Molecular characterization of two insect nematode species (Oxyurida: Thelastomatidae) using small subunit (18S) ribosomal DNA sequence and secondary-structure analyses A. Chaudhary, N. Singh and H.S. Singh* Molecular Taxonomy Laboratory, Department of Zoology, University Road, Chaudhary Charan Singh University, Meerut, Uttar Pradesh, India 250004 (Received 20 June 2012; Accepted 23 January 2013) Abstract Nematodes of the family Thelastomatidae are parasitic in the alimentary tract of many arthropods, including Periplaneta americana L. In Meerut, Uttar Pradesh, India, two nematode species, namely Hammerschmidtiella indicus and Thelastoma icemi, belonging to this family have been reported. In the present study, the molecular phylogeny of these two nematode species was derived using small subunit (18S) sequence and secondary-structure analyses. The small subunit sequence analyses were carried out to explore the validation and systematics of these species. Phylogenetic analyses were performed for primary sequence data as well as using neighbour-joining and maximum-parsimony approaches. In contrast, the inferred secondary structures for the two species, using free-energy modelling, showed structural identities. As well as this, motif sequences were also found to be a promising tool for nematode species identification. The study provides molecular characterization based on primary sequence data of the 18S ribosomal DNA region of the nematodes along with secondary-structure data and motif sequences for inferences at higher taxonomic levels. Introduction During the general survey of entomopathogenic nematodes of the Meerut region, two of the most important and prevalent species, namely Hammerschmid- tiella indicus Singh & Malti, 2003 and Thelastoma icemi (Schwenck, 1926) Travassos, 1929, were found infecting the cockroach Periplaneta americana L. Nematodes are one of the most abundant group of parasites, with over 20,000 species currently described (Anderson, 1984). The super- family Thelastomatoidea is one of the two superfamilies of the order Oxyurida; these nematodes are parasitic or commensal in saprophytic terrestrial arthropods (Shah, 2007). They live within the hindgut of the host and usually feed on the bacterial microfauna found there (Adamson, 1994; Jex et al., 2005). Several studies have been conducted to detect nematodes harboured by P. americana L. (Rao, 1958; Biswas & Chakravarty, 1963; Singh & Kaur, 1988; Shah, 2007); it has been found that along with the species of other genera, Hammerschmidtiella and Thelastoma are common in the hindgut of this insect (Blanco et al., 2012). Adamson & Van Waerebeke (1992a, b, c) divided the superfamily into five families, of which one was Thelastomatidae. Nematodes of the family Thelastomatidae Travassos, 1929 have been reported from a wide range of species and genera of arthropods (Diplopoda, Dioctyoptera, Orthoptera, Coleoptera, Diptera and, rarely, Lepidoptera) and have also been indicated by various checklists and monographs (Skrjabin et al., 1951, 1954, 1966; Basir, 1956; Leibersperger, 1960; Jarry, 1964; Kloss, 1965; Jarry & Jarry, 1968; Adamson & *E-mail: hirdaya_singh@rediffmail.com Journal of Helminthology , page 1 of 11 doi:10.1017/S0022149X13000072 q Cambridge University Press 2013