BackgroundPlant parasitic nematodes are unusual Metazoans as they are equipped with genes that allow for symbiont-independent degradation of plant cell walls. Among the cell wall-degrading enzymes, glycoside hydrolase family 5 (GHF5) cellulases are relatively well characterized, especially for high impact parasites such as root-knot and cyst nematodes. Interestingly, ancestors of extant nematodes most likely acquired these GHF5 cellulases from a prokaryote donor by one or multiple lateral gene transfer events. To obtain insight into the origin of GHF5 cellulases among evolutionary advanced members of the order Tylenchida, cellulase biodiversity data from less distal family members were collected and analyzed.ResultsSingle nematodes were used to obtain (partial) genomic sequences of cellulases from representatives of the genera Meloidogyne, Pratylenchus, Hirschmanniella and Globodera. Combined Bayesian analysis of ≈ 100 cellulase sequences revealed three types of catalytic domains (A, B, and C). Represented by 84 sequences, type B is numerically dominant, and the overall topology of the catalytic domain type shows remarkable resemblance with trees based on neutral (= pathogenicity-unrelated) small subunit ribosomal DNA sequences. Bayesian analysis further suggested a sister relationship between the lesion nematode Pratylenchus thornei and all type B cellulases from root-knot nematodes. Yet, the relationship between the three catalytic domain types remained unclear. Superposition of intron data onto the cellulase tree suggests that types B and C are related, and together distinct from type A that is characterized by two unique introns.ConclusionsAll Tylenchida members investigated here harbored one or multiple GHF5 cellulases. Three types of catalytic domains are distinguished, and the presence of at least two types is relatively common among plant parasitic Tylenchida. Analysis of coding sequences of cellulases suggests that root-knot and cyst nematodes did not acquire this gene directly by lateral genes transfer. More likely, these genes were passed on by ancestors of a family nowadays known as the Pratylenchidae.
Foliar nematodes, plant-parasitic representatives of the genus Aphelenchoides, constitute a minority in a group dominated by fungivorous species. Distinction between (mostly harmless) fungal feeding Aphelenchoides species and high impact plant parasites such as A. besseyi, A. fragariae, A. ritzemabosi, and A. subtenuis is severely hampered by the scarcity of informative morphological characters, some of which are only observable in specific developmental stages. Poor description of a number of non-plant-parasitic Aphelenchoides species further complicates identification. Based on (nearly) full-length small subunit ribosomal DNA (SSU rDNA) sequences (≈1,700 bp), a phylogenetic tree was generated, and the four target species appeared as distinct, well-supported groups. Notably, this genus does not constitute a monophyletic group: A. besseyi and A. ritzemabosi cluster together and they are phylogenetically isolated from A. fragariae, A. subtenuis, and most other fungivorous species. A phylum-wide SSU rDNA framework was used to identify species-specific DNA motifs. For the molecular detection of four plant-parasitic Aphelenchoides species, polymerase chain reaction primers were developed with high, identical annealing temperatures (63°C). Within the molecular framework presented here, these primers can be used for the rapid screening of plant material and soil for the presence of one or multiple foliar nematode species.
In 2000 Siddiqi formulated a hypothesis stating that root-knot nematodes (Meloidogyne spp.) constitute a branch arising from yet atiother important group of plant parasites, the tnigratory Pratylenchidae. This hypothesis was solely based on morphological characteristics. Ribosomal DNA (rDNA) sequence analysis supports this hypothesis in its broad sense, but the more precise question about the identity of a migratory Pratylenchidae representative being closest to the most basal Meloidogyne species could not be addressed due to a lack of backbone resolution ). Here we present an extended small subunit rDNA sequence analysis and a data set of partial RNA polymerase II sequences from Pratylenchidae and basal Meloidogynidae. Our data point at members of the genus Pratylenchus as being closest to the common ancestor of the root-knot nematodes, but it was not possible unequivocally to identify a candidate lesion nematode species. Pratylenciius is a species-rich genus (ca 70 valid species), and we suggest that the species closest to the most basal root-knot nematode should be sought outside of the group of relatively well-characterised, agronomicaUy relevant, species.Within the phylum Nematoda, the order Tylenchida harbours the largest and agronomicaUy most important groups of plant parasites. However, plant parasitism is not the only trophic group in this speciose clade as the insect and mite-parasitic suborder Hexatylina is nested within. In terms of economic impact, three tylenchid groups are most relevant, viz., root-knot {Meloidogyne spp.), lesion (Pratylenchus spp.) and cyst nematodes (Heterodera and Globodera spp.). Root-knot and cyst nematodes are sedentary endoparasites that induce the formation of a feeding site in the roots of their host plant, whereas lesion nematodes are migratory endoparasites that continuously enter and leave the plant root. Also, from an applicationoriented point of view, it is relevant to pinpoint in more detail the evolutionary relationship between the three families, Meloidogynidae, Pratylenchidae and Heteroderidae, in which these genera currently reside.On the basis of (nearly) full-length small subunit ribosomal DNA (SSU rDNA) sequences, a sister relationship was inferred between members of the Meloidogynidae and Pratylenchidae on the one hand, and Heteroderidae and Hoplolaimidae on the other (Holterman et ai, 2009). Despite the fact that more and more data point to a close evolutionary relationship between lesion and rootknot nematodes, we do not have a detailed picture yet, as most of the research efforts are dedicated to the more distal members of Meloidogyne such as M. incognita, M. hapla and M. chitwoodi.
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