Phytochemicals as Biopesticides against the Pinewood Nematode Bursaphelenchus xylophilus: A Review on Essential Oils and Their Volatiles

The impacts of a rapidly changing environment together with the growth in global trade activities has promoted new plant pest pandemic events in forest ecosystems. The pinewood nematode (PWN), Bursaphelenchus xylophilus, causes strong worldwide economic and ecological impacts. Direct control is performed through trunk injection of powerful nematicides, however many of these (hemi)synthetic compounds have raised ecological and human health concerns for affecting non-target species and accumulating in food products. As sustainable alternatives, essential oils (EOs) have shown very promising results. In this work, available literature on the direct activity of EOs against PWN is reviewed, as a contribution to advance the search for safer and greener biopesticides to be used in sustainable PWD pest management strategies. For the first time, important parameters concerning the bioassays performed, the PWNs bioassayed, and the EOs used are summarized and comparatively analyzed. Ultimately, an overview of the chemical composition of the most active EOs allowed to uncover preliminary guidelines for anti-PWN EO efficiency. The analysis of important information on the volatile phytochemicals composing nematicidal EOs provides a solid basis to engineer sustainable biopesticides capable of controlling the PWN under an integrated pest management framework and contributes to improved forest health.

Sub-THz spectroscopic characterization identification for pine wood nematode ribosomal DNA

This paper, we introduced Sub-terahertz (Sub-THz) technology to identify nematode DNA sequence. First, data mining technology and restriction enzyme digestion were used to cut out two corresponding sequences, each containing about 100 base pairs that could represent the characteristic fragments of Bursaphelenchus xylophilus (Bx) and Bursaphelenchus mucronatus (Bm) rDNA in internal transcribed spacer 1 (ITS1) region. Then, vibration spectra of the two enzyme-cut sequences were measured by Fourier transform infrared spectroscopy (FTIR). Meanwhile, the spectrum was analyzed by molecular dynamics method. It was found that the calculated and experimental spectra of the two enzyme-cut sequences were consistent, although the differences of the sequences could not be well reflected in the spectra. The vibration modes corresponding to diverse absorption peaks in the spectra were quite different, which were closely related to the internal bases sequencing. This can be used as an indicator for identifying Bx and Bm DNA. Moreover, the normal mode analysis (NMA) method was first adopted for spectral attribution analysis of DNA long sequences. Finally, the vibration spectra of shorter sequences predicted by second-order Markov chains and Monte Carlo method were studied. To some extent, the predicted short sequences can represent the complete sequence as the initial calculation structure.

Widespread prevalence but contrasting patterns of intragenomic rRNA polymorphisms in nematodes: Implications for phylogeny, species delimitation and life history inference

Ribosomal RNA genes have long been a favoured locus in phylogenetic and metabarcoding studies. Within a genome, rRNA loci are organized as tandem repeated arrays and the copies are homogenized through the process of concerted evolution. However, some level of rRNA variation (intragenomic polymorphism) is known to persist and be maintained in the genomes of many species. In nematode worms, the extent of rRNA polymorphism (RP) across species and the evolutionary and life history factors that contribute to the maintenance of intragenomic RP is largely unknown. Here, we present an extensive analysis across 30 terrestrial nematode species representing a range of free-living and parasitic taxa isolated worldwide. Our results indicate that RP is common and widespread, ribosome function appears to be maintained despite mutational changes, and intragenomic variants are stable in the genome and neutrally evolving. However, levels of variation were varied widely across rRNA locus and species, with some taxa observed to lack RP entirely. Higher levels of RP were significantly correlated with shorter generation time and high reproductive rates, and population-level factors may play a role in the geographic and phylogenetic structuring of rRNA variants observed in genera such as Rotylenchulus and Pratylenchus. Although RP did not dramatically impact the clustering and recovery of taxa in mock metabarcoding analyses, the present study has significant implications for global biodiversity estimates of nematode species derived from environmental rRNA amplicon studies, as well as our understanding of the evolutionary and ecological factors shaping genetic diversity across the nematode Tree of Life.

Genetic diversity of Bursaphelenchus cocophilus in South America

Molecular characterisation ofBursaphelenchus cocophilus, the causal agent of ‘red ring disease’, is imperative for efficient identification procedures in Brazil and Colombia, because quarantine species such asB. xylophilusandB. mucronatusare already listed in both countries. ITS-1/2 region and D2-D3 segment of LSU rDNA were used to characterise isolates ofB. cocophilusobtained from coconut plantations in Brazil and Colombia. Results from ITS-1/2 and LSU rDNA regions showed that all isolates ofB. cocophilusfrom Brazil and Colombia formed a monophyletic group. The LSU rDNA region indicated that all isolates formed a single monophyletic group with high Bayesian posterior probability (100%). This is the first study on ITS-1/2 for the characterisation ofB. cocophiluspopulations. A species-specific primer was designed for identification ofB. cocophilus.

