A survival-reproduction trade-off in entomopathogenic nematodes mediated by their bacterial symbionts

In this work, we investigate the investment of entomopathogenic Steinernema nematodes (Rhabditidae) in their symbiotic association with Xenorhabdus bacteria (Enterobacteriaceae). Their life cycle comprises two phases: (1) a free stage in the soil, where infective juveniles (IJs) of the nematode carry bacteria in a digestive vesicle and search for insect hosts, and (2) a parasitic stage into the insect where bacterial multiplication, nematode reproduction, and production of new IJs occur. Previous studies clearly showed benefits to the association for the nematode during the parasitic stage, but preliminary data suggest the existence of costs to the association for the nematode in free stage. IJs deprived from their bacteria indeed survive longer than symbiotic ones. Here we show that those bacteria-linked costs and benefits lead to a trade-off between fitness traits of the symbiotic nematodes. Indeed IJs mortality positively correlates with their parasitic success in the insect host for symbiotic IJs and not for aposymbiotic ones. Moreover mortality and parasitic success both positively correlate with the number of bacteria carried per IJ, indicating that the trade-off is induced by symbiosis. Finally, the trade-off intensity depends on parental effects and, more generally, is greater under restrictive environmental conditions.     

Aggregative group behavior in insect parasitic nematode dispersal

Movement behavior of foraging animals is critical to the determination of their spatial ecology and success in exploiting resources. Individuals sometimes gain advantages by foraging in groups to increase their efficiency in garnering these resources. Group movement behavior has been studied in various vertebrates. In this study we explored the propensity for innate group movement behavior among insect parasitic nematodes. Given that entomopathogenic nematodes benefit from group attack and infection, we hypothesised that the populations would tend to move in aggregate in the absence of extrinsic cues. Movement patterns of entomopathogenic nematodes in sand were investigated when nematodes were applied to a specific locus or when the nematodes emerged naturally from infected insect hosts; six nematode species in two genera were tested (Heterorhabditis bacteriophora, Heterorhabditis indica, Steinernema carpocapsae, Steinernema feltiae, Steinernema glaseri and Steinernema riobrave). Nematodes were applied in aqueous suspension via filter paper discs or in infected insect host cadavers (to mimic emergence in nature). We discovered that nematode dispersal resulted in an aggregated pattern rather than a random or uniform distribution; the only exception was S. glaseri when emerging directly from infected hosts. The group movement may have been continuous from the point of origin, or it may have been triggered by a propensity to aggregate after a short period of random movement. To our knowledge, this is the first report of group movement behavior in parasitic nematodes in the absence of external stimuli (e.g., without an insect or other apparent biotic or abiotic cue). These findings have implications for nematode spatial distribution and suggest that group behavior is involved in nematode foraging.     

Entomopathogenic nematodes,a potential microbial biopesticide: mass production and commercialisation status – a mini review

Parasitic nematodes have several important attributes that make them excellent candidates for biological control of soil insects. These nematodes can be produced by in vivo by baiting technique on insects and commercially by in vitro solid/liquid culturing. Numerous insect pests on many different crops are being controlled by these insect parasitic nematodes, including root weevils, flea beetles, mint root borer, colorado potato beetle, white grubs, caterpillars and plant parasitic root nematode, e.g. root-knot nematodes. Utilisation of entomopathogenic nematodes (EPN) has raised intense interest and has been a growing concern globally mainly because of its potential efficiency, exemption from registration and other impressive attributes for utilising against the control of soil dwelling pests. This review highlights the mass production, commercialisation and utilisation of EPN as microbial biopesticide in bio-intensive pest management programmes.     

Nematodes, bacteria, and flies: a tripartite model for nematode parasitism

More than a quarter of the world's population is infected with nematode parasites, and more than a hundred species of nematodes are parasites of humans [1-3]. Despite extensive morbidity and mortality caused by nematode parasites, the biological mechanisms of host-parasite interactions are poorly understood, largely because of the lack of genetically tractable model systems. We have demonstrated that the insect parasitic nematode Heterorhabditis bacteriophora, its bacterial symbiont Photorhabdus luminescens, and the fruit fly Drosophila melanogaster constitute a tripartite model for nematode parasitism and parasitic infection. We find that infective juveniles (IJs) of Heterorhabditis, which contain Photorhabdus in their gut, can infect and kill Drosophila larvae. We show that infection activates an immune response in Drosophila that results in the temporally dynamic expression of a subset of antimicrobial peptide (AMP) genes, and that this immune response is induced specifically by Photorhabdus. We also investigated the cellular and molecular mechanisms underlying IJ recovery, the developmental process that occurs in parasitic nematodes upon host invasion and that is necessary for successful parasitism. We find that the chemosensory neurons and signaling pathways that control dauer recovery in Caenorhabditis elegans also control IJ recovery in Heterorhabditis, suggesting conservation of these developmental processes across free-living and parasitic nematodes.     

