Chemo- and thermosensory neurons: structure and function in animal parasitic nematodes

Microscopy and the helminth parasite

Microscopy has a long and distinguished history in the study of helminth parasites and has made a singularly outstanding contribution to understanding how these complex animals organise their lives and relate to their hosts. Increasingly, the microscope has been used as a powerful investigative tool in multidisciplinary approaches to parasitological problems, placing emphasis on functional correlates rather than anatomical detail. In doing so, microscopy has also uncovered a number of attributes of parasites that are of wider significance in the field of biology. Parasite surfaces have understandably demanded most of the attention of microscopists, largely as a result of the pioneering studies using transmission electron microscopy. Their findings focused the attention of physiologists and immunologists on the tegument and cuticle of helminths and in doing so helped unravel the complex molecular exchanges that are fundamental to understanding host-parasite interactions. Scanning electron microscopy succeeded in augmenting these data by revealing novel microtopographical features of the host-parasite relationship, as well as proving invaluable in helminth taxonomy and in assessing the efficacy of test substances in drug screens. Control of helminth parasites has never been more critical: problems of drug resistance demand urgent action to identify exploitable targets for new generation anthelmintics. In this regard, the neuropeptide signalling system of helminths is envisioned as central to nerve-muscle function, and thereby a crucial regulatory influence on their motility, alimentation and reproduction. The use of immunocytochemistry interfaced with confocal scanning laser microscopy has not only been instrumental in discovering the peptidergic system of helminths and its potential for chemotherapeutic exploitation, but through increasingly sophisticated bio-imaging technologies has continued to help dissect and analyse the molecular dynamics of this and other cellular systems within these important parasites.

High prevalence of seropositivity to a major allergen of Anisakis simplex, Ani s 1, in dyspeptic patients

Finding evidence of anisakidosis requires invasive methods. We have developed a serological assay based on the detection of an immunoglobulin E (IgE) specifically directed against Ani s 1 protein, a major parasite allergen of Anisakis simplex, which has shown a high level of accuracy in the diagnosis of anisakidosis. We used this tool to determine the prevalence of anti-Ani s 1 IgE in dyspeptic patients and to investigate if its seropositivity could be related to epidemiological factors other than raw fish consumption. A total of 174 dyspeptic patients who submitted to upper digestive tract endoscopy were studied. Specific IgE against Ani s 1 was determined by immunoblotting. Quantitative information on smoking, alcohol consumption, and fish consumption as well as a history of gastric surgery was recorded. Twenty-four (13.8%) patients were seropositive for Ani s 1 protein. The seroprevalence of anti-Ani s 1 IgE increased with age in patients who were less than 62 years old (P = 0.047). Seropositivity to Ani s 1 was associated with the consumption of fish in vinegar (P < 0.001), raw fish (P = 0.001), and smoked fish (P = 0.007). There was no relationship between anti-Ani s 1 IgE seropositivity and the number of cigarettes smoked (P = 0.098) or alcohol intake (P = 0.179). Five patients had undergone previous gastric surgery, and three of those patients were seropositive for Ani s 1 (P = 0.019). In multivariate analysis, the consumption of fish in vinegar (P = 0.006), raw fish (P = 0.017), and smoked fish (P = 0.002) and a history of gastric surgery (P = 0.005) were independent factors associated with anti-Ani s 1 IgE detection. In conclusion, at present, anisakidosis might frequently be underdiagnosed, and it might have a clinical role in patients with upper dyspeptic symptoms. Uncooked-fish ingestion and previous gastric surgery were associated with seropositivity for Ani s 1 protein.

