An 18S rDNA Perspective on the Classification of Criconematoidea

Abstract

In the nematode family Criconematidae, a taxonomy primarily based on cuticle characters has created classifications that are notoriously volatile. Molecular characters may lead to their stabilization. A phylogenetic tree of Criconematoidea was constructed using 166 new near full-length 18S rDNA sequences and 58 sequences from GenBank. Bayesian and maximum likelihood (ML) analyses produced trees with similar topologies. Major features include a strongly supported clade that includes Criconematidae and Hemicycliophoridae, excluding Paratylenchidae and Tylenchulidae. Another well-supported clade groups Criconema, Ogma, Crossonema, and Hemicriconemoides plus Xenocriconemella, combining nematodes with cuticular scales with those without scales at any life stage. Mesocriconema, Discocriconemella limitanea, Hemicaloosia, and Lobocriconema are recognized as monophyletic groups, but Criconemoides is paraphyletic. Both trees support an unexpected sister relationship between Bakernema and Hemicycliophora. The 18S rDNA dataset was insufficient for distinguishing genus boundaries between Criconema, Ogma, and Crossonema. The relationships depicted by the 18S rDNA phylogeny suggest that key morphological characters used in the classification of Criconematidae are not homologous.

Twenty Years after De Ley and Blaxter—How Far Did We Progress in Understanding the Phylogeny of the Phylum Nematoda?

“…Phylogenomics involving two of the families, namely Anguinidae and Neotylenchidae, provided strong support for the monophyly of the superfamily as well as that of the respective families, while 18S rRNA data implies both Anguinidae (T10 in Figure 5 a) [ 58 ] and Neotylenchidae (T11 in Figure 5 a) [ 13 ] to be paraphyletic, which, taking into consideration the limited taxon sampling in phylogenomic analysis [ 15 ], is reflected in Figure 5 a. While the most recent analyses of the 18S rRNA gene provided support for the monophyly of the superfamily Criconematoidea [ 13 ], two of its three families, Tylenchulidae (T12 in Figure 5 a) and Criconematidae (T13 in Figure 5 a), are consistently shown to be paraphyletic [ 13 , 21 , 62 ]. Hemicyliophoridae, represented mostly by a single genus, is recovered as monophyletic and nested within paraphyletic Criconematidae.…”

Species Diversity of Pin Nematodes (Paratylenchus spp.) from Potato Growing Regions of Southern Alberta, Canada

“…Nevertheless, such morphological complexes can be resolved using molecular data. Several taxonomic issues have been successfully addressed with molecular studies, such as the validity and differentiation of Radopholoides from Hoplotylus and Radopholus [45], the transfer of Tylaphelenchus jiaae to the genus Pseudaphelenchus as P. jiaae [46], the revision and species synonymization in Laimaphelenchus [47], the species delimitation in members of Criconematoidea [48][49][50][51], and the resolution of the cryptic diversity and species complexes in Longidoridae [52][53][54].…”

A Proposed New Species Complex within the Cosmopolitan Ring Nematode Criconema annuliferum (de Man, 1921) Micoletzky, 1925

“…The genus Criconema displays a great phenotypic plasticity and absence of clear diagnostic characters, and for this reason, molecular taxonomy and DNA barcoding is providing accurate and useful tools for species identification in recent years [ 13 , 14 , 15 , 16 , 17 ]. Subbotin et al [ 13 ] analysed the phylogenetic relationships of the main lineages recognised in Criconematidae based on D2-D3 expansion segments of the 28S nuclear ribosomal RNA, and their data support the monophyly of Criconema .…”