Dauer juvenile longevity and stress tolerance in natural populations of entomopathogenic nematodes: is there a relationship?

Preferences for livestock bedding as a development substrate of the stable fly, Stomoxys calcitrans L. (Diptera: Muscidae), and potential application of entomopathogenic nematodes for controlling stable fly larvae

The stable fly, Stomoxys calcitrans L. (Diptera: Muscidae), is a significant insect pest with global veterinary implications due to its capacity to both cause nuisance and transmit disease-causing pathogens to livestock. This study aimed to determine the livestock bedding preferred for use as a development substrate by S. calcitrans larvae and field-collected adults. The result showed that S. calcitrans larvae exhibited a preference (26.7%) for 7-day-old cow manure. Gravid females displayed a pronounced preference (55.0%) for fresh cow manure. As there were eight choices, indifference would result in 12.5% for each bedding substrate. Furthermore, the efficacy of four entomopathogenic nematodes (EPNs), namely Heterorhabditis bacteriophora (Poinar), Heterorhabditis indica Poinar, Karunakar & David (Rhabditida: Heterorhabditidae), Steinernema siamkayai Poinar, Karunakar & David and Steinernema carpocapsae (Weiser) (Rhabditida: Steinernematidae), against S. calcitrans larvae and the persistence after application to livestock bedding substrates were evaluated under laboratory conditions. In filter paper bioassays, all four EPNs caused 76.7%-100.0% mortality in the second instar larvae of S. calcitrans when applied at 50 and 100 infective juveniles (IJs)/cm2 within 5 days after exposure. For the third instar larvae of S. calcitrans, only H. indica induced high mortalities of 86.6% when applied at 100 IJs/cm2 within 5 days after exposure, while the other EPNs resulted in mortalities of less than 70%. The data further demonstrated that H. bacteriophora, H. indica and S. siamkayai remained present in the substrates linked to S. calcitrans larvae for as long as 7 days after the application of EPNs. This study demonstrates the potential of EPNs as a biologically based control agent against larvae of S. calcitrans, a serious pest and significant vector for various livestock animals.

The evolution of entomopathogeny in nematodes

Understanding how parasites evolved is crucial to understand the host and parasite interaction. The evolution of entomopathogenesis in rhabditid nematodes has traditionally been thought to have occurred twice within the phylum Nematoda: in Steinernematidae and Heterorhabditidae families, which are associated with the entomopathogenic bacteria Xenorhabdus and Photorhabdus, respectively. However, nematodes from other families that are associated with entomopathogenic bacteria have not been considered to meet the criteria for "entomopathogenic nematodes." The evolution of parasitism in nematodes suggests that ecological and evolutionary properties shared by families in the order Rhabditida favor the convergent evolution of the entomopathogenic trait in lineages with diverse lifestyles, such as saprotrophs, phoretic, and necromenic nematodes. For this reason, this paper proposes expanding the term "entomopathogenic nematode" considering the diverse modes of this attribute within Rhabditida. Despite studies are required to test the authenticity of the entomopathogenic trait in the reported species, they are valuable links that represent the early stages of specialized lineages to entomopathogenic lifestyle. An ecological and evolutionary exploration of these nematodes has the potential to deepen our comprehension of the evolution of entomopathogenesis as a convergent trait spanning across the Nematoda.

Stress tolerance in entomopathogenic nematodes: Engineering superior nematodes for precision agriculture

Entomopathogenic nematodes (EPNs) are soil-dwelling parasitic roundworms commonly used as biocontrol agents of insect pests in agriculture. EPN dauer juveniles locate and infect a host in which they will grow and multiply until resource depletion. During their free-living stage, EPNs face a series of internal and environmental stresses. Their ability to overcome these challenges is crucial to determine their infection success and survival. In this review, we provide a comprehensive overview of EPN response to stresses associated with starvation, low/elevated temperatures, desiccation, osmotic stress, hypoxia, and ultra-violet light. We further report EPN defense strategies to cope with biotic stressors such as viruses, bacteria, fungi, and predatory insects. By comparing the genetic and biochemical basis of these strategies to the nematode model Caenorhabditis elegans, we provide new avenues and targets to select and engineer precision nematodes adapted to specific field conditions.

Moroccan entomopathogenic nematodes as potential biocontrol agents against Dactylopius opuntiae (Hemiptera: Dactylopiidae)