An investigation of chemotaxis in the insect parasitic nematode Heterorhabditis bacteriophora

We tested the chemotactic responses of dauer juvenile stages (DJs) of the insect parasitic nematode Heterorhabditis bacteriophora to a variety of compounds that are known to be highly attractive or highly repellent to Caenorhabditis elegans. While H. bacteriophora DJs respond to alcohols and some aromatic compounds as well as to host metabolites such as uric acid and CO2, the most notable difference in the responses of these two nematodes is that H. bacteriophora DJs are unresponsive to a large number of compounds which C. elegans finds highly attractive. The latter compounds are typical by-products of bacterial metabolism and include aldehydes, esters, ketones and short-chain alcohols. While C. elegans finds long-chain alcohols (e.g. 1-heptanol and 1-octanol) repellent and short-chain alcohols highly attractive, H. bacteriophora DJs are strongly attracted to 1-heptanol, 1-octanol and 1-nonanol and find short-chain alcohols to be only slightly attractive. Parasitic-stage H. bacteriophora nematodes show a very weak chemotactic response to volatile molecules that DJs find highly attractive. Our results suggest that, associated with the adoption of a parasitic mode of life by Heterorhabditis, there was an adaptive change in chemotactic behaviour of the infective stages, resulting in a decreased sensitivity to volatile by-products of bacterial metabolism and an increased sensitivity towards long-chain alcohols and other insect-specific volatiles and possibly also to herbivore-induced plant volatiles.

Molecular biology of reproduction and development in parasitic nematodes: progress and opportunities

Molecular biological research on the development and reproduction of parasites is of major significance for many fundamental and applied areas of medical and veterinary parasitology. Together with knowledge of parasite biology and epidemiology, the application of molecular tools and technologies provides unique opportunities for elucidating developmental and reproductive processes in helminths. This article focuses specifically on recent progress in studying the molecular mechanisms of development, sexual differentiation and reproduction in parasitic nematodes of socio-economic importance and comparative analyses, where appropriate, with the free-living nematode Caenorhabditis elegans. It also describes the implications of such work for understanding reproduction, tissue migration, hypobiosis, signal transduction and host-parasite interactions at the molecular level, and for seeking new means of parasite intervention.

Behaviour of filariae: morphological and anatomical signatures of their life style within the arthropod and vertebrate hosts

This paper attempts to pinpoint the most original morphological anatomical features of the biology of filariae per se and those which are or could be important for triggering regulatory processes in the arthropod vector and uncontrolled pathogenic processes in the vertebrate hosts. The following stages are considered: the motile egg or newly-hatched larva, the microfilaria, in the lymphatic or blood vessels of its vertebrate host; the larva, its migrations and its intrasyncitial development in the hematophagous arthropod subverted as vector; its transfer to the vertebrate host, migratory properties through the lymphatic system, maturation, mating and, finally, egg laying in the tissues they reach. This synthesis is based on parasite morphological features and their functional interpretation, histological features in the different niches the filariae reach, and on quantitative analyses of filarial development at its different phases, as well as on the rare and valuable observations of living parasites in situ. Data have been drawn from various species of Onchocercidae from amphibians, reptiles, birds and mammals. These comparative analyses have revealed the major constraints to which the filariae, including those parasitizing humans, have been subjected during their evolution from their ancestors, the oviparous and heteroxenic spirurids. Emphasis is placed on mechanical events: resistance of the microfilariae to the currents in the blood or lymph vessels, regulatory processes induced in the vector mesenteron by the movements of the ingested microfilariae, transient disruption by the microfilarial cephalic hook of the vectors' tissues and cell membranes during microfilarial translocation, attachment of males to females during mating by means of 'non-slip' systems, etc. Like other nematodes, filariae are equipped with sensory organs and a locomotor system, composed of the muscles and of the original osmoregulatory-excretory cell. Any change in one of these elements will result in the destruction of the filaria, at some stage of its development. In the vertebrate host, the intravascular stages will no longer be able to resist being carried passively towards the organs of destruction such as the lymph nodes or the lungs.

