Morphology of theDaphnianervous system: A comparative study onDaphnia pulex,Daphnia lumholtzi, andDaphnia longicephala

Unveiling the multilevel impact of four water-soluble polymers on Daphnia magna: From proteome to behaviour (a case study)

The ubiquitous presence of water-soluble polymers (WSPs) in freshwater environments raises concerns regarding potential threats to aquatic organisms. This study investigated, for the first time, the effects of widely used WSPs -polyvinyl alcohol (PVA), polyvinylpyrrolidone (PVP), polyacrylic acid (PAA), and polyethylene glycol (PEG)- using a multi-level approach in the freshwater biological model Daphnia magna. This integrated assessment employed a suite of biomarkers, evaluation of swimming behaviour, and proteomic analysis to investigate the effects of three environmentally relevant concentrations (0.001, 0.5, and 1 mg/L) of the tested WSPs from molecular to organismal levels, assessing both acute and chronic effects. Our findings reveal that exposure to different WSPs induces specific responses at each biological level, with PEG being the only WSP inducing lethal effects at 0.5 mg/L. At the physiological level, although all WSPs impacted both swimming performance and heart rate of D. magna specimens, PAA exhibited the greatest effects on the measured behavioural parameters. Furthermore, proteomic analyses demonstrated altered protein profiles following exposure to all WSPs, with PVA emerging as the most effective.

On the origin and nature of nongenetic information in eumetazoans

Nongenetic information implies all the forms of biological information not related to genes and DNA in general. Despite the deep scientific relevance of the concept, we currently lack reliable knowledge about its carriers and origins; hence, we still do not understand its true nature. Given that genes are the targets of nongenetic information, it appears that a parsimonious approach to find the ultimate source of that information is to trace back the sequential steps of the causal chain upstream of the target genes up to the ultimate link as the source of the nongenetic information. From this perspective, I examine seven nongenetically determined phenomena: placement of locus-specific epigenetic marks on DNA and histones, changes in snRNA expression patterns, neural induction of gene expression, site-specific alternative gene splicing, predator-induced morphological changes, and cultural inheritance. Based on the available evidence, I propose a general model of the common neural origin of all these forms of nongenetic information in eumetazoans.

Distinct cell proliferation patterns underlying the development of defensive crests in Daphnia longicephala

The freshwater crustacean Daphnia is well known for its expression of morphological defenses in the presence of predators. Research into this phenomenon has mostly centered on the ecology and evolution of Daphnia defenses; information is limited on the cellular mechanisms that underlie site-specific tissue growth. We aimed to determine these cellular mechanisms, specifically those associated with the development of defensive crests in D. longicephala. With the help of a cell-proliferation assay we monitored changes in the epidermal tissue of naïve and predator-exposed D. longicephala. Based on our results, we propose that cell division is delayed in favor of cell growth, which results in crest formation. Further, we identify specific regions of proliferative activity in a time-dependent manner. Defense development starts in the ventral region, before extending in the cranial and then dorsal directions. We demonstrate that these cellular changes begin as early as 2 h after predator exposure. Our results provide new insights into the cellular processes underlying morphological defense expression in Daphnia.

Microbiota-Dependent and -Independent Production of l-Dopa in the Gut of Daphnia magna

Host-microbiome interactions are essential for the physiological and ecological performance of the host, yet these interactions are challenging to identify. Neurotransmitters are commonly implicated in these interactions, but we know very little about the mechanisms of their involvement, especially in invertebrates. Here, we report a peripheral catecholamine (CA) pathway involving the gut microbiome of the model species Daphnia magna. We demonstrate the following: (i) tyrosine hydroxylase and Dopa (3,4-dihydroxyphenylalanine) decarboxylase enzymes are present in the gut wall; (ii) Dopa decarboxylase gene is expressed in the gut by the host, and its expression follows the molt cycle peaking after ecdysis; (iii) biologically active l-Dopa, but not dopamine, is present in the gut lumen; (iv) gut bacteria produce l-Dopa in a concentration-dependent manner when provided l-tyrosine as a substrate. Impinging on gut bacteria involvement in host physiology and ecologically relevant traits, we suggest l-Dopa as a communication agent in the host-microbiome interactions in daphnids and, possibly, other crustaceans.

