Role Of Nematodes Research Articles

Neonicotinoid Seed Treatments Influence Soil Nematode Taxonomic Composition and the Soil Microbial Cooccurrence Networks

Neonicotinoid insecticides are widely used to control early-season and foliar-feeding pests. Some studies have revealed their nontarget impacts on pollinators and other invertebrates but few investigated their effects on soil microbiota. Given the crucial role of soil prokaryotic and eukaryotic microbial communities in agroecosystem regulation and their contribution to soil fertility, it is critical to understand their structure and changes in response to disturbances such as pesticide application. Among these communities, free-living nematodes have the potential to indicate the ecological changes in soil caused by environmental stress and have a key role in forming and modulating soil microbial composition and function by feeding on other soil microorganisms or interacting with them. Here, we used 18S ribosomal RNA gene amplicon sequencing to characterize the effects of neonicotinoids on soil nematode communities in a 3-year soybean-corn crop rotation in Quebec, Canada. We also quantified the changes in nematode-bacteria cooccurrence networks in soil exposed to neonicotinoids. We found that neonicotinoid seed treatment significantly explained variation in nematode community composition and affected the relative abundance of some nematode families, such as a decrease in the omnivorous family Dorylaimidae in neonicotinoid-treated samples. Moreover, neonicotinoids altered the patterns of nematode-bacteria cooccurrence, including the structure and taxonomic composition of the networks. However, it is unclear whether neonicotinoids affected bacterial cooccurrence networks directly, or indirectly by affecting nematodes that feed on bacteria. Further research is needed to understand how neonicotinoids affect nematodes and the role of nematodes in microbial network variation in soil exposed to neonicotinoids.

An exploration of nematode assemblages in the hydroponic media beds of a commercial aquaponics system

Summary Nematodes perform important ecosystem services in soils and are well studied in traditional farming, but their role in newer soil-less food production systems is unstudied. We determined the nematode assemblages from substrates within the hydroponic media beds of a commercial aquaponics system. Three substrates were studied: coir, compost and lightweight expanded clay aggregate (LECA), with the nematode assemblages ascertained via traditional morphological analysis. All substrates were suitable for nematodes but abundance was lower compared with traditional compost. Compost from within the hydroponic media beds had nematode assemblages with similar maturity indices (MI) to compost external to the system, and both composts housed nematodes with a high proportion of bacterivores, a high degree of enrichment and no structure. LECA that were regularly disturbed and had little organic build up resembled compost – highly enriched and lacking structure. LECA that had been left undisturbed for longer and allowed to accumulate more organic matter had highly structured assemblages and showed less enrichment than the other substrates. Nematode assemblages within the coir showed very little enrichment but had higher MI and were more structured than compost. Problematic plant-parasitic nematodes were not found in any of the samples. We present the first glimpse into the role of nematodes in aquaponics systems and encourage future studies to explore how nematodes in these systems affect yield and plant health.

In planta transformation of Polianthes tuberosa for concomitant knockdown of flp-1, flp-12 and flp-18 genes induced root-knot nematode resistance

Polianthes tuberosa is a lucrative crop owing to its huge demand in floriculture, perfume and pharmaceutical industry. Root-knot nematodes, especially Meloidogyne incognita poses a major impediment on tuberose yield. Currently nematode management relies on the use of hazardous nematicides and other inefficacious means viz. crop rotation and soil solarization. The ongoing search for an ecologically sustainable and target specific approach, led to the progression of genetic modification based techniques to target genes critical for nematode survival. FMRFamide like peptide (FLPs) genes have drawn interest as prospective targets because of their diverse physiological roles in nematodes. Here, the effect of concomitant silencing of three vital nematode FLP genes (Mi-flp-1, Mi-flp-12 and Mi-flp-18) using host induced RNAi in tuberose was assessed against M. incognita. Firstly, the fusion (flp-1, flp-12 and flp-18) gene cassette was functionally validated using host-delivered RNAi in tobacco model system. Transgenic tobacco expressing dsRNA of fusion gene significantly reduced the nematode multiplication factor (MF) by 82.6%. Since, genetic transformation of tuberose has been neglected so far, and transgenic development is an inevitable step for host-generated RNAi, an Agrobacterium mediated transformation protocol was optimized in tuberose. Tuberose tubers were pricked at the apical meristem of the axis and at the tiny white meristematic buds emerging from them, followed by infection with Agrobacterium hosting pCAMBIA 3301 vector. Maximum transformation efficiency was achieved with 200 μM acetosyringone as observed from the GUS staining. For HIGS, fusion transgenic events were developed in tuberose and molecular analysis confirmed the presence and integration of fusion transgene using PCR, qRT-PCR and Southern blot analysis. Bioefficacy analysis of transgenic plants against M. incognita resulted in 73.75% reduction in nematode fecundity, thereby demonstrating the effectiveness of the three flp genes for nematode management in tuberose. This study established first ever report of in planta Agrobacterium-mediated transformation of tuberose for host delivered RNAi for M. incognita resistance.

