Three-dimensional fine structural reconstruction of the nose sensory structures of Acrobeles complexus compared to Caenorhabditis elegans (Nematoda: Rhabditida)

The role of carbon dioxide in nematode behaviour and physiology

Carbon dioxide (CO2) is an important sensory cue for many animals, including both parasitic and free-living nematodes. Many nematodes show context-dependent, experience-dependent and/or life-stage-dependent behavioural responses to CO2, suggesting that CO2 plays crucial roles throughout the nematode life cycle in multiple ethological contexts. Nematodes also show a wide range of physiological responses to CO2. Here, we review the diverse responses of parasitic and free-living nematodes to CO2. We also discuss the molecular, cellular and neural circuit mechanisms that mediate CO2 detection in nematodes, and that drive context-dependent and experience-dependent responses of nematodes to CO2.

Evolution of neuronal anatomy and circuitry in two highly divergent nematode species

The nematodes C. elegans and P. pacificus populate diverse habitats and display distinct patterns of behavior. To understand how their nervous systems have diverged, we undertook a detailed examination of the neuroanatomy of the chemosensory system of P. pacificus. Using independent features such as cell body position, axon projections and lipophilic dye uptake, we have assigned homologies between the amphid neurons, their first-layer interneurons, and several internal receptor neurons of P. pacificus and C. elegans. We found that neuronal number and soma position are highly conserved. However, the morphological elaborations of several amphid cilia are different between them, most notably in the absence of ‘winged’ cilia morphology in P. pacificus. We established a synaptic wiring diagram of amphid sensory neurons and amphid interneurons in P. pacificus and found striking patterns of conservation and divergence in connectivity relative to C. elegans, but very little changes in relative neighborhood of neuronal processes. These findings demonstrate the existence of several constraints in patterning the nervous system and suggest that major substrates for evolutionary novelty lie in the alterations of dendritic structures and synaptic connectivity.

Nerve cells, also called neurons, are responsible both for sensing signals from the environment and for determining how organisms react. This means that the unique features of an animal’s nervous system underpin its characteristic behaviors. Comparing the anatomy of the nervous systems in different animals could therefore yield valuable insights into how structural and behavioral differences emerge over time.

Behavioral variation often occurs even in similar-looking animals. One example is a group of microscopic worms, called nematodes. Although many nematode species exist, their overall body plans are the same, and the worms of each species contain a fixed number of cells. Despite these apparent similarities, different species of nematodes inhabit a variety of environments and may respond differently to the same signals.

The main sensory organs in nematodes are called the amphid sensilla. They are used to detect chemicals, as well as other inputs from the environment such as temperature and pheromones from other nematodes. Although researchers have often speculated that the number of cells in these organs and their arrangement are broadly the same across species, their anatomy had not been studied in detail.

Hong, Riebesell et al. compared the detailed structure and genetic features of the sensory systems in two distantly related species of nematode worms, Pristionchus pacificus and Caenorhabditis elegans. These two species behave in different ways, for example, P. pacificus is usually found in association with different species of beetles, while C. elegans is free-living and usually found on rotting fruit. By comparing the two, Hong, Riebesell et al. wanted to determine whether the diverse behaviors observed in the two species could be determined by differences between their sensory systems.

Experiments using electron microscopy yielded several thousand high resolution images spanning the entire sensory organ. These images were then used to create detailed reconstructions of the sensory nervous system in each worm species, demonstrating that both species had the same number of sensory nerve cells, allowing one-to-one comparisons between them.

Further analysis showed that while the overall structure of the neuronal connections remains the same between the two species, the neurons in P. pacificus made more diverse connections than those in C. elegans. Detailed studies of gene activity also revealed that neurons in each species switched on a slightly different group of genes, possibly indicating that each type of worm processes sensory signals in different ways.

These results shed new light on how nervous systems in related species can change over time without any change in neuron count. In the future, a better understanding of these changes could link the evolution of the nervous system to the emergence of different behaviors, in both simple and more complex organisms.

A targeted 3D EM and correlative microscopy method using SEM array tomography

Using electron microscopy to localize rare cellular events or structures in complex tissue is challenging. Correlative light and electron microscopy procedures have been developed to link fluorescent protein expression with ultrastructural resolution. Here, we present an optimized scanning electron microscopy (SEM) workflow for volumetric array tomography for asymmetric samples and model organisms (Caenorhabditis elegans, Drosophila melanogaster, Danio rerio). We modified a diamond knife to simplify serial section array acquisition with minimal artifacts. After array acquisition, the arrays were transferred to a glass coverslip or silicon wafer support. Using light microscopy, the arrays were screened rapidly for initial recognition of global anatomical features (organs or body traits). Then, using SEM, an in-depth study of the cells and/or organs of interest was performed. Our manual and automatic data acquisition strategies make 3D data acquisition and correlation simpler and more precise than alternative methods. This method can be used to address questions in cell and developmental biology that require the efficient identification of a labeled cell or organelle.