Dactylopius opuntiae (Cockerell) (Hemiptera: Dactylopiidae) or prickly pear cochineal, is the most damaging pest on cactus species with heavy economic losses worldwide. The efficacy of two Moroccan EPN isolates; Steinernema feltiae (Filipjev) (Rhabditida: Steinernematidae) and Heterorhabditis bacteriophora (Poinar) (Rhabditida: Heterorhabditidae) (applied at 25, 50, and 75 IJs cm⁻²) against D. opuntiae nymphs and young females were evaluated under both laboratory bioassays and field conditions. Results showed that S. feltiae was more effective, causing higher mortality of nymphs and adult females (98.8% and 97.5%, respectively) after 8 days of exposure, resulting in an LT₅₀ value of 5.9 days (nymph) and 6.0 days (young female). While, H. bacteriophora had lower mortalities (83.8% for nymph and 81.3% for adult females). For the cochineal nymphs and adult females, no significant difference was observed among S. feltiae at 25, 50, and 75 IJs cm⁻², and the positive control, d-limonene applied at 0.5 g/L which was used due to its high effectiveness against nymphs and females of D. opuntiae. In the field experiment, d-limonene at 0.5 g/L and S. feltiae applied at 75 IJs cm⁻² were effective in reducing nymph and adult female populations by 85.3–93.9% at 12 days of post exposure period. To our knowledge, this work is the first report on the use of EPNs to control D. opuntiae. Thus, in addition to d-limonene, both Moroccan EPN isolates S. feltiae, and H. bacteriophora could be used as part of the integrated pest management strategy against D. opuntiae. Many factors such as temperature can affect the establishment and effectiveness of EPNs under field conditions. Therefore, additional studies under field conditions are needed.

Survival and efficacy of entomopathogenic nematodes on exposed surfaces

Entomopathogenic nematodes (EPN) species differ in their capability to withstand rapid desiccation (RD). Infective juveniles of Steinernema carpocapsae are a better adaptable and tolerant than Steinernema feltiae or Heterorhabditis bacteriophora as, an optimal RH of > 90% is required by S. feltiae and H. bacteriophora while maintaining RH equivalent to 74% could sustain survival of S. carpocapsae under RD. Our findings from infectivity suggest that following application, shrunk IJs are acquired passively by the larvae, probably rehydrate and resume infection within the insect gut. Water loss rate is a key factor affecting survival of S. carpocapsae on exposed surfaces. The present study provides the foundation for characterizing mechanism of rapid rate of water loss in EPN. ATR-FTIR is a rapid and reliable method for analysis of water loss. Changes in peak intensity was observed at 3100-3600 cm-1 (OH bonds of water), 2854 cm-1 (CH stretching of symmetric CH2, acyl chains), 2924 cm-1 (CH stretching of anti-symmetric CH2, lipid packing heterogeneity), 1634 cm-1 (amide I bonds) indicate major regions for hydration dependent changes in all EPN species. FTIR data also indicates that, S. carpocapsae contains strong water interacting regions in their biochemical profile, which could be an influencing factor in their water holding capacity under RD. ATR-FTIR were correlated to water content determined gravimetrically by using Partial Least square -Regression and FTIR multivariate method, which could be used to screen a formulation's potential to maintain or delay the rate of water loss in a rapid and efficient manner.

Ecological Characterisation of Native Isolates of Heterorhabditis indica from Viti Levu, Fiji Islands

Entomopathogenic nematodes (EPNs) in the families Steinernematidae and Heterorhabditidae are obligate parasites of soil inhibiting insects. EPNs are being widely researched as promising biocontrol agents for a wide range of agricultural pests. It is known that strains of EPNs isolated from different geographical regions differ in their attributes, such as host-finding ability, host range, infectivity, reproduction, and environmental stress tolerance. A precise knowledge of these factors is therefore an essential pre-requisite for devising successful strategies to use these nematodes in biological control programmes. Thus, ecological characterisation of the EPN Heterorhabditis indica (Rhabditida: Heterorhabditidae) newly isolated and representing the only species of EPN reported from the island of Viti Levu, Fiji was carried out using Galleria mellonella larvae (L) (Pyralidae: Galleriinae) as hosts to allow comparisons between bioassays conducted in different laboratories around the world. Temperature data showed that native isolates of H. indica are warm-adapted nematodes with thermal range for infectivity between 15˚C and 35˚C and can reproduce between 20˚C and 30˚C. They are highly virulent with LC50 values against G. mellonella ranging from 2.8 IJ to 3.8 IJ/larva. However, they showed poor desiccation tolerance and fail to infect hosts in soil with moisture levels below 8%. They showed a moderate level of hypoxic tolerance and can be stored at 15˚C for 4 months. Results also showed great variability within the selected native isolates of H. indica. Beneficial traits for selected isolates were added up to identify a superior candidate. The current study also suggested that the thermal niche breadth for infection can differ among conspecific strains of an EPN species. The results of this experimental study on ecological aspects of these native isolates of H. indica should form a basis for their potential use in biological control of insect pests in Fiji.

Quantification of pH tolerance levels among entomopathogenic nematodes

Soil pH affects the availability of nutrients, which impacts plant growth and development. Similarly, soil pH may also influence microorganisms in the soil, either beneficial or nonbeneficial. One such group of beneficial microorganisms is entomopathogenic nematodes (EPN), parasites of soil-inhabiting insects. Entomopathogenic nematodes have a number of attributes that make them good alternatives to chemical insecticides. The objective of this study was to investigate pH tolerance of 11 steinernematids and six heterorhabditids post exposure to different pH levels. Entomopathogenic nematode populations were exposed to varying pH levels (pH2 to pH11) made up from two different chemical solutions (ammonium-acetate and citrate-phosphate). Entomopathogenic nematode populations are expected to have varying tolerance to different pH levels. The highest infective juvenile survival was obtained from pH3 to pH10 in citrate-phosphate, where all populations displayed >50% survival. Steinernema carpocapsae populations had >90% survival at pH3 to pH11 in citrate-phosphate solutions. Overall, the steinernematids had a higher survival range in ammonium-acetate pH solutions compared with the heterorhabditids. Moreover, Steinernema spp., S. carpocapsae (ScCxrd, ScAll, and ScItalian) and S. riobrave showed consistently higher survival in both acidic and alkaline solutions, when compared to the other steinernematids, suggesting that they may be applied in both acidic and alkaline soils. These findings can be of use when selecting EPNs for biological control purposes in the two countries, respectively.

