Cue reliability, risk sensitivity and inducible morphological defense in a marine snail

Reliable cues that communicate current or future environmental conditions are a requirement for the evolution of adaptive phenotypic plasticity, yet we often do not know which cues are responsible for the induction of particular plastic phenotypes. I examined the single and combined effects of cues from damaged prey and predator cues on the induction of plastic shell defenses and somatic growth in the marine snail Nucella lamellosa. Snails were exposed to chemical risk cues from a factorial combination of damaged prey presented in isolation or consumed by predatory crabs (Cancer productus). Water-borne cues from damaged conspecific and heterospecific snails did not affect plastic shell defenses (shell mass, shell thickness and apertural teeth) or somatic growth in N. lamellosa. Cues released by feeding crabs, independent of prey cue, had significant effects on shell mass and somatic growth, but only crabs consuming conspecific snails induced the full suite of plastic shell defenses in N. lamellosa and induced the greatest response in all shell traits and somatic growth. Thus the relationship between risk cue and inducible morphological defense is dependent on which cues and which morphological traits are examined. Results indicate that cues from damaged conspecifics alone do not trigger a response, but, in combination with predator cues, act to signal predation risk and trigger inducible defenses in this species. This ability to “label” predators as dangerous may decrease predator avoidance costs and highlights the importance of the feeding habits of predators on the expression of inducible defenses.     

Behavioural and Growth Responses of a Territorial Fish (Atlantic Salmon, Salmo salar, L.) to Multiple Predatory Cues

Antipredatory behaviours are important fitness components. The probability of survival decreases if animals fail to respond to signs of danger, but in contrast, energetic costs increase if the response to the threat is exaggerated. We conducted a laboratory experiment designed to examine the behavioural and growth responses of a territorial fish (Atlantic salmon fry, Salmo salar, L.) to different predatory cues (no predatory cues, chemical cues alone, physical cues alone and combined chemical and physical cues). We evaluated the response of Atlantic salmon, focussing on behaviours linked to predator avoidance and to other fitness‐enhancing activities (territory defence and energy acquisition) both during the day and the night. The cost of such responses in terms of growth was assessed and we compared the relative contributions of behaviours in explaining individual growth rate, according to each predation treatment. We demonstrated that the magnitude and nature of behavioural modification varied according to the response variables we considered. An index of predator avoidance and the distance from the food source were affected in an additive fashion by predatory cues (interaction term, p = 0.469 and p = 0.888 for the index of predator avoidance and the distance from the food source respectively); the effect of physical cues was stronger than the effect of chemical cues and the effect of the combined cues was highest. An index of territoriality was affected in a threshold‐like fashion (interaction term, p = 0.040); chemical or physical cues alone had no effect but when both cues were combined, Atlantic salmon significantly reduced their territorial defence. An index of foraging activity was not significantly affected by predatory cues (alone or combined). We detected no effect on the growth rate of Atlantic salmon (p = 0.328). Finally, we found that the relative contribution of behaviours in explaining individual growth rate changed according to the treatments we considered. Overall, these results demonstrated that fish were able to accurately integrate multiple predatory cues and that this information was used to modulate their antipredatory response. Behaviours involved in the response were relatively independent of each other, allowing fish to adopt behavioural tactics that maximized the ratio of net energy gain to predator avoidance.     

Regulation of growth cone actin filaments by guidance cues

The motile behaviors of growth cones at the ends of elongating axons determine pathways of axonal connections in developing nervous systems. Growth cones express receptors for molecular guidance cues in the local environment, and receptor-guidance cue binding initiates cytoplasmic signaling that regulates the cytoskeleton to control growth cone advance, turning, and branching behaviors. The dynamic actin filaments of growth cones are frequently targets of this regulatory signaling. Rho GTPases are key mediators of signaling by guidance cues, although much remains to be learned about how growth cone responses are orchestrated by Rho GTPase signaling to change the dynamics of polymerization, transport, and disassembly of actin filaments. Binding of neurotrophins to Trk and p75 receptors on growth cones triggers changes in actin filament dynamics to regulate several aspects of growth cone behaviors. Activation of Trk receptors mediates local accumulation of actin filaments, while neurotrophin binding to p75 triggers local decrease in RhoA signaling that promotes lengthening of filopodia. Semaphorin IIIA and ephrin-A2 are guidance cues that trigger avoidance or repulsion of certain growth cones, and in vitro responses to these proteins include growth cone collapse. Dynamic changes in the activities of Rho GTPases appear to mediate responses to these cues, although it remains unclear what the changes are in actin filament distribution and dynamic reorganization that result in growth cone collapse. Growth cones in vivo simultaneously encounter positive and negative guidance cues, and thus, growth cone behaviors during axonal pathfinding reflect the complex integration of multiple signaling activities.     

