Ditylenchus destructor and Ditylenchus dipsaci

A fast and sensitive method for the simultaneous identification of three important nematode species of the genus Ditylenchus

BACKGROUND

Nematodes of the genus Ditylenchus are parasites of a wide range of hosts, including higher plants. The most destructive of these species are D. dipsaci and D. destructor, two frequently quarantined pests. No rapid molecular method is available for unambiguous detection and distinguishing of these species from each other or from D. gigas, a pest of Vicia faba, either by multiplex PCR or real-time PCR.

RESULTS

By aligning all D. dipsaci, D. destructor and D. gigas rDNA sequences, the authors found a constant-sequence region that could be used as a universal 5' primer and constant regions in the ITS1 regions of the rDNAs that could be used as species-specific 3' primers for PCR detection of these nematodes. A standardised protocol was developed for both singleplex- and triplex-mode PCR that yields a single product of distinct length for each of the species. The PCR protocol has also been adapted for real-time PCR.

CONCLUSION

The present diagnostic PCR protocol is the only method that can identify all three species with the use of a triplex and/or a singleplex PCR assay. Importantly, the 3' primer for D. destructor ITS1 rDNA was designed so that it would hybridise all haplotypes.

Effect of initial population densities of Ditylenchus destructor and D. dipsaci on potato tuber damage and nematode reproduction

Glasshouse experiments were conducted to evaluate the effect of initial population densities () of Ditylenchus destructor and D. dipsaci on potato tuber damage and nematode reproduction. Ditylenchus destructor did not influence tuber numbers but influenced tuber weight at high levels. Ditylenchus dipsaci influenced tuber numbers and weights at a level of 14.29 (g growing medium)−1. Tolerance limit estimates according to the Seinhorst model were very low indicating both nematode species have a major impact on potato tuber weight. External and internal tuber rot caused by both species increased with levels. Ditylenchus destructor caused more tuber rot than D. dipsaci at all levels. Reproduction rates of D. destructor were higher at all levels studied compared to D. dipsaci. The equilibrium density of 1.3 and 0.6 for D. destructor and D. dipsaci, respectively, was observed at level of 14.29 (g growing medium)−1.

Banning of methyl bromide for seed treatment: could Ditylenchus dipsaci again become a major threat to alfalfa production in Europe?

In Europe, the stem and bulb nematode Ditylenchus dipsaci has been listed as a quarantine pest by EPPO: without any control, it may cause complete failure of alfalfa crops. Movement of nematodes associated with seeds is considered to be the highest-risk pathway for the spread of this pest. Since the 2010 official withdrawal of methyl bromide in Europe, and in the absence of any alternative chemical, fumigation of contaminated seed batches is no longer possible, which makes the production of nematode-free alfalfa seeds difficult to achieve and leads to unmarketable seed batches. Thermotherapy is being considered as a realistic alternative strategy, but its efficiency still remains to be validated. The combination of the currently available methods (i.e. use of resistant cultivars, seed production according to a certification scheme, mechanical sieving, seed batch inspection) could significantly reduce the likelihood of seed contamination. However, it does not guarantee a total eradication of the nematode. Although it is already widely distributed all over Europe, reclassification of D. dipsaci as a regulated non-quarantine pest to reduce the possibility of further introductions and the rate of spread of this pest appears to be a risky strategy because of the lack of up-to-date documented data to evaluate damage thresholds and determine acceptable tolerance levels.

Characterisation and functional importance of β-1,4-endoglucanases from the potato rot nematode, Ditylenchus destructor

Plant-parasitic nematodes have developed a series of enzymes to degrade the rigid plant cell wall; β-1,4-endoglucanase is a very important component. Ditylenchus destructor is a migratory endoparasite for which few molecular data have been published. Two novel β-1,4-endoglucanases (Dd-eng-1a and Dd-eng-2) were cloned and characterised from D. destructor. The DD-ENG-1A putative protein consists of a signal peptide, a catalytic domain and a carbohydrate-binding module (CBM). By contrast, the CBM domain is absent from DD-ENG-2. The exon/intron structure and phylogenetic tree indicate that both cellulase genes could have evolved from common ancestral genes. Southern blotting confirmed that the β-1,4-endoglucanases were of nematode origin and a member of a small multi-gene family. In situ hybridisation localised the expression of Dd-eng-1a and Dd-eng-2 to the subventral pharyngeal glands. RT-PCR showed that both genes were expressed in the adult female and second-stage juvenile. The stylet secretions of D. destructor showed clear cellulase activity in carboxymethylcellulose (CMC) plate assay, and similar results were observed in total homogenates and DD-ENG-1A and DD-ENG-2 recombinant proteins. These results demonstrated that D. destructor can produce and secrete functional cellulases. Silencing the putative β-1,4-endoglucanases by double-stranded RNA (dsRNA) resulted in an average decrease in infection of 50%. Successful RNAi in vitro was demonstrated in this study, which confirmed that Dd-eng-1a and Dd-eng-2 play important roles in nematode parasitism.

Exploring the host parasitism of the migratory plant-parasitic nematode Ditylenchus destuctor by expressed sequence tags analysis

The potato rot nematode, Ditylenchus destructor, is a very destructive nematode pest on many agriculturally important crops worldwide, but the molecular characterization of its parasitism of plant has been limited. The effectors involved in nematode parasitism of plant for several sedentary endo-parasitic nematodes such as Heterodera glycines, Globodera rostochiensis and Meloidogyne incognita have been identified and extensively studied over the past two decades. Ditylenchus destructor, as a migratory plant parasitic nematode, has different feeding behavior, life cycle and host response. Comparing the transcriptome and parasitome among different types of plant-parasitic nematodes is the way to understand more fully the parasitic mechanism of plant nematodes. We undertook the approach of sequencing expressed sequence tags (ESTs) derived from a mixed stage cDNA library of D. destructor. This is the first study of D. destructor ESTs. A total of 9800 ESTs were grouped into 5008 clusters including 3606 singletons and 1402 multi-member contigs, representing a catalog of D. destructor genes. Implementing a bioinformatics' workflow, we found 1391 clusters have no match in the available gene database; 31 clusters only have similarities to genes identified from D. africanus, the most closely related species to D. destructor; 1991 clusters were annotated using Gene Ontology (GO); 1550 clusters were assigned enzyme commission (EC) numbers; and 1211 clusters were mapped to 181 KEGG biochemical pathways. 22 ESTs had similarities to reported nematode effectors. Interestedly, most of the effectors identified in this study are involved in host cell wall degradation or modification, such as 1,4-beta-glucanse, 1,3-beta-glucanse, pectate lyase, chitinases and expansin, or host defense suppression such as calreticulin, annexin and venom allergen-like protein. This result implies that the migratory plant-parasitic nematode D. destructor secrets similar effectors to those of sedentary plant nematodes. Finally we further characterized the two D. destructor expansin proteins.