IMPORTANCE Neurotransmitters are commonly implicated in host-microbiome communication, yet the molecular mechanisms of this communication remain largely elusive. We present novel evidence linking the gut microbiome to host development and growth via neurotransmitter l-Dopa in Daphnia, the established model species in ecology and evolution. We found that both Daphnia and its gut microbiome contribute to the synthesis of the l-Dopa in the gut. We also identified a peripheral pathway in the gut wall, with a molt stage-dependent dopamine synthesis, linking the gut microbiome to the daphnid development and growth. These findings suggest a central role of l-Dopa in the bidirectional communication between the animal host and its gut bacteria and translating into the ecologically important host traits suitable for subsequent testing of causality by experimental studies.

The Crustacean Antennule: A Complex Organ Adapted for Lifelong Function in Diverse Environments and Lifestyles

AbstractThe crustacean first antenna, or antennule, has been an experimental model for studying sensory biology for over 150 years. Investigations have led to a clearer understanding of the functional organization of the antennule as an olfactory organ but also to a realization that the antennule is much more than that. Across the Crustacea, the antennules take on many forms and functions. As an example, the antennule of reptantian decapods has many types of sensilla, each with distinct structure and function and with hundreds of thousands of chemosensory neurons expressing hundreds of genes that code for diverse classes of receptor proteins. Together, these antennular sensilla represent multiple chemosensory pathways, each with its own central connections and functions. The antennule also has a diversity of sensors of mechanical stimuli, including vibrations, touch, water flow, and the animal's own movements. The antennule likely also detects other environmental cues, such as temperature, oxygen, pH, salinity, and noxious stimuli. Furthermore, the antennule is a motor organ-it is flicked to temporally and spatially sample the animal's chemo-mechanical surroundings-and this information is used in resolving the structure of chemical plumes and locating the odor source. The antennule is also adapted to maintain lifelong function in a changing environment. For example, it has specific secretory glands, grooming structures, and behaviors to stay clean and functional. Antennular sensilla and the annuli on which they reside are also added and replaced, leading to a complete turnover of the antennule over several molts. Thus, the antennule is a complex and dynamic sensory-motor integrator that is intricately engaged in most aspects of the lives of crustaceans.

Characterization of neurotransmitters and related metabolites in Daphnia magna juveniles deficient in serotonin and exposed to neuroactive chemicals that affect its behavior: A targeted LC-MS/MS method

Neurotransmitters are endogenous metabolites that play a crucial role within an organism, at the chemical synapses. There is a growing interest in their analytical determination for understanding the neurotoxic effect of contaminants. Daphnia magna represents an excellent aquatic model for these environmental studies, due to its similarities with vertebrates in several neurotransmitters and related gene pathways and because of its wide application in ecotoxicological studies. Within this study, an accurate and sensible method of analysis of 17 neurotransmitters and related precursors and metabolites was developed. The method was validated in terms of sensitivity, reproducibility, precision, and accuracy, and also matrix effect was evaluated. As an independent probe of method validation and applicability, the method was applied to two different scenarios. First, it was used for the study of neurotransmitter levels in genetically mutated tryptophan hydrolase D. magna clones, confirming the absence of serotonin and its metabolite 5-HIAA. Additionally, the method was applied for determining the effects of chemical compounds known to affect different neurotransmitter systems and to alter Daphnia behavior. Significant changes were observed in 13 of the analyzed neurotransmitters across treatments, which were related to the neurotransmitter systems described as being affected by these neurochemicals. These two studies, which provide results on the ways in which the neurotransmitter systems in D. magna are affected, have corroborated the applicability of the presented method, of great importance due to the suitability of this organism for environmental neurotoxicity studies.