The non-consumptive effects of predators and personality on prey growth and mortality

Individual organisms vary in personality, and the ecological consequences of that variation can affect the strength of predator–prey interactions. Prey with bolder tendencies can mitigate the strength of species interactions by altering growth and initiating ontogenetic niche shifts (ONS). While the link between personality and growth has been established, recent research has highlighted the important interplay between ONS and predator cues in community ecology. The objective of this study was to evaluate the effects of prey personality and predator cues on prey growth and ONS. We predicted growth–mortality trade-offs among personalities with higher survival, larger size, and accelerated ONS for bold individuals in comparison with shy individuals. To evaluate this objective, we conducted behavioral assays and a mesocosm experiment to test how southern leopard frog (Rana sphenocephala) tadpole personality and predatory fish (bluegill, Lepomis macrochirus) cues affects tadpole growth and metamorphosis. On average, bold tadpoles had higher mortality across all treatments in comparison with shy tadpoles. The effects of fish cues were dependent on tadpole personality with shy tadpoles metamorphosing significantly later than bold tadpoles. Bold tadpoles were larger than shy tadpoles at metamorphosis; however, that pattern reversed with fish cues as shy individuals metamorphosed larger than bold individuals. Our results suggest personality may be useful for predicting growth and life history for some prey species with predators. Specifically, the threat of predation can interact with personality to incur a benefit (earlier ONS) while also incurring a cost (size at metamorphosis). Hence by incorporating predator cues with personality, ecologists will be able to elucidate growth–mortality trade-offs mediated by personality.     

Predator-induced changes in morphology of a prey fish: the effects of food level and temporal frequency of predation risk

In a series of experiments, we investigated the effects of food availability and risk frequency on the dynamics of predator-induced changes in growth and morphology of prey fish using goldfish (Carassius auratus) as our test species. In experiment 1, we fed goldfish high or low food rations and exposed them to either alarm cues from conspecifics, cues from swordtails or a water control. After 60 days, goldfish in the alarm cue treatment significantly increased their body depth and body weight but had smaller body length than goldfish exposed to swordtails cues or water, likely reducing their vulnerability to gape-limited predators. Importantly, food level had an impact on the amplitude of the morphological changes. In experiment 2, goldfish were exposed to two different frequencies of predation cues or a water control for 50 days. The cues were either continued or discontinued from day 51 to 100, and all cues were resumed from day 101 to 150. We found that goldfish exposed to predation cues increased their depth and weight at a faster rate than did the goldfish exposed to water, and of particular significance was the fact that frequency of risk had an effect on the amplitude of the change. When the cues were interrupted, the increase in growth rate parameters was reduced to the level of the goldfish exposed to water. However, when the cues were resumed, the rate increased to match the growth rate of the goldfish that were continuously exposed to the cues. Finally, we staged encounters between goldfish of differing morphologies and yellow perch (Perca flavescens) and found that deep-bodied goldfish had better survival than the shallow-bodied ones. These experiments illustrate the dynamic nature of inducible morphological defences.     

Cellular Strategies of Axonal Pathfinding

Axons follow highly stereotyped and reproducible trajectories to their targets. In this review we address the properties of the first pioneer neurons to grow in the developing nervous system and what has been learned over the past several decades about the extracellular and cell surface substrata on which axons grow. We then discuss the types of guidance cues and their receptors that influence axon extension, what determines where cues are expressed, and how axons respond to the cues they encounter in their environment.This article provides an overview of how growth cones respond to the cellular substrata and molecular cues they encounter as they extend through the developing nervous system. It elaborates on the primer by Kolodkin and Tessier-Lavigne (2010) and touches on many of the topics covered in greater detail in the articles that follow. The first sections describe how axons extend in a directed manner, the substrata on which they grow, interactions between pioneer and follower axons, and growth cone behaviors in emerging tracts and at decision points. The subsequent sections discuss examples of specific cues, their distributions, how their distributions are determined, and how growth cones integrate multiple cues during pathfinding.     

