excessive cell death and an increased number of cells in S-phase in the posterior eye imaginal disc. Furthermore, the knockdown of SerT in the eye disc suppressed the activation of Akt, and the introduction of PI3K effectively rescued this phenotype. These results suggested that SerT plays a role in the healthy eye development of D.melanogaster by controlling cell death through the regulation of the PI3K/Akt pathway.

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.

Trehalose metabolism confers developmental robustness and stability in Drosophila by regulating glucose homeostasis

Organisms have evolved molecular mechanisms to ensure consistent and invariant phenotypes in the face of environmental fluctuations. Developmental homeostasis is determined by two factors: robustness, which buffers against environmental variations; and developmental stability, which buffers against intrinsic random variations. However, our understanding of these noise-buffering mechanisms remains incomplete. Here, we showed that appropriate glycemic control confers developmental homeostasis in the fruit fly Drosophila. We found that circulating glucose levels are buffered by trehalose metabolism, which acts as a glucose sink in circulation. Furthermore, mutations in trehalose synthesis enzyme (Tps1) increased the among-individual and within-individual variations in wing size. Whereas wild-type flies were largely resistant to changes in dietary carbohydrate and protein levels, Tps1 mutants experienced significant disruptions in developmental homeostasis in response to dietary stress. These results demonstrate that glucose homeostasis against dietary stress is crucial for developmental homeostasis.

Eiger/TNFα-mediated Dilp8 and ROS production coordinate intra-organ growth in Drosophila

Coordinated intra- and inter-organ growth during animal development is essential to ensure a correctly proportioned individual. The Drosophila wing has been a valuable model system to reveal the existence of a stress response mechanism involved in the coordination of growth between adjacent cell populations and to identify a role of the fly orthologue of p53 (Dmp53) in this process. Here we identify the molecular mechanisms used by Dmp53 to regulate growth and proliferation in a non-autonomous manner. First, Dmp53-mediated transcriptional induction of Eiger, the fly orthologue of TNFα ligand, leads to the cell-autonomous activation of JNK. Second, two distinct signaling events downstream of the Eiger/JNK axis are induced in order to independently regulate tissue size and cell number in adjacent cell populations. Whereas expression of the hormone dILP8 acts systemically to reduce growth rates and tissue size of adjacent cell populations, the production of Reactive Oxygen Species—downstream of Eiger/JNK and as a consequence of apoptosis induction—acts in a non-cell-autonomous manner to reduce proliferation rates. Our results unravel how local and systemic signals act concertedly within a tissue to coordinate growth and proliferation, thereby generating well-proportioned organs and functionally integrated adults.

The coordination of growth between the parts of a given developing organ is an absolute requirement for the generation of functionally integrated structures during animal development. Although this question has fascinated biologists for centuries, the molecular mechanisms responsible have remained elusive to date. In this work, we used the developing wing primordium of Drosophila to identify the molecular mechanisms and signaling molecules that mediate communication between adjacent cell populations upon a targeted reduction of growth rate. We first present evidence that the activation of Dmp53 in the growth-depleted territory induces the expression of the fly TNF ligand Eiger, which activates the JNK stress signaling pathway in a cell-autonomous manner. While JNK-dependent expression of the systemic hormone dILP8 reduces the growth and final size of adjacent territories, the production of Reactive Oxygen Species downstream of JNK and the apoptotic machinery act locally to regulate the proliferation of adjacent epithelial cells. Our data reveal how different signals, acting both locally and systemically, can regulate tissue growth and cell proliferation in an independent manner to coordinate the tissue size and cell number of different parts of an organ, ultimately giving rise to well-proportioned adult structures.