Infective Juveniles of the Entomopathogenic Nematode Steinernema scapterisci Are Preferentially Activated by Cricket Tissue

TARANTOBELUS JEFFDANIELSI N. SP. (PANAGROLAIMOMORPHA; PANAGROLAIMIDAE), A NEMATODE PARASITE OF TARANTULAS

Multiple tarantula deaths for a wholesale breeder were reported in 2018. The breeder noticed white discharge in the oral cavities of the tarantulas. Upon inspection, it was discovered that the white discharge was a large group of nematodes intertwined inside the tarantula's oral cavity. We examined the nematodes and propose a new species, Tarantobelus jeffdanielsi n. sp., in the currently monotypic genus Tarantobelus based on a combination of morphological and morphometrical data and unique nuclear rDNA 28S and 18S sequences. Based on phylogenetic analyses, the previously described Tarantobelus arachnicida was relocated, along with T. jeffdanielsi, into the family Panagrolaimidae. We also provide evidence of the ability of T. jeffdanielsi to parasitize Galleria mellonella larvae and the tarantula Grammostola pulchra. The life span and fecundity of the new species were also assessed, resulting in an 11.2-d average life span, and a total fertility rate of 158 nematodes/adult.

Earthworms and their cutaneous excreta can modify the virulence and reproductive capability of entomopathogenic nematodes and fungi

Earthworms are ecological engineers that can contribute to the displacement of biological control agents such as the entomopathogenic nematodes (EPNs) and fungi (EPF). However, a previous study showed that the presence of cutaneous excreta (CEx) and feeding behavior of the earthworm species Eisenia fetida (Haplotaxida: Lumbricidae) compromise the biocontrol efficacy of certain EPN species by reducing, for example, their reproductive capability. Whether this phenomenon is a general pattern for the interaction of earthworms-entomopathogens is still unknown. We hypothesized that diverse earthworm species might differentially affect EPN and EPF infectivity and reproductive capability. Here we investigated the interaction of different earthworm species (Eisenia fetida, Lumbricus terrestris, and Perionyx excavatus) (Haplotaxida) and EPN species (Steinernema feltiae, S. riojaense, and Heterorhabditis bacteriophora) (Rhabditida) or EPF species (Beauveria bassiana and Metarhizium anisopliae) (Hypocreales), in two independent experiments. First, we evaluated the application of each entomopathogen combined with earthworms or their CEx in autoclaved soil. Hereafter, we studied the impact of the earthworms' CEx on entomopathogens applied at two different concentrations in autoclaved sand. Overall, we found that the effect of earthworms on entomopathogens was species-specific. For example, E. fetida reduced the virulence of S. feltiae, resulted in neutral effects for S. riojaense, and increased H. bacteriophora virulence. However, the earthworm P. excavates increased the virulence of S. feltiae, reduced the activity of H. bacteriophora, at least at specific timings, while S. riojaense remained unaffected. Finally, none of the EPN species were affected by the presence of L. terrestris. Also, the exposure to earthworm CEx resulted in a positive, negative or neutral effect on the virulence and reproduction capability depending on the earthworm-EPN species interaction. Concerning EPF, the impact of earthworms was also differential among species. Thus, E. fetida was detrimental to M. anisopliae and B. bassiana after eight days post-exposure, whereas Lumbricus terrestris resulted only detrimental to B. bassiana. In addition, most of the CEx treatments of both earthworm species decreased B. bassiana virulence and growth. However, the EPF M. anisopliae was unaffected when exposed to L. terrestris CEx, while the exposure to E. fetida CEx produced contrasting results. We conclude that earthworms and their CEx can have positive, deleterious, or neutral impacts on entomopathogens that often coinhabit soils, and that we must consider the species specificity of these interactions for mutual uses in biological control programs. Additional studies are needed to verify these interactions under natural conditions.

A putative UDP-glycosyltransferase from Heterorhabditis bacteriophora suppresses antimicrobial peptide gene expression and factors related to ecdysone signaling

