Dictyanthus unicornus Pío-León & L.O.Alvarado, 2023

Pío-León, Juan Fernando & Alvarado-Cárdenas, Leonardo O., 2023, Another remarkable new species of Dictyanthus (Apocynaceae, Asclepiadoideae, Asclepiadeae, Gonolobinae) with a long stylar head from Mexico, Phytotaxa 632 (2), pp. 175-183 : 176-177

Dictyanthus unicornus Pío-León & L.O.Alvarado , sp. nov. ( Figs 1 , 2 ).

Type:— MEXICO. Sinaloa: Municipio Cosalá. Carretera Cosalá-Reserva Nuestra Señora , 6.8 km al E de Cosalá, 2.3 km al NO de la Reserva, 24°24’71.82’’N, 106°37’27.7’’W; 522 m, 25 September 2022, J.F. Pío-León & M.G. Millán-Otero 375 (holotype: CIIDIR! , isotypes: FCME! , MEXU! ) .

Dictyanthus unicornus is similar to D. nichongus , both sharing an exerted stylar head and the gynostegial corona lobes exceeding the length of the corolla tube. The new species is characterized by its flowers of 3.0– 4.5 cm diameter (vs. ca. 2 cm in D. nichongus ), longer and thinner stylar head, 3.48–4 × ca. 1 mm, (vs. 2.7–3.2 × ca. 2 mm in D. nichongus ), non-revolute corolla lobes (vs. revolute in D. nichongus ), and brown, spatulate corona lobes (vs. yellowish white, rhombic in D. nichongus ).

Twining vines 1–1.5 m long, root not observed. Stems with mixed indumentum of long and short trichomes, the short ones unicellular, homogeneously distributed all over the stem, ca. 0.1 mm long, the long ones multicellular, with dark septa, on a single line heterogeneously separated, 0.32–0.8 mm long, denser at the nodes, associated to the base of the petiole. Leaves petiolate, petioles 4.0–7.0 cm long, with mixed indumentum, short trichomes homogeneously distributed, long trichomes restricted to upper part of petiole, close to insertion with lamina, multicellular with dark septa; lamina membranaceous, 8–10 × 6.5–8.5 cm, ovate, 6–8 secondary veins, abaxial surface with unicellular trichomes over the entire surface, multicellular trichomes denser on veins, abaxial surface with multicellular trichomes heterogeneously distributed over surface, denser on veins; margin ciliate, with multicellular trichomes 0.2–0.5 mm long; base lobed to cordate, lobes 1.9–10.5 mm long, apex acuminate to mucronate; colleters yellow, 6–8 at base of lamina, 0.2–0.3 mm long. Inflorescence racemiform-cymose, with 2–3 flowers, indumentum similar to stem; peduncle ca. 1.6 cm long; bracts 1.8–5.3 × 0.2–0.8 mm, linear to lanceolate; pedicels 1.2–1.6 cm long. Flowers pentamerous, 5-lobed, with valvate estivation. Sepals 1–1.2 × 0.3–0.4 cm, lanceolate, with heterogeneously distributed trichomes on abaxial side, ca. 0.1 mm long, glabrous on adaxial surface, with a colleter between each lobe, 1.5–2.0 mm long. Corolla 3.0– 4.5 cm diam, campanulate-rotulate, sacciform, yellowish with reddish-brown reticulation, pubescent, with unicellular trichomes ca. 0.1 mm long, scattered over entire surface, tube 5.2–6.8 × 8.9– 1.3 mm, with a pattern of vertical to partially reticulate parallel lines, limb 3.5–5.2 mm long, lobes 0.8–1.7 × 1.0– 1.3 cm, triangular, patent, apex acute to slightly rounded, limb and lobes with reddish-brown reticulate pattern, adaxially and abaxially with short trichomes ca. 0.1 mm long; gynostegial corona 5-lobed, lobes 4.5–6.0 × 3.0– 3. 6 mm, exerted, spatulate, fleshy, apex rounded, brownish, apparently secretory, lobes connate to corolla tube, adnate basally to gynostegial stipe; gynostegium ca. 1.2 × 2.5 mm, pentagonal, stipe 1.2–1.3 mm long; corpuscle ca. 0.2 × 0.09 mm, sagittate, brownish; pollinia 0.59–0.6 × 0.3–0.4 mm, apically sterile; apex of stylar head 3.48–4 × 1 mm, purple at base, yellow or white toward apex, base pentagonal widening, apex narrowly conical, exserted, slightly bilobed; ovaries apocarpic, warty, 0.8–0.9 × 1.5–1.6 mm. Follicles 8–10 × 2.5–3 cm, brown, fusiform, prickles 4–6 mm long, curved to nearly erect, acuminate. Seeds 5–7 × ca. 4 mm, dark brown, obovate to suborbicular, surfaces verrucose, margin ca. 1 mm long, crenulate to irregularly dentate, coma 1.5–1.8 cm long.

