Molecular and morphological characterization of Hemicriconemoides vietnamensis sp. n. (Nematoda: Criconematidae) associated with rose (Rosa gallica L.) in Vietnam.

Hemicriconemoides spp. are sheathoid, root-ectoparasitic nematodes with complex taxonomy due to overlapping morphology and cryptic diversity. In this study, an integrative taxonomic approach combining morphology and molecular data supports the characterization of Hemicriconemoides vietnamensis sp. n. recovered from the rhizosphere of roses in Vietnam. The new species females are distinguished by a slightly ventrally curved body with a closely fitting sheath lacking anastomoses, a lip region with two annuli and a protruding first annulus, a long (62-68 μm) mostly straight stylet with rounded, posteriorly sloping knobs, and an excretory pore located 5-7 annuli posterior to the pharyngeal base. The vulva is slit-like without vulval flaps, the vagina is oblique, and the tail is bluntly to pointed conical with a rounded terminus. Males were not found. Morphologically, the new species is most similar to H. litchi, H. rotundoides, and H. strictathecatus, but can be differentiated by a combination of morphological and molecular characterizations. Molecular analyses of ITS rDNA, D2-D3 expansion segments of 28S rDNA, and COI mtDNA consistently support the distinct status of H. vietnamensis sp. n. Bayesian phylogenetic analyses place the new species in a well-supported clade with H. phoenicis and H. strictathecatus, forming a distinct lineage across all markers. Species delimitation analyses (bPTP, GMYC, and ASAP) consistently recover Hemicriconemoides vietnamensis sp. n. as an independent species. These results provide clear evidence for the establishment of Hemicriconemoides vietnamensis sp. n. and expand the documented diversity of Hemicriconemoides in Vietnam, where molecular data for the genus remain limited.