Natural enemies of herbivores maintain their biological control potential under short-term exposure to future CO2, temperature, and precipitation patterns

Climate change will profoundly alter the physiology and ecology of plants, insect herbivores, and their natural enemies, resulting in strong effects on multitrophic interactions. Yet, manipulative studies that investigate the direct combined impacts of changes in CO2, temperature, and precipitation on the third trophic level remain rare. Here, we assessed how exposure to elevated CO2, increased temperature, and decreased precipitation directly affect the performance and predation success of species from four major groups of herbivore natural enemies: an entomopathogenic nematode, a wolf spider, a ladybug, and a parasitoid wasp. A four-day exposure to future climatic conditions (RCP 8.5), entailing a 28% decrease in precipitation, a 3.4°C raise in temperature, and a 400 ppm increase in CO2 levels, slightly reduced the survival of entomopathogenic nematodes, but had no effect on the survival of other species. Predation success was not negatively affected in any of the tested species, but it was even increased for wolf spiders and entomopathogenic nematodes. Factorial manipulation of climate variables revealed a positive effect of reduced soil moisture on nematode infectivity, but not of increased temperature or elevated CO2. These results suggest that natural enemies of herbivores may be well adapted to short-term changes in climatic conditions. These findings provide mechanistic insights that will inform future efforts to disentangle the complex interplay of biotic and abiotic factors that drive climate-dependent changes in multitrophic interaction networks.

Potential of Moroccan entomopathogenic nematodes for the control of the Mediterranean fruit fly Ceratitis capitata Wiedemann (Diptera: Tephritidae)

The Mediterranean fruit fly, Ceratitis capitata Wiedemann, is a deleterious pest worldwide affecting fruit production. The entomopathogenic nematodes (EPNs) are a potential biocontrol agent that could be effectively used to control this Mediterranean fruit fly. In this study, five EPN strains reported from different fields in Morocco were evaluated for their efficacy against C. capitata. In laboratory assays, Steinernema feltiae-SF-MOR9, S. feltiae-SF-MOR10 and Heterorhabditis bacteriophora-HB-MOR7 strains showed significantly higher infectivity and penetration rates when compared to the other strains. S. feltiae-SF-MOR9 caused the highest larval mortality rate (80%) at 50 infective juveniles (IJs) cm⁻². However, additional results showed that both S. feltiae strains were significantly effective in controlling C. capitata larvae in apricot (Prunus armeniaca) fruits on soil surface with high mortality rate at 50 and 100 IJs cm⁻². Different soil textures and moisture levels resulted in a significant variation in EPN strain virulence against C. capitata. Sandy clay loam soil in combination with 50 IJs cm⁻² of S. feltiae (SF-MOR9 or SF-MOR10) caused a higher mortality rate of C. capitata larvae. Furthermore, applying these EPN strains at 50–100 IJs cm⁻² in combination with 10–15% moisture level showed optimal results against C. capitata larvae. Therefore, those two Moroccan EPN strains could be used as promising eco-friendly biological agents against C. capitata.

Characterization of the phenotypic and genotypic tolerance to abiotic stresses of natural populations of Heterorhabditis bacteriophora

Entomopathogenic nematodes are effective biocontrol agents against arthropod pests. However, their efficacy is limited due to sensitivity to environmental extremes. The objective of the present study was to establish a foundation of genetic-based selection tools for beneficial traits of heat and desiccation tolerance in entomopathogenic nematodes. Screening of natural populations enabled us to create a diverse genetic and phenotypic pool. Gene expression patterns and genomic variation were studied in natural isolates. Heterorhabditis isolates were phenotyped by heat- and desiccation-stress bioassays to determine their survival rates compared to a commercial line. Transcriptomic study was carried out for the commercial line, a high heat-tolerant strain, and for the natural, low heat-tolerant isolate. The results revealed a higher number of upregulated vs. downregulated transcripts in both isolates vs. their respective controls. Functional annotation of the differentially expressed transcripts revealed several known stress-related genes and pathways uniquely expressed. Genome sequencing of isolates with varied degrees of stress tolerance indicated variation among the isolates regardless of their phenotypic characterization. The obtained data lays the groundwork for future studies aimed at identifying genes and molecular markers as genetic selection tools for enhancement of entomopathogenic nematodes ability to withstand environmental stress conditions.

