Molecular phylogeny and morphology support recognition of a new species
Considering ecological traits, both Vincetoxicum sangyojarniae and V. flexuosum var. flexuosum grow in lowland. The former is found at 150 m. The latter is more widespread, occurring from near sea level up to 800 m. In one location (Surin province), it has been found growing near V. sangyojarniae. The evidence from molecular phylogenetic analysis and comprehensive morphological comparisons clearly showed that V. sangyojarniae is a well-defined entity and clearly distinct from V. flexuosum var. flexuosum and all the other varieties of this species. Morphologically, these two taxa are similar in several aspects. They possess a slender twining stem and branches, and ovate to lanceolate leaves that are glabrous on both surfaces. They produce branched cymes with zigzag rachises, and small red or reddish-brown flowers (5–6 mm in diameter) with a broadly conical gynostegium. The new species, however, can be clearly distinguished from V. flexuosum var. flexuosum by several consistent characters. It has broadly pyramidal flower buds with an obtuse apex, and corolla lobes that are pubescent on the adaxial surface, the distal half of which is covered with dense long white trichomes. On the contrary, V. flexuosum var. flexuosum has oblate flower buds with a rounded apex. Its corolla lobes are always completely glabrous throughout on both surfaces. These characteristics were found to be consistent in all samples from the eight populations studied. Detailed comparisons of the two species for each character are provided in Table 2.
Molecular phylogenetic relationships revealed that Vincetoxicum sangyojarniae is evolutionarily more closely related to several African species, i.e., the climber V. lycioides, and the non-twiners V. caffrum and V. fleckii, than to all varieties of V. flexuosum. Therefore, the similarity in morphology between V. sangyojarniae and V. flexuosum var. flexuosum observed in Thailand is likely the result of convergence. Overall, V. flexuosum var. flexuosum and the other varieties of this species included in this phylogenetic study were placed with high support in three distinct clades: (1) V. flexuosum var. flexuosum from different locations in Thailand, (2) V. flexuosum var. tenuis from Thailand and Borneo, and (3) V. flexuosum var. flexuosum from Malaysia and the Philippines and V. flexuosum var. perrottetianum from the Philippines. This result suggests that the taxonomic status as an infraspecific rank of V. flexuosum var. tenuis (Schneidt 1999; Liede et al. 2002; Liede-Schumann et al. 2012) might need to be reconsidered. Morphological examination of samples of this taxon obtained throughout its distribution range in Thailand revealed its substantial differences from V. flexuosum var. flexuosum both in ecological and floral traits (Fig. 5). While V. flexuosum var. flexuosum usually occurs in mixed deciduous to evergreen forests, V. flexuosum var. tenuis is rheophytic and restricted to wetlands. The former has oblate flower bud with a rounded apex (Fig. 5d). Its flowers are consistently characterized by the distally thickened inner corolla tube. This thickening forms a continuous ring at the corolla throat around the gynostegial corona (Fig. 5c). The latter, on the other hand, has flower buds that are oval in shape and obtuse at the apex (Fig. 5e). Its flowers lack the thickened ring on the inner surface around the corolla tube (Fig. 5f). For each of the two taxa, these qualitative traits are constant in all populations studied in Thailand. However, additional sampling in the other countries within the areas of distribution of these taxa is required to confirm whether the species rank of V. flexuosum var. tenuis should be reinstated.
Taxonomic treatment
Vincetoxicum sangyojarniae A. Kidyoo, sp. nov. :—TYPE: Thailand, Surin province, Muang district, Rajamangala University of Technology Isan Surin Campus, 150 m a. s. l., 31 Oct 2021, A. Kidyoo 80 (holotype: BKF!; isotype: BCU!) (Figs. 3 and 4)
Diagnosis: Vincetoxicum sangyojarniae can be clearly distinguished from the similar V. flexuosum var. flexuosum by its broadly pyramidal flower bud with an obtuse apex and corolla lobes that are adaxially densely pubescent.
Etymology: The specific epithet ‘sangyojarniae’ is given in honour of Asst. Prof. Pacharaporn Sangyojarn, the senior botanist of the Faculty of Agriculture and Technology, Rajamangala University of Technology Isan, who first discovered and drew our attention to the plant.