A novel ascaroside controls the parasitic life cycle of the entomopathogenic nematode Heterorhabditis bacteriophora

Entomopathogenic nematodes survive in the soil as stress-resistant infective juveniles that seek out and infect insect hosts. Upon sensing internal host cues, the infective juveniles regurgitate bacterial pathogens from their gut that ultimately kill the host. Inside the host, the nematode develops into a reproductive adult and multiplies until unknown cues trigger the accumulation of infective juveniles. Here, we show that the entomopathogenic nematode Heterorhabditis bacteriophora uses a small-molecule pheromone to control infective juvenile development. The pheromone is structurally related to the dauer pheromone ascarosides that the free-living nematode Caenorhabditis elegans uses to control its development. However, none of the C. elegans ascarosides are effective in H. bacteriophora, suggesting that there is a high degree of species specificity. Our report is the first to show that ascarosides are important regulators of development in a parasitic nematode species. An understanding of chemical signaling in parasitic nematodes may enable the development of chemical tools to control these species.     

Influence of Nematodes on Resource Utilization by Bacteria—an in vitro Study

The positive influence of bacterial feeding nematodes on bacterial mediated processes such as organic matter mineralization and nutrient cycling is widely accepted, but the mechanisms of these interactions are not always apparent. Both transport of bacteria by nematodes, and nutritional effects caused by nematode N excretion are thought to be involved, but their relative importance is not known because of the difficulties in studying these interactions in soil. We developed a simple in vitro assay to study complex nematode/bacterial interactions and used it to conduct a series of experiments to determine the potential influence of nematode movement and nutritional effects on bacterial resource use. The system used bacterial feeding and nonfeeding insect parasitic nematodes, and luminescent bacteria marked with metabolic reporter genes. Both nutritional enhancement of bacterial activity and bacterial transport were observed and we hypothesize that in nature, the relative importance of transport is likely to be greater in bulk soil, whereas nematode excretion may have greater impact in the rhizosphere. In both cases, the ability of nematodes to enhance bacterial resource utilization has implications for soil components of biogeochemical cycling.     

Cloning and biochemical characterization of blisterase,a subtilisin-like convertase from the filarial parasite,Onchocerca volvulus

Blisterase is a subtilisin-like proprotein convertase of nematodes. The enzyme is named after the blistered cuticle found in Caenorhabditis elegans with the bli-4 e937 mutation. The critical role of the enzyme in cuticle production makes it a potential drug target for parasitic nematodes. We have cloned and expressed blisterase from the parasitic nematode Onchocerca volvulus, a major cause of blindness in Africa. The catalytic domain of the protease exhibits 84% identity with the corresponding domain of its closest homologue, C. elegans blisterase. O. volvulus blisterase expressed in insect cells has maximal activity in 1 mm calcium at neutral pH. The protease is inhibited by EDTA, the suicide substrate decanoyl-RVKR-chloromethylketone, alpha1-antitrypsin Portland and by its own propeptide. Substrate assays with fluorescent peptides show that O. volvulus blisterase requires a P4 arginine and a basic amino acid at P1 for cleavage. The kcat of blisterase on the peptide substrate, t-butyloxycarbonyl-RVRR-4-methylcoumaryl-7-amide was determined to be 0.018 s-1. In vitro cleavage studies with the nematode polyprotein antigen demonstrated that blisterase cleaved at tetrabasic (RRKR) but not at dibasic (KR) sites. This report describes the first biochemical characterization of the nematode specific protease, blisterase.     

Above- and belowground insect herbivores differentially affect soil nematode communities in species-rich plant communities

