Location: National Germplasm Resources LaboratoryTitle: Molecular and morphological characterisation of Xiphinema americanum group species (Nematoda:Dorylaimida)from California and other regions and co-evolution of bacteria from the genus Candidata Xiphinemobacter with nematodes Author
Submitted to: Nematology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/27/2016
Publication Date: 9/15/2016
Citation: Orlando, V., Chitambar, J., Dong, K., Chizov, V., Mollov, D.S., Bert, W., Subbotin, S. 2016. Molecular and morphological characterisation of Xiphinema americanum group species (Nematoda:Dorylaimida)from California and other regions and co-evolution of bacteria from the genus Candidata Xiphinemobacter with nematodes. Nematology. 18:1015-1043. Interpretive Summary: The dagger nematode, Xiphinema species, parasitizes many cultivated and naturally occurring landscape plants. Furthermore, Xiphinema is also known to vector an economically important group of plant viruses from the genus Nepovirus. The taxonomy of Xiphinema currently relies primarily on morphological characteristics, and partial sequences of ribosomal DNA from the nematode. However, these two approaches are often insufficient to distinguish among species. In this paper we described the association of nematodes with its endosymbiotic bacterium, Candidatus Xiphinematobacter. Our results indicate there is a very strong correlation between the bacteria and the Xiphinema species at the DNA level. This information provides another means of determining nematode species, and suggests that the bacteria and its nematode host evolved in a symbiotic relationship. The application of this technique led to the description of four previously uncharacterized nematode species. This research supports nematode identification and their association with specific host plants, which ultimately will aid in better pest management practices.
Technical Abstract: The Xiphinema americanum group is a large species complex containing more than two dozen nematode species. They are economically important because they are vectors of nepoviruses. The species differentiation of X. americanum group is problematic because the species share similar morphological characters. In the present study, we collected nematode samples from different locations in the USA, Italy and Russia. Six valid species, X. americanum, X. brevicolle, X. californicum, X. pachtaicum, X. rivesi, X. simile and four unidentified putative Xiphinema species were characterised by morphology and sequencing of the D2-D3 of 28S rRNA, ITS rRNA and coxI mtDNA genes. One hundred and forty seven new sequences of nematodes were generated. Phylogenetic relationships of the Xiphinema americanum group species reconstructed by Bayesian inference for the D2-D3 of 28S rRNA gene sequences did not provide clear species delimitation of the samples studied, although the mtDNA presented interspecific variations useful for demarcation among species. Xiphinema americanum, X. californicum, X. pachtaicum, X. rivesi, and two unidentified Xiphinema species were found in seventy one soil samples from California. We also reconstructed the phylogenetic relationships using partial 16S rRNA gene sequences within endosymbiotic bacteria of the genus Candidatus Xiphinematobacter and provided solid evidence for distinguishing seventeen species of this genus based on the analysis of new and previously published sequences. Fifty five new bacterial sequences were obtained in the present study and comparison of the bacterial 16S rRNA and nematode coxI phylogenies revealed a high level of cospeciation events between host and symbiont.