Distribution, habitat and ecology:— The new species has only been collected from two populations at the type locality, in semi-deciduous tropical forest, just on the outer edge of the Mineral de Nuestra Señora de la Candelaria state reserve in Sinaloa, 700 m SE of a mining area ( Fig. 3 ). A second locality was documented for Puerto Vallarta, Jalisco, through a photograph on a plant identification group on Facebook (Botánica en México: https://bit.ly/3neYTBa) by user Roberto Hornelas, but no collection was made, and no information was provided.

During the collection, larvae of a tiger moth of the genus Euchaetes ( Erebidae , Arctiinae) were observed feeding on the foliage and flower buds, and some individuals of the new species had also been foraged, probably by cattle or wildlife.At the time of plant collection, ants were observed visiting the flowers, foraging in the corona and gynostegium area, but without extracting pollinia.

Phenology: —Flowering occurs from late July to September and fruiting in September. This period is the wettest and rainiest months in the region.

Etymology: —The specific epithet “unicornus ” is proposed because of the distinctive attribute of the style apex of the gynostegium, which is very pronounced and looks like a horn. The style apex is a character of taxonomic importance not only within Dictyanthus , but in many genera of Apocynaceae .

IUCN conservation assessment: —This species has an area of occupancy (AOO) of 12 km 2, and an extend of occurrence (EOO) of 330 km 2, considering both Sinaloa and Jalisco localities. Plant and habitat data are known only for the Sinaloa population. The known population is located at the outher boundary of the area of the Nuestra Señora Reserve, in the municipality of Cosalá; however, the individuals of this species grow in an anthropized area with a great deal of mining and cattle ranching activity. No more than five individuals have been found in the study area. Based on this, we suggest placing this species in the category of Endangered (En) B1 ab(iii) + 2 ab(iii).

Additional specimens examined: — MEXICO. Sinaloa: Cosalá, noreste de Cosalá, cerca de la mina Escorpio, bosque tropical subcaducifolio, suelo gris pedregoso , 24°24’43’’N, 106°38’03’’W, 580 m, 23 September 2021, R. Vega-Aviña, F. Delgado-Vargas and J. F. Pío-León 12888 ; carretera Cosalá-Reserva Nuestra Señora , 6.8 km al E de Cosalá, 2.3 km al NO de la Reserva, 24°24’71.82’’N, 106°37’27.7’’W, 522 m, 23 July 2022, L. O. Alvarado Cárdenas and J. F. Pío-León s.n. ( FCME ) .

Figures

FIGURE 1. Dictyanthus unicornus A. Branch segment with inflorescence and flower bud. B. Detail of leaf base with colleters. C. Flower. D. Longitudinal section of the flower. E. Apex of the stylar head. F. Pollinaria. G. Fruit. Illustration of Quiyahuitl Colibrí Fernández Armendáriz, based on holotype J. F. Pío-León & M. G. Millán-Otero 375.

FIGURE 2. Dictyanthus unicornus A. Branch with leaves. B. Leaves and flower bud. C. Top view of flower. D. Lateral view of flower. E. Fruit. Credits: J. F. Pío-León.

FIGURE 3. Distribution map of Dictyanthus unicornus and morphologically similar species.