Insect pathogens have adopted an array of mechanisms to subvert the immune pathways of their respective hosts. Suppression may occur directly at the level of host-pathogen interactions, for instance phagocytic capacity or phenoloxidase activation, or at the upstream signaling pathways that regulate these immune effectors. Insect pathogens of the family Baculoviridae, for example, are known to produce a UDP-glycosyltransferase (UGT) that negatively regulates ecdysone signaling. Normally, ecdysone positively regulates both molting and antimicrobial peptide production, so the inactivation of ecdysone by glycosylation results in a failure of host larvae to molt, and probably a reduced antimicrobial response. Here, we examine a putative ecdysteroid glycosyltransferase, Hba_07292 (Hb-ugt-1), which was previously identified in the hemolymph-activated transcriptome of the entomopathogenic nematode Heterorhabditis bacteriophora. Injection of recombinant Hb-ugt-1 (rHb-ugt-1) into Drosophila melanogaster flies resulted in diminished upregulation of antimicrobial peptides associated with both the Toll and Immune deficiency pathways. Ecdysone was implicated in this suppression by a reduction in Broad Complex expression and reduced pupation rates in r Hb-ugt-1-injected larvae. In addition to the finding that H. bacteriophora excreted-secreted products contain glycosyltransferase activity, these results demonstrate that Hb-ugt-1 is an immunosuppressive factor and that its activity likely involves the inactivation of ecdysone.

Study of Steinernema hermaphroditum (Nematoda, Rhabditida), from the West Uttar Pradesh, India

INTRODUCTION

The entomopathogenic nematodes have been reported from all continents (except Antarctica) and almost all regions of the world. Surveys of EPNs in India has resulted in the recovery of several isolates of Steinernema. Among one of them, isolate CS34 was identified as S. hermaphroditum Stock, Griffin & Chaerani, 2004. We investigated the identification and the pathogenicity of S. hermaphroditum in District Meerut of Western Uttar Pradesh, India.

MATERIALS AND METHODS

The Steinernema was examined for its pathogenicity and accurate identification by the mean of morphological and molecular technique and its geographical distribution was mapped based on meta-analysis of the ITS GenBank records.

RESULTS

The surveys of agricultural soils of district Meerut, India, resulted in the isolation of one strain from entomopathogenic nematode labelled CS34 through Galleria baiting technique. Morphological characters and morphometrical analysis indicated that the strain CS34 was closely related to the "glaseri" group of Steinernema spp. The Nblast results indicated that ITS rDNA sequence had no nucleotide differences in comparison with the S. hermaphroditum (JQ687355). However, one variation in the D2-D3 segment of 28S rDNA was observed in comparison with the AY598358. The phylogenetic analysis using ITS and 28S rDNA indicated that the Indian S. hermaphroditum could be placed together with other S. hermaphroditum, with strong posterior probability. Besides, the PCA analysis demonstrated some variability within the test populations. The distribution of S. hermaphroditum based on meta-analysis of the GenBank records showed its presence in the three Asian countries-India, Thailand and Indonesia. The Indian strain of S. hermaphroditum also tested positively for its virulence against three major pests, namely, Galleria mellonella, Helicoverpa armigera, and Spodoptera litura, with resultant which showed good efficacy on the mortalities.

CONCLUSIONS

In conclusion, the economy of India is agriculture-based, but there are huge losses due to different insect pests infesting different crops. Steinernema hermaphroditum CS34 is an indigenous species to Indian subcontinent and efforts should be made to evaluate its virulence and pathogenicity against the other agricultural pests hampering productivity throughout the country. This may lead to incorporate S. hermaphroditum strain CS34 as a regular biological control agent against important lepidopteran pest in integrated pest management programs in the future.

Heterorhabditis bacteriophora Excreted-Secreted Products Enable Infection by Photorhabdus luminescens Through Suppression of the Imd Pathway

Upon entering the hemocoel of its insect host, the entomopathogenic nematode Heterorhabditis bacteriophora releases its symbiotic bacteria Photorhabdus luminescens, which is also a strong insect pathogen. P. luminescens is known to suppress the insect immune response independently following its release, but the nematode appears to enact its own immunosuppressive mechanisms during the earliest phases of an infection. H. bacteriophora was found to produce a unique set of excreted-secreted proteins in response to host hemolymph, and while basal secretions are immunogenic with regard to Diptericin expression through the Imd pathway, host-induced secretions suppress this expression to a level below that of controls in Drosophila melanogaster. This effect is consistent in adults, larvae, and isolated larval fat bodies, and the magnitude of suppression is dose-dependent. By reducing the expression of Diptericin, an antimicrobial peptide active against Gram-negative bacteria, the activated excreted-secreted products enable a more rapid propagation of P. luminescens that corresponds to more rapid host mortality. The identification and isolation of the specific proteins responsible for this suppression represents an exciting field of study with potential for enhancing the biocontrol of insect pests and treatment of diseases associated with excessive inflammation.