Touch and go: guidance cues signal to the growth cone cytoskeleton

Growth cones, the highly motile tips of growing axons, guide axons to their targets by responding to molecular cues. Growth cone behaviors such as advancing, retracting, turning and branching are driven by the dynamics and reorganization of the actin and microtubule cytoskeleton through signaling pathways linked to guidance cue receptors. Actin filaments play a major part in growth cone motility, and because of their peripheral locations were thought to be the primary target of molecular cues. However, recent studies have shown that dynamic microtubules can penetrate the growth cone periphery where guidance molecules can influence them directly. Moreover, guidance cues can regulate growth cone steering by modulating dynamic actin-microtubule interactions.     

Effects of fish chemical cues on the interactions between tadpoles and crayfish

We studied the effects of predatory crayfish ( Pacifastacus leniusculus ), the non-lethal effects of fish chemical cues ( Oncorhynchus mykiss ), and the combined effects of crayfish and fish chemical cues on the performance of tadpoles of two co-existing anuran species, Rana temporaria and Bufo bufo , in experimental pools. We also examined grazing effects on periphyton, the main food source for the tadpoles. Crayfish significantly reduced tadpole survival, particularly by feeding on Bufo . Rana benefited from reduced numbers of competitors, resulting from crayfish predation, by increased growth rate, whereas the growth rate of Bufo was unaffected by crayfish. The proportion of Rana in refuges (in relation to the number of survivors at the end of the experiment) was unaffected by crayfish, whereas proportionally more Bufo stayed in refuges in the presence of crayfish, relative to controls. Fish cues had no effect on tadpole survival of either species. During the entire larval period, Rana responded to fish cues by increasing the use of refuges relative to controls, whereas Bufo did not show any significant behavioural response to fish cues. In accordance with these observations, the proportion of Rana in refuges at the end of the experiment was high in the presence of fish cues, whereas the use of refuges by Bufo was not affected by fish cues. Predatory crayfish and fish chemical cues had additive effects on tadpole survival, growth and refuge use. Tadpoles in all treatments reduced periphyton biomass. Both crayfish and fish cues had positive indirect effects on periphyton biomass. The positive indirect effect of fish cues on periphyton was likely an effect of reduced grazing from Rana . Thus lethal, as well as non-lethal, predator effects on prey populations can influence lower trophic levels.     

The role of repulsive guidance molecules in the embryonic and adult vertebrate central nervous system

During the development of the nervous system, outgrowing axons often have to travel long distances to reach their target neurons. In this process, outgrowing neurites tipped with motile growth cones rely on guidance cues present in their local environment. These cues are detected by specific receptors expressed on growth cones and neurites and influence the trajectory of the growing fibres. Neurite growth, guidance, target innervation and synapse formation and maturation are the processes that occur predominantly but not exclusively during embryonic or early post-natal development in vertebrates. As a result, a functional neural network is established, which is usually remarkably stable. However, the stability of the neural network in higher vertebrates comes at an expensive price, i.e. the loss of any significant ability to regenerate injured or damaged neuronal connections in their central nervous system (CNS). Most importantly, neurite growth inhibitors prevent any regenerative growth of injured nerve fibres. Some of these inhibitors are associated with CNS myelin, others are found at the lesion site and in the scar tissue. Traumatic injuries in brain and spinal cord of mammals induce upregulation of embryonic inhibitory or repulsive guidance cues and their receptors on the neurites. An example for embryonic repulsive directional cues re-expressed at lesion sites in both the rat and human CNS is provided with repulsive guidance molecules, a new family of directional guidance cues.     

Unique Responses of Differentiating Neuronal Growth Cones to Inhibitory Cues Presented by Oligodendrocytes

During central nervous system development, neurons differentiate distinct axonal and dendritic processes whose outgrowth is influenced by environmental cues. Given the known intrinsic differences between axons and dendrites and that little is known about the response of dendrites to inhibitory cues, we tested the hypothesis that outgrowth of differentiating axons and dendrites of hippocampal neurons is differentially influenced by inhibitory environmental cues. A sensitive growth cone behavior assay was used to assess responses of differentiating axonal and dendritic growth cones to oligodendrocytes and oligodendrocyte- derived, myelin-associated glycoprotein (MAG). We report that >90% of axonal growth cones collapsed after contact with oligodendrocytes. None of the encounters between differentiating, MAP-2 positive dendritic growth cones and oligodendrocytes resulted in growth cone collapse. The insensitivity of differentiating dendritic growth cones appears to be acquired since they develop from minor processes whose growth cones are inhibited (nearly 70% collapse) by contact with oligodendrocytes. Recombinant MAG(rMAG)-coated beads caused collapse of 72% of axonal growth cones but only 29% of differentiating dendritic growth cones. Unlike their response to contact with oligodendrocytes, few growth cones of minor processes were inhibited by rMAG-coated beads (20% collapsed). These results reveal the capability of differentiating growth cones of the same neuron to partition the complex molecular terrain they navigate by generating unique responses to particular inhibitory environmental cues.     

