Genomic and genetic research on bursate nematodes: significance, implications and prospects

Molecular genetic research on parasitic nematodes (order Strongylida) is of major significance for many fundamental and applied areas of medical and veterinary parasitology. The advent of gene technology has led to some progress for this group of nematodes, particularly in studying parasite systematics, drug resistance and population genetics, and in the development of diagnostic assays and the characterisation of potential vaccine and drug targets. This paper gives an account of the molecular biology and genetics of strongylid nematodes, mainly of veterinary socio-economic importance, indicates the implications of such research and gives a perspective on genome research for this important parasite group, in light of recent technological advances and knowledge of the genomes of other metazoan organisms.     

The biology and ecology of lotic nematodes

• 1 Morphological structures for identifying freshwater nematodes, e.g. buccal cavity, sensory receptors, oesophagus, reproductive organs and tail are described.
• 2 Most freshwater nematodes belong to the Adenophorea and are characterised by the presence of setae, adhesive glands and conspicuous amphids.
• 3 Methods for collecting nematodes from the sediments of running water (e.g. corer, pumps), within plants and aufwuchs are listed. Methods for fixation, extracting and preparing nematodes for identification are described.
• 4 Life history parameters (e.g. generation time, eggs per female) are not available for lotic nematodes but are summarised for free‐living nematodes in soil, lakes and seas. Field studies indicate that, in contrast to laboratory experiments with nematode cultures, many species will have a generation time of several months.
• 5 Abundance and species diversity of nematodes of lotic habitats are provided; more than 100 nematode species inhabit lotic habitats and densities can reach 230 individuals per ml.
• 6 Links between meiobenthic nematodes and the micro‐ and macrobenthos are unclear at present. Evidence such as the increased bacterial activity due to nematode grazing suggests that such interactions may be significant.

     

Three sons of fortune: early embryogenesis, evolution and ecology of nematodes

Comparative analysis of nematode development has revealed considerable variations in how the fates of embryonic cells are specified. Such early variations seem enigmatic as they do not influence the resultant structure or performance of the emerging animal. Three different nematode species are used to consider why alternative ways to reach the same goal may have been established during evolution and why early steps of embryogenesis are particularly variable. A scenario is sketched with a shift from late to early cell specification, along with an increase in maternal contribution and developmental tempo and a decrease in regulative potential expressing different developmental strategies. Future studies of larger numbers of species are needed to assess the extent of such variations and to understand more fully the underlying mechanisms, rules and driving forces.     

The role of weed hosts and the distribution and activity of vector nematodes in the ecology of tobacco rattle virus

At a site in eastern Scotland, nine common species of arable weeds were infected with tobacco rattle virus (TRV), and some of these, notably Viola arvensis and Stellaria media, comprised an overwintering reservoir of the virus. TRV was seed-borne both in naturally and in experimentally infected V. arvensis (2–10%), and occasionally in other weed species. In the glasshouse at 20 ^oC a naturally infective population of vector nematodes (Tricho-dorus spp.) kept in soil free of plants retained its infectivity for 20 wk, although few Trichodorus survived for this period. In the field, the incidence of TRV infection in potato (spraing disease) in plots kept free of weeds for 1–5 years was 3–4 times that in weed-infested plots but Trichodorus numbers did not differ appreciably between the two treatments. Presumably the virus is retained for long periods in its vectors and these feed on potato more frequently when other hosts are not available. Weeds are probably important in the long term as hosts of both TRV and its vectors, but in the short term weed control seems unlikely to prevent potato spraing because of the long persistence of TRV in vector populations. In the field, Trichodorus accumulated near the interface between topsoil and subsoil, and the incidence of spraing was greatest where the topsoil was shallowest. When cucumber seedlings were exposed to virus-carrying Trichodorus, TRV reached a greater concentration in roots at 20 ^oC than at 24 ^oC, and the virus was not detected in roots at 29 ^oC. In a sandy soil, TRV was transmitted only when the water content exceeded 15%, and at least 30 % water was needed for maximum transmission. Annual records of rainfall and spraing disease suggest that spraing is most prevalent when the summer is wettest. TRV is not confined to cultivated land. Stabilized sand dunes supporting a pure stand of Ammophila armaria were colonized by Trichodorus pachyder-mus, but TRV was detected only where the plant community had enlarged to include V. arvensis and other dicotyledons. In such situations, TRV may be introduced in the seed of V. arvensis, and the movement of soil by wind probably contributes to the dispersal of Trichodorus.     