A highly divergent Wolbachia with a tiny genome in an insect-parasitic tylenchid nematode.

Wolbachia symbionts are the most successful host-associated microbes on the planet, infecting arthropods and nematodes. Their role in nematodes is particularly enigmatic, with filarial nematode species either 100% infected and dependent on symbionts for reproduction and development, or not at all infected. We have discovered a highly divergent strain of Wolbachia in an insect-parasitic tylenchid nematode, Howardula sp., in a nematode clade that has not previously been known to harbour Wolbachia. While this nematode is 100% infected with Wolbachia, we did not detect it in related species. We sequenced the Howardula symbiont (wHow) genome and found that it is highly reduced, comprising only 550 kilobase pairs of DNA, approximately 35% smaller than the smallest Wolbachia nematode symbiont genomes. The wHow genome is a subset of all other Wolbachia genomes and has not acquired any new genetic information. While it has lost many genes, including genes involved in cell wall synthesis and cell division, it has retained the entire haem biosynthesis pathway, suggesting that haem supplementation is critical. wHow provides key insights into our understanding of what are the lower limits of Wolbachia cells, as well as the role of Wolbachia symbionts in the biology and convergent evolution of diverse parasitic nematodes.

Detection and distribution of Stephanofilaria sp. in buffalo flies and buffalo fly lesions in north Australian beef cattle

Buffalo flies (Haematobia irritans exigua) are ectoparasites of major animal health and production concern in north Australian beef herds. Skin lesions associated with buffalo fly infestation, cause hide damage and welfare issues and are manifested as dermatitis or ulcerated areas found most commonly near the medial canthus of the eye, along the lateral and ventral neck and on the abdomen. Buffalo flies can transmit a nematode, Stephanofilaria sp., which has been considered the main aetiological agent for buffalo fly lesions, but the role of nematodes in the development of the lesions has not been defined. To investigate the geographical distribution of Stephanofilaria, swabs were taken from the surface of lesions and buffalo flies were netted from the backs of beef cattle from 20 properties located in northern, central and southern Queensland. Swabs and buffalo flies were then tested for the presence of Stephanofilaria by qPCR. In northern and central Queensland, all properties except one, tested positive for the presence of Stephanofilaria in either buffalo flies or swabs, or in both. The infection rate varied amongst sites ranging from 0% to 100% in lesions and 0–34% in female buffalo flies. No nematodes were found in male buffalo flies. In contrast, none of the 66 lesion swabs or 1220 buffalo flies collected from southern Queensland tested positive for Stephanofilaria infection despite the frequent occurrence of lesions in the herds from which samples were collected. These findings suggest that infection with Stephanofilaria, although frequently detected, is not essential for the development of buffalo fly lesions and other factors may contribute to the initiation of lesions. This study also confirmed the potential for using surface swabs as a quicker and less invasive means of sampling lesions than dermal biopsies when testing for the presence of Stephanofilaria by qPCR, but further studies will be required to estimate the sensitivity of this technique. Understanding the pathogenesis of buffalo fly lesions will aid the development of optimal treatment and control strategies.