Interactions between above‐ and belowground invertebrate herbivores alter plant diversity, however, little is known on how these effects may influence higher trophic level organisms belowground. Here we explore whether above‐ and belowground invertebrate herbivores which alter plant community diversity and biomass, in turn affect soil nematode communities. We test the hypotheses that insect herbivores 1) alter soil nematode diversity, 2) stimulate bacterial‐feeding and 3) reduce plant‐feeding nematode abundances. In a full factorial outdoor mesocosm experiment we introduced grasshoppers (aboveground herbivores), wireworms (belowground herbivores) and a diverse soil nematode community to species‐rich model plant communities. After two years, insect herbivore effects on nematode diversity and on abundance of herbivorous, bacterivorous, fungivorous and omni‐carnivorous nematodes were evaluated in relation to plant community composition. Wireworms did not affect nematode diversity despite enhanced plant diversity, while grasshoppers, which did not affect plant diversity, reduced nematode diversity. Although grasshoppers and wireworms caused contrasting shifts in plant species dominance, they did not affect abundances of decomposer nematodes at any trophic level. Primary consumer nematodes were, however, strongly promoted by wireworms, while community root biomass was not altered by the insect herbivores. Overall, interaction effects of wireworms and grasshoppers on the soil nematodes were not observed, and we found no support for bottom‐up control of the nematodes. However, our results show that above‐ and belowground insect herbivores may facilitate root‐feeding rather than decomposer nematodes and that this facilitation appears to be driven by shifts in plant species composition. Moreover, the addition of nematodes strongly suppressed shoot biomass of several forb species and reduced grasshopper abundance. Thus, our results suggest that nematode feedback effects on plant community composition, due to plant and herbivore parasitism, may strongly depend on the presence of insect herbivores.     

A survey of insect parasitic nematodes in Northern Ireland

A survey of insect parasitic nematodes was carried out in Northern Ireland between November 1986 and June 1987. A total of 1093 soils were examined using Galleria mellonella larvae as bait. Nematodes were recovered from 41 of these soils. The rate of recovery declined from February onwards and possible reasons for this are discussed. Clay and clay loam types accounted for 56·1% of the survey samples but only 9·8% of the recovered nematode populations. Loams comprised 39·6% of samples and 85·4% of nematodes, indicating a greater likelihood of recovering insect parasitic nematodes from lighter soils. The nematodes were also recovered from peat soils (4·3% of samples and 4·9% of nematodes). All the recovered isolates were Neoaplectana bibionis.     

Trade-offs shape the evolution of the vector-borne insect pathogen Xenorhabdus nematophila

Our current understanding on how pathogens evolve relies on the hypothesis that pathogens' transmission is traded off against host exploitation. In this study, we surveyed the possibility that trade-offs determine the evolution of the bacterial insect pathogen, Xenorhabdus nematophila. This bacterium rapidly kills the hosts it infects and is transmitted from host cadavers to new insects by a nematode vector, Steinernema carpocapsae. In order to detect trade-offs in this biological system, we produced 20 bacterial lineages using an experimental evolution protocol. These lineages differ, among other things, in their virulence towards the insect host. We found that nematode parasitic success increases with bacteria virulence, but their survival during dispersal decreases with the number of bacteria they carry. Other bacterial traits, such as production of the haemolytic protein XaxAB, have a strong impact on nematode reproduction. We then combined the result of our measurements with an estimate of bacteria fitness, which was divided into a parasitic component and a dispersal component. Contrary to what was expected in the trade-off hypothesis, we found no significant negative correlation between the two components of bacteria fitness. Still, we found that bacteria fitness is maximized when nematodes carry an intermediate number of cells. Our results therefore demonstrate the existence of a trade-off in X. nematophila, which is caused, in part, by the reduction in survival this bacterium causes to its nematode vectors.     

Corethrellonema grandispiculosum n. gen., n. sp. and Aproctonema chapmani n. sp. (Nematoda: Tetradonematidae), Parasites of the Dipterous Insect Genera,Corethrella and Culicoides in Louisiana

Two new nematodes of the family Tetradonematidae, parasitic in aquatic dipterous insects in Louisiana, are presented. Corethrellonema grandispiculosum n. gen., n. sp., from the chaoborid fly, Corethrella brakeleyi Coquillett, and Aproctonema chapmani n. sp., from the sand fly, Culicoides arboricola Root and Hoffman, are described and illustrated. The biology and life histories of these nematodes show that the adults occur in the last larval instar of the insect host. The adult nematodes mate in the body cavity of the insect, and later the female nematode, replete with eggs, exits from the larval fly causing the death of the insect. Male nematodes usually remain in the insect cadaver.     