A core set of venom proteins is released by entomopathogenic nematodes in the genus Steinernema

Parasitic helminths release molecular effectors into their hosts and these effectors can directly damage host tissue and modulate host immunity. Excreted/secreted proteins (ESPs) are one category of parasite molecular effectors that are critical to their success within the host. However, most studies of nematode ESPs rely on in vitro stimulation or culture conditions to collect the ESPs, operating under the assumption that in vitro conditions mimic actual in vivo infection. This assumption is rarely if ever validated. Entomopathogenic nematodes (EPNs) are lethal parasites of insects that produce and release toxins into their insect hosts and are a powerful model parasite system. We compared transcriptional profiles of individual Steinernema feltiae nematodes at different time points of activation under in vitro and in vivo conditions and found that some but not all time points during in vitro parasite activation have similar transcriptional profiles with nematodes from in vivo infections. These findings highlight the importance of experimental validation of ESP collection conditions. Additionally, we found that a suite of genes in the neuropeptide pathway were downregulated as nematodes activated and infection progressed in vivo, suggesting that these genes are involved in host-seeking behavior and are less important during active infection. We then characterized the ESPs of activated S. feltiae infective juveniles (IJs) using mass spectrometry and identified 266 proteins that are released by these nematodes. In comparing these ESPs with those previously identified in activated S. carpocapsae IJs, we identified a core set of 52 proteins that are conserved and present in the ESPs of activated IJs of both species. These core venom proteins include both tissue-damaging and immune-modulating proteins, suggesting that the ESPs of these parasites include both a core set of effectors as well as a specialized set, more adapted to the particular hosts they infect.

Sand crickets (Gryllus firmus) have low susceptibility to entomopathogenic nematodes and their pathogenic bacteria

The entomopathogenic nematode, Steinernema scapterisci, a specialist parasite of crickets, has been successfully used to combat the southern mole cricket, Neoscapteriscus borellii, which is an invasive pest of turf grass. As an entomopathogenic nematode, S. scapterisci causes rapid death of the insects it infects and uses bacteria to facilitate its parasitism. However, our understanding of the relative contributions of the nematode, S. scapterisci, and its bacterial symbiont, Xenorhabdus innexi, to parasitism remains limited. Here we utilized the sand cricket, Gryllus firmus, as a model host to evaluate the contributions of the EPNs S. scapterisci and S. carpocapsae, as well as their symbiotic bacteria, X. innexi and X. nematophila, respectively, to the virulence of the nematode-bacterial complex. We found that G. firmus has reduced susceptibility to infection from both S. scapterisci and the closely related generalist parasite S. carpocapsae, but that S. scapterisci is much more virulent than S. carpocapsae. Further, we found that N. borellii has reduced susceptibility to X. nematophila, and that G. firmus has reduced susceptibility to X. nematophila, X. innexi, and Serratia marcescens, much more so than other insects that have been studied. We found that the reduced susceptibility of G. firmus to bacterial infection is dependent on development, with adults being less susceptible to infection than nymphs. Our data provide evidence that unlike other EPNs, the virulence of S. scapterisci to crickets is dependent on the nematode rather than the bacterial symbiont that it carries and we speculate that S. scapterisci may be evolving independence from X. innexi.

Transmission Success of Entomopathogenic Nematodes Used in Pest Control

Entomopathogenic nematodes from the two genera Steinernema and Heterorhabditis are widely used as biological agents against various insect pests and represent a promising alternative to replace pesticides. Efficacy and biocontrol success can be enhanced through improved understanding of their biology and ecology. Many endogenous and environmental factors influence the survival of nematodes following application, as well as their transmission success to the target species. The aim of this paper is to give an overview of the major topics currently considered to affect transmission success of these biological control agents, including interactions with insects, plants and other members of the soil biota including conspecifics.

Host-Specific Activation of Entomopathogenic Nematode Infective Juveniles

Entomopathogenic nematodes (EPNs) are potent insect parasites and have been used for pest control in agriculture. Despite the complexity of the EPN infection process, hosts are typically killed within 5 days of initial infection. When free-living infective juveniles (IJs) infect a host, they release their bacterial symbiont, secrete toxic products, and undergo notable morphological changes. Collectively, this process is referred to as “activation” and represents the point in a nematode’s life cycle when it becomes actively parasitic. The effect of different host tissues and IJ age on activation, and how activation itself is related to virulence, are not well understood. Here, we employed a recently developed bioassay, which quantifies IJ activation, as a tool to address these matters. Appreciating that activation is a key part of the EPN infection process, we hypothesized that activation would positively correlate to virulence. Using the EPNs Steinernema carpocapsae and S. feltiae we found that EPN activation is host-specific and influenced by infective juvenile age. Additionally, our data suggest that activation has a context-dependent influence on virulence and could be predictive of virulence in some cases such as when IJ activation is especially low.