Description: Slender climber, 1–5 m long with clear exudate in all vegetative parts. Stems and branches twining, terete; internodes 5–20 cm long, 1–2 mm in diameter, green to greyish green, with trichomes arranged in one row. Leaves opposite; petiole terete, slightly canaliculate and sparsely pubescent along the upper side, 8–16 mm long, 1.3–1.7 mm in diameter; blade membranous, ovate, ovate-oblong to oblong-lanceolate, 3.1–7.5 × 0.9–3.9 cm; leaf margins entire to ciliate; leaf base obtuse, rounded to subcordate, bearing 1–2 ovate-lanceolate glands on the adaxial surface, leaf apex acute to acuminate; adaxial surface green to dark green, glabrous except for scattered trichomes along the midrib, abaxial surface paler, glabrous; midrib prominent on the abaxial surface, lateral veins slightly visible on both surfaces, 7–9 on each side, at acute (45–60°) angles to the midrib. Inflorescence branched with 1-4 zigzag rachises, extra-axillary, 20–many-flowered; peduncle tiny, slender, 1.2–3 cm long, 0.7–0.9 mm in diameter, green, greenish brown to reddish brown, glabrous; pedicels green, greenish brown to reddish brown, 6–8 mm long, 0.4–0.5 mm in diameter, glabrous; bracts minute, attached at the base of the pedicel, triangular-lanceolate with acute apex, green to greenish brown. Flower bud broadly pyramidal with obtuse apex. Calyx lobes ovate-triangular with acute to acuminate apex, 0.7–0.9 × 0.6–0.7 mm, greenish brown, glabrous, bearing an ovate-oblong gland inside each lobe sinus. Corolla rotate, 6–6.5 mm in diameter, pinkish brown to reddish brown; tube 0.6–0.8 mm long, glabrous on both surfaces; lobes ovate-triangular, 2.2–2.5 × 1.8–1.9 mm, apex acute and slightly twisted; the distal half of adaxial surface densely covered with long white trichomes, abaxial surface glabrous; margins recurved. Gynostegium conical, 1.5–1.6 mm in diameter, 0.7–0.9 mm in length, sessile. Staminal corona lobes 5, inserted at the base of the stamens, fleshy, broadly ovate with rounded apex, half as long as the anther, 0.5–0.6 × 0.5–0.6 mm, reddish brown. Anthers obovate, yellow, tinted reddish brown at the center, with white membranous appendages, attached around the style-head. Pollinaria 5, pendulous, almost horizontally disposed with respect to the filament; pollinia ovoid, yellow, 0.16–0.19 × 0.12–0.13 mm; translator arms hyaline, 0.04–0.05 mm long; corpusculum ovate, reddish brown, 0.08–0.09 × 0.04–0.05 mm. Carpels ovoid, glabrous, 0.8–0.9 mm in length, ca. 0.3 mm in diameter; style-head truncate, pentagonal. Follicles cylindric-fusiform, green when young and turning brown with age, 7–8 × 0.7–0.9 cm. Seeds brown, ovate-oblong, 4–5 × 2.5–3 mm; coma white, 2–2.5 cm.
Phenology: Flowering from September to January.
Distribution and habitat: This new species is known only from the type locality. It grows in loam soil along roads in shady areas of disturbed mixed deciduous forest (Fig. 3a).
Additional specimens examined: Vincetoxicum sangyojarniae A. Kidyoo: THAILAND, Surin province: Muang district, Rajamangala University of Technology Isan Surin Campus, 150 m elevation, 31 October 2021, A. Kidyoo 81 (paratype: BCU!).
Conservation status: Vincetoxicum sangyojarniae is apparently rare, known from a single locality in Thailand. About 20 to 30 mature individuals were found growing in an area of 1500 m2 that is frequently disturbed by human activities. Following IUCN (2019), a provisional conservation assessment of Endangered: B1ab(iii) is assigned owing to its extent of occurrence much smaller than 5,000 km2, its restricted geographic distribution, and the small number of locations, which are susceptible to threats in the near future.