Evolution of Parasitism in Insect-transmitted Plant Nematodes

Nematode-insect associations have evolved many times in the phylum Nematoda, but these lineages involve plant parasitism only in the Secernentean orders Aphelenchida and Tylenchida. In the Aphelenchida (Aphelenchoidoidea), Bursaphelenchus xylophilus (Pine wood nematode), B. cocophilus (Red ring or Coconut palm nematode) (Parasitaphelenchidae), and the many potential host-specific species of Schistonchus (fig nematodes) (Aphelenchoididae) nematode-insect interactions probably evolved independently from dauer-forming, mycophagous ancestors that were phoretically transmitted to breeding sites of their insect hosts in plants. Mycophagy probably gave rise to facultative or obligate plant-parasitism because of opportunities due to insect host switches or peculiarities in host behavior. In the Tylenchida, there is one significant radiation of insect-associated plant parasites involving Fergusobia nematodes (Fergusobiinae: Neotylenchidae) and Fergusonina (Fergusoninidae) flies as mutualists that gall myrtaceous plant buds or leaves. These dicyclic nematodes have different phases that are parasitic in either the insect or the plant hosts. The evolutionary origin of this association is unclear.     

Delivery of macromolecules to plant parasitic nematodes using a tobacco rattle virus vector

Plant parasitic nematodes cause significant damage to crops on a worldwide scale. These nematodes are often soil dwelling but rely on plants for food and to sustain them during reproduction. Complex interactions occur between plants and nematodes during the nematode life cycle with plant roots developing specialized feeding structures through which nematodes withdraw nutrients. Here we describe a novel method for delivering macromolecules to feeding nematodes using a virus-based vector [tobacco rattle virus (TRV)]. We show that the parasitic nematode Heterodera schachtii will ingest fluorescent proteins transiently expressed in plant roots infected with a TRV construct carrying the appropriate protein sequence. A prerequisite for this delivery is the presence of replicating virus in root tips prior to the formation of nematode-induced syncytia. We show also that TRV vectors expressing nematode gene sequences can be used to induce RNAi in the feeding nematodes.     

The bacterial community of entomophilic nematodes and host beetles

Insects form the most species‐rich lineage of Eukaryotes and each is a potential host for organisms from multiple phyla, including fungi, protozoa, mites, bacteria and nematodes. In particular, beetles are known to be associated with distinct bacterial communities and entomophilic nematodes. While entomopathogenic nematodes require symbiotic bacteria to kill and reproduce inside their insect hosts, the microbial ecology that facilitates other types of nematode–insect associations is largely unknown. To illuminate detailed patterns of the tritrophic beetle–nematode–bacteria relationship, we surveyed the nematode infestation profiles of scarab beetles in the greater Los Angeles area over a five‐year period and found distinct nematode infestation patterns for certain beetle hosts. Over a single season, we characterized the bacterial communities of beetles and their associated nematodes using high‐throughput sequencing of the 16S rRNA gene. We found significant differences in bacterial community composition among the five prevalent beetle host species, independent of geographical origin. Anaerobes Synergistaceae and sulphate‐reducing Desulfovibrionaceae were most abundant in Amblonoxia beetles, while Enterobacteriaceae and Lachnospiraceae were common in Cyclocephala beetles. Unlike entomopathogenic nematodes that carry bacterial symbionts, insect‐associated nematodes do not alter the beetles' native bacterial communities, nor do their microbiomes differ according to nematode or beetle host species. The conservation of Diplogastrid nematodes associations with Melolonthinae beetles and sulphate‐reducing bacteria suggests a possible link between beetle–bacterial communities and their associated nematodes. Our results establish a starting point towards understanding the dynamic interactions between soil macroinvertebrates and their microbiota in a highly accessible urban environment.     

The Natural Host Range of Steinernema and Heterorhabditis spp. and Their Impact on Insect Populations

The natural host range of entomopathogenic nematodes of the genera Steinernema and Heterorhabditis can be defined as the range of insects which indigenous nematode populations use for propagation. Information on the natural host range is rare. However, based on records of insects found to be naturally infected with nematodes, some conclusions regarding the natural host range of some Steinernema spp. and Heterorhabditis spp. are presented. Reports of indigenous nematode populations impacting on insect populations can be divided between relatively balanced, long-lasting nematode-host associations and unbalanced, short-lasting epizootics. Examples of the augmentation and inoculative introduction of nematodes in agriculture and forestry ecosystems are presented. Based on current knowledge, nematode reproduction strategies are discussed and indications of the risk involved in the release of non-indigenous nematodes are given.     