Ultrastructural plasticity in the plant-parasitic nematode, Bursaphelenchus xylophilus

Phenotypic plasticity is one of the most important strategies used by organisms with low mobility to survive in fluctuating environments. Phenotypic plasticity plays a vital role in nematodes because they have small bodies and lack wings or legs and thus, cannot move far by themselves. Bursaphelenchus xylophilus, the pathogenic nematode species that causes pine wilt disease, experiences fluctuating conditions throughout their life history; i.e., in both the phytophagous and mycetophagous phases. However, whether the functional morphology changes between the life phases of B. xylophilus remains unknown. Our study revealed differences in the ultrastructure of B. xylophilus between the two phases. Well-developed lateral alae and atrophied intestinal microvilli were observed in the phytophagous phase compared with the mycetophagous phase. The ultrastructure in the phytophagous phase was morphologically similar to that at the dauer stage, which enables the larvae to survive in harsh environments. It suggests that the living tree represents a harsh environment for B. xylophilus, and ultrastructural phenotypic plasticity is a key strategy for B. xylophilus to survive in a living tree. In addition, ultrastructural observations of obligate plant-parasitic species closely related to B. xylophilus revealed that B. xylophilus may be in the process of adapting to feed on plant cells.

Fusarium-nematode wilt disease complexes, etiology and mechanism of development

Plant parasitic nematodes constitute an important group of pathogens and cause 7–15% yield loss to agricultural crops. The nematodes, besides causing direct damage to plants, also possess capability to synergise other pathogens leading to development of disease complex. The nematode invasion may enhance severity of fungal or bacterial diseases or may break host resistance against a disease. Fusarium-nematode disease complexes involving root-knot nematodes, Meloidogyne spp. are widely prevalent and highly damaging to vegetable and pulse crops in India as well as in other countries. The present chapter critically analyzes the information on Fusarium-nematode wilt disease complex for it occurrence and economic significance. The role of nematodes as a biopredisposing agent in promoting infection of plants by Fusarium spp. and the mechanism of development of wilt disease complexes are discussed in detail.

Nematodes in Meiobenthos and Periphyton of the Rybinsk Reservoir (Review)

This paper presents data on changes in the taxonomic composition and abundance of nematodes in meiobenthos and periphyton of the Rybinsk Reservoir in the long term. The species composition of roundworms in bottom communities is significantly richer and their number is manyfold higher than among fouling organisms. These differences are determined by the different habitat conditions at the bottom and hard substrate surfaces in the water column. In modern anthropogenically disturbed zones of various waterbodies, the biocenotic and functional roles of nematodes depend to a larger extent on food availability than on some other environmental factors limiting the development of worms. Roundworms, living under a wide range of ecological conditions, are one of the key groups of invertebrates critical for preserving aquatic life under anthropogenic stresses.

Community structure of soil nematodes under different drought conditions

The study aims to investigate the effects of different drought treatments on the distribution of soil nematode community under the background of global warming. Light drought, moderate drought, severe drought treatments and control plot were set in the experiment. Experimental results showed that a total of 32 genera of nematodes from 18 families were identified under the four drought treatments, including 13 genera of bacterivores, 10 genera of plant-parasites, 4 genera of fungivores and 5 genera of omnivores-predators. Eucephalobus and Helicotylenchus were dominant genera under drought treatments. The quantity, genera and diversity of soil nematodes in drought treatments were significantly lower than those in control plots. The structure of the nematode community degenerated and the role of nematodes in soil food web was weakened in the agro-ecosystem after long-term drought. The study provides soil zoological basis for agro-ecosystem responding to extreme drought under climate change.

Transgenic Nicotiana benthamiana plants expressing a hairpin RNAi construct of a nematode Rs-cps gene exhibit enhanced resistance to Radopholus similis

Burrowing nematodes (Radopholus similis) cause severe harm in many agronomic and horticultural crops and are very difficult to manage. Cathepsin S is one of the most important cysteine proteinases and plays key roles in nematodes and many other parasites. To evaluate the effect of in planta RNAi on the control of this nematode, a specific fragment from the protease gene, cathepsin S (Rs-cps), was cloned into the binary vector pFGC5941 in the forward and reverse orientations to construct recombinant plant RNAi vectors. Transgenic Nicotiana benthamiana plants expressing Rs-cps dsRNA were obtained and studied. The transcript abundance of Rs-cps dsRNA appeared to be diverse in the different transgenic lines. Moreover, the bioassay results revealed that Rs-cps transgenic N. benthamiana plants were resistant to R. similis and the transcription level of Rs-cps in R. similis was drastically decreased. In addition, the reproduction and hatching rate of R. similis isolated from the Rs-cps transgenic plants were also significantly reduced. Our results suggest that Rs-cps is essential for the reproduction and pathogenicity of R. similis. This is the first study to employ in planta RNAi approach to target the Rs-cps gene for the control of plant parasitic nematodes.

