Taxonomy of the genus Rhyssocolpus

EVIDENCE DERIVED FROM MORPHOLOGICAL COMPARATIVE ANALYSIS

When describing Enchodelus morgensis Loof, 1988, Loof compared and discussed several diagnostic characters of the genera Enchodelus and Rhyssocolpus, concluding that the latter was identical to the former, and transferred most Rhyssocolpus species to Enchodelus. Nevertheless, Loof’s actions were not followed by other authors (Jairajpuri & Ahmad, 1992; Andrássy, 2009). Loof (1988) noted that two relevant features, namely the odontostyle length and the nature of the cuticle in the vulval area, were different in both genera, but considered that these differences had “no more than specific value”. The conical-tailed members of Enchodelus, i.e., those comparable to Rhyssocolpus species, were recently separated from rounded-tailed forms by Andrássy (2009, 2011), who grouped them under the (resurrected) genus Heterodorus – until then a junior synonym of Enchodelus – and retained the rounded-tailed species under the (true) genus Enchodelus. Consequently, Rhyssocolpus is herein compared with Heterodorus.

Three features might be relevant to separate Rhyssocolpus from Heterodorus using a morphological approach: odontostyle length, uterus morphology, and the presence/absence of cuticular irregularities around the vulval area. Rhyssocolpus representatives have a comparatively small odontostyle, up to 13 µ m long, often only up to 10 µ m long and always shorter than the lip region diam., whereas Heterodorus species have a larger odontostyle, never less than 10 µ m long and very often easily surpassing this value (reaching up to 70 µ m), and almost always longer than, only very exceptionally equal to (never shorter than), the lip region diam. The uterus is bipartite in both genera, but the distal part close to the sphincter bears some differentiation (refractive granules in its lumen, hyaline cells surrounding it, etc.) in many members of Heterodorus such as H. arcuatus (Thorne, 1939) Andrássy, 2009, H. brevidentatus, H. geraldi (Winiszweska, 1987) Andrássy, 2009, H. magnificus Altherr, 1952 (the type species), H. morgensis, H. porosus Guerrero, Liébanas & Peña-Santiago, 2007 and H. veletensis Guerrero, Liébanas & Peña-Santiago, 2007, which have recently been studied in some detail (cf., Guerrero & Peña-Santiago, 2007; Guerrero et al., 2007). This type of uterine differentiation has not been reported in Rhyssocolpus forms. The presence of very strong cuticular irregularities in the vulval area is a remarkably common feature of Rhyssocolpus species and has not been described with comparable development in Heterodorus forms, although it is known to occur in isolated species of several genera such as Eudorylaimus and Mesodorylaimus Andrássy, 1959. In conclusion, from a morphological perspective, Heterodorus and Rhyssocolpus show relevant differences that allow their differentiation into two groups.

EVIDENCE DERIVED FROM MOLECULAR ANALYSIS

The results obtained from partial 18S rDNA analysis (Fig. 5), including new sequences of four species (see Material and methods section), show that representatives of the Nordiidae are split into three major subgroups with variable clade support. The first group consists of sequences of Pungentus Thorne & Swanger, 1936 plus one sequence of Californidorus Robbins & Weiner, 1978. The second includes sequences of Enchodelus as well as several sequences of four non-nordiid genera: Prodorylaimus Andrássy, 1959 in Dorylaimidae de Man, 1876, and Crassolabium Yeates, 1967, Eudorylaimus and Epidorylaimus Andrássy, 1986 in Qudsianematidae. The third group is formed by sequences of the members of Heterodorus and Rhyssocolpus, along with other species from the Qudsianematidae and Nordiidae (see above). These results generally agree with those published by Holterman et al. (2008) and Pedram et al. (2011), and confirm that the Nordiidae is not a natural (monophyletic) taxon and that the relationships between dorylaimid families are still far from being well established. The evolutionary relationships between the members of the third clade are not well resolved in the tree and hence serious uncertainties persist concerning the systematics of its members, especially due to the inclusion of the qudsianematid taxa and because the sequences of Heterodorus species do not group together. Thus, as yet there is insufficient molecular evidence to elucidate the nature of the relationship between Heterodorus and Rhyssocolpus (cf., Pedram et al., 2011), although it is relevant that the two sequences of Rhyssocolpus are the closest to each other within the clade.

