Plant-cyst nematode and plant-root-knot nematode interactions

Root-knot nematodes and cyst nematodes are obligate plant parasites that cause extensive damage to the agriculture of both temperate and tropical countries. In this review, Andreas Niebel, Godelieve Gheysen and Marc Van Montagu describe how, in the past decade, the use of molecular techniques has provided new insights in the complex interactions between these sedentary plant-parasitic nematodes and their infected host plants. They give an account of the progress in our understanding of both the parasite and the host during compatible and incompatible interactions. They also outline the importance of a new model host system. Arabidopsis thaliana.     

Plant nematode communities

Most plant-parasitic nematodes live in polyspecific communities. They are abundant in most habitats. Their numbers and diversities are influenced greatly by the host and diversity of the habitats. Although populations are affected by interactions with other organisms working through the host plant, it is doubtful that there is much direct competition among plant-parasitic nematodes. Except were damage is severe as a result of parasitism, there usually is an abundance for food. Damage to plants may be additive or partly so by two or more species of nematodes.     

Post-genomic nematode parasitology

The future direction of post-genomic nematode parasitology should focus on the function of the genes that are defined by large-scale expressed sequence tag sequencing and on broader questions about the genetic basis of parasitism. Functional characterisation will require the application of high throughput technologies that have been developed in other fields, including genome mapping strategies and DNA microarray analysis. These will be greatly aided by the development and application of appropriate model organisms. It is crucial that the field make the transition from a narrow focus on one or a few genes at a time to a focus on whole genomes in order to fully realise the potential of the expressed sequence tag and other genomic projects currently under way.     

The molecular size of nematode acetylcholinesterases and their separation from nematode allergens

The molecular size of nematode acetylcholinesterases, and their separation from nematode allergens. International Journal for Parasitology 3: 735–741. Acetylcholinesterases and allergens were derived from two parasitic nematodes, Nippostrongylus brasiliensis which parasitises rats and Trichostrongylus colubriformis which infects sheep and guinea pigs.

Chromatographic studies indicated that the mol. wt of nematode acetylcholinesterases lies between 65,000 and 75,000. The acetylcholinesterases of both species were separated from nematode allergens whose mol. wt is in the range of 10,000–50,000.

γG binding antigens of T. colubriformis were located in fractions with a mol. wt range of 30,000–150,000. γE binding activity was confined to allergenic material with a mol. wt of less than 70,000.     

Novel nematode amber suppressors

Nine amber suppressor mutations were isolated in the nematode Caenorhabditis elegans by reverting amber alleles of a sex-determining gene, tra-3. One suppressor maps to a known locus, sup-5 III , but the other eight map to three new loci, sup-21 X (five alleles), sup-22 IV (two alleles) and sup-23 IV (one allele). Amber alleles of tra-3 and of a dumpy gene, dpy-20, were used to measure the efficiency of suppression; the sup-21 and the sup-22 alleles were both shown to be heterogeneous and generally weaker suppressors than sup-5 alleles, which are homogeneous. The spectrum of mutations suppressed by a strong sup-21 allele, e1957, was investigated and compared to the spectra for the amber suppressors sup-5 III and sup-7 X, using amber alleles in 13 assorted genes. Some of the differences between these spectra may be due to limited tissue specificity in sup-21 expression.—Suppression of dpy-20 was used to show that the sex-linked suppressors sup-7 and sup-21 are not dosage compensated in male (XO) relative to hermaphrodite (XX).—Several uses of amber suppressors are critically discussed: for identifying null mutations, for varying levels of gene activity and for detecting maternal mRNA.     

Probing the nematode surface

The surface of parasitic nematodes has been well studied with respect to its structural and immunological properties, but little is known about its biophysical nature and the role this plays in the host-parasite relationship. In this article, Clare Roberts and Jay Modha highlight some biophysical features of nematode surfaces and discuss their recent findings regarding mechanisms controlling surface-associated biophysical phenomena observed in parasitic nematodes during infection or culture in medium simulating the mammalian host environment. The nematode surface is distinct from the plasma membrane, nevertheless some parallel features exist and are described.     

Evolution of nematode development

Multiple evolutionary variations occur in the cellular and genetic programming of nematode development. Many changes involve alterations of inductive interactions. Surprisingly, inductive processes vary during evolution, irrespective of changes in the final cell lineages and morphological structures. Genetic studies in some nematodes also shed light on the underlying mechanisms of evolutionary change.     