Taxonomic and trophic structures of dominants in the zooperiphyton of waterbodies and watercourses of the upper Volga basin

All the main trophic groups of invertebrates are presented in the compositions of dominant assemblages of zooperiphyton in the studied waterbodies and watercourses. The strong pollution of water by suspended solids of anthropogenic origin in the water of the upper Volga reservoirs leads to the suppression of typically sessile filter-feeding invertebrates (sponges, bryozoans, and Dreissena) able to form the highest biomass of zooperiphyton using natural food items. The conditions of the disturbed structure of food resources are favorable for the development of detritivores-gatherers. In small rivers receiving industrial wastewaters, excessive amounts of food in the water column and in the form of sediments on the substrates suppresses invertebrates-biofowlers and facilitates the expansion of the benthic larvae of chironomids of g. Chironomus (combining the filtration and gathering of food particles on the substrates) to the vacant spatial niches. At the same time, the role of nematodes and oligochaetes with prevailing detritophages increases. In the acidified lakes of the Darwinsky National Reserve, the decrease in the total species diversity of zooperiphyton is accompanied by decreases in the role of filter-feeding invertebrates. The chironomid larvae and oligochaetes prevailed in the poor zoocenoses. The roles of predatory caddisfly larvae, black gnats, and nematodes increased. The labile structural characteristics of zooperiphyton with the conservation of the historically formed methods of foraging are the main modern way that biofouling communities adapt to changes in the quality and quantity of food resources upon modern anthropogenic pollution of the aquatic environment.

Gene induction by desiccation stress in the entomopathogenic nematode Steinernema carpocapsae reveals parallels with drought tolerance mechanisms in plants

The dauer juvenile (DJ) stage of the insect parasitic nematode Steinernema carpocapsae is the only stage in the life cycle which is capable of surviving outside its host and it is adapted for tolerating environmental stresses and for host finding. We have isolated 45 unique expressed sequence tags (ESTs) that are up-regulated in response to desiccation in S. carpocapsae DJs. The majority of these ESTs were co-expressed in response to desiccation and osmotic stress and were generally not induced in response to heat and cold stress. Thirty-two ESTs showed similarity to known sequences. Among these were sequences which encode putative signalling molecules or transcription factors, sequences which detoxify reactive oxygen species, two C-type lectin sequences, ESTs which encode membrane-associated proteins and seven distinct late embryogenic abundant (LEA) sequences. We also isolated 13 novel ESTs. These data show that the molecular response to desiccation stress in entomopathogenic nematode DJs is complex and parallels many of the adaptive changes which occur in drought tolerant plants during exposure to desiccation and osmotic stress. A notable feature of the desiccation response of plants is the number and diversity of hydrophilic LEA proteins synthesised in response to desiccation. All of the LEA sequences detected in animals to date, including those reported in this study, belong to LEA3 group. We show that S. carpocapsae expresses several novel sequences which encode putative hydrophilic and natively unfolded proteins. It is likely that these novel and putative proteins play an important role in desiccation tolerance, possibly by carrying out analogous roles in nematodes to those carried out by the other LEA protein classes in plants.

Role of nematodes in decomposition processes: Does within-trophic group diversity matter?