Sensory Ecology of Predator-Induced Phenotypic Plasticity

Ecological communities are organized in trophic levels that share manifold interactions forming complex food webs. Infochemicals can further modify these interactions, e.g., by inducing defenses in prey. The micro-crustacean Daphnia is able to respond to predator-specific chemical cues indicating an increased predation risk. Daphnia shows plastic responses by adapting its morphology, behavior, and physiology, increasing organism, and population fitness. This stabilizes community structures. This review will describe the progress that has been made in understanding the high degree of plasticity observed in the model crustacean Daphnia. I summarize current knowledge on the processes of predator detection, ranging from the nature of biologically active chemical cues to the underlying neurophysiological mechanisms. With this, I aim to provide a comprehensive overview on the molecular mechanisms of ad hoc environmental phenotypic adaptation. In times of climate change and pollution understanding information transfer in aquatic systems is valuable as it will allow us to predict whether and how community structures are being affected.

Scan, extract, wrap, compute-a 3D method to analyse morphological shape differences

Quantitative analysis of shape and form is critical in many biological disciplines, as context-dependent morphotypes reflect changes in gene expression and physiology, e.g., in comparisons of environment-dependent phenotypes, forward/reverse genetic assays or shape development during ontogenesis. 3D-shape rendering methods produce models with arbitrarily numbered, and therefore non-comparable, mesh points. However, this prevents direct comparisons. We introduce a workflow that allows the generation of comparable 3D models based on several specimens. Translocations between points of modelled morphotypes are plotted as heat maps and statistically tested. With this workflow, we are able to detect, model and investigate the significance of shape and form alterations in all spatial dimensions, demonstrated with different morphotypes of the pond-dwelling microcrustacean Daphnia. Furthermore, it allows the detection even of inconspicuous morphological features that can be exported to programs for subsequent analysis, e.g., streamline- or finite-element analysis.

Biomechanical properties of predator-induced body armour in the freshwater crustacean Daphnia

The freshwater crustacean Daphnia is known for its ability to develop inducible morphological defences that thwart predators. These defences are developed only in the presence of predators and are realized as morphological shape alterations e.g. 'neckteeth' in D. pulex and 'crests' in D. longicephala. Both are discussed to hamper capture, handling or consumption by interfering with the predator's prey capture devices. Additionally, D. pulex and some other daphniids were found to armour-up and develop structural alterations resulting in increased carapace stiffness. We used scanning transmission electron microscopy (STEM) and confocal laser scanning microscopy (CLSM) to identify predator-induced structural and shape alterations. We found species specific structural changes accompanying the known shape alterations. The cuticle becomes highly laminated (i.e. an increased number of layers) in both species during predator exposure. Using nano- and micro-indentation as well as finite element analysis (FEA) we determined both: the structure's and shape's contribution to the carapace's mechanical resistance. From our results we conclude that only structural alterations are responsible for increased carapace stiffness, whereas shape alterations appear to pose handling difficulties during prey capture. Therefore, these defences act independently at different stages during predation.

Mechanisms underlying the control of responses to predator odours in aquatic prey

In aquatic systems, chemical cues are a major source of information through which animals are able to assess the current state of their environment to gain information about local predation risk. Prey use chemicals released by predators (including cues from a predator's diet) and other prey (such as alarm cues and disturbance cues) to mediate a range of behavioural, morphological and life-history antipredator defences. Despite the wealth of knowledge on the ecology of antipredator defences, we know surprisingly little about the physiological mechanisms that control the expression of these defensive traits. Here, we summarise the current literature on the mechanisms known to specifically mediate responses to predator odours, including dietary cues. Interestingly, these studies suggest that independent pathways may control predator-specific responses, highlighting the need for greater focus on predator-derived cues when looking at the mechanistic control of responses. Thus, we urge researchers to tease apart the effects of predator-specific cues (i.e. chemicals representing a predator's identity) from those of diet-mediated cues (i.e. chemicals released from a predator's diet), which are known to mediate different ecological endpoints. Finally, we suggest some key areas of research that would greatly benefit from a more mechanistic approach.

