Dictyanthus stevensii Gonz., 2019

González-Martínez, César Adrián, Lozada-Pérez, Lucio & Alvarado-Cárdenas, Leonardo O., 2019, Dictyanthus stevensii (Apocynaceae; Asclepiadoideae; Gonolobinae), a new species from Chimalapas region, Oaxaca, Mexico, Phytotaxa 394 (1), pp. 79-88 : 81-86

Dictyanthus stevensii Gonz. -Martínez, Lozada-Pérez et L.O. Alvarado sp. nov. ( Figs. 1–4 )

Dictyanthus stevensii is morphologically related to D. eximius ( Stevens 1988: 1552) Stevens (2000: 243) but differs by having a row of uncinated and interpetiolar trichomes, the indumentum of young stems consists of uncinated and straight trichomes only, corolla green with undulate margin, the lobes of the corona are oblong with spatulate, rounded apex or slightly bilobulated, the apex is exerted from the corolla tube and with the anthers vertices slightly exerted.

Type: — Mexico. Oaxaca: Municipio San Miguel Chimalapa, Arroyo El Caracol , parte inferior al NO de Benito Juárez, ca. 39 ‒ 40 km en línea recta al N de San Pedro Tapanatepec, 900 ‒ 1,100 m, 16°43’ N, 94°9’ W, 24 June 1986, Salomón Maya J. 3463 (fl.) (holotype MEXU!; isotype CHAPA!) .

Vine 0.6 ‒ 1.4 mm wide, twining, the base suberosed or slightly lignified, root not observed, indumentum made of uncinated trichomes (long and short) and straight trichomes (long and short), yellowish, the uncinated trichomes are scarce in mature stems and abundant in herbaceous regions of the plant. Young stems lignified, pubescent with short straight trichomes and scarce uncinated trichomes, the interpetiolar region with a horizontal row of uncinated trichomes, the buds with a tuft of straight long trichomes. Leaves with blades (3.1 ‒)3.6 ‒ 5.6 × 2 ‒ 4.3(‒ 5) cm, widely ovate, the based with lobes 0.3 ‒ 1.3 cm long; the apex acute or slightly acuminated; membranaceous, velvety, with (5 ‒)7 ‒ 8 secondary veins, the blade olive green, the underside with lighter tones of olive green; upperside and underside scarcely pubescent with both long and short uncinated trichomes distributed on the blade and veins, with 2 ‒ 4 colleters at the base of the blade, copperish or orange; the petioles 1.6 ‒ 4.6 cm long, puberulent with short straight trichomes, distally with scarce uncinated trichomes. Inflorescence a monochasium with 2 flowers; the peduncle (3 ‒)5.3 ‒ 7.3(‒ 8) mm long, puberulent; the bracts (1.2 ‒)1.8 ‒ 2.8 × 0.2 ‒ 0.5 mm, lanceolate, pubescent with both long and short uncinated trichomes; the pedicels 1.3 ‒ 5.3 mm long, puberulent with scarce uncinated trichomes or absent. Calyx with lobes 4 ‒ 5.5 × 0.9 ‒ 1.5(‒ 1.7) mm, lanceolate, the upperside glabrate, the underside pubescent with both long and short uncinated trichomes, mostly at the margin, with 1 colleter between the lobes. Corolla urceolate ‒ campanulate, sacciform, 0.7 ‒ 1.1 cm in diameter, the tube (2.8 ‒)3.5 ‒ 4.4 × (3.8 ‒) 4.2 ‒ 5.3 mm, the limb (0.8 ‒) 1.3 ‒ 1.5 mm long, the lobes 3.2 ‒ 4.5 × (2.6 ‒) 3.5 ‒ 4 mm, patent or ascendent, acute apex, ondulate margin, the corolla green, the venation of the tube with vertical and parallel lines between the corona lobes, the limb and the base of the lobes with concentric venation and slightly reticulated towards the apex of the lobes, with brown veins and short straight trichomes on the veins of the limb, lobes and margin of the corolla. Corona gynostegial, 5-palmately lobed, staminal elements 1.6 ‒ 2 × (0.5 ‒) 0.7 ‒ 0.8 mm, oblong, the apex spatulate, rounded or slightly bilobulate, the base widened and mucronate, with short straight trichomes on the margin, the corona purple, the lobes partially connated at the corolla tube, sometimes the apex exerting the corolla tube and adnated basally to the stipe of the gynostegium through a smooth and thick septum. Gynostegium ca. 1.3 × 2.1 ‒ 2.3 mm, slightly exerted, pentagonal, each corner of the anthers or vertices elevated 1 ‒ 1.2 mm long, the stipe 0.8 ‒ 1 mm long; corpusculum ca. 0.5 × 0.2 mm, slightly sagittate, dark brown; pollinia 0.9 ‒ 1 × ca. 0.3 mm, apically sterile. The apex of the style head pentagonal and concave. Follicle and seeds not observed.

Etymology: —The specific epithet honors Dr.Warren Douglas Stevens(b.1944), botanist, collector and investigator at the Missouri Botanical Garden. Stevens has greatly contributed to the knowledge and taxonomy of the subfamily Asclepiadoidae . Among his contributions, his participation as editor in chief of the Flora of Nicaragua is one of the most important. He has also described over 130 new species from many genera of Asclepiadoideae and other families, mostly from Mexico and Central America. He carried out the most exhaustive taxonomic revision of Dictyanthus ( Stevens 1975, 1988) and recognized it as an independent genus from Matalea s.l., based on morphological evidence ( Stevens 1999, 2000). In addition, he described six new species for the genus and provided new information about this genus in the Mexican Flora ( Stevens 1988).

