Research Article: New evidence of nematode-endosymbiont bacteria coevolution based on one new and one known dagger nematode species of Xiphinema americanum-group (Nematoda, Longidoridae)

Date Published: June 26, 2019

Publisher: Public Library of Science

Author(s): Mahyar Mobasseri, Matthew C. Hutchinson, Farahnaz Jahanshahi Afshar, Majid Pedram, Ulrich Melcher.


Three populations of Xiphinema primum n. sp. and two populations of X. pachtaicum were recovered from natural forests and cultural regions of northern Iran. Both species belong to the X. americanum-group and were characterized by their morphological, morphometric and molecular data. The new species, which was recovered in three locations, belongs to the X. brevicolle-complex and is characterized by 2124–2981 μm long females with a widely rounded lip region separated from the rest of the body by a depression, 103–125 μm long odontostyle, two equally developed genital branches with endosymbiont bacteria inside the ovary, which are visible under light microscope (LM), vulva located at 51.8–58.0%, the tail is 26–37 μm long with a bluntly rounded end and four juvenile developmental stages. It was morphologically compared with nine similar species viz. X. brevicolle, X. diffusum, X. incognitum, X. himalayense, X. luci, X. parabrevicolle, X. paramonovi, X. parataylori and X. taylori. The second species, X. pachtaicum, was recovered in two geographically distant points close to city of Amol. Molecular phylogenetic studies of the new species were performed using partial sequences of the D2-D3 expansion segments of the large subunit ribosomal RNA gene (LSU rDNA D2-D3), the internal-transcribed spacer rDNA (ITS = ITS1+5.8S+ITS2), and the mitochondrial cytochrome c oxidase I gene (COI mtDNA) regions. The Iranian population of X. pachtaicum was also phylogenetically studied based upon its LSU rDNA D2-D3 sequences. Both species were also inspected for their putative endosymbiont bacteria. Candidatus Xiphinematobacter sp. was detected from two examined populations of the new species, whereas the second endosymbiont bacterium, detected from three examined isolates of X. pachtaicum, was related to the plant and fungal endosymbionts of the family Burkholderiaceae. The phylogenetic analyses of the two endosymbiont bacteria were performed using partial sequences of 16S rDNA. In cophylogenetic analyses, significant levels of cophylogenetic signal were observed using both LSU rDNA D2-D3 and COI mtDNA markers of the host nematodes and 16S rDNA marker of the endosymbiont bacteria.

Partial Text

The nematode species of the genus Xiphinema Cobb, 1913 [1] are migratory ectoparasites of roots in a wide range of plants. They cause damage through direct feeding on the root cells of host plants and also through the transmission of some pathogenic plant nepoviruses [2]. Species of this genus are traditionally and historically divided into two groups, those of the americanum-group, and those of non-americanum-group [3–6].

The nematodes of the Xiphinema americanum-group are infamous for their difficulty in species delimitation using traditional criteria. The conserved morphology of even phylogenetically distant species or the close morphometric data ranges further complicate their identification. Using the mean values of the morphometric data as proposed by Orlando et al. [10] and applied in this study (see Differential and Diagnosis), could help primary species comparisons.

Hereby, we studied two Iranian populations of Xiphinema pachtaicum and described a new species, both of which belonged to the X. americanum-group, using an integrated approach. Significant cophylogeny between the newly generated sequences of nematode LSU rDNA D2-D3 and COI mtDNA sequences and the 16S rDNA sequences of the endosymbiont bacteria were also observed.




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