Sensory neuroanatomy of Parastrongyloides trichosuri, a nematode parasite of mammals: Amphidial neurons of the first-stage larva

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.

Terror in the dirt: Sensory determinants of host seeking in soil-transmitted mammalian-parasitic nematodes

Infection with gastrointestinal parasitic nematodes is a major cause of chronic morbidity and economic burden around the world, particularly in low-resource settings. Some parasitic nematode species, including the human-parasitic threadworm Strongyloides stercoralis and human-parasitic hookworms in the genera Ancylostoma and Necator, feature a soil-dwelling infective larval stage that seeks out hosts for infection using a variety of host-emitted sensory cues. Here, we review our current understanding of the behavioral responses of soil-dwelling infective larvae to host-emitted sensory cues, and the molecular and cellular mechanisms that mediate these responses. We also discuss the development of methods for transgenesis and CRISPR/Cas9-mediated targeted mutagenesis in Strongyloides stercoralis and the closely related rat parasite Strongyloides ratti. These methods have established S. stercoralis and S. ratti as genetic model systems for gastrointestinal parasitic nematodes and are enabling more detailed investigations into the neural mechanisms that underlie the sensory-driven behaviors of this medically and economically important class of parasites.

Temperature-dependent behaviors of parasitic helminths

Parasitic helminth infections are the most common source of neglected tropical disease among impoverished global communities. Many helminths infect their hosts via an active, sensory-driven process in which environmentally motile infective larvae position themselves near potential hosts. For these helminths, host seeking and host invasion can be divided into several discrete behaviors that are regulated by both host-emitted and environmental sensory cues, including heat. Thermosensation is a critical sensory modality for helminths that infect warm-blooded hosts, driving multiple behaviors necessary for host seeking and host invasion. Furthermore, thermosensory cues influence the host-seeking behaviors of both helminths that parasitize endothermic hosts and helminths that parasitize insect hosts. Here, we discuss the role of thermosensation in guiding the host-seeking and host-infection behaviors of a diverse group of helminths, including mammalian-parasitic nematodes, entomopathogenic nematodes, and schistosomes. We also discuss the neural circuitry and molecular pathways that underlie thermosensory responses in these species.

Unexpected Variation in Neuroanatomy among Diverse Nematode Species

Nematodes are considered excellent models for understanding fundamental aspects of neuron function. However, nematodes are less frequently used as models for examining the evolution of nervous systems. While the habitats and behaviors of nematodes are diverse, the neuroanatomy of nematodes is often considered highly conserved. A small number of nematode species greatly influences our understanding of nematode neurobiology. The free-living species Caenorhabditis elegans and, to a lesser extent, the mammalian gastrointestinal parasite Ascaris suum are, historically, the primary sources of knowledge regarding nematode neurobiology. Despite differences in size and habitat, C. elegans and A. suum share a surprisingly similar neuroanatomy. Here, we examined species across several clades in the phylum Nematoda and show that there is a surprising degree of neuroanatomical variation both within and among nematode clades when compared to C. elegans and Ascaris. We found variation in the numbers of neurons in the ventral nerve cord and dye-filling pattern of sensory neurons. For example, we found that Pristionchus pacificus, a bacterial feeding species used for comparative developmental research had 20% fewer ventral cord neurons compared to C. elegans. Steinernema carpocapsae, an insect-parasitic nematode capable of jumping behavior, had 40% more ventral cord neurons than C. elegans. Interestingly, the non-jumping congeneric nematode, S. glaseri showed an identical number of ventral cord neurons as S. carpocapsae. There was also variability in the timing of neurodevelopment of the ventral cord with two of five species that hatch as second-stage juveniles showing delayed neurodevelopment. We also found unexpected variation in the dye-filling of sensory neurons among examined species. Again, sensory neuron dye-filling pattern did not strictly correlate with phylogeny. Our results demonstrate that variation in nematode neuroanatomy is more prevalent than previously assumed and recommend this diverse phylum for future "evo-devo-neuro" studies.

The dauer hypothesis and the evolution of parasitism: 20 years on and still going strong

How any complex trait has evolved is a fascinating question, yet the evolution of parasitism among the nematodes is arguably one of the most arresting. How did free-living nematodes cross that seemingly insurmountable evolutionary chasm between soil dwelling and survival inside another organism? Which of the many finely honed responses to the varied and harsh environments of free-living nematodes provided the material upon which natural selection could act? Although several complementary theories explain this phenomenon, I will focus on the dauer hypothesis. The dauer hypothesis posits that the arrested third-stage dauer larvae of free-living nematodes such as Caenorhabditis elegans are, due to their many physiological similarities with infective third-stage larvae of parasitic nematodes, a pre-adaptation to parasitism. If so, then a logical extension of this hypothesis is that the molecular pathways which control entry into and recovery from dauer formation by free-living nematodes in response to environmental cues have been co-opted to control the processes of infective larval arrest and activation in parasitic nematodes. The molecular machinery that controls dauer entry and exit is present in a wide range of parasitic nematodes. However, the developmental outputs of the different pathways are both conserved and divergent, not only between populations of C. elegans or between C. elegans and parasitic nematodes but also between different species of parasitic nematodes. Thus the picture that emerges is more nuanced than originally predicted and may provide insights into the evolution of such an interesting and complex trait.

Strongyloides stercoralis age-1: a potential regulator of infective larval development in a parasitic nematode

Infective third-stage larvae (L3i) of the human parasite Strongyloides stercoralis share many morphological, developmental, and behavioral attributes with Caenorhabditis elegans dauer larvae. The ‘dauer hypothesis’ predicts that the same molecular genetic mechanisms control both dauer larval development in C. elegans and L3i morphogenesis in S. stercoralis. In C. elegans, the phosphatidylinositol-3 (PI3) kinase catalytic subunit AGE-1 functions in the insulin/IGF-1 signaling (IIS) pathway to regulate formation of dauer larvae. Here we identify and characterize Ss-age-1, the S. stercoralis homolog of the gene encoding C. elegans AGE-1. Our analysis of the Ss-age-1 genomic region revealed three exons encoding a predicted protein of 1,209 amino acids, which clustered with C. elegans AGE-1 in phylogenetic analysis. We examined temporal patterns of expression in the S. stercoralis life cycle by reverse transcription quantitative PCR and observed low levels of Ss-age-1 transcripts in all stages. To compare anatomical patterns of expression between the two species, we used Ss-age-1 or Ce-age-1 promoter::enhanced green fluorescent protein reporter constructs expressed in transgenic animals for each species. We observed conservation of expression in amphidial neurons, which play a critical role in developmental regulation of both dauer larvae and L3i. Application of the PI3 kinase inhibitor LY294002 suppressed L3i in vitro activation in a dose-dependent fashion, with 100 µM resulting in a 90% decrease (odds ratio: 0.10, 95% confidence interval: 0.08–0.13) in the odds of resumption of feeding for treated L3i in comparison to the control. Together, these data support the hypothesis that Ss-age-1 regulates the development of S. stercoralis L3i via an IIS pathway in a manner similar to that observed in C. elegans dauer larvae. Understanding the mechanisms by which infective larvae are formed and activated may lead to novel control measures and treatments for strongyloidiasis and other soil-transmitted helminthiases.