Molecular systematics, phylogeny and ecology of anisakid nematodes of the genus Anisakis Dujardin, 1845: an update

Advances in the taxonomy and ecological aspects concerning geographical distribution and hosts of the so far genetically recognised nine taxa of the nematodes belonging to genus Anisakis (i.e. A. pegreffii, A. simplex s.s., A. simplex C, A. typica, A. ziphidarum, Anisakis sp., A. physeteris, A. brevispiculata and A. paggiae) are here summarized. Genetic differentiation and phylogenetic relationships inferred from allozyme (20 enzyme-loci) and mitochondrial (sequences of cox-2 gene) markers, are revised and compared. The two genetic analyses are congruent in depicting their phylogenetic relationships. Two main clusters are showed to exist in the obtained trees, one encompassing the species A. pegreffii, A. simplex s.s., A. simplex C, A. typica, A. ziphidarum and Anisakis sp.; while, the second including A. physeteris, A. brevispiculata and A. paggiae. The existence of two clades is also supported by their morphological differentiation in adult and larval morphology. Comparison of phylogenetic relationships among Anisakis spp. with those currently available for their cetacean definitive hosts suggests parallelism between host and parasite phylogenetic tree topologies. Preliminary data for reconstruction of a possible co-evolutionary scenario between cetacean hosts and their Anisakis endoparasites suggests that cospeciation and host-switching events may have accompanied the evolution of this group of parasites. Finally, genetic/molecular markers for the identification of the so far genetically recognized taxa of Anisakis at any life-stage and both sexes were given also in relation to human anisakiosis is discussed.     

The host plant and the nematodes

Parasite selection by the host would be expected and is frequently found amongst the specialized plant parasitic nematodes and on perennial plants. Hijink & Oostenbrink (1968) showed that annual cropping could establish distinct nematode communities and such communities can become quickly established (Green, 1975). In carrot crops Aphelenchoides and Rotylenchus species became a larger proportion of the community very early in the growth of the crop (Green, 1976). Stabilization of communities in this way indicates a very close relationship between the nematodes and the host plants even when specialized feeding is not apparent. Cropping alters the plant community so that even these nematodes with a wide host range become dependant on the one host, provided weed control is efficient. Spacing of the crop further restricts the nematodes so they become more dependant on individual plants. Nematode attacks on crops are notable for two features. The hosts appear under-exploited, that is there are few individual hosts with many parasites but many with few and, the hosts are rarely killed.     

Hsp-90 and the biology of nematodes

Background  

Hsp-90 from the free-living nematode Caenorhabditis elegans is unique in that it fails to bind to the specific Hsp-90 inhibitor, geldanamycin (GA). Here we surveyed 24 different free-living or parasitic nematodes with the aim of determining whether C. elegans Hsp-90 was the exception or the norm amongst the nematodes. We combined these data with codon evolution models in an attempt to identify whether hsp-90 from GA-binding and non-binding species has evolved under different evolutionary constraints.     

Application and commercialization of nematodes

While nematodes are most commonly known for their negative impact on plants, animals, and humans, there are a number of species which are commercially explored. This review highlights some of the most important success stories for the application of nematodes. They are used as bioindicators in ecological and toxicity studies, as model organisms for elucidating fundamental biological questions and for high throughput screening of drugs. Besides these indirect uses, direct applications include the use of Beddingia siricidicola against a major forest pest and the commercialization of Steinernema, Heterorhabditis, and Phasmarhabditis as biological pest control products. New directions for the commercialization of nematodes are the use as living food, specifically loaded with essential nutrients for various fish and shrimp larvae. Even human parasites or closely related species have been successfully used for curing autoimmune disorders and are currently in the process of being developed as drugs. With the striving development of life sciences, we are likely to see more applications for nematodes in the future. A prerequisite is that we continue to explore the vast number of yet undiscovered nematode species.