Gas sensing in nematodes

Nearly all animals are capable of sensing changes in environmental oxygen (O2) and carbon dioxide (CO2) levels, which can signal the presence of food, pathogens, conspecifics, predators, or hosts. The free-living nematode Caenorhabditis elegans is a powerful model system for the study of gas sensing. C. elegans detects changes in O2 and CO2 levels and integrates information about ambient gas levels with other internal and external cues to generate context-appropriate behavioral responses. Due to its small nervous system and amenability to genetic and genomic analyses, the functional properties of its gas-sensing microcircuits can be dissected with single-cell resolution, and signaling molecules and natural genetic variations that modulate gas responses can be identified. Here, we discuss the neural basis of gas sensing in C. elegans, and highlight changes in gas-evoked behaviors in the context of other sensory cues and natural genetic variations. We also discuss gas sensing in other free-living nematodes and parasitic nematodes, focusing on how gas-sensing behavior has evolved to mediate species-specific behavioral requirements.

Origin and evolution of dishevelled

Dishevelled (Dsh or Dvl) is an important signaling protein, playing a key role in Wnt signaling and relaying cellular information for several developmental pathways. Dsh is highly conserved among metazoans and has expanded into a multigene family in most bilaterian lineages, including vertebrates, planarians, and nematodes. These orthologs, where explored, are known to have considerable overlap in function, but evidence for functional specialization continues to mount. We performed a comparative analysis of Dsh across animals to explore protein architecture and identify conserved and divergent features that could provide insight into functional specialization with an emphasis on invertebrates, especially nematodes. We find evidence of dynamic evolution of Dsh, particularly among nematodes, with taxa varying in ortholog number from one to three. We identify a new domain specific to some nematode lineages and find an unexpected nuclear localization signal conserved in many Dsh orthologs. Our findings raise questions of protein evolution in general and provide clues as to how animals have dealt with the complex intricacies of having a protein, such as Dsh, act as a central messenger hub connected to many different and vitally important pathways. We discuss our findings in the context of functional specialization and bring many testable hypotheses to light.

A sensory code for host seeking in parasitic nematodes

Parasitic nematode species often display highly specialized host-seeking behaviors that reflect their specific host preferences. Many such behaviors are triggered by host odors, but little is known about either the specific olfactory cues that trigger these behaviors or the underlying neural circuits. Heterorhabditis bacteriophora and Steinernema carpocapsae are phylogenetically distant insect-parasitic nematodes whose host-seeking and host-invasion behavior resembles that of some devastating human- and plant-parasitic nematodes. We compare the olfactory responses of Heterorhabditis and Steinernema infective juveniles (IJs) to those of Caenorhabditis elegans dauers, which are analogous life stages. The broad host range of these parasites results from their ability to respond to the universally produced signal carbon dioxide (CO(2)), as well as a wide array of odors, including host-specific odors that we identified using thermal desorption-gas chromatography-mass spectroscopy. We find that CO(2) is attractive for the parasitic IJs and C. elegans dauers despite being repulsive for C. elegans adults, and we identify a sensory neuron that mediates CO(2) response in both parasitic and free-living species, regardless of whether CO(2) is attractive or repulsive. The parasites' odor response profiles are more similar to each other than to that of C. elegans despite their greater phylogenetic distance, likely reflecting evolutionary convergence to insect parasitism.

Resolving phylogenetic incongruence to articulate homology and phenotypic evolution: a case study from Nematoda

Modern morphology-based systematics, including questions of incongruence with molecular data, emphasizes analysis over similarity criteria to assess homology. Yet detailed examination of a few key characters, using new tools and processes such as computerized, three-dimensional ultrastructural reconstruction of cell complexes, can resolve apparent incongruence by re-examining primary homologies. In nematodes of Tylenchomorpha, a parasitic feeding phenotype is thus reconciled with immediate free-living outgroups. Closer inspection of morphology reveals phenotypes congruent with molecular-based phylogeny and points to a new locus of homology in mouthparts. In nematode models, the study of individually homologous cells reveals a conserved modality of evolution among dissimilar feeding apparati adapted to divergent lifestyles. Conservatism of cellular components, consistent with that of other body systems, allows meaningful comparative morphology in difficult groups of microscopic organisms. The advent of phylogenomics is synergistic with morphology in systematics, providing an honest test of homology in the evolution of phenotype.

