The evolutionary relationships of 82 species of tylenchid and aphelenchid nematodes were evaluated by use of sequence data of the D2 and D3 expansion fragments of the 28S ribosomal RNA genes. Nine automatic and one culled sequence alignments were analysed using maximum parsimony and Bayesian inference approaches. The molecular data sets showed that the order Tylenchida comprises lineages that largely correspond to two suborders, Hoplolaimina and Criconematina, and other taxonomic divisions as proposed by Siddiqi (2000). Several significant results also derived from our study include: i) the basal position of groups that include entomoparasitic nematodes within tylenchid trees; ii) paraphyly of the superfamily Dolichodoroidea sensu Siddiqi (2000); iii) evidence for a Pratylenchus, Hirschmanniella and Meloidogyne clade; and iv) lack of support for widely held traditional placement of Radopholus within Pratylenchidae and placement of this genus within Hoplolaimidae or Heteroderidae. Congruence and incongruence of molecular phylogeny and traditional classifications and morphological-based hypotheses of phylogeny of tylenchids are discussed.The order Tylenchida Thorne, 1949 includes the largest and most economically important group of plant-parasitic nematodes. As plant parasites they have diverged to exploit all plant parts including foliage, flowers and seeds, but mostly they attack roots. The order also includes mycophagous species, and a large group parasitises the haemocoel of insects and mites. Several classifications and phylogenies have been proposed for ty-

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.

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Morphological identification of spiral nematodes of the genus Helicotylenchus is a difficult task because most characters used for their diagnosis vary within species. In this paper we provide morphological and molecular characterisations of several spiral nematodes, H. broadbalkiensis, H. digonicus, H. dihystera, H. microlobus, H. paxilli and H. pseudorobustus, collected in different geographical areas of USA, Switzerland, Italy, New Zealand, Spain, UK, South Korea and Russia. We suggest that H. microlobus and H. pseudorobustus are valid species separated from each other morphologically and molecularly. Seven species with distinct molecular characteristics are also distinguished, but are not ascribed morphologically to any specific taxon because of the low number of specimens available. Phylogenetic relationships of H. pseudorobustus with other Helicotylenchus species are given as inferred from the analyses of 154 sequences of the D2-D3 of 28S rRNA gene and 37 sequences of ITS rRNA gene.

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