Integrated Management of Chive Gnats (Bradysia odoriphaga Yang & Zhang) in Chives Using Entomopathogenic Nematodes and Low-Toxicity Insecticides

Bradysia odoriphaga is a major pest that causes damage to chive production, and which has developed resistance to highly toxic chemical insecticides. Entomopathogenic nematodes (EPN) show a high potential for B. odoriphaga control. This study aimed to develop an effective management method against B. odoriphaga larvae, using EPN with low-toxicity insecticides. Fourteen selected insecticides had no significant effects on the survival and infectivity of Steinernema feltiae SN and Heterorhabditis indica LN2. Synergistic interactions were observed for imidacloprid and osthole with S. feltiae SN against B. odoriphaga larvae. Steinernema feltiae SN was more effective than H. indica LN2 against B. odoriphaga at 15 and 20 °C, and the addition of imidacloprid at 1/10 recommended concentration (RC) significantly increased the efficacy of S. feltiae SN. The year-round occurrence of the B. odoriphaga larvae in chive fields treated by EPN and imidacloprid at 1/10 RC was studied. Results showed that the application of EPN with imidacloprid at 1/10 RC successfully suppressed larval populations of B. odoriphaga in chive fields, thus significantly increasing the yield of chive. The practical method of applying EPN–imidacloprid combinations provided a cost-effective and environmental safety strategy for controlling B. odoriphaga larvae in chive production, which can reduce the usage of toxic chemical insecticides.

Applying inbreeding, hybridization and mutagenesis to improve oxidative stress tolerance and longevity of the entomopathogenic nematode Heterorhabditis bacteriophora

Poor shelf-life and sensitivity to environmental stress of entomopathogenic nematodes (EPNs) are traits, which deserve attention for improvement. Recently, a strong positive correlation between oxidative stress tolerance and longevity of Heterorhabditis bacteriophora dauer juveniles (DJs) has been reported. In this study, the improvement of H. bacteriophora DJ longevity was achieved by hybridization and mutagenesis. A hybrid pool deriving from two oxidative stress tolerant and long-living parental strains was generated. This hybrid AU1 × HU2 survived 2.6 days and 18 days longer than its best parent under oxidative stress and control conditions, respectively. In addition to the natural genetic variability, an EMS-mutant pool (M-OXI) with high longevity was generated and one of the derived mutagenized inbred lines (MOX-IL6) survived 5.8 days and 28.4 days longer than its donor line (IL3) under oxidative stress and control conditions, respectively. A genetic cross between the mutagenized inbred line and its donor line (MOX-IL × IL3) still survived 2.5 days and 18.5 days longer than the donor line under oxidative stress and control conditions, respectively. Concerning virulence and reproductive potential, trade-off effects were not observed as a result of hybridization and mutagenesis. These results underline the potential of classical genetic approaches for trait improvement in the nematode H. bacteriophora.

Phenotyping dauer juvenile oxidative stress tolerance, longevity and persistence within wild type and inbred lines of the entomopathogenic nematode Heterorhabditis bacteriophora

The commercial use of the entomopathogenic nematodeHeterorhabditis bacteriophoraas a biocontrol agent against noxious insects is limited due to its relatively short shelf-life. Longevity of dauer juveniles (DJ) during storage and in transit to end users is considerably restricted by environmental stresses. As a derivative stress triggered by environmental factors, oxidative stress causes a strong internal metabolic imbalance leading to lifespan reduction. In this study, the relation between DJ oxidative stress tolerance and longevity inH. bacteriophorawas investigated at 25 and 7°C. A strong and significant correlation between DJ oxidative stress tolerance and longevity during storage in Ringer’s solution ( at 7°C; at 25°C) was recorded. Phenotyping of these traits was performed for 40H. bacteriophorawild type strain and inbred line collections. At 25°C, the mean time survived in Ringer’s by 50% of the DJ (MTS50) ranged from 21 to 57 days, whereas under oxidative stress, survival was from 3 to 22 days. At 7°C, a maximum MTS50of 94 days was assessed when DJ were stored in Ringer’s, while the maximum MTS50was only 25 days with oxidative stress induction. The heritability of DJ tolerance to oxidative stress, determined by using homozygous inbred lines, is high (), an indication of a high probability for successful selective breeding. In a subset of preselectedH. bacteriophorainbred lines, DJ oxidative stress tolerance correlated with the DJ survival (persistence) after application to sand (). The study provides fundamental data required for a genetic breeding programme to produce hybrids with improved stress tolerance and prolonged shelf-life and soil persistence.

Mechanisms of animal diapause: recent developments from nematodes, crustaceans, insects, and fish

Life cycle delays are beneficial for opportunistic species encountering suboptimal environments. Many animals display a programmed arrest of development (diapause) at some stage(s) of their development, and the diapause state may or may not be associated with some degree of metabolic depression. In this review, we will evaluate current advancements in our understanding of the mechanisms responsible for the remarkable phenotype, as well as environmental cues that signal entry and termination of the state. The developmental stage at which diapause occurs dictates and constrains the mechanisms governing diapause. Considerable progress has been made in clarifying proximal mechanisms of metabolic arrest and the signaling pathways like insulin/Foxo that control gene expression patterns. Overlapping themes are also seen in mechanisms that control cell cycle arrest. Evidence is emerging for epigenetic contributions to diapause regulation via small RNAs in nematodes, crustaceans, insects, and fish. Knockdown of circadian clock genes in selected insect species supports the importance of clock genes in the photoperiodic response that cues diapause. A large suite of chaperone-like proteins, expressed during diapause, protects biological structures during long periods of energy-limited stasis. More information is needed to paint a complete picture of how environmental cues are coupled to the signal transduction that initiates the complex diapause phenotype, as well as molecular explanations for how the state is terminated. Excellent examples of molecular memory in post-dauer animals have been documented in Caenorhabditis elegans It is clear that a single suite of mechanisms does not regulate diapause across all species and developmental stages.

