Molecular and morphological characterisation of Xiphinema americanum-group species (Nematoda: Dorylaimida) from California, USA, and other regions, and co-evolution of bacteria from the genus Candidatus Xiphinematobacter with nematodes

Abstract

The Xiphinema americanum-group is a large species complex containing more than 50 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 s. str., X. brevicolle, X. californicum, X. pachtaicum, X. rivesi and X. simile, and four unidentified putative Xiphinema species were characterised by morphology and sequencing of D2-D3 of 28S rRNA, ITS1 rRNA and mitochondrial COI genes. New nematode sequences generated totalled 147. Phylogenetic relationships of the X. americanum-group species reconstructed by Bayesian inference for 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 s. str., X. californicum, X. pachtaicum, X. rivesi, and two unidentified Xiphinema species were found in 72 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 17 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 COI phylogenies revealed a high level of co-speciation events between host and symbiont.

First report of the dagger nematode Xiphinema pachtaicum on onion in Morocco

“…Morphometric measurements of Xiphinema pachtaicum are listed in Table 1. The morphometric data of described Moroccan specimens were perfectly fit within the two populations of Xiphinema pachtaicum recorded by Orlando et al (2016) from Italy.…”

The utility of mtDNA and rDNA for barcoding and phylogeny of plant-parasitic nematodes from Longidoridae (Nematoda, Enoplea)

“…1. However, these species comprise species complexes that must be further studied, as recently proposed by Orlando et al ., because some of them may have been misidentified 26 . In contrast, intra-specific molecular variability detected in X. italiae and X. adenohystherum was accurate and correct.…”

“…for the X. americanum group (Fig. S1 ), as one X. rivesi sequence (AM086697) was considered as X. floridae (AM086696) 26 . In addition, Xiphinema sp.…”

Pest categorisation of Xiphinema californicum

“…Based on molecular (by sequencing of D2–D3 of 28S rRNA, ITS rRNA and mitochondrial COI genes) and morphological characterization of the X. americanum group species, Orlando et al. () recognised X. californicum as a distinct taxonomic entity.…”

“…Because of the difficulties in the morphological identification of species of X. americanum sensu lato, the use of molecular approaches to discriminate species within the X. americanum group is recommended (Brown et al, 1995;Lamberti et al, 2000). Based on molecular (by sequencing of D2-D3 of 28S rRNA, ITS rRNA and mitochondrial COI genes) and morphological characterization of the X. americanum group species, Orlando et al (2016) recognised X. californicum as a distinct taxonomic entity.…”