Metaxonchium Coomans & Nair, 1975

Peña-Santiago, R., Niknam, G., Álvarez-Ortega, S. & Jabbari, H., 2014, Metaxonchium persicum sp. n. from Iran (Nematoda, Dorylaimida, Belondiridae), with an updated taxonomy of the genus, Zootaxa 3785 (4), pp. 501-517 : 508

Taxonomy of Metaxonchium Coomans & Nair, 1975

= Axonchium (Metaxonchium) Cobb, 1920 ( Coomans & Nair, 1975)

= Axonchium (Discaxonchium) Cobb, 1920 ( Coomans & Nair, 1975)

= Axonchium (Epaxonchium) Cobb, 1920 ( Coomans & Nair, 1975)

= Axonchium (Spiculaxonchium) Cobb, 1920 ( Ahmad & Jairajpuri, 1982; but see below)

Brief historical outline. In the last paper of their excellent revision of the genus Axonchium Cobb, 1920 , Coomans and Nair (1975; see also Jairajpuri & Ahmad, 1992) proposed its division into nine “groups or subgenera” (p. 325), one of them Metaxonchium , with three species, namely Axonchium (Metaxonchium) coronatum (de Man, 1907) Thorne & Swanger, 1936, which was proposed as type species, A. (M.) leptocephalum Altherr, 1953 and A. (M.) vaginatum Jairajpuri, 1965 . The subgenus Metaxonchium was characterized and separated from its relatives by having, among other features, lip region offset, fusiform odontostyle, anterior portion of pharynx quite muscular and lacking a spindle-shaped portion, pars refringens vaginae present (= “vagina sclerotized”), and males rare with small spicules. In describing two new Axonchium species from the Iberian Peninsula, Peña-Santiago and Coomans (1990) were aware that the differences between Metaxonchium and the subgenus Epaxonchium , also proposed by Coomans and Nair (op. cit.), were not significant and regarded the latter as a junior synonym of the former. Andrássy (1991) raised the rank of Metaxonchium to generic level and stated (see also Andrássy, 2009) that the subgenera Discaxonchium Coomans & Nair, 1975 , Epaxonchium and Spiculaxonchium Ahmad & Jairajpuri, 1982 may be also classified under it. In 1996, the same author described the new species M. echinulatum from Hungary and provided an updated list of 19 valid species.

Morphological characterization. Metaxonchium is a relatively homogeneous taxon, mainly characterized by its lip region with separate lips and offset by constriction, odontostyle short and fusiform, both pharyngeal sections separated by an isthmus-like narrowing, pars refringens vaginae well developed, caudal region short and rounded to convex conoid in both sexes, and spaced ventromedian supplements without hiatus. A few species, however, do not totally fit this general pattern and introduce some heterogeneity to the group: M. bihariense lacks a perceptible isthmus-like narrowing between both pharyngeal regions; M. choristum ( Thorne, 1939) Andrássy, 1996 and M. serpens ( Thorne, 1939) Andrássy, 1996 bear a series of 19–26 and 22–30 contiguous ventromedian supplements with and without hiatus, respectively; M. spiculum shows a peculiar combination of characters (deep constriction rather than an isthmus-like narrowing between both pharyngeal sections, which are abutting, spicules with very atypical morphology (bearing a very short median piece), and weakly developed ventromedian supplements arranged in two separate groups), which raises serious doubt about its belonging to Metaxonchium ; M. tacitum ( Ahmad & Jairajpuri, 1982) Andrássy, 1996 has abutting pharyngeal sections as well; and M. thornei ( Hechler, 1969) Andrássy, 1996 bears 13–15 contiguous ventromedian supplements with hiatus. A few other morphological features display significant variation: presence/absence of valve-like elements in a swelling of the anterior pharyngeal section, presence/absence of apophyses in the uterus, and shape of pars refringens vaginae. Accordingly, the separation of most species is principally based on morphometrics.

Diagnosis (emended). Medium to large-sized nematodes, body 1.6–4.2 mm long. Cuticle bi- or tri-layered, with fine transverse striation and especially thick at caudal region. Lip region offset by constriction, with separate lips. Amphid openings encircling most the lip region base. Odontostyle fusiform, 8–23 µm long or about equal to lip region diameter. Guiding ring simple, visibly refractive. Odontophore rod-like, lacking any significant differentiation. Both pharyngeal sections typically separated by a isthmus-like narrowing, very occasionally by a constriction or without differentiation; basal expansion large, occupying up to three-fourths of total neck length. Cardia tongue-like, well developed. Female genital system mono-opisthodelphic, with anterior branch reduced to a more or less (but usually well-) developed uterine sac plus a vestigial terminal cell mass, posterior uterus long and often tripartite with apophyses (echinophorous uterus, cf. Andrássy, 1991) in several species; pars refringens vaginae well (very occasionally less) developed; vulva a transverse slit. Tail similar in both sexes, short and rounded to convex conoid. Males often frequent, with variably sized (39–107 µm long) and shaped spicules, and 7– 17 spaced ventromedian supplements (but 19–30 contiguous in two species), with or without hiatus.

Relationships. Within the subfamily Axonchiinae Thorne, 1964, which contains eight genera (cf. Andrássy, 2009), Metaxonchium resembles the genera Axonchium , Dactyluraxonchium Coomans & Nair, 1975 and Syncheilaxonchium Coomans & Nair, 1975 , but it can easily separated from all of them in the presence of well developed pars refringens vaginae (vs totally absent). Besides, it differs from Dactyluraxonchium in vaginal morphology (vs lumen first very wide and elliptical) and caudal region (vs conical subdigitate, distinctly longer than anal body diameter); and from Syncheilaxonchium in the morphology of lip region (vs non-offset, with amalgamated lips).