Remarks on phylogeny and molecular variations of criconematid species (Nematoda: Criconematidae) with case studies from Vietnam

The family Criconematidae is a remarkable group of nematodes, containing roughly 600 nominal root-ectoparasitic species, of which many species are known to be significant agricultural pests. Strikingly, our phylogenetic analyses based on 18S, D2-D3 of 28S rRNA, and COI mtDNA sequences of criconematid species, supported by tree topology tests (SH and AU tests), revealed that almost all studied genera, including Criconema, Ogma, Crossonema, Discocriconema, Hemicriconemoides, Criconemoides, Mesocriconema, and Lobocriconema, are not monophyletic groups, a finding that is partly contrary to those of previous studies on these groups. Our results suggest that key morphological characters used in the classification of Criconematidae are the consequence of convergent evolution. It is clear from our studies that the species status of at least 40 sequences of criconematid species from GenBank must be either revised or reconsidered, with analyses based on a polyphasic approach that includes different tree- and distance-based molecular species-delimitation methods (bPTP, GMYC, ABGD1, and ABGD2). Our studies found the ABGD2 output of the automatic barcode method to agree remarkably well with established species delimitations, while in general, the four species-delimitation results corresponding to three barcode regions forwarded significantly more putative species compared to those originally considered. This study also characterised for the first time the populations of Criconemoides myungsugae and Discocriconemella hensungica associated with Vietnamese ginseng, one of the most precious and rare ginseng varieties in the world. Although these populations are morphologically in agreement with the original descriptions of C. myungsugae and D. hengsungica, their molecular data display notable variations compared to the sequences deposited in GenBank. These species demonstrate clearly the immense molecular variations that can be observed in several species of the family Criconematidae.