Melanesobasis bicellulare Donnelly, 1984

Saxton, Natalie A., Powell, Gareth S., Rashni, Bindiya, Jensen, Colin R., Beaty, Christopher D., Polhemus, Dan, Marinov, Milen & Bybee, Seth M., 2025, Rediscovery of Melanesobasis Donnelly, 1984 (Odonata: Coenagrionidae) in Vanuatu reveals a patern of vicariance in insular damselflies, Zoological Journal of the Linnean Society 204 (3) : -

Melanesobasis bicellulare Donnelly ( Figs 2–4)

Melanesobasis bicellulare Donnelly 1984: 100 .

Tope material examined: Holotype (one ♂, BPBM), ‘ NEW HEBRIDES [Vanuatu]: Maewo I: Sounwari : 0–360 m, 4–5. IX.1979 ’ ‘ G.M. Nishida Coll. / BISHOP Museum / Acc. No. 1979.380’.

Additional material examined: (one ♂, BYU), ‘ VANUATU: Pentecost Is., / Ranmawat , May 30, 2019 ; / −15.8126, 168.1770 / Coll: SM Bybee, GS Powell / #BYU-VU-2019’ ‘OD1706 ’.

(one ♀, BYU), ‘ VANUATU: Malekula Is., / Wiaru River , May 13 th 2019 / −16.0787, 167.2726 / Coll. SM Bybee, GS Powell / BYU-VU-2019’ ‘OD1705 ’.

Description of female

Head: Labium beige; labrum light yellow with darker posterolateral corners, medial posterior edge with dark brown spot; anteclypeus and genae green, mandibles (except for reddish tips) yellow; postclypeus, frons, and scape black with bronze shimmer; antennae with black scape, pedicels yellow, flagella dark brown and darkening apically ( Fig. 4D); vertex and rear of head black except for pale yellow–white patches extending from the base of the scape to the lateral ocelli; pale occipital bar present; eyes dark green on dorsal two-thirds and lighter on basal one-third.

Torax : Prothorax dorsally black with strong bronze shimmer; laterally black with pale stripes; hind lobe with a straight medially edge; mesostigmal plates light brown, roughly triangular, and without a prominent caudal depression, lateral corner broadly rounded ( Fig. 4B). Pterothorax with black carina; laterally black with bronze shimmer except for a pale stripe along the mesopleural suture, a pale spot on the apical corner of the mesinfraepisternum, a short pale stripe on the posterior one-third of the interpleural suture, and a pale stripe that runs along the ventral edge of the metepisternum but reaches the metapleural suture on its posterior half; anterior carina pale along the metepisternum and metepimeron. Coxae black; legs light brown ventrally and dark brown dorsally; trochanters and femora with dark brown spines; tibiae with lighter spines than that of the femora; tarsi reddish-brown and small recurved spines; claws reddish brown.

Wings: Hyaline, with dark brown venation; brown pterostigma; three post-discoidal cells. Nodal index: forewing 18/2–2/15; hindwing 17/2–2/14.

Abdomen: Overall black with bronze shimmer; S8 reddish-brown dorsally; S9 and S10 black ( Fig. 4E); cerci dark brown and roughly limuloid, lightening apically; paraproct dark brown on dorsal side, fading to a brownish red on ventral side; valve with excision on dorsal apex forming small tooth, stylus extending beyond cerci, narrow and broadly rounded, gonapophysis reddish-brown ( Fig. 4A, C).

Measurements: Total length 41 mm, abdomen 33 mm, and hindwing 24 mm.

Distribution: Vanuatu (Maewo, Pentecost, and Malekula).

Molecular phylogenetics and divergence time estimates

In total, we included 28 specimens in the maximum likelihood phylogeny ( Table 1), composing 2596 bp. Seven of the eight known species of Melanesobasis were included, in addition to one subspecies. Currently M. c. marginata is treated as a subspecies of M. c. corniculata . Te maximum likelihood phylogeny recovered a monophyletic Melanesobasis (98% BS). Vanuatu specimens of Melanesobasis bicellulare were recovered as sister to all Fijian species. Congruent with previous analyses (i.e. Beaty et al. 2017), we recovered a well-supported ‘c orniculata ’ clade (100% BS) and ‘ simmondsi ’ clade (100% BS). Of the species tested for monophyly, Melanesobasis c. corniculata is recovered as non-monophyletic (51% BS), with undescribed species Melanesobasis sp2 sister to the subspecies M. c. marginata (71% BS) ( Fig. 5).