Environmental calcium modifies induced defences in snails

Inducible defences are adaptive phenotypes that arise in response to predation threats. Such plasticity incurs costs to individuals, but there has been little interest in how such induced traits in animals may be constrained by environmental factors. Here, we demonstrate that calcium availability interacts with predation cues to modify snail shell growth and form. Small snails increased their growth and were heavier when exposed to fish chemical cues, but this response was calcium limited. There was also an interactive effect of fish cues and calcium on the shell growth of larger snails, but shell strength and aperture narrowness were affected by calcium alone. For small snails, behavioural avoidance was greatest for snails exhibiting least morphological plasticity, suggesting a trade-off. There was no trade-off of somatic growth with plasticity. We suggest that the expression of defensive traits in molluscs can be constrained by calcium availability, which has implications for molluscan ecology and evolution.     

Microtubule behavior during guidance of pioneer neuron growth cones in situ

The growth of an axon toward its target results from the reorganization of the cytoskeleton in response to environmental guidance cues. Recently developed imaging technology makes it possible to address the effect of such cues on the neural cytoskeleton directly. Although high resolution studies can be carried out on neurons in vitro, these circumstances do not recreate the complexity of the natural environment. We report here on the arrangement and dynamics of microtubules in live neurons pathfinding in response to natural guidance cues in situ using the embryonic grasshopper limb fillet preparation. A rich microtubule network was present within the body of the growth cone and normally extended into the distal growth cone margin. Complex microtubule loops often formed transiently within the growth cone. Branches both with and without microtubules were regularly observed. Microtubules did not extend into filopodia. During growth cone steering events in response to identified guidance cues, microtubule behaviour could be monitored. In turns towards guidepost cells, microtubules selectively invaded branches derived from filopodia that had contacted the guidepost cell. At limb segment boundaries, microtubules displayed a variety of behaviors, including selective branch invasion, and also invasion of multiple branches followed by selective retention in branches oriented in the correct direction. Microtubule invasion of multiple branches also was seen in growth cones migrating on intrasegmental epithelium. Both selective invasion and selective retention generate asymmetrical microtubule arrangements within the growth cone, and may play a key role in growth cone steering events.     

Regulation of axonal outgrowth and pathfinding by integrin-ECM interactions

Developing neurons use a combination of guidance cues to assemble a functional neural network. A variety of proteins immobilized within the extracellular matrix (ECM) provide specific binding sites for integrin receptors on neurons. Integrin receptors on growth cones associate with a number of cytosolic adaptor and signaling proteins that regulate cytoskeletal dynamics and cell adhesion. Recent evidence suggests that soluble growth factors and classic axon guidance cues may direct axon pathfinding by controlling integrin-based adhesion. Moreover, because classic axon guidance cues themselves are immobilized within the ECM and integrins modulate cellular responses to many axon guidance cues, interactions between activated receptors modulate cell signals and adhesion. Ultimately, growth cones control axon outgrowth and pathfinding behaviors by integrating distinct biochemical signals to promote the proper assembly of the nervous system. In this review, we discuss our current understanding how ECM proteins and their associated integrin receptors control neural network formation.     

Microtubules and growth cone function

It has been recognized for a long time that the neuronal cytoskeleton plays an important part in neurite growth and growth cone pathfinding, the mechanism by which growing axons find an appropriate route through the developing embryo to their target cells. In the growth cone, many intracellular signaling pathways that are activated by guidance cues converge on the growth cone cytoskeleton and regulate its dynamics. Most of the research effort in this area has focussed on the actin, microfilament cytoskeleton of the growth cone, principally because it underlies growth cone motility, the extension and retraction of filopodia and lamellipodia, and these structures are the first to encounter guidance cues during growth cone advance. However, more recently, it has become apparent that the microtubule cytoskeleton also has a role in growth cone pathfinding and is also regulated by guidance cues operating through intracellular signaling pathways via engagement with cell membrane receptors. Furthermore, recent work has revealed an interaction between these two components of the growth cone cytoskeleton that is probably essential for growth cone turning, a fundamental growth cone behavior during pathfinding. In this short review I discuss recent experiments that uncover the function of microtubules in growth cones, how their behavior is regulated, and how they interact with the actin filaments.     