The importance of biodiversity for the functioning of ecosystems is still very unclear. Many hypotheses, mainly based on terrestrial studies, have been formulated, focussing on the plant diversity-productivity relationship. In this study, diversity-related and species-specific effects of bac- terivorous nematodes on the decomposition rate of cordgrass detritus Spartina anglica and on the associated microbial community were investigated using laboratory microcosm experiments. Four bacterivorous nematode species (Diplolaimelloides meyli, Diplolaimelloides oschei, Diplolaimella dievengatensis and Panagrolaimus paetzoldi) were added either separately or in combinations of 2 or 3 species to the microcosms. In contrast with previous reports, no stimulation of the decomposition process was observed in the presence of nematodes. Still, clear differences in process rates were found between nematode treatments. P. paetzoldi, reaching considerably higher densities than the other nematode species, suppressed bacterial activity and diversity, probably due to overgrazing. This was, however, not translated into slower decomposition. Within treatments exclusively contain- ing monhysterid nematode species (D. meyli, D. oschei and D. dievengatensis), differences in micro- bial activity and decomposition rates were found, but again no link was observed between activity and decomposition. Our data did not support any hypothesis predicting enhancement of process rates with an (initial) increase in numbers of nematode species, or redundancy among the studied species. Rather, we obtained support for an idiosyncratic diversity model, as differences in the effect of bac- terivore nematode species and species combinations could not be predicted at the start of the exper- iment. This could be explained by the inhibitory interactions between nematode species.

Prediction of Mammalian MicroRNA Targets

MicroRNAs (miRNAs) can play important gene regulatory roles in nematodes, insects, and plants by basepairing to mRNAs to specify posttranscriptional repression of these messages. However, the mRNAs regulated by vertebrate miRNAs are all unknown. Here we predict more than 400 regulatory target genes for the conserved vertebrate miRNAs by identifying mRNAs with conserved pairing to the 5′ region of the miRNA and evaluating the number and quality of these complementary sites. Rigorous tests using shuffled miRNA controls supported a majority of these predictions, with the fraction of false positives estimated at 31% for targets identified in human, mouse, and rat and 22% for targets identified in pufferfish as well as mammals. Eleven predicted targets (out of 15 tested) were supported experimentally using a HeLa cell reporter system. The predicted regulatory targets of mammalian miRNAs were enriched for genes involved in transcriptional regulation but also encompassed an unexpectedly broad range of other functions.

Functional characterization of alternatively spliced 5-HT2 receptor isoforms from the pharynx and muscle of the parasitic nematode, Ascaris suum.

Serotonin (5-HT) receptors play key regulatory roles in nematodes and alternatively spliced 5-HT2 receptor isoforms have been identified in the parasitic nematode, Ascaris suum. 5-HT2As1 and 5-HT2As2 contain different C-termini, and 5-HT2As1Delta4 lacks 42 amino acids at the C-terminus of the third intracellular loop. 5-HT2As1 and 5-HT2As2 exhibited identical pharmacological profiles when stably expressed in human embryonic kidney (HEK) 293 cells. Both 5-HT2As isoforms had higher affinity for 5-HT than their closely related Caenorhabditis elegans homolog (5-HT2Ce). This increased 5-HT affinity was not related to the substitution in 5-HT2As1 of F120 for Y in the highly conserved DRY motif found in the second intracellular loop of other 5-HT receptors, since a 5-HT2As1F120Y mutant actually exhibited increased 5-HT affinity compared with that of 5-HT2As1. As predicted, cells expressing either 5-HT2As1 or 5-HT2As2 exhibited a 5-HT-dependent increase in phosphatidylinositol (PI) turnover. In contrast, although 5-HT2As1Delta4 displayed a 10-fold higher affinity for 5-HT and 5-HT agonists than either 5-HT2As1 or 5-HT2As2, 5-HT2As1Delta4 did not couple to either PI turnover or adenyl cyclase activity. Based on RT-PCR, 5-HT2As1 and 5-HT2As2 were more highly expressed in pharynx and body wall muscle and 5-HT2As1Delta4 in nerve cord/hypodermis. This is the first report of different alternatively spliced 5-HT2 receptor isoforms from any system.

