RNA interference and plant parasitic nematodes

RNA interference (RNAi) has recently been demonstrated in plant parasitic nematodes. It is a potentially powerful investigative tool for the genome-wide identification of gene function that should help improve our understanding of plant parasitic nematodes. RNAi should help identify gene and, hence, protein targets for nematode control strategies. Prospects for novel resistance depend on the plant generating an effective form of double-stranded RNA in the absence of an endogenous target gene without detriment to itself. These RNA molecules must then become available to the nematode and be capable of ingestion via its feeding tube. If these requirements can be met, crop resistance could be achieved by a plant delivering a dsRNA that targets a nematode gene and induces a lethal or highly damaging RNAi effect on the parasite.     

Neurobiology of plant parasitic nematodes

The regulatory constraints imposed on use of chemical control agents in agriculture are rendering crops increasingly vulnerable to plant parasitic nematodes. Thus, it is important that new control strategies which meet requirements for low toxicity to non-target species, vertebrates and the environment are pursued. This would be greatly facilitated by an improved understanding of the physiology and pharmacology of these nematodes, but to date, these microscopic species of the Phylum Nematoda have attracted little attention in this regard. In this review, the current information available for neurotransmitters and neuromodulator in the plant parasitic nematodes is discussed in the context of the more extensive literature for other species in the phylum, most notably Caenorhabditis elegans and Ascaris suum. Areas of commonality and distinctiveness in terms of neurotransmitter profile and function between these species are highlighted with a view to improving understanding of to what extent, and with what level of confidence, this information may be extrapolated to the plant parasitic nematodes.     

Amphids in Strongyloides stercoralis and other parasitic nematodes

In this review, Francis Ashton and Gerhard Schad examine the ultrastructure of the amphids of several animal parasitic nematodes. These structures are the main chemosensory organs of these worms and probably play an important role in host-finding behavior and the control of development. Reconstructions made from serial micrographs of the neurons in the amphids of the threadworm Strongyloides stercoralis are shown. These stereo images permit three-dimensional visualization of these complex sense organs. The association between each amphidial neuron and its cell body has not been made previously for a parasitic nematode; however, this has been done for the free-living nematode Caenorhabditis elegans, which served as a model for these studies. Recognition of the cell bodies will provide a point of departure for laser microbeam ablation studies to determine individual neuronal function.     

Mucosal immunity against parasitic gastrointestinal nematodes

The last two decades witnessed significant advances in the efforts of immunoparasitologists to elucidate the nature and role of the host mucosal defence mechanisms against intestinal nematode parasites. Aided by recent advances in basic immunology and biotechnology with the concomitant development of well defined laboratory models of infection, immunoparasitologists have more precisely analyzed and defined the different immune effector mechanisms during the infection; resulting in great improvement in our current knowledge and understanding of protective immunity against gastrointestinal (GI) nematode parasites. Much of this current understanding comes from experimental studies in laboratory rodents, which have been used as models of livestock and human GI nematode infections. These rodent studies, which have concentrated on Heligmosomoides polygyrus, Nippostrongylus brasiliensis, Strongyloides ratti/S. venezuelensis, Trichinella spiralis and Trichuris muris infections in mice and rats, have helped in defining the types of T cell responses that regulate effector mechanisms and the effector mechanisms responsible for worm expulsion. In addition, these studies bear indications that traditionally accepted mechanisms of resistance such as eosinophilia and IgE responses may not play as important roles in protection as were previously conceived. In this review, we shall, from these rodent studies, attempt an overview of the mucosal and other effector responses against intestinal nematode parasites beginning with the indices of immune protection as a model of the protective immune responses that may occur in animals and man.     

Optimal timing of first reproduction in parasitic nematodes

The time between infection and the onset of reproduction (maturation time) is a key determinant of body size, fecundity and generation time in parasitic nematodes. An optimality model for maturation time is developed centred on prematurational growth, the duration of which has opposing consequences for fecundity and for survival to reproductive age. The maturation time favoured by natural selection is found to be inversely proportional to prematurational mortality rate. The product of maturation time and mortality rate is predicted to be an invariant number equal to the allometric slope linking daily fecundity to maturation time. Predictions are tested using comparative data on mammalian gastrointestinal nematode taxa. Half the cross-species variation in prepatent period (the time from infection until propagules are shed from the host) is accounted for by this adaptive trade-off hypothesis, even though many biological details are not explicitly modelled. These results are discussed in the light of previous studies and in the context of general models of life history evolution.     

