Helicotylenchus coreanus n. sp. (Nematoda: Hoplolaimidae): a new spiral nematode associated with Abies koreana Wils. from the Republic of Korea

A new species of the genus Helicotylenchus extracted from the rhizospheric soils of dead Abies koreana was described using morphological characters, morphometrics, and molecular DNA barcodes. Helicotylenchus coreanus n. sp. is characterized by its lateral fields with non-areolated bands, modified at vulval region by widening on vulval side, inner lateral field incisures on tail end fused distally into a Y- or U-shape, phasmids located 0–7 annuli posterior to anal opening, rarely three annuli anterior to anal opening, lip region hemispherical, continuous with body contour, marked by six or seven distinct annuli, stylet 3.4–4.0 times as long as lip region diam., stylet knobs rounded to flattened, spermatheca rounded, empty, tail mostly longer than anal body diam., with 16–31 annuli; rarely less than 15 annuli on ventral contour, tail terminal region with a well-demarcated hyaline portion, 3.0–5.5 μm long, terminus notched, sometimes with two small notches; in the more conoid individuals, terminus is truncate or rounded. The new species was morphologically compared with the six related species of the genus, including H. indenticaudatus, H. holguinensis, H. parapteracercus, H. ussuriensis, H. belli, and H. oscephalus. The phylogenetic relationships among species were reconstructed using 18S-rRNA, 28S-rRNA, and COI gene sequences. Inferences from the more informative 28S-rRNA and COI genes supported sister relations of Helicotylenchus coreanus n. sp. with H. oscephalus.