INTEGRATIVE APPROACH

Both morphological and molecular (with currently available data) evidence support the conclusion that Rhyssocolpus species form a natural group. On the one hand, the morphological differences between Rhyssocolpus and Heterodorus are small but certainly significant, and can be interpreted under a cladistic approach. The very short and attenuate odontostyle along with the presence of strong cuticular irregularities at vulval level are considered apomorphic conditions of their respective characters, and synapomorphies which define the Rhyssocolpus pattern. The differentiations observed in the distal part of the uterus in members of Heterodorus are equally regarded as an apomorphic state characterising the Heterodorus pattern. On the other hand, in spite of available molecular data not satisfactorily defining the evolutionary relationships of Rhyssocolpus and Heterodorus, the two available Rhyssocolpus sequences lie close together in the tree. Thus, contrary to Loof’s (1988) opinion, and following the opinions of other authors (Jairajpuri & Ahmad, 1992; Vinciguerra, 2006; Andrássy, 2009), Enchodelus, Heterodorus and Rhyssocolpus are considered as valid genera, with Heterodorus and Rhyssocolpus probably sharing a common recent ancestor and being distinctly separated from Enchodelus. A revised taxonomic characterisation of Rhyssocolpus therefore follows.

Genus Rhyssocolpus Andrássy, 1971

DIAGNOSIS (EMENDED)

Nordiidae. Small- to medium-sized nematodes, 0.70- 1.98 mm long. Cuticle dorylaimoid. Lip region continuous or offset by depression, exceptionally by constriction. Odontostyle small and attenuate, never longer and usually shorter than lip region diam., with narrow lumen and minute aperture. Guiding ring double. Odontophore rod-like. Pharynx dorylaimoid, with gradual enlargement and pharyngeal expansion occupying up to two-fifths (27- 40%) of total neck length; S 1 N (AS) very reduced. Female genital system didelphic-amphidelphic, uterus bipartite, pars refringens vaginae present, vulval aperture a transverse or longitudinal slit. Strong cuticular irregularities (wrinkles, folds, etc., and occasionally flaps) present near vulval area. Caudal region conical, regularly curved ventrad in both sexes. Spicules dorylaimoid, 37-60 µ m long. Male ventromedian supplements 2-11, spaced, with hiatus.

REMARKS

Rhyssocolpus is a rather homogeneous dorylaimid taxon from a morphological perspective, its species being separated by means of small and subtle differences (see below). It seems to be a genuine representative of the Holarctic nematode fauna and is widely spread in Eurasia and North America (see geographical distribution in Table 2).

TYPE SPECIES

R. vulvostriatus (Stefański, 1924) Andrássy, 1971

= Dorylaimus vulvostriatus Stefański, 1924

= Eudorylaimus vulvostriatus (Stefański, 1924) Andrássy, 1959

= Enchodelus vulvostriatus (Stefański, 1924) Loof, 1988

= Dorylaimus gracilis apud Micoletzky (1925) nec de Man (1884) [syn. by Andrássy (1971)]

OTHER SPECIES

R. aquilonius Andrássy, 2003

R. arcticus Ebsary, 1984

= Enchodelus ebsaryi Loof, 1988

R. fluviatilis Gagarin, 1985

R. iuventutis Andrássy, 1971

= Enchodelus iuventutis (Andrássy, 1971) Loof, 1988

R. microdorus (Schiemer, 1965) Andrássy, 1971

= Enchodelus microdorus Schiemer, 1965

R. repis (Brzeski, 1992) Vinciguerra, 2006

= Enchodelus repis Brzeski, 1992

R. vinciguerrae Pedram, Pourjam, Robbins, Ye & Peña-Santiago, 2011

SPECIES TRANSFERRED TO, OR RETAINED UNDER, OTHER GENERA

R. alleni (Brzeski, 1962) Andrássy, 1986, retained under Eudorylaimus

= Eudorylaimus alleni Brzeski, 1962

R. brasiliensis (Meyl, 1956) Andrássy, 1986, transferred to Eudorylaimus (see below)

= Dorylaimus brasiliensis Meyl, 1956

= Prodorylaimus brasiliensis (Meyl, 1956) Andrássy, 1959

R. paradoxus (Loof, 1975) Andrássy, 1986, retained under Eudorylaimus

= Eudorylaimus paradoxus Loof, 1975

NOTES ON SOME SPECIES

Rhyssocolpus alleni: Loof (1988) suggested that this species “probably... does not belong to the Enchodelus complex either”, and several of its diagnostic features support its retention under Eudorylaimus: large size (body length 2.5-2.7 mm, see Brzeski (1962) and Loof (1971)), odontostyle 21-23 µ m long, pharyngeal expansion occupying about one-half of total neck length, and absence of strong cuticular irregularities in the vulval area.

