Morphological, Morphometrical and Molecular Characterization of Oscheius siddiqii Tabassum and Shahina, 2010 (Rhabditida, Rhabditidae) from India with Its Taxonomic Consequences for the Subgenus Oscheius Andrassy, 1976

Simple Summary Due to their potential entomopathogenicity, nematodes of the genus Oscheius have been in the spotlight of the scientific interest in recent years. Unfortunately, some of these species are poorly described and have inadequate or no molecular support. This fact complicates the systematics of the group, and a revision of these species is necessary to elucidate their taxonomic status. In the present study, we provide a detailed description based on the morphological, morphometrical, and molecular characteristics of Oscheius siddiqii from Uttar Pradesh, India, including the first scanning electron microscopy (SEM) studies of the species. Furthermore, based on morphological and molecular data, the status of some Oscheius species is discussed, and several synonymisations are proposed. An insect parasitic nematode belonging to the genus Oscheius was recovered from the agricultural soils from the Hapur district in western Uttar Pradesh, India. Morphological studies on this species exhibited its high resemblance with two Pakistani species: Oscheius siddiqii and O. niazii. No molecular data are available for these taxa but, morphologically, both species do not differ significantly from our strains and each other. Hence, these nematodes can be considered conspecific, and the correct name for this taxon is O. siddiqii, the first described species. The phylogenetic analyses of the ITS-, 18S-, and the 28S rDNA sequences showed that O. siddiqii is a sister taxon to the group formed by Oscheius microvilli, O. myriophilus, O. safricanus, and several unidentified Oscheius species. Additionally, our analyses show that based on molecular and morphological data, the species Oscheius rugaoensis and O. microvilli cannot be distinguished from O. chongmingensis and O. myriophilus, respectively, and are thus considered junior synonyms of these taxa. Furthermore, the available data are not sufficient to evaluate the status of Oscheius basothovii and O. safricanus, which are, in consequence, considered species inquirendae. These findings highlight the necessity of the proper morphological and molecular characterisation of the described Oscheius species.