The avermectin receptors of Haemonchus contortus and Caenorhabditis elegans

Most of the recent evidence suggests that the avermectin/milbemycin family of anthelmintics act via specific interactions with glutamate-gated chloride channels. These channels are encoded by a small family of genes in nematodes, though the composition of the gene family and the function of the individual members of the family may vary between species. We review our current knowledge concerning the properties of the glutamate-gated chloride channels from Caenorhabditis elegans and the related parasite, Haemonchus contortus. We conclude that the biological effects of the avermectins/milbemycins can be largely explained by the known pharmacology and distribution of the glutamate-gated chloride channels and that differences between the glutamate-gated chloride channels from different nematodes may underlie species-specific variations in anthelmintic action.

Distribution of glutamate-gated chloride channel subunits in the parasitic nematode Haemonchus contortus

Glutamate-gated chloride channels (GluCl) are related to gamma-aminobutyric acid-A (GABA(A)) receptors and are the target sites for the avermectin/milbemycin (A/M) anthelmintics, drugs that cause paralysis of the somatic and pharyngeal muscles in nematodes. We have previously identified four GluCl subunits, HcGluClalpha, HcGluClbeta, HcGluClalpha3A, and HcGluClalpha3B from the sheep parasite Haemonchus contortus. We raised specific antisera against all of these subunits and used them in immunofluorescence experiments on adult parasites. All of the subunits were expressed in the motor nervous system, especially motor neuron commissures. Double-immunostaining experiments suggested that HcGluClalpha and HcGluClbeta were expressed on the same commissures; these were also stained with an anti-GABA antibody, suggesting that they may be inhibitory motor neurons. The HcGluClbeta subunit was also detected in lateral and sublateral nerve cords. The HcGluClalpha3A and -B subunits, products of an alternatively spliced gene, were expressed in different neurons. We found HcGluClalphaA in a pair of sensory, possibly amphid, neurons in the head, in addition to the motor neuron commissures. HcGluClalpha3B was detected in three cell bodies, probably of pharyngeal neurons, and to ventral and lateral cords. These results indicate that the GluCl are widely distributed in the H. contortus nervous system and suggest that they have critical roles controlling locomotion, pharyngeal function, and possibly sensory processing in parasitic nematodes. They also provide an explanation for the observed effects of the A/M anthelmintics.

XOL-1, primary determinant of sexual fate in C. elegans, is a GHMP kinase family member and a structural prototype for a class of developmental regulators

In Caenorhabditis elegans, an X chromosome-counting mechanism specifies sexual fate. Specific genes termed X-signal elements, which are present on the X chromosome, act in a concerted dose-dependent fashion to regulate levels of the developmental switch gene xol-1. In turn, xol-1 levels determine sexual fate and the activation state of the dosage compensation mechanism. The crystal structure of the XOL-1 protein at 1.55 A resolution unexpectedly reveals that xol-1 encodes a GHMP kinase family member, despite sequence identity of 10% or less. Because GHMP kinases, thus far, have only been characterized as small molecule kinases involved in metabolic pathways, for example, amino acid and cholesterol synthesis, XOL-1 is the first member that controls nonmetabolic processes. Biochemical investigations demonstrated that XOL-1 does not bind ATP under standard conditions, suggesting that XOL-1 acts by a mechanism distinct from that of other GHMP kinases. In addition, we have cloned a XOL-1 ortholog from Caenorhabditis briggsae, a related nematode that diverged from C. elegans approximately 50-100 million years ago. These findings demonstrate an unanticipated role for GHMP kinase family members as mediators of sexual differentiation and dosage compensation and, possibly, other aspects of differentiation and development.

Amphidial structure of ivermectin-resistant and susceptible laboratory and field strains of Haemonchus contortus