A transcriptomic insight into the infective juvenile stage of the insect parasitic nematode, Heterorhabditis indica

Background

Nematodes are the most numerous animals in the soil. Insect parasitic nematodes of the genus Heterorhabditis are capable of selectively seeking, infecting and killing their insect-hosts in the soil. The infective juvenile (IJ) stage of the Heterorhabditis nematodes is analogous to Caenorhabditis elegans dauer juvenile stage, which remains in ‘arrested development’ till it finds and infects a new insect-host in the soil. H. indica is the most prevalent species of Heterorhabditis in India. To understand the genes and molecular processes that govern the biology of the IJ stage, and to create a resource to facilitate functional genomics and genetic exploration, we sequenced the transcriptome of H. indica IJs.

Results

The de-novo sequence assembly using Velvet-Oases pipeline resulted in 13,593 unique transcripts at N50 of 1,371 bp, of which 53 % were annotated by blastx. H. indica transcripts showed higher orthology with parasitic nematodes as compared to free living nematodes. In-silico expression analysis showed 30 % of transcripts expressing with ≥100 FPKM value. All the four canonical dauer formation pathways like cGMP-PKG, insulin, dafachronic acid and TGF-β were active in the IJ stage. Several other signaling pathways were highly represented in the transcriptome. Twenty-four orthologs of C. elegans RNAi pathway effector genes were discovered in H. indica, including nrde-3 that is reported for the first time in any of the parasitic nematodes. An ortholog of C. elegans tol-1 was also identified. Further, 272 kinases belonging to 137 groups, and several previously unidentified members of important gene classes were identified.

Conclusions

We generated high-quality transcriptome sequence data from H. indica IJs for the first time. The transcripts showed high similarity with the parasitic nematodes, M. hapla, and A. suum as opposed to C. elegans, a species to which H. indica is more closely related. The high representation of transcripts from several signaling pathways in the IJs indicates that despite being a developmentally arrested stage; IJs are a hotbed of signaling and are actively interacting with their environment.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-2510-z) contains supplementary material, which is available to authorized users.

Isolation of Radiation-Resistant Bacteria from Mars Analog Antarctic Dry Valleys by Preselection, and the Correlation between Radiation and Desiccation Resistance

Extreme radiation-resistant microorganisms can survive doses of ionizing radiation far greater than are present in the natural environment. Radiation resistance is believed to be an incidental adaptation to desiccation resistance, as both hazards cause similar cellular damage. Desert soils are, therefore, promising targets to prospect for new radiation-resistant strains. This is the first study to isolate radiation-resistant microbes by using gamma-ray exposure preselection from the extreme cold desert of the Antarctic Dry Valleys (a martian surface analogue). Halomonads, identified by 16S rRNA gene sequencing, were the most numerous survivors of the highest irradiation exposures. They were studied here for the first time for both their desiccation and irradiation survival characteristics. In addition, the association between desiccation and radiation resistance has not been investigated quantitatively before for a broad diversity of microorganisms. Thus, a meta-analysis of scientific literature was conducted to gather a larger data set. A strong correlation was found between desiccation and radiation resistance, indicating that an increase in the desiccation resistance of 5 days corresponds to an increase in the room-temperature irradiation survival of 1 kGy. Irradiation at -79°C (representative of average martian surface temperatures) increases the microbial radiation resistance 9-fold. Consequently, the survival of the cold-, desiccation-, and radiation-resistant organisms isolated here has implications for the potential habitability of dormant or cryopreserved life on Mars.

KEY WORDS

Extremophiles-Halomonas sp.-Antarctica-Mars-Ionizing radiation-Cosmic rays.

Stressful environments can indirectly select for increased longevity

Longevity is modulated by a range of conserved genes in eukaryotes, but it is unclear how variation in these genes contributes to the evolution of longevity in nature. Mutations that increase life span in model organisms typically induce trade-offs which lead to a net reduction in fitness, suggesting that such mutations are unlikely to become established in natural populations. However, the fitness consequences of manipulating longevity have rarely been assessed in heterogeneous environments, in which stressful conditions are encountered. Using laboratory selection experiments, we demonstrate that long-lived, stress-resistant Caenorhabditis elegans age-1(hx546) mutants have higher fitness than the wild-type genotype if mixed genotype populations are periodically exposed to high temperatures when food is not limited. We further establish, using stochastic population projection models, that the age-1(hx546) mutant allele can confer a selective advantage if temperature stress is encountered when food availability also varies over time. Our results indicate that heterogeneity in environmental stress may lead to altered allele frequencies over ecological timescales and indirectly drive the evolution of longevity. This has important implications for understanding the evolution of life-history strategies.