Comparative, three-dimensional anterior sensory reconstruction of Aphelenchus avenae (nematoda: Tylenchomorpha)

The anterior sensory anatomy (not including amphids) of the nematode Aphelenchus avenae (Tylenchomorpha) has been three-dimensionally reconstructed from serial, transmission electron microscopy thin sections. Models, showing detailed morphology and spatial relationships of cuticular sensilla and internal sensory receptors, are the first computerized reconstruction of sensory structures of a Tylenchomorpha nematode. Results are analyzed with respect to similarly detailed reconstructions of Rhabditida outgroup nematodes, Acrobeles complexus (Cephalobomorpha) and Caenorhabditis elegans (Rhabditomorpha). Homologies identified in A. avenae demonstrate the general conservation of the anterior sensory system between freeliving nematodes and the largely plant parasitic Tylenchomorpha. A higher degree of similarity is shown between A. avenae and A. complexus, with common features including: the presence of a second, internal outer labial dendrite (OL1); a second cephalic dendrite in the female (CEP2/CEM); an accessory process loop of inner labial dendrite 1; and terminus morphology and epidermal associations of internal sensory receptors BAG and URX. Unique to A. avenae is a pair of peripheral, lateral neurons of unknown homology but with axial positions and intercellular relationships nearly identical to the "posterior branches" of URX in A. complexus. Knowledge of homologies and connectivity of anterior sensory structures provides a basis for expansion of the experimental behavioral model of C. elegans to the economically important nematodes of Tylenchomorpha.

Three-dimensional reconstruction of the stomatostylet and anterior epidermis in the nematode Aphelenchus avenae (Nematoda: Aphelenchidae) with implications for the evolution of plant parasitism

A three-dimensional model of the stomatostylet and associated structures has been reconstructed from serial thin sections of Aphelenchus avenae, a representative of Tylenchomorpha, a group including most plant parasitic nematodes. The reconstruction is compared with previous work on bacteriovorous cephalobids and rhabditids to better understand the evolution of the stylet and its associated cells. Two arcade syncytia ("guide ring") line the stylet shaft, supporting the hypothesis that the stylet shaft and cone (into which the shaft extends and which is not lined by syncytia) are homologous with the gymnostom of cephalobids, the sister taxon of tylenchids. Epidermal syncytia, HypA, HypB, HypC, and HypE, line the cephalic framework, vestibule, and vestibule extension, congruent with the hypothesis that these components are homologous with the cephalobid cheilostom. Relative to outgroups, HypC is expanded in A. avenae, enclosing sensilla that fill most of the cephalic framework. The homolog of syncytium HypD in the cephalobid Acrobeles complexus is not observed in A. avenae. Arcade syncytia are reduced compared with those of cephalobids. Stylet protractor muscles in A. avenae are homologous with the most anterior set of radial muscles of cephalobids. Observations to date test and verify our previous hypotheses of homology of the stomatostylet with respect to the stoma of bacteriovorous outgroups. Reconstruction of the stegostom and pharynx will provide further tests of homology and evolution of feeding structure adaptations for plant parasitism.

Three-dimensional reconstruction of the amphid sensilla in the microbial feeding nematode, Acrobeles complexus (Nematoda: Rhabditida)

Amphid sensilla are the primary olfactory, chemoreceptive, and thermoreceptive organs in nematodes. Their function is well described for the model organism Caenorhabditis elegans, but it is not clear to what extent we can generalize these findings to distantly related nematodes of medical, economic, and agricultural importance. Current detailed descriptions of anatomy and sensory function are limited to nematodes that recent molecular phylogenies would place in the same taxonomic family, the Rhabditidae. Using serial thin-section transmission electron microscopy, we reconstructed the anatomy of the amphid sensilla in the more distantly related nematode, Acrobeles complexus (Cephalobidae). Amphid structure is broadly conserved in number and arrangement of cells. Details of cell anatomy differ, particularly for the sensory neurite termini. We identify an additional sensory neuron not found in the amphid of C. elegans and propose homology with the C. elegans interneuron AUA. Hypotheses of homology for the remaining sensory neurons are also proposed based on comparisons between C. elegans, Strongyloides stercoralis, and Haemonchus contortus.