The development of anthelmintic resistance by nematode parasites is a growing problem for veterinarians, pet owners, and producers. The intensive use of the macrocyclic lactones for the treatment of a variety of parasitic diseases has hastened the development of resistance to this family of parasiticides. As a result, resistance to ivermectin, moxidectin, nemadectin, and doramectin by Haemonchus contortus has been documented throughout the world. Sensory neurons located in the cephalic end of nematodes are in close contact with the external environment. Through these neurons, important chemical and thermal cues are gathered by the parasite. Examination of serial electron micrographs of ivermectin-susceptible and ivermectin-resistant H. contortus allows for comparison of neuronal structure, arrangement of neurons within the amphidial channel, and distance of the tip of the dendritic processes to the amphidial pore. The latter of these characteristics provides a useful means by which to compare the association between the neurons and the external environment of the worm. Comparison of parental laboratory strains of ivermectin-susceptible strains of H. contortus with related selected, ivermectin-resistant strains and with a wild-type ivermectin-susceptible field strain of H. contortus from Louisiana reveal that the ivermectin-resistant worms examined have markedly shorter sensory cilia than their ivermectin-susceptible parental counterparts. Additionally, the amphidial neurons of ivermectin-resistant worms are characterized by generalized degeneration and loss of detail, whereas other neurons outside of the channels, such as the labial and cephalic neurons, are normal in structure. These findings raise a number of questions regarding the relationship between amphidial structure and ivermectin resistance as well as the role of amphids as a means of entry for ivermectin. While shortened amphidial sensilla are associated with ivermectin resistance, it remains unclear if such a structural modification facilitates survival of nematodes exposed to macrocyclic lactones.

Molecular phylogenetic analysis of the genus Strongyloides and related nematodes

Strongyloides spp., parasitic nematodes of humans and many other terrestrial vertebrates, display an unusual heterogonic lifecycle involving alternating parasitic and free-living adult reproductive stages. A number of other genera have similar lifecycles, but their relationships to Strongyloides have not been clarified. We have inferred a phylogeny of 12 species of Strongyloides, Parastrongyloides, Rhabdias and Rhabditophanes using small subunit ribosomal RNA gene (SSU rDNA) sequences. The lineage leading to Strongyloides appears to have arisen within parasites of terrestrial invertebrates. Inferred lifecycle evolution was particularly dynamic within these nematodes. Importantly, the free-living Rhabditophanes sp. KR3021 is placed within a clade of parasitic taxa, suggesting that this species may represent a reversion to a non-parasitic lifecycle. Species within the genus Strongyloides are very closely related, despite the disparity of host species parasitised. The highly pathogenic human parasite Strongyloides fuelleborni kelleyi is not supported as a subspecies of the primate parasite S. fuelleborni fuelleborni, but is most likely derived from a local zoonotic source.

Orientation of Onchocerca lienalis stiles (Filarioidea: Onchocercidae) microfilariae to black fly saliva

Black flies (Simulium spp.) are intermediate hosts and vectors of parasitic nematodes belonging to the genus Onchocerca (Filarioidea Onchocercidae). Infection and subsequent transmission of infective third-stage larvae occur at the vertebrate host-skin interface. Experimental evidence presented here demonstrates that Onchocerca lienalis Stiles microfilariae orient to one or more components (microfilarial orientation factor [s]; MOF) in black fly saliva. MOFs may serve as a means for microfilariae to find and infect black flies during the act of blood-feeding. Directed movement through the host's skin to the bite site is necessary because Onchocerca spp. microfilariae do not circulate in the blood. The substance directing microfilarial orientation appears to be a salivary protein, but it is not the Simulium vittatum Zetterstedt erythema protein (SVEP) described from New World Simulium spp. These results support earlier field observations that associated increased numbers of cutaneous microfilariae with black fly feeding and indicate that a fundamental molecular mechanism linked to vector saliva may be key for the maintenance of the life cycle of Onchocerca spp. Salivary molecules that induce orientation of microfilariae to the bite site are potential targets for use in transmission-blocking vaccines to uncouple this primary vector infection step.

Transforming growth factor-beta and insulin-like signalling pathways in parasitic helminths

The signal transduction pathways involved in regulating developmental arrest in the free-living nematode, Caenorhabditis elegans, are fairly well characterised. However, much less is known about how these processes may influence the developmental timing and maturation in helminth parasites. Here, we provide an overview of two signalling pathways implicated in the regulation of dauer larva formation in C. elegans, the insulin-like signalling pathway and the transforming growth factor-beta pathway, and explore what is known about these signalling pathways in a variety of parasitic helminths. Understanding the differences about how these pathways are affected by environmental cues in free-living versus parasitic species of helminths may provide insights into novel mechanisms for the control or prevention of helminth-induced disease.