Characterization in biological traits of entomopathogenic nematodes isolated from North China

In a series of bioassays, thirty-one isolates that were collected from diverse locations in northern China and the laboratory kept isolate Steinernema carpocapsae All, were compared in order to select superior isolates for biological control of Bradysia odoriphaga. Virulence of the isolates against B. odoriphaga was significantly different among nematode isolates. Tolerance of infective juveniles (IJs) to heat, cold, and desiccation differed significantly among and within species. Strains from S. carpocapsae, S. ceratophorum, S. longicaudum, Heterorhabditis indica, and H. bacteriophora were more heat tolerant than strains from S. feltiae, S. hebeiense, S. monticolum, and H. megidis. Heterorhabditis megidis, H. bacteriophora, and S. carpocapsae showed better cold tolerance than the other species. High desiccation tolerance was recorded for S. carpocapsae, S. hebeiense, and S. ceratophorum. The infectivity of IJ of these species against Galleria mellonella larvae was not significantly different between the treated and non-treated IJ after the nematodes had been exposed to 40°C for 2 h, -5°C for 8 h or 25% glycerin for 72 h. Nematode survival was significantly affected by exposure time and IJ concentration when exposed to 40°C or -5°C. All nematode isolates lost their infectivity against G. mellonella after exposure to -5°C for 16 h, except for H. megidis LFS10, which had a low infectivity of 3.3%. A hierarchical classification analysis classified the isolates in four main clusters. The fourth cluster, composed of 13 isolates, grouped the isolates that scored well for most traits.

A lover and a fighter: the genome sequence of an entomopathogenic nematode Heterorhabditis bacteriophora

Heterorhabditis bacteriophora are entomopathogenic nematodes that have evolved a mutualism with Photorhabdus luminescens bacteria to function as highly virulent insect pathogens. The nematode provides a safe harbor for intestinal symbionts in soil and delivers the symbiotic bacteria into the insect blood. The symbiont provides virulence and toxins, metabolites essential for nematode reproduction, and antibiotic preservation of the insect cadaver. Approximately half of the 21,250 putative protein coding genes identified in the 77 Mbp high quality draft H. bacteriophora genome sequence were novel proteins of unknown function lacking homologs in Caenorhabditis elegans or any other sequenced organisms. Similarly, 317 of the 603 predicted secreted proteins are novel with unknown function in addition to 19 putative peptidases, 9 peptidase inhibitors and 7 C-type lectins that may function in interactions with insect hosts or bacterial symbionts. The 134 proteins contained mariner transposase domains, of which there are none in C. elegans, suggesting an invasion and expansion of mariner transposons in H. bacteriophora. Fewer Kyoto Encyclopedia of Genes and Genomes Orthologies in almost all metabolic categories were detected in the genome compared with 9 other sequenced nematode genomes, which may reflect dependence on the symbiont or insect host for these functions. The H. bacteriophora genome sequence will greatly facilitate genetics, genomics and evolutionary studies to gain fundamental knowledge of nematode parasitism and mutualism. It also elevates the utility of H. bacteriophora as a bridge species between vertebrate parasitic nematodes and the C. elegans model.

Differences in Immune Defense Evasion of Selected Inbred Lines of Heterorhabditis Bacteriophora in Two White Grub Species

We determined virulence of seven Heterorhabditis bacteriophora strain GPS11 inbred lines possessing superior infective juvenile longevity, and heat and ultra violet radiation tolerance against white grubs Popillia japonica and Cyclocephala borealis. At 1 and 2 weeks after treatment, inbred line A2 was significantly more virulent towards P. japonica compared to the parent strain GPS11 and inbred lines A7, A8, A12 and A21; and line A2 caused significantly higher C. borealis mortality than lines A6 and A12. Penetration, encapsulation and survival of two inbred lines, A2 and A12, that showed the highest and lowest virulence against both grub species were then assessed. There were no differences between the two lines for the total number of nematodes penetrated in either P. japonica or C. borealis within the first 24 h, but a significantly higher percentage of penetrated nematodes were alive in line A2 compared to the line A12 in both grub species. P. japonica immune response over time to hemocoel-injected nematodes of A2, A12 and the parent strain was further investigated. While all injected nematodes were encapsulated at 6 h post injection, non-encapsulated living nematodes were detected at 12 and 24 h post injection, showing the breakage out of encapsulation. A higher percentage of non-encapsulated living nematodes and a lower percentage of dead nematodes were found in line A2 as compared to the line A12 after 12 h post injection. These data suggest that virulence differences in the studied H. bacteriophora inbred lines are not due to differences in nematode penetration or recognition by the grub immune system, but are related to the ability of the infective juveniles to break out of encapsulation.