The insect pathogenic bacterium Xenorhabdus innexi has attenuated virulence in multiple insect model hosts yet encodes a potent mosquitocidal toxin

BACKGROUND

Xenorhabdus innexi is a bacterial symbiont of Steinernema scapterisci nematodes, which is a cricket-specialist parasite and together the nematode and bacteria infect and kill crickets. Curiously, X. innexi expresses a potent extracellular mosquitocidal toxin activity in culture supernatants. We sequenced a draft genome of X. innexi and compared it to the genomes of related pathogens to elucidate the nature of specialization.

RESULTS

Using green fluorescent protein-expressing X. innexi we confirm previous reports using culture-dependent techniques that X. innexi colonizes its nematode host at low levels (~3-8 cells per nematode), relative to other Xenorhabdus-Steinernema associations. We found that compared to the well-characterized entomopathogenic nematode symbiont X. nematophila, X. innexi fails to suppress the insect phenoloxidase immune pathway and is attenuated for virulence and reproduction in the Lepidoptera Galleria mellonella and Manduca sexta, as well as the dipteran Drosophila melanogaster. To assess if, compared to other Xenorhabdus spp., X. innexi has a reduced capacity to synthesize virulence determinants, we obtained and analyzed a draft genome sequence. We found no evidence for several hallmarks of Xenorhabdus spp. toxicity, including Tc and Mcf toxins. Similar to other Xenorhabdus genomes, we found numerous loci predicted to encode non-ribosomal peptide/polyketide synthetases. Anti-SMASH predictions of these loci revealed one, related to the fcl locus that encodes fabclavines and zmn locus that encodes zeamines, as a likely candidate to encode the X. innexi mosquitocidal toxin biosynthetic machinery, which we designated Xlt. In support of this hypothesis, two mutants each with an insertion in an Xlt biosynthesis gene cluster lacked the mosquitocidal compound based on HPLC/MS analysis and neither produced toxin to the levels of the wild type parent.

CONCLUSIONS

The X. innexi genome will be a valuable resource in identifying loci encoding new metabolites of interest, but also in future comparative studies of nematode-bacterial symbiosis and niche partitioning among bacterial pathogens.

Activated entomopathogenic nematode infective juveniles release lethal venom proteins

Entomopathogenic nematodes (EPNs) are unique parasites due to their symbiosis with entomopathogenic bacteria and their ability to kill insect hosts quickly after infection. It is widely believed that EPNs rely on their bacterial partners for killing hosts. Here we disproved this theory by demonstrating that the in vitro activated infective juveniles (IJs) of Steinernema carpocapsae (a well-studied EPN species) release venom proteins that are lethal to several insects including Drosophila melanogaster. We confirmed that the in vitro activation is a good approximation of the in vivo process by comparing the transcriptomes of individual in vitro and in vivo activated IJs. We further analyzed the transcriptomes of non-activated and activated IJs and revealed a dramatic shift in gene expression during IJ activation. We also analyzed the venom proteome using mass spectrometry. Among the 472 venom proteins, proteases and protease inhibitors are especially abundant, and toxin-related proteins such as Shk domain-containing proteins and fatty acid- and retinol-binding proteins are also detected, which are potential candidates for suppressing the host immune system. Many of the venom proteins have conserved orthologs in vertebrate-parasitic nematodes and are differentially expressed during IJ activation, suggesting conserved functions in nematode parasitism. In summary, our findings strongly support a new model that S. carpocapsae and likely other Steinernema EPNs have a more active role in contributing to the pathogenicity of the nematode-bacterium complex than simply relying on their symbiotic bacteria. Furthermore, we propose that EPNs are a good model system for investigating vertebrate- and human-parasitic nematodes, especially regarding the function of excretory/secretory products.

Steinernema carpocapsae belongs to a special group of insect-parasitic nematodes known as entomopathogenic nematodes (EPNs). These differ from other insect parasites in at least two ways; first they kill their hosts quickly (within 2–3 days), and second they associate with bacteria to facilitate their parasitic lifestyle. The infective stage of these parasites, the infective juvenile (IJ) stage, is the only free-living stage and these IJs are developmentally arrested and only reinitiate development once they are inside a suitable host. Little is known about the early stages of parasitism and how these parasites initiate the parasitic phase of their life cycle and reinitiate development. Here we characterized the changes that occur to the nematodes' physical morphology, gene expression, and the release of protein molecules that accompany the transition from developmentally arrested IJ to active, developing parasite. We showed that contrary to long-held assumptions, the nematodes are not merely transporting pathogenic bacteria but that the nematodes contribute to parasitism by releasing toxic proteins into the host. Many of the S. carpocapsae toxins are also found in species of human-parasitic nematodes, and S. carpocapsae may serve as a valuable model for understanding the specific function of these toxins.