Diplogasteroides nix n. sp. (Nematoda: Diplogastridae), a cryptic species related to D. andrassyi, isolated from Monochamus urussovii (Coleoptera: Cerambycidae) from Hokkaido, Japan, with remarks on body surface structures

Diplogasteroides nix n. sp. is described based on its typological characteristics, molecular profile and mating experiments. Diplogasteroides nix n. sp. is characterised by its tube-like stoma with three small dorsal teeth and two subventral ridges, spicule clearly ventrally bent at one-third from the anterior end, gubernaculum with a somewhat squared anterior end and sharply pointed distal end in a lateral view and nine pairs of genital papillae with an arrangement of ⟨v1, (v2, v3d)/v4, ad, ph, (v5, v6, v7), pd⟩, with the very small v5 and v6 protruding from socket-like bases, and v6 has a tripartite tip on males and well developed receptaculum seminis in females. In addition to the general morphological characters, the new species has several small secretory pore-like openings connected to secretory cells just beneath the surface cuticle (tentatively termed as ‘lateral glands’) in both males and females, and two pairs of button-like surface structures with clear internal connections in females. The new species is typologically and molecularly very similar to its tentatively assigned closest relative, D. andrassyi . These two species are morphologically almost identical, forming a cryptic species complex with some very minor morphological differences, e.g. , the morphology of gubernaculum somewhat roundish squared vs clearly rounded anterior edge. Molecularly, these two species have identical small subunit sequences and have only 5 bp difference in the D2-D3 extension segments of the large subunit. However, the new species can be distinguished from D. andrassyi by mating incompatibility, yielding only F1 progenies in both male × female and female × male combinations. Biologically, these two species and another close relative were isolated from different carrier Monochamus longhorn beetles, i.e. , D. nix n. sp., D. andrassyi and D. asiaticus have been found from M. urussovii , M. grandis and M. alternatus , respectively. However, because these beetle species share the distribution range and host tree species, further collection and identification are necessary to understand their host (carrier) interactions and specificity.