Effect of temperature on activity of third-stage larvae of Angiostrongylus vasorum

To evaluate the effect of temperature on the activity and mortality of the L3 of Angiostrongylus vasorum, 1,500 L3 were isolated from experimentally infected snails and distributed into five equal groups. Three groups were incubated at 37°C, 27°C, and 5°C. The remaining two groups were incubated at 27°C and 5°C for 10 days, at which time the temperature for the 27°C group was reduced to 5°C and the 5°C group increased to 27°C. Larva activity was observed daily and inactive larvae were removed. At 37°C, larvae survived up to 8 days. At 27°C, larvae were active until day 6. When subjected to a reduction in temperature from 27°C to 5°C beginning on day 10, the number of active larvae increased until day 13. Only on day 17 did the number of active larvae decline to zero. At 5°C, larvae remained active until day 15, surviving to 24 days. When temperature was increased from 5°C to 27°C beginning on day 10, larvae were found active until day 12 and maintained an intermediate level of activity to day 21. Survival of larvae was greater at lower temperatures, while high temperatures were associated with higher mortality.

Aquatic molluscs as auxiliary hosts for terrestrial nematode parasites: implications for pathogen transmission in a changing climate

Nematodes are common parasites of molluscs but are often overlooked. Both metastrongyloid and rhabditoid species dominate the fauna within land snail and slug populations. Nevertheless, a key characteristic of many laboratory studies is the ability of these terrestrial nematodes to utilize aquatic molluscs as auxiliary hosts. The significance of this to the ecology of the parasite has never been evaluated. There is increasing concern as to the impact of climate change on the epidemiology of many parasitic diseases. In particular, it has been suggested that host switching may increase under the pressure of extreme climatic conditions. It is therefore timely to assess the role that aquatic molluscs may play in transmitting terrestrial nematodes, which include species of medical and veterinary importance such as Angiostrongylus cantonensis, A. vasorum, and Muellerius capillaris. The present review assesses the mechanisms of terrestrial nematode transmission through aquatic molluscs focusing on metastrongyloid and rhabditoid species, the importance of variable susceptibility of molluscan hosts, field studies on natural occurrence within aquatic habitats, and the impact of extreme climatic events (floods and droughts) that may increase in frequency under climate change.

HOST AVAILABILITY AND THE EVOLUTION OF PARASITE LIFE-HISTORY STRATEGIES

Parasites exploit an inherently patchy resource, their hosts, which are discrete entities that may only be available for infection within a relatively short time window. However, there has been little consideration of how heterogeneities in host availability may affect the phenotypic or genotypic composition of parasite populations or how parasites may evolve to cope with them. Here we conduct a selection experiment involving an entomopathogenic nematode (Steinernema feltiae) and show for the first time that the infection rate of a parasite can evolve rapidly to maximize the chances of infecting within an environment characterized by the rate of host availability. Furthermore, we show that the parasite's infection rate trades off with other fitness traits, such as fecundity and survival. Crucially, the outcome of competition between strains with different infection strategies depends on the rate of host availability; frequently available hosts favor "fast" infecting nematodes, whereas infrequently available hosts favor "slow" infecting nematodes. A simple evolutionarily stable strategy (ESS) analysis based on classic epidemiological models fails to capture this behavior, predicting instead that the fastest infecting phenotype should always dominate. However, a novel model incorporating more realistic, discrete bouts of host availability shows that strain coexistence is highly likely. Our results demonstrate that heterogeneities in host availability play a key role in the evolution of parasite life-history traits and in the maintenance of phenotypic variability. Parasite life-history strategies are likely to evolve rapidly in response to changes in host availability induced by disease management programs or by natural dynamics in host abundance. Incorporating parasite evolution in response to host availability would therefore enhance the predictive ability of current epidemiological models of infectious disease.

GENETIC VARIATION AND RELATIONSHIPS BETWEEN ISOLATES AND SPECIES OF THE ENTOMOPATHOGENIC NEMATODE GENUS HETERORHABDITIS DECIPHERED THROUGH ISOZYME PROFILES

We studied variation in isozyme patterns of 8 metabolic enzymes in 5 species of Heterorhabditis (H. bacteriophora, H. indica, H. marelata, H. megidis, and H. zealandica) comprising 18 isolates. Isozyme banding patterns of all the 8 enzymes were species specific; however, 3 enzymes, i.e., arginine kinase, fumarate hydratase, and malate dehydrogenase, displayed distinct patterns among all the 18 isolates. Phylogenetic analysis of the isozyme patterns produced dendrograms depicting a high degree of genetic variation among Heterorhabditis species, with the average pairwise distance of 0.2000. Trees constructed using different phylogenetic methods showed a relatively close genetic relationship between H. megidis and H. zealandica and between H. bacteriophora and H. indica. Also, H. bacteriophora HP88 was the most distant species from H. megidis (UK isolate), H. marelatus (Oregon isolate), and H. zealandica (X1 isolate) with pairwise distance of 0.1957, 0.2228, and 0.2120, respectively. Phylogenetic analysis also revealed genetic variation among H. bacteriophora isolates with the average pairwise distance of 0.1507. GPS2 and GPS3 were the most closely related isolates with the average distance of only 0.0870, followed by GPS1 and GPS2 with average distance of 0.1087. In contrast, KMD19 and HP88, OH25, and HP88, and OH25 and Acows isolates were the most divergent populations with a pairwise distance of 0.2011 and 37 character differences. Pairwise distance analysis also revealed that genetic divergence among populations of H. bacteriophora is relatively independent of geographic distance. Overall, these results demonstrate strong subspecies structuring in H. bacteriophora.