Evolutionary variation in neural gene expression in the developing sense organs of the crustacean Daphnia magna

Arthropods have numerous sense organs, which are adapted to their habitat. While some sense organs are similar in structure and function in all arthropod groups, structural differences in functionally related sense organs have been described, as well as the absence of particular sense organ subtypes in individual arthropod groups. Here we address the question of how the diverse structures of arthropod sense organs have evolved by analysing the underlying molecular developmental processes in a crustacean, an arthropod group that has been neglected so far. We have investigated the development of four types of chemo- and mechanosensory sense organs in the branchiopod Daphnia magna (Cladocera) that either cannot be found in arthropods other than crustaceans or represent adaptations to an aquatic environment. The formation of the sensory organ precursors shows greater similarity to the arthropod taxa Chelicerata and Myriapoda than to the more closely related insects. All analysed sense organ types co-express the proneural genes ASH and atonal regardless of their structure and function. In contrast, in Drosophila melanogaster, ASH and atonal expression does not overlap and the genes confer different sense organ subtype identities. We performed experimental co-expression studies in D. melanogaster and found that the combinatorial expression of ato and ASH can change the external structure of sense organs. Our results indicate a central role for ASH and Atonal family members in the emergence of structural variations in arthropod sense organs.

Push or Pull? The light-weight architecture of the Daphnia pulex carapace is adapted to withstand tension, not compression

Daphnia (Crustacea, Cladocera) are well known for their ability to form morphological adaptations to defend against predators. In addition to spines and helmets, the carapace itself is a protective structure encapsulating the main body, but not the head. It is formed by a double layer of the integument interconnected by small pillars and hemolymphatic space in between. A second function of the carapace is respiration, which is performed through its proximal integument. The interconnecting pillars were previously described as providing higher mechanical stability against compressive forces. Following this hypothesis, we analyzed the carapace structure of D. pulex using histochemistry in combination with light and electron microscopy. We found the distal integument of the carapace to be significantly thicker than the proximal. The pillars appear fibrous with slim waists and broad, sometimes branched bases where they meet the integument layers. The fibrous structure and the slim-waisted shape of the pillars indicate a high capacity for withstanding tensile rather than compressive forces. In conclusion they are more ligaments than pillars. Therefore, we measured the hemolymphatic gauge pressure in D. longicephala and indeed found the hemocoel to have a pressure above ambient. Our results offer a new mechanistic explanation of the high rigidity of the daphniid carapace, which is probably the result of a light-weight construction consisting of two integuments bound together by ligaments and inflated by a hydrostatic hyper-pressure in the hemocoel. J. Morphol. 277:1320-1328, 2016. © 2016 Wiley Periodicals, Inc.

Dopamine is a key regulator in the signalling pathway underlying predator-induced defences in Daphnia

The waterflea Daphnia is a model to investigate the genetic basis of phenotypic plasticity resulting from one differentially expressed genome. Daphnia develops adaptive phenotypes (e.g. morphological defences) thwarting predators, based on chemical predator cue perception. To understand the genomic basis of phenotypic plasticity, the description of the precedent cellular and neuronal mechanisms is fundamental. However, key regulators remain unknown. All neuronal and endocrine stimulants were able to modulate but not induce defences, indicating a pathway of interlinked steps. A candidate able to link neuronal with endocrine responses is the multi-functional amine dopamine. We here tested its involvement in trait formation in Daphnia pulex and Daphnia longicephala using an induction assay composed of predator cues combined with dopaminergic and cholinergic stimulants. The mere application of both stimulants was sufficient to induce morphological defences. We determined dopamine localization in cells found in close association with the defensive trait. These cells serve as centres controlling divergent morphologies. As a mitogen and sclerotization agent, we anticipate that dopamine is involved in proliferation and structural formation of morphological defences. Furthermore, dopamine pathways appear to be interconnected with endocrine pathways, and control juvenile hormone and ecdysone levels. In conclusion, dopamine is suggested as a key regulator of phenotypic plasticity.