Spirocerca lupi and dogs: the role of nematodes in carcinogenesis

Response from Luis A. Herrera and Patricia Ostrosky-WegmanOne of the major objectives of our article 1xDo helminths play a role in carcinogenesis?. Herrera, L.A. and Ostrosky-Wegman, P. Trends Parasitol. 2001; 17: 172–175Abstract | Full Text | Full Text PDF | PubMed | Scopus (22)See all References1 was to attract the attention of scientists to the question: is chronic inflammation the only mechanism whereby parasites induce cancer? The answer seems to be no. Host–parasite relationships are rather complex, they are not limited to the development of inflammation. Parasites can interfere with cellular processes that are relevant to carcinogenesis, such as intercellular communication, signal transduction, or even secret molecules with carcinogenic potential. However, evidence supporting any alternative mechanisms is scanty, and in an attempt to stimulate research on this topic, we postulated several other possibilities by which helminths might participate in malignant transformation of host cells. Thus, we agree with Melendez and Suarez-Pellin that there are still more questions than answers on this subject. We also considered that future studies should: (1) find more epidemiological evidence for potential associations between helminthiases and cancer; this should imply the development or improvement of technologies for the detection and identification of parasites in tissue samples from patients with cancer, in particular, for the detection of past infections; and (2) use animal models in which pathological changes induced by specific parasites in humans could be reproduced and correlated with the multi-step carcinogenic process (e.g. in the mouse–Schistosoma mansoni model, the development of immature worms up to the adult stage in the murine liver is similar to the development of S. mansoni or Schistosoma haematobium in humans. In fact, this model has been used extensively to investigate the effects of schistosomiasis on hepatic carcinogen metabolism 2xRecovery of the hepatic carcinogen-metabolising capacity of schistosome-infected mice after treatment with the antischistosomal praziquantel. Sheweita, S.A. and Mostafa, M.H. Oncol. Rep. 1995; 2: 155–159PubMedSee all References, 3xRelationship between schistosomiasis and bladder cancer. Mostafa, M.H. et al. Clin. Microb. Rev. 1999; 12: 97–111PubMedSee all References.Although the Spirocerca lupi–dog model is not representative of some human helminthiasis, this model could be suitable for research into the mechanisms involved in carcinogenesis that are induced by parasites. Therefore, we strongly encourage Melendez and Suarez-Pellin to explore the interesting possibilities they described in their letter that will certainly help scientists to understand the role of helminths in human carcinogenesis.

Physiological and pharmacological studies on nematodes

Classical transmitters and neuroactive peptides act as transmitters or modulators within the central and peripheral nervous systems of nematodes, for example Ascaris suum and Caenorhabditis elegans. Acetylcholine (ACh) and gamma-aminobutyric acid (GABA) are respectively the excitatory and inhibitory transmitters onto somatic body wall muscle while 5-hydroxytrypamine (5-HT) is the excitatory transmitter onto pharyngeal muscle. 5-HT also reduces ACh-induced contractions of somatic muscle and this action of 5-HT is mediated through activation of adenylate cyclase while that on pharyngeal muscle is mediated through inositol phosphate activation. Glutamate, dopamine and octopamine also have transmitter roles in nematodes. Neuroactive peptides of the RFamide family can excite somatic muscle, for example, AF-1 (KNEFIRFamide), AF-2 (KHEYLRFamide), AF-3 (AVPGVLRFamide) and AF-4 (GDVPGVLRFamide) or inhibit and relax this muscle, for example, PF-1 (SDPNFLRFamide), PF-2 (SADPNFLRFamide) and PF-4 (KPNlRFamide). In addition PF-3 (AF-8) (KSAYMRFamide) has a biphasic action on pharyngeal muscle, excitation followed by inhibition while AF-1 only inhibits this muscle. The peptide effects can be either pre- or postsynaptic or both and are likely to be mediated through second messenger systems. In addition these peptides modulate the action of classical transmitters, particularly ACh.