Abbreviations: LRD = lip region diam.; Odont. = odontostyle; Ph.exp. = pharyngeal expansion; Ve.sup = ventral supplements; Distrib. = geographical distribution. References: 1: Andrássy (2003); 2: Ebsary (1984); 3: Gagarin (1985); 4: Andrássy (1971); 5: Present paper; 6: Schiemer (1965); 7: Vinciguerra & De Francisci (1973); 8: Brzeski (1992); 9: Pedram et al. (2011); 10: Stefański (1924); 11: Thorne & Swanger (1936); 12: Micoletzky (1925); 13: Altherr (1952); 14: Zullini (1970); 15: Zullini (1971).

∗ Values calculated from other measurements and/or illustrations.

Rhyssocolpus arcticus and R. iuventutis: the separation of these two species is problematic. The presence and absence, respectively, of vulval flaps is the only relevant morphological difference, whereas the ranges of their morphometrics widely overlap.

Rhyssocolpus brasiliensis: the true identity of this species remains obscure, but the available information suggests that it is not compatible with the Rhyssocolpus pattern: odontostyle longer (1.3 times) than lip region diam. and comparatively stronger, pharyngeal expansion occupying ca 45% of total neck length, and 13- 14 contiguous ventromedian supplements. It is provisionally transferred to Eudorylaimus as E. brasiliensis (Meyl, 1956) comb. n.

Rhyssocolpus microdorus and R. repis: the separation of these two species is also problematic. The absence and presence, respectively, of vulval flaps is the only relevant morphological difference, whereas the ranges of their morphometrics widely overlap.

Rhyssocolpus fluviatilis: the original description of this species is not very detailed and some doubts persist as to the identity as the lip region is distinctly angular and offset by a constriction (see Gagarin’s Figure 3.3) and the odontostyle is as long as the lip region diam. – two features that do not fit the pattern of the genus. Nevertheless, other characters (attenuate odontostyle, guiding ring double and the presence of coarse cuticle irregularities in the vulval area) support its classification under this genus.

Rhyssocolpus paradoxus: as Loof (1988) pointed out, this species does not fit either the Enchodelus pattern, or the Rhyssocolpus pattern because of its larger general size (body length 2.37-3.13 mm), odontostyle 28-35 µ m long, pharyngeal expansion occupying more than two-fifths of total neck length, absence of strong cuticle irregularities near the vulval area, and higher number (22- 27) of contiguous ventromedian supplements.

Key to species

1 Lip region distinctly angular and offset by constriction; male unknown............................ fluviatilis

Lip region rounded or weakly angular and continuous or offset by depression; male known............... 2

2 Smaller general size, body length up to 1.1 mm long, spicules up to 41 µ m long.........................3

Larger general size, body length over 1.0 mm long, spicules 43 µ m long or more...................... 4

3 Vulval flaps absent...................... microdorus Vulval flaps present........................... repis

4 Vulval flaps present.............................. 5

Vulval flaps absent............................... 6

5 Tail longer (c′ = 1.3-1.6), strongly curved ventrad, hook-like; most posterior ventromedian supplement anterior to anterior end of spicules, far from adcloacal pair and close to second-most posterior ventromedian supplement................................ arcticus

Tail shorter, about as long as anal body diam., weakly curved ventrad, not hook-like; most posterior ventromedian supplement anterior to anterior end of spicules, mid-way between adcloacal pair and second-most posterior ventromedian supplement......... vulvostriatus

6 Male bearing 11 ventromedian supplements.................................................. aquilonius

Male bearing 5-8 ventromedian supplements........ 7

7 Larger general size, body 1.31-1.66 mm long; vulva transverse; spicules 54-60 µ m long......... iuventutis

Smaller general size, body 1.04-1.37 mm long; vulva longitudinal; spicules 43-51 µ m long.... vinciguerrae