Endopolyploidy as a potential driver of animal ecology and evolution

Endopolyploidy - the existence of higher-ploidy cells within organisms that are otherwise of a lower ploidy level (generally diploid) - was discovered decades ago, but remains poorly studied relative to other genomic phenomena, especially in animals. Our synthetic review suggests that endopolyploidy is more common in animals than often recognized and probably influences a number of fitness-related and ecologically important traits. In particular, we argue that endopolyploidy is likely to play a central role in key traits such as gene expression, body and cell size, and growth rate, and in a variety of cell types, including those responsible for tissue regeneration, nutrient storage, and inducible anti-predator defences. We also summarize evidence for intraspecific genetic variation in endopolyploid levels and make the case that the existence of this variation suggests that endopolyploid levels are likely to be heritable and thus a potential target for natural selection. We then discuss why, in light of evident benefits of endopolyploidy, animals remain primarily diploid. We conclude by highlighting key areas for future research such as comprehensive evaluation of the heritability of endopolyploidy and the adaptive scope of endopolyploid-related traits, the extent to which endopolyploid induction incurs costs, and characterization of the relationships between environmental variability and endopolyploid levels.

Development of an efficient RNA interference method by feeding for the microcrustacean Daphnia

Background

RNA interference (RNAi) is an important molecular tool for analysis of gene function in vivo. Daphnia, a freshwater microcrustacean, is an emerging model organism for studying cellular and molecular processes involved in aging, development, and ecotoxicology especially in the context of environmental variation. However, in spite of the availability of a fully sequenced genome of Daphnia pulex, meaningful mechanistic studies have been hampered by a lack of molecular techniques to alter gene expression. A microinjection method for gene knockdown by RNAi has been described but the need for highly specialized equipment as well as technical expertise limits the wider application of this technique. In addition to being expensive and technically challenging, microinjections can only target genes expressed during embryonic stages, thus making it difficult to achieve effective RNAi in adult organisms.

Results

In our present study we present a bacterial feeding method for RNAi in Daphnia. We used a melanic Daphnia species (Daphnia melanica) that exhibits dark pigmentation to target phenoloxidase, a key enzyme in the biosynthesis of melanin. We demonstrate that our RNAi method results in a striking phenotype and that the phenoloxidase mRNA expression and melanin content, as well as survival following UV insults, are diminished as a result of RNAi.

Conclusions

Overall, our results establish a new method for RNAi in Daphnia that significantly advances further use of Daphnia as a model organism for functional genomics studies. The method we describe is relatively simple and widely applicable for knockdown of a variety of genes in adult organisms.

Daphnia longicephala neuropeptides: morphological description of crustacean cardioactive peptide (CCAP) and periviscerokinins in the Ctenodaphnia central nervous system

The publication of the Daphnia genome has driven research in this ecologically relevant model organism in many directions. However, information on this organism's physiology and the relevant controlling factors is limited. In this regard, especially neuropeptides are important biochemical regulators that control a variety of cellular processes, which in combination influence physiological conditions and allow the adaptation of the internal physiological state to external conditions. Thus, neuropeptides are prime in understanding an organism's physiology. We here aimed to detect and describe the distribution of evolutionary conserved neuropeptides including the crustacean cardioactive peptide (CCAP) and peptides of the family periviscerokinins (PVKs) in the central nervous system and the periphery of the Daphnia longicephala head region. We were able to identify a large pair of CCAP immunoreactive cells within central nervous system. In addition, in the periphery we found CCAP immunoreactive cells in the epidermis of the head with processes indicating cuticular secretion. Furthermore, we were able to identify and describe a complex neuronal circuit of PVK neuropeptides in the central nervous system. The data obtained in this study will provide important background information for future investigations aiming to unravel the cellular, neuronal and physiological pathways in a highly adaptive organism such as Daphnia.