Role of meiofauna in estuarine soft‐bottom habitats*

Meiofauna are ubiquitous in estuaries worldwide averaging 106 m−2. Abundance and species composition are controlled primarily by three physical factors: sediment particle size, temperature and salinity. While meiofauna are integral parts of estuarine food webs, the evidence that they are biologically controlled is equivocal. Top down (predation) control clearly does not regulate meiofaunal assemblages. Meiofauna reproduce so rapidly and are so abundant that predators cannot significantly reduce population size. Food quantity (bottom up control) also does not appear to limit meiofaunal abundance; there is little data on the effect of food quality. In estuarine sediments meiofauna: (i) facilitate biomineralization of organic matter and enhance nutrient regeneration; (ii) serve as food for a variety of higher trophic levels; and (iii) exhibit high sensitivity to anthropogenic inputs, making them excellent sentinels of estuarine pollution. Generally mineralization of organic matter is enhanced and bacterial production stimulated in the presence of meiofauna. Tannins from mangrove detritus in northern Queensland appear to inhibit meiofaunal abundance and therefore the role of meiofauna in breakdown of the leaves. Meiofauna, particularly copepods, are known foods for a variety of predators especially juvenile fish and the meiofaunal copepods are high in the essential fatty acids required by fish. The copepod’s fatty acid composition is like that of the microphytobenthos they eat; bacterial eaters (nematodes?) do not have the essential fatty acids necessary for fish. Most contaminants in estuaries reside in sediments, and meiofauna are intimately associated with sediments over their entire life‐cycle, thus they are increasingly being used as pollution sentinels. Australian estuarine meiofauna research has been concentrated in Queensland, the Hunter River estuarine system in New South Wales, and Victoria’s coastal lagoons. Studies in northern Queensland have primarily concentrated on the role of nematodes in mineralization of organic matter, whereas those from southern Queensland have concentrated on the role of meiofauna as food for fish and as bacterial grazers. The New South Wales studies have concentrated on the Hunter River estuary and on the structure and function of marine nematode communities. In Victoria, several fish have been shown to eat meiofauna. The Australian world of meiofaunal research has hardly been touched; there are innumerable opportunities for meiofaunal studies. In contaminated estuarine sediments reduced trophic coupling between meiofauna and juvenile fish is a basic ecological question of habitat suitability, but also a question with relevance to management of estuarine resources. Because meiofauna have short lifecycles, the effects of a contaminant on the entire life‐history can be assessed within a relatively short time. The use of modern molecular biology techniques to assess genetic diversity of meiofauna in contaminated vs uncontaminated sediments is a promising avenue for future research. Much of the important meiofaunal functions take place in very muddy substrata; thus, it is imperative to retain mudflats in estuaries.

Nematodes as bioindicators of dry pasture recovery after temporary rye cultivation

Soil and plant parasitic nematodes were sampled from the top 10 cm of a luvisol in dry grassland north of Madrid (Central Spain). A natural pasture and two grazed pastures which had been cropped with rye 1 and 6 yr before, respectively, were compared by multivariate statistical analysis of the nematode fauna. Results from stepwise discriminant analysis demonstrated the influence of agricultural management on the abundance of the nematode community of these pasture soils. Specifically, it was found that Paratylenchus and Rhabditidae can have a discriminative value of the recovery of these soil ecosystems. Among the indices used to analyze the abundance, diversity and maturity of the nematode community, only plant parasite index (PPI) detected significant differences among pastures, decreasing its values as recovery time after human intervention increased, with the significantly highest value in the recently cropped (C) pasture. It is concluded that length of time after cropping influences the abundance more than the diversity of the nematodes present in pastures of these areas, and that multivariate statistical analyses are the most useful tests to detect differences. The role of nematodes as biological indicators of the degree of recovery of these soils after human disturbance is noted.

The role of nematodes, fungi, bacteria, and abiotic factors in the etiology of apple replant problems in the Granite Belt of Queensland

Investigations of apple replant failure in the Granite Belt suggested that the problem had a complex etiology. Soil fertility was an important factor because apple seedlings grew best in replant soils with high levels of nitrogen, phosphorus, and potassium. Consistent improvements in the growth of apple seedlings were obtained when typical orchard soils were treated with fenamiphos, confirming that lesion nematode was also an important component of the disease complex. Pratylenchus penetrans had been recognised as a pathogen of apples, and pathogenicity tests showed that P. jordanensis, another species widely distributed in the Granite Belt, had similar effects. Growth responses of apple seedlings were greater when soil was pasteurised than when it was treated with fenamiphos, suggesting that root pathogens other than nematodes were involved in apple replant failure. However, the primary cause probably differed between orchards because soils did not respond in the same manner to pasteurisation and nematicide treatments. Pathogenicity tests with 14 bacteria associated with apple roots showed no effect on the growth of apple seedlings. However, Fusarium tricinctum, Cylindrocarpon destructans, and Pythium sp. were implicated in the problem because they were consistently recovered from discoloured roots. In a factorial experiment involving nematodes and fungi in pots, P. jordanensis, P. penetrans, E. tricinctum, and C. destructans reduced the dry weight of apple roots but there was no interaction between nematodes and fungi.