Retinal axon guidance: novel mechanisms for steering

Axons from the retina traverse different molecular territories as they navigate to the tectum. A single territory might span only a few cell diameters and harbour multiple guidance cues, many of which are beginning to be characterized. Also present in the pathway are 'modulators' that influence a growth cone's response to a coincident signal but do not guide growth directly. An emerging principle is that the growth cone, itself, changes molecularly as it journeys through the visual pathway. Growing retinal axons contain mRNAs, ubiquitinating and apoptotic enzymes, translation and degradation machinery. Guidance cues can trigger rapid and local synthesis, degradation and endocytosis of proteins, providing a fast and flexible way for growth cones to respond to cues in their microenvironment and to alter their responsiveness. The data raise the idea that the localized synthesis and downregulation of proteins might help to steer retinal axon growth and, further, might contribute to the changing character of a growth cone as it ages.     

Accumulation of actin in subsets of pioneer growth cone filopodia in response to neural and epithelial guidance cues in situ

Directed outgrowth of neural processes must involve transmission of signals from the tips of filopodia to the central region of the growth cone. Here, we report on the distribution and dynamics of one possible element in this process, actin, in live growth cones which are reorienting in response to in situ guidance cues. In grasshopper embryonic limbs, pioneer growth cones respond to at least three types of guidance cues: a limb axis cue, intermediate target cells, and a circumferential band of epithelial cells. With time-lapse imaging of intracellularly injected rhodamine-phalloidin and rhodamine-actin, we monitored the distribution of actin during growth cone responses to these cues. In distal limb regions, accumulation of actin in filopodia and growth cone branches accompanies continued growth, while reduction of actin accompanies withdrawal. Where growth cones are reorienting to intermediate target cells, or along the circumferential epithelial band, actin selectively accumulates in the proximal regions of those filopodia that have contacted target cells or are extending along the band. Actin accumulations can be retrogradely transported along filopodia, and can extend into the central region of the growth cone. These results suggest that regulation and translocation of actin may be a significant element in growth cone steering.     

生长素介导环境信号调控植物的生长发育

植物是一类营固着生活的自养型生物, 如何更好地适应周围环境对植物的生存至关重要。生长素是调控植物生长发育的重要激素之一。近年来的研究发现, 生长素不仅能够响应内在的发育信号, 而且能够介导各种环境信号, 参与植物生长发育和生长反应的调控。该文主要从光信号、温度信号、重力信号、营养元素和金属离子信号等方面重点阐述生长素如何介导上述各种不同的环境信号, 从而调控植物的生长发育。     

Guiding neuronal growth cones using Ca2+ signals

Pathfinding by growing axons in the developing or regenerating nervous system is guided by gradients of molecular guidance cues. The neuronal growth cone, located at the ends of axons, uses surface receptors to sense these cues and to transduce guidance information to cellular machinery that mediates growth and turning responses. Cytoplasmic Ca2+ signals have key roles in regulating this motility. Global growth cone Ca2+ signals can regulate cytoskeletal elements and membrane dynamics to control elongation, whereas Ca2+ signals localized to one side of the growth cone can cause asymmetric activation of effector enzymes to steer the growth cone. Modulating Ca2+ levels in the growth cone might overcome inhibitory signals that normally prevent regeneration in the central nervous system.     

Substrate-cytoskeletal coupling as a mechanism for the regulation of growth cone motility and guidance

Growth cones are highly motile structures at the end of neuronal processes, capable of receiving multiple types of guidance cues and transducing them into directed axonal growth. Thus, to guide the axon toward the appropriate target cell, the growth cone carries out different functions: it acts as a sensor, signal transducer, and motility device. An increasing number of molecular components that mediate axon guidance have been characterized over the past years. The vast majority of these molecules include proteins that act as guidance cues and their respective receptors. In addition, more and more signaling and cytoskeleton-associated proteins have been localized to the growth cone. Furthermore, it has become evident that growth cone motility and guidance depends on a dynamic cytoskeleton that is regulated by incoming guidance information. Current and future research in the growth cone field will be focussed on how different guidance cues transmit their signals to the cytoskeleton and change its dynamic properties to affect the rate and direction of growth cone movement. In this review, we discuss recent evidence that cell adhesion molecules can regulate growth cone motility and guidance by a mechanism of substrate-cytoskeletal coupling.     

Predation risk affects egg mortality and carry over effects in the larval stages in damselflies

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