中华卵索线虫的体外培养

在研究中华卵索线虫的体外培养方法的同时,对其在不同培养基中的生长发育情况进行了观察。结果表明：以培养基TC－199加20％热灭活胎牛血清的培养效果较为理想,大多数线虫可存活3个月,最大虫体长55．1mm,宽204．13um,其发育程度大致与该种索线虫在宿主粘虫体内寄生8－9天的情况相近,培养期间观察到2次蜕皮;第一次蜕皮在卵内,第二次在培养6－8天之后,口针消失,虫体内滋养物体发育明显,尾部附器已经形成,没有观察到生殖原基的发育。     

The role of parasite release in invasion of the USA by European slugs

Several species of European slugs are invasive in the USA, threatening native species and damaging agricultural and horticultural crops. One possible explanation for the success of these invaders is parasite release. To test this hypothesis we collected European slugs in part of their native range (United Kingdom) and in the USA and compared prevalence, distribution and species richness of their nematode parasites. All slugs were dissected and examined for the presence of nematodes. In the UK, nematodes were present at 93% of study sites and 16.4% of all slugs examined were associated with nematodes whereas in the USA the respective figures were 34% of sites and 5.4% of slugs. Nematode species richness was greater in the UK with 12 species being found, seven of which were thought to be truly parasitic as opposed to being phoretic or necromenic. Nine species of nematode were found in the USA, four of which were truly parasitic. Four of the ten European slug species examined in the USA, were entirely free of truly parasitic nematodes whereas all were infected by nematodes in some sites in the UK. There was a significant difference in the prevalence of truly parasitic nematodes in five of these species when comparing their home versus invasive range. A significant difference in parasite prevalence was observed when comparing native and introduced slug species in the USA, however, this was not significant in the UK. Our data support a role for parasite release during the invasion of the USA by European slugs.     

Do plant parasitic nematodes have differential effects on the productivity of a fast- and a slow-growing grass species?

We have examined the interaction between plant parasitic nematodes and plant species from different stages of grassland succession. In these grasslands, fertiliser application was stopped in order to restore the former nutrient-poor ecosystems. This management resulted in a reversed succession of high- to low-productivity. Nematodes isolated from a high-productive early-successional field and a low-productive late-successional field were inoculated to sterilised soil planted with seedlings of either Lolium perenne (a fast-growing early-successional species) or Festuca rubra (a slow-growing late-successional species). The experiment was performed at low and high supply rates of nutrients. We hypothesised that at a low nutrient supply rate the growth of L. perenne will be more reduced by nematode herbivory than the growth of F. rubra. Furthermore, we hypothesised that higher numbers of plant parasitic nematodes will develop under L. perenne. We found no support for our first hypothesis, because nematodes did not affect plant growth. Our results suggest that changes in the nutrient availability rather than plant parasitic nematodes affect plant succession in impoverished grasslands. On the other hand, plant species and nutrient supply rate significantly affected the density and composition of the plant parasitic nematode community. In line with our second hypothesis, plant parasitic nematodes reproduced better on the fast-growing L. perenne than on the slow-growing F. rubra. Our results, therefore, suggest that the succession of the plant parasitic nematode community is probably more affected by changes in the plant community than the other way round.     

线虫耐寒性研究进展

对于分布在温带和寒带的线虫,它们只有战胜冬季寒冷的挑战,才能有利于种群的存在与发展。因此,耐寒性是线虫生物学研究中不可忽视的内容。综述了关于线虫在低温胁迫下的耐寒性测定方法、耐寒对策及耐寒机制等方面的研究进展。线虫的耐寒性和昆虫一样,可通过过冷却点和低温存活率两种指标进行评价,但在具体的实验方法上,线虫耐寒性研究有其不同之处。线虫的耐寒对策和耐寒机制具有多样化。耐寒对策主要有耐冻和避冻,二者能共同渗透于线虫的耐寒过程中。耐寒机制包括特殊发育阶段的形成、低温驯化作用、低分子量抗冻物质的聚集、以及高分子量抗冻蛋白和热休克蛋白的产生,等等。此外,还强调应从多个角度研究线虫的耐寒性,如寒冷敏感型线虫的研究、寄生线虫的耐寒对策研究以及交叉胁迫的研究。     

Biological control potential of plant growth‐promoting rhizobacteria suppression of Meloidogyne incognita on cotton and Heterodera glycines on soybean: A review

Over the past decade, we have seen an increasing market for biopesticides and an increase in number of microbial control studies directed towards plant‐parasitic nematodes. This literature survey provides an overview of research on biological control of two economically important plant‐parasitic nematodes, Meloidogyne incognita (Kofoid & White) Chitwood (southern root‐knot nematode) and Heterodera glycines Ichinohe (soybean cyst nematode) using spore‐forming plant growth‐promoting rhizobacteria (PGPR). In this review, the current biological control strategies for the management of those cotton and soybean nematodes, the mechanism of using BacillusPGPR for biological control of plant‐parasitic nematode including induced systemic resistance and antagonism and the future of biological control agents on management of plant‐parasitic nematodes are covered.