An 18S rDNA Perspective on the Classification of Criconematoidea.

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 , , , and plus , combining nematodes with cuticular scales with those without scales at any life stage. , , , and are recognized as monophyletic groups, but is paraphyletic. Both trees support an unexpected sister relationship between and . The 18S rDNA dataset was insufficient for distinguishing genus boundaries between , , and . The relationships depicted by the 18S rDNA phylogeny suggest that key morphological characters used in the classification of Criconematidae are not homologous.