Morphology, development stages, and phylogeny of the Rhabditolaimus ulmi (Nematoda: Diplogastridae), a phoront of the bark beetle Scolytus multistriatus from the elm Ulmus glabra Huds. in Northwest Russia

The nematode Rhabditolaimus ulmi was found in galleries, adults, and larvae of Scolytus multistriatus, the vector of the Dutch elm disease, in St. Petersburg parks. This nematode co-occurred with Bursaphelenchus ulmophilus, which is another phoretic partner of S. multistriatus. Nematodes were cultured on the fungus Botryotinia fuckeliana in potato sugar agar (PA) and used for morphological analyses of adults, juveniles, eggs, and dauers. Nematode females showed a didelphic female genital tract rather than a monoprodelphic gonad as reported in the original description. Male bursa peloderan, caudal papillae include three preanal pairs and one precloacal unpaired papillae; seven postanal papilla pairs, among which one is pore-like and possibly the phasmid homolog, one subdorsal, and a pair of three closely situated posteriorly at bursa alae. The juvenile stages differ in size and structure of their sexual primordia. Sex of juveniles may be identified from the third stage. The dauer juvenile is a phoretic third juvenile stage (DJ3), which enters and remains localized in the buccal cavity of beetle adults and last-instar larvae and also under the elytra and in the ovipositor's cavity of pupae and imagoes. The first molt J1-J2 occurred inside the eggshell. Adult females laid eggs in early stages of embryonic development or containing molted J2. The propagative non-phoretic J2 inside the egg and J3 have a long and well-developed median bulb. The phoretic dauer DJ3 has a small spherical bulb like the J1 juvenile within the egg. In a sterile fungal culture, the nematodes feed on both mycelium and their unidentified ecto-symbiotic bacteria, located on nematode surface coat and multiplying in PA. Diagnosis and tabular key to the Rhabditolaimus species are given. Phylogenetic analysis of the D2-D3 of 28S rRNA gene sequences resulted in the Bayesian consensus tree with the highly supported clade of the Rhabditolaimus species.