Response to carbon dioxide by the infective larvae of three species of parasitic nematodes

The response of infective third-stage larvae (L3) of three species of parasitic nematodes, Ancylostoma caninum, Strongyloides stercoralis, and Haemonchus contortus to carbon dioxide (CO(2)) at physiological concentrations was investigated. L3 of the skin-penetrating species, A. caninum and S. stercoralis, were stimulated by CO(2) at the concentration found in human breath (3.3-4%); these larvae responded by crawling actively, but not directionally. Crawling was not stimulated by breath passed through a CO(2)-removing "scrubber" or by "bench air". Both A. caninum and S. stercoralis L3 stopped crawling when exposed to 5% CO(2) for 1 min. L3 of A. caninum became active 9-14 min after exposure to 5% CO(2) ended, but activity resumed more rapidly (10-15 s) if larvae were subsequently exposed to breath or breath through the scrubber. L3 of S. stercoralis resumed crawling 30-35 s after exposure to 5% CO(2), but resumed crawling within a very few seconds when exposed to breath or breath through the scrubber. Thus, while 5% CO(2) was inhibitory, lower concentrations of this gas stimulated L3 of both species. Apparently, exposing immobilized larvae to breath or breath through the scrubber causes the environmental CO(2) concentration to drop to a level that is stimulatory. The L3 of H. contortus ceased crawling and coiled when exposed to human breath or to 1% CO(2), but continued to move within the coil in both cases. The crawling response of the L3 of the two skin-penetrating species, A. caninum and S. stercoralis, to stimulation by CO(2) probably relates to their active host-finding behavior, while the cessation response elicited by CO(2) in H. contortus larvae may relate to the fact that they rely on passive ingestion by a ruminant host.

Caenorhabditis elegans: how good a model for veterinary parasites?

The organism about which most is known on a molecular level is a nematode, the free-living organism Caenorhabditis elegans. This organism has served as a reasonable model for the discovery of anthelmintic drugs and for research on the mechanism of action of anthelmintics. Useful information on mechanisms of anthelmintic resistance has also been obtained from studies on C. elegans. Unfortunately, there has not been a large-scale extension of genetic techniques developed in C. elegans to research on parasitic species of veterinary (or human) parasites. Much can be learned about the essentials of nematode biology by studying C. elegans, but discovering the basic biology of nematode parasitism can only be gained through comparative studies on multiple parasitic species.

In vitro reactivation of Ancylostoma caninum tissue-arrested third-stage larvae by transforming growth factor-beta

Developmental arrest in Ancylostoma caninum is associated with preparasitic, free-living third-stage (L3) larvae, as well as anthelmintic-resilient hypobiotic L3 larvae within the tissues of an infected dog. With the tissue-arrested larvae, pregnancy and, more specifically, the hormonal effects of estrogen and prolactin mediate reactivation resulting in transmammary transmission of infection to nursing puppies. Estrogen and prolactin have been shown to be critically involved in upregulation of transforming growth factor (TGF)-beta2 during pregnancy, and studies on the soil nematode Caenorhabditis elegans further implicate TGF-beta and insulin-like signaling pathways with larval arrest and reactivation. In this report, an in vitro assay was used to show that neither estrogen, prolactin, nor insulin had a direct effect on the feeding/reactivation response of tissue-arrested larvae; however, TGF-beta isoforms 1 and 2 both had significant stimulatory effects that were comparable to the effects of dog serum. The stimulatory effects of serum could be blocked by preincubation with anti-TGF-beta antibodies. Taken together, the results support the hypothesis that during pregnancy, host-derived TGF-beta can signal a parasite-encoded receptor to trigger the reactivation of tissue-arrested larvae. TGF-beta had no effect on preparasitic larvae, suggesting that different signals may be involved in reactivation of the 2 different arrested forms of A. caninum L3 larvae.