Cell cycle activation by plant parasitic nematodes

Sedentary nematodes are important pests of crop plants. They are biotrophic parasites that can induce the (re)differentiation of either differentiated or undifferentiated plant cells into specialized feeding cells. This (re)differentiation includes the reactivation of the cell cycle in specific plant cells finally resulting in a transfer cell-like feeding site. For growth and development the nematodes fully depend on these cells. The mechanisms underlying the ability of these nematodes to manipulate a plant for its own benefit are unknown. Nematode secretions are thought to play a key role both in plant penetration and feeding cell induction. Research on plant-nematode interactions is hampered by the minute size of cyst and root knot nematodes, their obligatory biotrophic nature and their relatively long life cycle. Recently, insights into cell cycle control in Arabidopsis thaliana in combination with reporter gene technologies showed the differential activation of cell cycle gene promoters upon infection with cyst or root knot nematodes. In this review, we integrate the current views of plant cell fate manipulation by these sedentary nematodes and made an inventory of possible links between cell cycle activation and local, nematode-induced changes in auxin levels.     

New approaches to control plant parasitic nematodes

Plant parasitic nematodes are a serious threat for crop production worldwide. This review summarizes our understanding of plant nematode interactions and presents new alternatives for nematode control in the field. Breeding for resistance has been a major goal for many important crop species like soybean, potato, tomato and sugar-beet. As a result numerous nematode-resistance genes have been identified, two of which have been cloned recently, Hs1 ^pro-1 from sugar-beet, giving resistance to the beet cyst nematode Heterodera schachtii, and Mi from tomato, giving resistance to the root-knot nematode Meloidogyne incognita. Also artificial resistance genes, coding for nematotoxic proteins or causing rapid death of feeding cells, have been elucidated. In the future, genetic engineering of nematode resistance will become more and more important for plant breeding. Transformation techniques will allow genes to be quickly introduced into susceptible breeding lines and then combined with each other to produce plant varieties with durable resistance. Received: 26 August 1998 / Received revision: 16 December 1998 / Accepted: 21 December 1998     

Production-based control of parasitic nematodes of cattle

Nematode parasite control in cattle is the goal of the parasitologist and the cattle producer. However, the language used to express the impact of that control has been a source of confusion between the two groups. Veterinary parasitologists speak in terms of reduction in worms or worm eggs, and cattle producers in terms of weight gain, milk production or calving rate. During the development of doramectin for cattle in temperate climates worldwide, the point came when we began to look for a different set of parameters to guide trial design and to communicate the results. In this paper, a series of published papers resulting from the yearling portion of this development programme are reviewed from the viewpoint of weight gain in relation to forage/feed availability. A pattern emerged that indicated that yearling cattle, when parasite control was effective (as indicated by egg counts) and forage was sufficient (as indicated by weather patterns), gained from 0.75 to 0.95 kg day(-1) in trials from the USA, Europe and Argentina. When parasite control or forage supply or both were insufficient, these rates of weight gain were significantly reduced. If more attention is spent on forage availability and weight-gain parameters when parasite-control programmes are designed, then researchers might communicate more meaningful information to producers on the value of parasite control.     

Carbonic acid as the host signal for the development of parasitic stages of nematodes

Petronijevic T., Rogers W. P. and Sommerville R. I. 1985. Carbonic acid as the host signal for the development of parasitic stages of nematodes. International Journal for Parasitology15: 661–667. This paper gives results on which may be based an identification of the component of the system CO₂ + H₂O ai H₂CO₃ ai H⁺ HCO₃^? which acts as the stimulus from the animal host for some nematodes. Using infective juveniles of Nematospiroides dubius and Haemonchus contortus, the effects on exsheathment of (1) low pCO₂ values, (2) the presence of carbonic anhydrase in the stimulating medium, and (3) the inhibition of carbonic anhydrase within the juveniles have been examined. The results lead to the suggestion that it is the “readily available” undissociated H₂CO₃, or H₂CO₃ + HCO₃^? which is the critical factor in the stimulus for development. The wide range of [H⁺]s over which “readily available” H₂CO₃ is present in physiological environments suggests that this host signal may be important for infection with many species.     

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.