To facilitate the process of spiral nematode species delineation, populations of Helicotylenchus canadensis, H. pseudorobustus, and H. varicaudatus deriving from various locations and diverse natural and anthropogenic environments from Poland were investigated and characterized. For the first time, 28S rDNA sequences are reported for H. canadensis and H. varicaudatus, whereas new mtCOI sequences were acquired for all three analyzed species. A Bayesian phylogenetic analysis of the 28S rDNA fragments revealed that H. canadensis and H. varicaudatus are members of a clade that is a sister group to all other Helicotylenchus species; however, the closest known sister group to H. canadensis is H. vulgaris type A. Both 28S rDNA- and mtCOI-based phylogenetic results suggest that this clade excludes H. pseudorobustus, whose most recent common ancestor with the former species was inferred to be the ancestor of all Helicotylenchus species. Moreover, within the mtCOI sequences obtained from H. pseudorobustus, unlike from the other two, a simultaneous presence of TAG and TAA codons was identified. This may indicate mitochondrial genetic code alterations or other genomic rearrangements in H. pseudorobustus.