Ancylostoma caninum: the finger cell neurons mediate thermotactic behavior by infective larvae of the dog hookworm

Bhopale, V. M., Kupprion, E. K., Ashton, F. T., Boston, R., and Schad, G. A. 2001. Ancylostoma caninum: The finger cell neurons mediate thermotactic behavior by infective larvae of the dog hookworm. Experimental Parasitology 97, 70-76. In the amphids (anteriorly positioned, paired sensilla) of the free-living nematode Caenorhabditis elegans, the so-called finger cells (AFD), a pair of neurons, each of which ends in a cluster of microvilli-like projections, are known to be the primary thermoreceptors. A similar neuron pair in the amphids of the parasitic nematode Haemonchus contortus is also known to be thermoreceptive. The hookworm of dogs, Ancylostoma caninum, has apparent structural homologs of finger cells in its amphids. The neuroanatomy of the amphids of A. caninum and H. contortus is strikingly similar, and the amphidial cell bodies in the lateral ganglia of the latter nematode have been identified and mapped. When the lateral ganglia of first-stage larvae (L1) of A. caninum are examined with differential interference contrast microscopy, positional homologs of the recognized amphidial cell bodies in the lateral ganglia of H. contortus L1 are readily identified in A. caninum. The amphidial neurons in A. caninum were consequently given the same names as those of their apparent homologs in H. contortus. It was hypothesized that the finger cell neurons (AFD) might mediate thermotaxis by the skin-penetrating infective larvae (L3) of A. caninum. Laser microbeam ablation experiments with A. caninum were conducted, using the H. contortus L1 neuronal map as a guide. A. caninum L1 were anesthetized and the paired AFD class neurons were ablated. The larvae were then cultured to L3 and assayed for thermotaxis on a thermal gradient. L3 with ablated AFD-class neuron pairs showed significantly reduced thermotaxis compared to control groups. The thermoreceptive function of the AFD-class neurons associates this neuron pair with the host-finding process of the A. caninum infective larva and shows functional homology with the neurons of class AFD in C. elegans and in H. contortus.

Sensory neuroanatomy of a passively ingested nematode parasite, Haemonchus contortus: amphidial neurons of the third-stage larva

The sensory neuronal ultrastructure of the amphids of the infective larva (L3) of Haemonchus contortus was analyzed, compared, and contrasted with that of the first-stage larva (L1). As in L1, each amphid of the L3 is innervated by 12 neurons. Thirteen ciliated dendritic processes of 10 neurons, 3 with double processes, lie in each amphidial channel. The dendritic process of each finger cell neuron ends in a large number of digitiform projections or "fingers," many more than in the L1. Processes of another pair of specialized neurons, probable homologs of wing cells in Caenorhabditis elegans, extend into the extreme anterior tip of the larva; they are much longer than those in L1. In L3, the neurons exit through the posterior wall of the amphidial chamber individually rather than in a bundle, as in L1. Cell constancy between L1 and L3 was confirmed, and the neurons were individually identified. Significant neuron-specific variations, presumably related to functional differences between the 2 stages were observed. In contrast, species-specific differences are surprisingly small. Haemonchus contortus is closely related to hookworms and has amphidial structure nearly identical to that in hookworms and similar to that in C. elegans, to which it is also closely related.

The neurons of class ALD mediate thermotaxis in the parasitic nematode, Strongyloides stercoralis