Distribution & habitat: — Dictyanthus stevensii is known only from the type specimen collected in the Chimalapas forest near to Arroyo El Caracol, part of the RTP “Selva Zoque - La Sepultura”. Also comprises Uxpanapa forest (Veracruz) at the NW, El Ocote (Chiapas) at the E and La Sepultura (Chiapas) at the SE (CONABIO 2004). This region is part of the biogeographic province of Soconusco, which comprises the Sierra Madre of Chiapas and of Guatemala ( Fig. 4 ), which holds great diversity and many endemic species ( Espinosa et al. 2008). The new species is scarce and grows in the forest according to information on the specimens label and can be found between 900 and 1,100 m elevation.

The type locality is in tropical semi deciduous forest with an annual precipitation of approximately 1,500 mm ( Espinosa et al. 2008). It was collected in steep and rocky slopes, associated to Bursera sp. Jacq. ex Linnaeus (1762: 471) ( Burseraceae ), Ficus sp. Linnaeus (1753: 1059) ( Moraceae ), and Cecropia sp. Loefling (1758: 272) ( Urticaceae ).

Phenology: —The specimens were collected in flower in June. The fruits have not been observed.

Conservation assessment: —The Zoque forest in the Chimalapas region, where the new species was collected, is not exempt from disturbances and it is under deforestation due to timber harvesting and farming ( Trejo 2006). The region has been also subject of social and environmental problems such as the fire of 1988, which consumed approximately 80% of the cloud forest ( Gallardo-Hernández & Lorea-Hernández 2010). Dictyanthus stevensii has not been collected in 30 years and little is known about its population, phenology, distribution in the Zoque forest in addition to its category of risk due to the fragmentation and land-use change. No attempt has been made to visit the type locality to collect the species because it represents an area of social, territorial, and resource conflicts. Considering that only the type collection is known, the occupation area (AOO) of the new species is 2 km 2 defined by the recommended grid width ( IUCN 2017), together with the fragmentation and change of land use, we suggest the category of Critically Endangered B2ab(ii) according to the red list criteria of the IUCN (2017).

Comments:— Dictyanthus stevensii is an interesting member of the genus due to the presence of new morphological features, including the small size of the corolla and the corona with oblong lobes, with a spatulate, rounded or slightly bilobated apex, sometimes free and exerting the tube of the corolla. The corona lobes have a great morphological variation in the other species and they are taxonomically important. For example, the lobes can be vescicular-like ( D. asper ( Miller 1768: unpaged) Stevens (1999: 564)), laminar ( D. sepicola ( Stevens 1975: 387) Stevens (2000: 243)) , linear ( D. tigrinus Conzatti & Standley (1924: 1183)) , and oblong with rugose spherical tip ( D. aeneus Woodson (1935: 691)) .

Dictyanthus stevensii presents the smallest urceolate-campanulate flower known for the genus. The new species shares with D. eximius , D. hamatus ( Stevens 1988: 1540) Stevens (2000: 243) and D. tuberosus Robinson (1893: 180) the presence of urceolate-campanulate corolla ( Fig. 3 , Tab. 1), but D. stevensii shows greater similarity with D. eximius , an endemic species of the Sierra Madre of Chiapas. Both species belong to the biogeographic province of Soconusco and showed similar inflorescences in monochasia with two flowers, urceolate-campanulate corolla, with vertical and parallel venation in the tube, but in a concentric pattern in the limb and reticulated towards the lobes of the corolla (see Fig. 12 of Stevens 1988). The new species can be separated because D. stevensii has corolla with length of the tube (2.8) 3.5 ‒ 4.4 mm, corona lobes oblong with spatulate, rounded or slightly bilobulated apex, and exerting the corolla tube. While D. eximius has length of the tube 16 ‒ 20 mm, corona lobes are linear-spatulate with acute apex, and are inserted in the corolla tube ( Stevens 1988). The new species inhabits tropical semideciduous forest between 900 and 1,100 m elevation, while D. eximius can be found in the ecotone of pine forest and tropical evergreen forest, between the 700 and 900 m elevation ( Tab. 1).

Figures

FIGURE 1. Dictyanthus stevensii. A. Habit of the plant with inflorescences; B. Flower; C. Corona and gynostegium; D. Pollinarium.A‒D based on Salomón Maya J. 3463 (MEXU) (Illustrated by César Adrián González Martínez).

FIGURE 2. Holotype of Dictyanthus stevensii. A. Herbarium specimen showing the inflorescences in monochasia and the size of flowers; bar 4 cm; B. Base of the blade abaxially, with three colleters and petiole with mixed indumentum of short straight trichomes and uncinated trichomes; C. Flower. The parallel vertical venation can be observed in the tube of the corolla; bar 2.5 mm; D. Corolla with concentric venation in the limb and reticulated towards the lobes with short straight trichomes on the veins; showing the gynostegium. The corpusculum and the apex of one lobe of the corona are shown exerting the corolla tube; bar 2.5 mm.

FIGURE 3. Morphological comparison of the flower of some species of Dictyanthus related to Dictyanthus stevensii, based on herbarium specimens. A. D. hamatus, Diego 6672 (FCME); B. D. eximius, Stevens 26651 (MEXU); C. D. tuberosus, Pringle 4482 (MEXU); E. D. stevensii, Salomón Maya J. 3463 (MEXU); bar 1 cm.

FIGURE 4. Known distribution of Dictyanthus stevensii (black dot), and species morphologically akin, D. eximius (green dots), D. hamatus (red dots), and D. tuberosus (blue dots). State of Oaxaca and location of the priority region (RTP) “Selva Zoque - La Sepultura” in box.