Divergence time analyses (D1 and D2) recovered overall similar median ages ( Table 2; Supporting Information, File S1). Both recovered the age of Melanesobasis around 23–26 Mya [9–40 Mya highest posterior density ( HPD)] and specifically the age of Fijian Melanesobasis at 18–24 Mya (7–33 Mya HPD). Most extant species are estimated to have originated within the last 5 Myr in both analyses. Te second dating strategy (D2) consistently recovered broader 95% HPD than the first dating strategy (D1), ofen with ranges spanning an additional 10–15 Myr.

Biogeography

Results of all models, including ancestral state probabilities for all internal nodes, can be found in the Supporting Information, File S2. Of all biogeographical models tested, DIVALIKE was recovered as the best fit for our two-area models, with an AIC score of 5.386 ( Table 2). In both DEC and DIVALIKE models, the MRCA of all Melanesobasis (node 10) was recovered as Fiji + Vanuatu ( AB) with>99% ancestral state probabilities, supporting a vicariance event in which descendants were split between Fiji (A) and Vanuatu (B). Te remaining internal nodes were recovered as Fiji (A) with>99% probability ( Fig. 6). Conversely, the BAYAREALIKE model ( AIC 15.49) recovered the MRCA of all Melanesobasis (node 10) as Fiji (A) (80%), with a subsequent dispersal event to Vanuatu. Te remaining internal nodes for this model recovered Fiji as the ancestral range with>90% probability. BAYAREALIKE+J recovered a similar result but with Fiji (A) and Vanuatu (B) as equally likely (50%) to be the ancestral state for Melanesobasis (node 10), with subsequent jump-dispersal events (founder-event speciation) to either Vanuatu or Fiji. All remaining internal nodes were reconstructed as Fiji (A) with>99% probability.

Using three-area models, the DIVALIKE+J model ( Fig. 7) was recovered as the best fit with an AIC score of 19.87. In this model, the MRCA of all Melanesobasis (node 10) was reconstructed with the highest probability (52%) to be southwest Fiji + Vanuatu ( AC). A vicariance event was reconstructed, with the descendants of node 10 being found in either southwest Fiji (A) or Vanuatu (C). Jump dispersal (founder-event speciation) was inferred for the species M. sp2 and M. c. marginata from southwest Fiji (A) to northeast Fiji (B). DIVALIKE reconstructed a highly similar scenario, in which vicariance was inferred with node 10 reconstructed as AC (85%) and the direct descendants being found in only A or C. Under this model, however, dispersal to northeast Fiji (B) occurred afer cladogenesis leading to a range expansion (nodes 14 and 15; AB), which was followed by vicariance in both major clades of Fijian Melanesobasis .

In the three-area DEC model, all areas ( ABC 46%) were recovered at node 10. Te descendants of the MRCA of Melanesobasis under this model had range contraction in northeast Fiji (B) (extinction) and vicariance into Fiji (A) and Vanuatu (C). Within the Fijian Melanesobasis there was back dispersal to northeast Fiji (B) leading to nodes 14 and 15 ( AB) followed by vicariance. DEC+J had a largely unresolved ancestral area for all Melanesobasis at node 10 ( AC 33%; A 20%; C 23%). Vicariance was recovered as the mostly likely scenario, with descendants of node 10 in area A or C. Tis was followed by jump-dispersal to northeast Fiji (B).

Under BAYAREALIKE, node 10 had the greatest probability to inhabit all three areas ( ABC 38%) or all of southwest and northeastern Fiji ( AB 31%). Tis reconstruction suggested either range contraction (extinction) leading to descendants in AB (node 11) and C (node 1), or subsequent dispersal to Vanuatu. Additional range contraction events were reconstructed in Fiji ( AB), leading to extant distributions in only southwestern Fiji (A) or northeastern Fiji (B). BAYAREALIKE+J recovered node 10 as C (50%) or A (42%). Based on the highest probabilities, jump-dispersal from Vanuatu to southwestern Fiji (A) occurred, with subsequent jump-dispersal to northeastern Fiji (B).

When the three-area model was time constrained to limit movement between Vanuatu and Fiji afer 10 Mya, we recovered almost identical results to the unconstrained three-area model (Supporting Information, File S2). DIVALIKE+J was recovered as the best-fiting model in this case, with an AIC score of 19.87. Te MRCA for all Melanesobasis was recovered as both Fiji and Vanuatu ( AC), followed by vicariance.