Stressed worms: Responding to the post-genomics era

Nematodes are among the most successful organisms in withstanding stress conditions associated with water loss, and viable individuals have been recovered from dry desert soils. Little is known about the biochemical and molecular events underpinning nematodes' physiological responses to dehydration. Post-genomics research in Caenorhabditis elegans may offer an opportunity to understand the stress response better. This review focuses on recent progress in understanding the molecular mechanisms of water-loss associated stress response in the model nematode C. elegans and in parasitic nematodes and discusses the scope for applying the knowledge and tools derived from a model organism for the study of wild, environmentally-adapted, parasitic nematodes, in the light of the emergence of genomics research of non-model organisms.

RAPID AGE-RELATED CHANGES IN INFECTION BEHAVIOR OF ENTOMOPATHOGENIC NEMATODES

Nonfeeding infective juvenile (IJ) entomopathogenic nematodes (EPNs) are used as biological agents to control soil-dwelling insects, but poor storage stability remains an obstacle to their widespread acceptance by distributors and growers as well as a frustration to researchers. Age is one factor contributing to variability in EPN efficacy. We hypothesized that age effects on the infectiousness of IJs would be evident within the length of time necessary for IJs to infect a host. The penetration behavior of "young" (<1-wk-old) and "old" (2- to 4-wk-old) Heterorhabditis bacteriophora (GPS 11 strain), Steinernema carpocapsae (All strain), and Steinernema feltiae (UK strain) IJs was evaluated during 5 "exposure periods" to the larvae of the wax moth, Galleria mellonella. Individual larvae were exposed to nematode-infested soil for exposure periods of 4, 8, 16, 32, and 64 hr. Cadavers were dissected after 72 hr, and the IJs that penetrated the larvae were counted. Larval mortality did not differ significantly between 72- and 144-hr "observation periods," or points at which larval mortality was noted, for any age class or species. However, age and species effects were noted in G. mellonella mortality and nematode penetration during shorter time periods. Initial mortality caused by S. carpocapsae and H. bacteriophora IJs declined with nematode age but increased with S. feltiae IJ age. Young S. carpocapsae IJs penetrated G. mellonella larvae at higher rates than old members of the species (27-45% vs. 1-4%). Conversely, old S. feltiae IJs had higher penetration rates than young IJs (approximately 8 to 57% vs. 4 to approximately 31%), whereas H. bacteriophora IJs had very low penetration rates regardless of age (3-5.6%). Our results show that the effect of age on IJ infectiousness can be detected in IJs aged only 2 wk by a 4-hr exposure period to G. mellonella. These results have important implications for storage and application of EPNs and suggest the possibility of shortening the time required to detect nematodes in the soil.

Acclimation of entomopathogenic nematodes to novel temperatures: trehalose accumulation and the acquisition of thermotolerance

The effect of thermal acclimation on trehalose accumulation and the acquisition of thermotolerance was studied in three species of entomopathogenic nematodes adapted to either cold or warm temperatures. All three Steinernema species accumulated trehalose when acclimated at either 5 or 35 degrees C, but the amount of trehalose accumulation differed by species and temperature. The trehalose content of the cold adapted Steinernema feltiae increased by 350 and 182%, of intermediate Steinernema carpocapsae by 146 and 122% and of warm adapted Steinernema riobrave by 30 and 87% over the initial level (18.25, 27.24 and 23.97 microg trehalose/mg dry weight, respectively) during acclimation at 5 and 35 degrees C, respectively. Warm and cold acclimation enhanced heat (40 degrees C for 8h) and freezing (-20 degrees C for 4h) tolerance of S. carpocapsae and the enhanced tolerance was positively correlated with the increased trehalose levels. Warm and cold acclimation also enhanced heat but not freezing tolerance of S. feltiae and the enhanced heat tolerance was positively correlated with the increased trehalose levels. In contrast, warm and cold acclimation enhanced the freezing but not heat tolerance of S. riobrave, and increased freezing tolerance of only warm acclimated S. riobrave was positively correlated with the increased trehalose levels. The effect of acclimation on maintenance of original virulence by either heat or freeze stressed nematodes against the wax moth Galleria mellonella larvae was temperature dependent and differed among species. During freezing stress, both cold and warm acclimated S. carpocapsae (84%) and during heat stress, only warm acclimated S. carpocapsae (95%) maintained significantly higher original virulence than the non-acclimated (36 and 47%, respectively) nematodes. Both cold and warm acclimated S. feltiae maintained significantly higher original virulence (69%) than the non-acclimated S. feltiae (0%) during heat but not freezing stress. In contrast, both warm and cold acclimated S. riobrave maintained significantly higher virulence (41%) than the non-acclimated (14%) nematodes during freezing, but not during heat stress. Our data indicate that trehalose accumulation is not only a cold associated phenomenon but is a general response of nematodes to thermal stress. However, the extent of enhanced thermal stress tolerance conferred by the accumulated trehalose differs with nematode species.