Molecular phylogenies provide insights into the evolutionary relationships of the Spirurida (Nematoda), with special emphasis on the superfamily Physalopteroidea.

backgroundNematodes of the order Spirurida are of significant veterinary, medical, and economic importance. However, current knowledge of the phylogenetic relationships within the order is far from comprehensive. Moreover, the monophyly of the Physalopteroidea/Physalopteridae, and the phylogenetic relationships of its three component subfamilies, remain uncertain due to inadequate sequence data.

methodsThe nuclear small ribosomal subunit (18S rRNA) and large ribosomal subunit (28S rRNA), plus the complete mitochondrial genomes of two physalopterid species, Thubunaea pudica (Thubunaeinae) and Abbreviata varani (Physalopterinae), are presented for the first time. Phylogenetic analyses of the Spirurida were performed using maximum likelihood and Bayesian inference on the basis of different concatenated datasets involving the most comprehensive subfamily-level taxon sampling of the superfamily Physalopteroidea to date to provide an initial understanding of the evolutionary relationships of major superfamilies within the order, with special emphasis on the Physalopteroidea/Physalopteridae.

resultsThe complete mitogenomes of T. pudica and A. varani are 13,645 bp and 13,730 bp in length, which both contain 36 genes and belong to the GA9 type gene arrangement. Molecular phylogenies based on different datasets all support a close affinity between the superfamilies Camallanoidea and Dracunculoidea. Our phylogenetic results also showed that the representatives of the Physalopteridae did not form a monophyletic group. The representative of the subfamily Proleptinae clustered together with the tetramerid species Crassicauda magna, and the representative of the subfamily Thubunaeinae formed a sister relationship with species of the subfamily Physalopterinae.

conclusionsThe mitogenome of T. pudica is the first for the subfamily Thubunaeinae, and that of A. varani is also the first for the genus Abbreviata. Molecular phylogenetic results suggest that the subfamily Proleptinae should be elevated to full family level, i.e., Proleptidae stat. nov., and that the genus Crassicauda (Habronematoidea: Tetrameridae) should be assigned to Proleptidae stat. nov. Our results also indicate that the Physalopteridae sensu stricto currently comprises two subfamilies, the Thubunaeinae and the Physalopterinae, and that the genus Skrjabinoptera should be transferred from the Physalopterinae to the Thubunaeinae.