Melomystrongylus somoroensis, , Smales & Heinrich, 2010

Durette-Desset, Marie-Claude & Digiani, María Celina, 2023, Revision of the genera of Heligmonellidae (Nematoda, Heligmosomoidea), parasitic in Muridae from New Guinea, Parasite (Paris, France) 30 (63), pp. 1-34 : 6-7

3.1.1.2 Melomystrongylus somoroensis ( Figs. 1E–1H ) 3.1.1.2.1 Synlophe (based on sections from five worms, sex not specified).

Sections analyzed herein are those within proximal part of body: male (Fig. 38/1E) and female (Fig. 40/1F), and at midbody: male (Fig. 42/1G), and female (Fig. 44/1H). In all sections lateral cords illustrated; ridges numbered in Figures 38/1E and 40/ 1F.

Within proximal body: in both sexes careen absent, synlophe with hypertrophied ventral ridge (ridge 5’, interpreted herein as comarete), and three large right ridges associated with right lateral cord; in both sections tips of ridges 1 and 1’ divergent; axis of orientation oblique.

Figure 1E (male section): 13 ridges almost regularly spaced: ridges on left-dorsal quadrant more widely spaced than those on right-dorsal quadrant. Gap between ridges 5’ and 4’ (arrowhead).

Fig. 1F (female section): 15 ridges irregularly spaced with left ventral ridges tighter than other ridges. Gap between ridges 6’ and 5’ (arrowhead).

At midbody: in both sexes careen absent, synlophe differing markedly from that in proximal part (comarete disappeared, number of ridges slightly increased). Ridges losing orientation after Smales & Heinrich [ 45].

Figure 1G (male section): ridges unequal in size and oriented from right to left on both dorsal and ventral sides, except tip of left ridge directed ventrally and convergent with tip of ridge immediately ventral.

Figure 1H (female section): ridges rather subequal in size, regularly spaced and oriented from right-ventral to left-dorsal side.

3.1.1.2.2 Bursa (illustrated in [ 45]: Fig. 49). Figure 49: bursa flattened out, orientation not specified; rays 2 and 3 longer than rays 4-6. From the written description [ 45]: bursa dissymmetrical with right lobe larger; pattern of type 3-2.

3.1.2 Comments

3.1.2.1 Synlophe

Size of ridges: the “midbody” sections of male and female seem to have been taken at different levels. Specifically, the female sections were probably taken towards the end of the mid-region of body (i.e., more distally than the male sections). This would explain the differences concerning the ridge size, since ridges usually become subequal towards the posterior end. This can be confirmed in the type species by noticing the male section at distal body (Fig. 46 in [ 45]) which is identical to the “midbody” female section ( Fig. 1H ). Unfortunately, in neither of the two species are there intermediate sections between the proximal body and the midbody. Consequently, it is not possible to know at which levels the ridges start decreasing in size, or new ridges appear. This means that it is not possible to establish any homologies between the synlophe at midbody and that within the proximal part of body.

Orientation of ridges: for both species, it was indicated [ 30, 45] that the orientation of the ridges is oblique within the proximal body and unclear in the rest of the body. Concerning the proximal part, we agree with the original statement, though this implies that on the four illustrated proximal sections, the tips of the ridges 1 and 1’ (which are supposed to be convergent) are incorrectly figured.

At midbody, the orientation of the ridges is unclear in the type species ( Figs. 1C and 1D ) but it is more evident in the female section of M. somoroensis where the ridges determine an oblique, almost subfrontal, axis of orientation ( Fig. 1H ). In the male section of M. somoroensis ( Fig. 1G ), the fact that the left ridge is oriented toward the ventral side means that the pair of ridges with convergent tips (curved arrows on the left) is situated ventrally. Such arrangement actually determines an axis of orientation right-dorsal to left-ventral, which is not at all the rule in the Heligmonellidae . We propose the reversion of the section on its frontal axis, which results in the left ridge directed dorsally and an axis of orientation directed from right-ventral to left-dorsal quadrant ( Fig. 1G’ ). Although without a clear inclination, this arrangement is similar to that observed in the female section ( Fig. 1H ).

As noted by Smales [ 30], Melomystrongylus is characterized by the absence of a careen and the presence of a hypertrophied mid-ventral ridge within the proximal third of the body. Among the Australasian Nippostrongylinae , there is only one genus without a careen and with hypertrophied ventral ridges: Hasanuddinia . Smales [ 30] distinguished Melomystrongylus from Hasanuddinia by the presence of a single hypertrophied ridge in the first third of body length, versus three hypertrophied ridges along the whole body in Hasanuddinia .

3.1.2.2 Bursa

M. sepikensis : judging from the illustration, the pattern of the left lobe is 1-4 with ray 3 diverging at same level as ray 6 from their common trunk. The pattern of the right lobe is not illustrated.

M. somoroensis : judging from the illustration, the pattern is 1-3-1 for both lobes with rays 3 and 6 diverging at same level from their common trunk.

In the Remarks concerning M. somoroensis, Smales & Heinrich [ 45] noted:

“ The generic diagnosis of the genus Melomystrongylus includes “bursa asymmetrical, left lobe larger”, the determination having been made after examining bursae that had not been completely rolled flat. In M. somoroensis the right lobe is the larger one and since this could also be the case for M. sepikensis the diagnosis needs to be reconsidered. ”

3.1.3 Conclusion

Despite the incomplete description of the synlophe, the presence of a hypertrophied ventral ridge (interpreted herein as a comarete) within the proximal third of the body allows the characterization of the genus. We consider Melomystrongylus a valid genus. Further studies will be needed, especially of the bursa of the type species, to provide a more complete generic definition.

3.1.4 Emended diagnosis

Melomystrongylus . Synlophe without careen. Within anterior third of body, 9–12 ridges; presence of one ventral comarete; ridges irregularly spaced; right ridge and dorsal adjacent ridge largest; other ridges unequal in size and small. At midbody, 14–17 ridges medium-sized to small, irregularly spaced. Axis of orientation oblique within proximal part, uncertain in midbody. Characteristic bursal pattern of type 1-3-1 (only known in M. somoroensis ).

Figures

Figure 1. Genus Melomystrongylus Smales, 2009. Body sections. A–D Melomystrongylus sepikensis Smales, 2009. A, B within proximal body. A male, B female; C, D at midbody. C male, D, female. E–H Melomystrongylus somoroensis Smales & Heinrich, 2010. E, F within proximal body. E male, F female. G, G’ at midbody, male. H female “at posterior end of midbody” (sic). Sources: A–D redrawn from [30]. E–H redrawn from [45]. G’ modified figure, reversed on its frontal axis with respect to the original.