Pathophysiology of nematode infections

Gastrointestinal nematodes are a major cause of reduced productivity in ruminants.An important feature of such infections is reduced feed intake, the aetiology of which remains uncertain. Alterations in gastrointestinal motility and digesta flow may be partially responsible for the inappetance as well as being associated with the occurrence of diarrhoea. However, digestion and absorption are, in general, not significantly impaired.The other distinctive feature of gastrointestinal parasitism is loss of proteins into the gastrointestinal tract and resultant changes in host metabolism account for much of the reduced protein and energy retention by infected animals. Mineral metabolism and water and electrolyte balance are also frequently disturbed. These pathophysiological effects may be modulated by a number of factors, including the nutritional and immunological status of the host. Details of the precise mechanisms involved in these interactions and several other aspects of the pathophysiology of nematode infections still require investigation.     

根结线虫放线菌及其生物防治活性研究

从感染植物根部的根结线虫卵和雌虫中,分离得到放线菌20株。形态、细胞壁氨基酸组分和16S rRNA基因序列分析表明:分离菌株分属于链霉菌属(Streptomyces)、诺卡氏菌属(Nocardia)和假诺卡氏菌属(Pseudonocardia),其中链霉菌占80.0%。分离菌株对根结线虫Meloidogynespp.卵的平均寄生率、卵的孵化率、幼虫死亡率分别为54.1%、40.4%和26.2%。根据体外测试的结果,选择具有高致病力的3株链霉菌(1-17,2-6,9-47)和1株诺卡氏菌(5-1)进行温室番茄防效测试,其生防效率分别为31.4%、37.7%、56.4%和42.4%。     

The environment and the nematode

It is customary to list the factors affecting animal population as edaphic, climatic and biotic. To these I would add a fourth, intrinsic.     

Stochasticity accelerates nematode egg development

Day-degrees models of nematode development assume that temperature stochasticity has no effect on the development rate of infective stages as long as the mean temperature is held constant. This assumption was tested in this study. Unembryonated Heterakis gallinarum eggs were subjected to nocturnal and diurnal daily temperature cycles at 12 and 17 C. respectively, and embryonation was compared with eggs subjected to similar stochastic daily cycles, in which random normal variations in the temperature were added to the 2 temperatures. The prediction that there is no effect of stochasticity was refuted. Embryonation of eggs subjected to variable daily cycles occurred significantly earlier than that of eggs subjected to deterministic daily cycles, suggesting that stochastic variation in temperature accelerated embryonation even though mean temperatures were the same. These findings show that the development time of H. gallnarum eggs is decreased by stochastic variation in temperature, which may have important implications for the effects of climate change on parasite availability.     

Caenorhabditis elegans-applications to nematode genomics

The complete genome sequence of the free-living nematode Caenorhabditis elegans was published 4 years ago. Since then, we have seen great strides in technologies that seek to exploit this data. Here we describe the application of some of these techniques and other advances that are helping us to understand about not only the biology of this important model organism but also the entire phylum Nematoda.     

Chromatin diminution in nematode development

Chromatin diminution in Parascaris and Ascaris represents the classical case of a developmentally programmed genome rearrangement. The process is very specific with respect to ontogenetic timing and chromosomal localization, and involves chromosomal breakage, new telomere formation and DNA degradation. Recent evidence from Ascaris lumbricoides var. suum suggests that chromatin diminution might have a function in gene regulation.     

Membrane flow during nematode spermiogenesis

Two distinct types of surface membrane rearrangement occur during the differentiation of Caenorhabditis elegans spermatids into amoeboid spermatozoa. The first, detected by the behavior of latex beads attached to the surface, is a nondirected, intermittent movement of discrete portions of the membrane. This movement starts when spermatids are stimulated to differentiate and stops when a pseudopod is formed. The second type of movement is a directed, continual flow of membrane components from the tip of the pseudopod to its base. Both membrane glycoproteins and fluorescent phospholipids inserted in the membrane flow backward at the same rate, approximately 4 micrometers/min, although their lateral diffusion coefficients in the membrane differ by at least a factor of 5. These observations suggest that pseudopodial membrane movement is due to bulk flow of membrane components away from the tip of the pseudopod.     

Nemoglobins: divergent nematode globins

Globins are proteins commonly associated with oxygen transport in vertebrate blood, but the invertebrate phyla display a wide variety of globin types that reflect their disparate life styles and evolutionary history. It has been known for over 100 years that parasitic nematodes contain globins, but recent molecular investigations are only now beginning to shed some light on their curious properties. Mark Blaxter here describes the diversity of the different globins found in nematodes, and reviews emerging data on their evolution and function.     

Plant nematode resistance genes.

Root-knot and cyst nematodes cause severe damage to crops throughout the world. Genes conferring resistance against nematodes have been identified in many plant species and several of these have been, or soon will be, cloned. Nematode biotypes that can infect resistant plants have been identified. Investigation of cloned resistance genes and of virulent nematodes is likely to lead to improved host resistance.