Oleaxonchium olearum gen. et sp. nov. (Nematoda, Dorylaimida) associated with an olive grove in the southern Iberian Peninsula, and new insights into the evolutionary relationships within Belondiridae

A new belondirid, dorylaimid taxon, Oleaxonchium olearumgen. et sp. nov., collected from an olive grove in the Andalusia region of Spain, is characterised, including its morphological description, morphometrics, SEM study, and molecular (18S-, 28S-rDNA) analyses. The new genus displays a unique combination of traits that distinguishes it from its closest genera: a rectangular lip region with sclerotized margins when observed in lateral view and visibly hexagonal in face view under SEM; a comparatively long cheilostom with thickened walls at its anterior part; a short isthmus-like section separating both pharyngeal regions; a mono-opistho-ovarian didelphic female genital system without pars refringens vaginae; and a short and rounded tail. The new species is characterised by its 2.44–2.87 mm long body, lip region 7–7.5 µm wide, odontostyle 10–10.5 µm long, neck 723–973 µm long, pharyngeal expansion occupying 63–72% of the total neck length, female anterior genital branch 4–6% of body length, tripartite posterior uterus 1.9–2.6 body diameters long, with a short intermediate section bearing sclerotized elements, vulva (V = 58–61) a transverse slit, caudal region 29–35 µm long (c = 74–89, c’ = 0.9–1.1), and male unknown. As derived from an integrative approach combining morphological and molecular data, the new genus is close to Metaxonchium, the polyphyly of Belondiridae is confirmed, and support is provided in favour of the monophyly of Axonchiinae.