Neoalloionema indicum n. sp. (Nematoda: Alloionematidae), a new alloionematid from India

A new nematode species belonging to the family Alloionematidae was originally isolated by Dr Marie Anne Félix on 16 December 2007 from a rotting coconut at the boat boarding point in Estuary island resort, Poovar, Kerala, India. Both morphologically and molecularly, the new isolate resembles Neoalloionema tricaudatum but, unlike N. tricaudatum this species has no phasmidial ‘horns’ in the juvenile stage, even though prominent structures in the mid-tail of dauer juveniles were observed. Females are characterised by a body length of 947 (869-1030) μm, vulva in the mid-body region, long, and tapering tail with a minute post-anal swelling. Males are 678 (606-757) μm long with a long tapering tail that is very thick at base. They are further characterised by seven pairs of genital papillae and a single genital papilla. Dauer juveniles are only 391 (364-434) μm long, thin and almost straight when heat-killed. Large, prominent phasmids are located laterally at the mid-tail region and lateral fields consisting of a single ridge are present. Several morphological and molecular differences encouraged us to describe the nematode as N. indicum n. sp. The ecology of the newly described species is unknown, but it is probably a saprophytic bacteriophagous nematode preferring rotting organic material.

Genome-wide variation in the pinewood nematode Bursaphelenchus xylophilus and its relationship with pathogenic traits

BACKGROUND

Bursaphelenchus xylophilus is an emerging pathogenic nematode that is responsible for a devastating epidemic of pine wilt disease across Asia and Europe. In this study, we report the first genome-wide variation analysis of the nematode with an aim to obtain a full picture of its diversity.

METHODS

We sequenced six key B. xylophilus strains using Illumina HiSeq sequencer. All the strains were isolated in Japan and have been widely used in previous studies. Detection of genomic variations were done by mapping the reads to the reference genome.

RESULTS

Over 3 Mb of genetic variations, accounting for 4.1 % of the total genome, were detected as single nucleotide polymorphisms or small indels, suggesting multiple introductions of this invaded species from its native area into the country. The high level of genetic diversity of the pine wood nematode was related to its pathogenicity and ecological trait differences. Moreover, we identified a gene set affected by genomic variation, and functional annotation of those genes indicated that some of them had potential roles in pathogenesis.

CONCLUSIONS

This study provides an important resource for understanding the population structure, pathogenicity and evolutionary ecology of the nematode, and further analysis based on this study with geographically diverse B. xylophilus populations will greatly accelerate our understanding of the complex evolutionary/epidemic history of this emerging pathogen.

Re-description of the slug-parasitic nematode Alloionema appendiculatum Schneider, 1859 (Rhabditida: Alloionematidae)

Alloionema appendiculatum is a common juvenile parasite of many terrestrial molluscs. Its third-stage juveniles (dauers) invade the foot muscle of snails and slugs and develop into fourth-stage juveniles, which then leave the host. Later they mature and reproduce in the soil. A population of A. appendiculatum was isolated from infected individuals of the invasive slug Arion vulgaris (= A. lusitanicus), collected in the city of České Budějovice, Czech Republic, and was designated as the AL strain. This nematode is also able to reproduce on pig kidney in laboratory culture. Our isolate was compared primarily with the description published by Mengert (1953) and in some aspects with the original description published by Schneider (1859). Insufficient morphology, morphometrics, phylogeny and an absence of pictorial material encouraged us to re-describe this frequent slug parasite and to add some new information on its life-cycle. The species is characterised by the absence, in adults, of ridges in the lateral fields. At this life stage the stoma is short, narrow and approximately twice as long as it is broad. Fourth-stage female juveniles produce a mucus-like substance from the phasmids. Males possess six pairs of papillae and a single inconspicuous papilla. The nematode has both parasitic and saprophytic life-cycles. Parasitic adults are bigger than the saprophytes and have a thick, digitate tail, whereas the smaller saprophytic generation has a filiform tail. The species is amphimictic, displaying a higher proportion of females, but males are frequent.

Using mitogenomic and nuclear ribosomal sequence data to investigate the phylogeny of the Xiphinema americanum species complex

Nematodes within the Xiphinema americanum species complex are economically important because they vector nepoviruses which cause considerable damage to a variety of agricultural crops. The taxonomy of X. americanum species complex is controversial, with the number of putative species being the subject of debate. Accurate phylogenetic knowledge of this group is highly desirable as it may ultimately reveal genetic differences between species. For this study, nematodes belonging to the X. americanum species complex, including potentially mixed species populations, were collected from 12 geographically disparate locations across the U.S. from different crops and in varying association with nepoviruses. At least four individuals from each population were analyzed. A portion of the 18S nuclear ribosomal DNA (rDNA) gene was sequenced for all individuals while the internal transcribed spacer region 1 (ITS1) of rDNA was cloned and 2 to 6 clones per individual were sequenced. Mitochondrial genomes for numerous individuals were sequenced in parallel using high-throughput DNA sequencing (HTS) technology. Phylogenetic analysis of the 18S rDNA revealed virtually identical sequences across all populations. Analysis of ITS1 rDNA sequences revealed several well-supported clades, with some degree of congruence with geographic location and viral transmission, but also numerous presumably paralogous sequences that failed to form clades with other sequences from the same population. Analysis of mitochondrial DNA (mtDNA) indicated the presence of three distinct monophyletic clades of X. americanum species complex nematodes. Two clades contained nematodes found in association with nepovirus and the third contained divergent mtDNA sequences from three nematode populations from the western U.S. where nepovirus was absent. The inherent heterogeneity in ITS1 rDNA sequence data and lack of informative sites in 18S rDNA analysis suggests that mtDNA may be more useful in sorting out the taxonomic confusion of the X. americanum species complex.