Strongyloides stercoralis, a skin-penetrating nematode parasite of homeotherms, migrates to warmth. In nematodes, the amphids, anteriorly positioned, paired sensilla, each contain a bundle of sensory neurons. In the amphids of the free-living nematode Caenorhabditis elegans, a pair of neurons, each of which ends in a cluster of microvilli-like projections, are known to be the primary thermoreceptors, and have been named the finger cells (class AFD). A similar neuron pair in the amphids of the parasite Haemonchus contortus is also known to be thermosensory. Strongyloides stercoralis lacks finger cells but, in its amphids, it has a pair of neurons whose dendrites end in a multi-layered complex of lamellae, the so-called lamellar cells (class ALD). Consequently, it was hypothesised that these lamellar cells might mediate thermotaxis by the skin-penetrating infective larva of this species. To investigate this, first stage S. stercoralis larvae were anaesthetised and the paired ALD class neurons were ablated with a laser microbeam. The larvae were then cultured to the infective third stage (L3) and assayed for thermotaxis on a thermal gradient. L3 with ablated ALD class neuron pairs showed significantly reduced thermotaxis compared with control groups. The thermoreceptive function of the ALD class neurons (i) associates this neuron pair with the host-finding process of S. stercoralis and (ii) demonstrates a functional similarity with the neurons of class AFD in C. elegans. The structural and positional characteristics of the ALD neurons suggest that these neurons may, in fact, be homologous with one pair of flattened dendritic processes known as wing cells (AWC) in C. elegans, while their florid development and thermosensory function suggest homology with the finger cells (AFD) of that nematode.

Thermotaxis and thermosensory neurons in infective larvae of Haemonchus contortus, a passively ingested nematode parasite

As a basis for studies of thermal behavior of infective larvae (L3) of Haemonchus contortus resulting from ablation of amphidial neurons, the locations of the amphidial cell bodies in the hatchling larva (L1) were compared with their locations in the L3. We sought to verify that killing each targeted cell body in L1 destroys the putative corresponding dendrite of the L3. These comparisons confirmed the predicted cell body-to-dendrite connections, as well as similarities in the general amphidial structure of the two stages. We then conducted a series of studies using laser microbeam ablation of amphidial cell bodies in the L1 to determine the role of specific neurons in the thermal behavior of the L3. In a thermal gradient, normal L3 of H. contortus migrate to the temperature at which they were cultured and/or maintained. Larvae grown at 16 degrees or 26 degrees C migrate appropriately to either of these temperatures. Larvae grown to the L3 stage at 16 degrees C and then moved to 26 degrees C become acclimated to this temperature and thereafter migrate to it. However, when the putative thermosensory neurons, the finger cell neurons (AFD), were ablated in hatchling larvae with a laser microbeam, and these were grown to the L3 stage and tested on a radial thermal gradient, they failed to migrate to their culture temperature. Instead, they moved actively and continuously over much of the assay plate surface, with no obviously oriented cryo- or thermotactic movement. Ablation-control larvae, those in which putatively chemosensory neuron classes ASE or AWC were killed, migrated normally to their culture temperature. When the RIA interneurons (identified by positional homology with those of Caenorhabditis elegans) were ablated, the operated larvae moved actively, but circled near the initial placement point; control larvae, in which other nonamphidial neurons were killed, migrated normally. These results indicate that the finger cell neurons (AFD) are the primary thermosensory class in H. contortus. The RIA-class neurons integrate thermal responses in H. contortus, as do their putative structural homologs in C. elegans, but the behavior of H. contortus subsequent to RIA ablation is strikingly different.

The LIM homeobox gene ceh-14 confers thermosensory function to the AFD neurons in Caenorhabditis elegans

In Caenorhabditis elegans three pairs of neurons, AFD, AIY, and AIZ, play a key role in thermosensation. The LIM homeobox gene ceh-14 is expressed in the AFD thermosensory neurons. ceh-14 mutant animals display athermotactic behaviors, although the neurons are still present and differentiated. Two other LIM homeobox genes, ttx-3 and lin-11, function in the two interneurons AIY and AIZ, respectively. Thus, the three key thermosensory neurons are specified by three different LIM homeobox genes. ceh-14 ttx-3 lin-11 triple mutant animals have a basic cryophilic thermotaxis behavior indicative of a second thermotaxis pathway. Misexpression of ceh-14 in chemosensory neurons can restore thermotactic behavior without impairing the chemosensory function. Thus, ceh-14 confers thermosensory function to neurons.