Dioscorea magnibracteata T.Clayton ex R.Couto & Cornejo, 2022

Couto, Ricardo Sousa & Cornejo, Xavier, 2022, Dioscorea magnibracteata (Dioscoreaceae), a new species from western Ecuador based on the unpublished work of Temple Clayton, Phytotaxa 542 (3), pp. 293-299 : 294-297

Dioscorea magnibracteata T.Clayton ex R.Couto & Cornejo sp. nov. ( Fig. 1 )

Similar to the species of Dioscorea sect. Monadelpha and D. margarethia G.M.Barroso. E.F. Guimaraes & Sucre (1970: 2) , the single species of D. sect. Margarethia , by being one of the few monoecious Dioscorea with staminate racemes at the lower portion of the stem and pistillate spikes at the apex of the stem, and some of the largest flowers (1.5–3 cm) in the genus. The new species is recognizable by the entire leaves, staminate and pistillate inflorescences with membranaceous wings (ca. 1 mm long) along the inflorescence axes, staminate flowers with six stamens in a central column, tepals widely obovate to very broadly obtrullate, pistillate inflorescences with flowers at the apex (occupying only 1–2 cm long), ovaries with membranous wing projected from the dorsal edge of each carpel and fruits with membranaceous wings along the valves and also between them. It is unique due to the solitary and colossal cymbiform bract (4.8–7.4 × 2.9–5.7 mm) at the base of the staminate flower, with toothed to lacerated margin.

Type:— ECUADOR. Prov. Guayas: Cerro Azul, W of Guayaquil , climbing in thicket, 10 February 1955, Erik Asplund 15394 (♂ ♀) (holotype S10-29250 !, isotype S-R-1578!) .

Twining vine, monoecious, geophyte, left-twining. Tuber discoid, somewhat flattened, ca. 6 cm in diam., 2 cm thick. Stem up to 6 m long, annual, glabrous, herbaceous, furrowed and ridged when dry but narrowly winged on the ridges when fresh, smooth to ridged, 1.5–3.3 mm diam., greenish when fresh, yellowish-green to light brownish when dry, nodes with conspicuous intumescences; aerial tubers absent in the axils. Leaves alternate, entire, monomorphic; petiole 3.1–10.3 cm long, glabrous to short-pilose, twisted at base, slightly swollen in the basal pulvinus, flattened to oval in cross section, 1.5–2.1 mm diam, slightly channeled above, laterally winged, without stipuliform expansions on stem insertion, color as on the stem, generally darker on upper third when dry; blade 8.2–16.8 × 5–13.1 cm, short-pilose on veins at base abaxially, otherwise glabrous on both sides, membranaceous, cordate to ovate-cordate, adaxially deepgreen, glossy when fresh, olive-green to yellowish, glossy when dry, macules absent, abaxially light-green, glossy when fresh, matte when dry, base cordate, sinus 1.4–3.2 cm long, acute to obtuse, basal lobes rounded, apex 0.8–2.3 cm long, attenuate to cuspidate, extra floral nectaries at the base, sometimes inconspicuous when fresh and blackish when dry, forerunner tip 2.9–3.7 mm long, margins entire to sometimes wavy from lateral view, 9–13 main veins, slightly protruding abaxially, with 2–3 restricted to basal lobes, usually bifurcated. Staminate inflorescence restricted to the medial portion of the stem in fully developed plants, 1–3 at each node, 7.3–19.5 cm long, in simple racemes, few-flowered, erect to more or less patent, flattened in cross section, with two membranaceous wings (ca. 1 mm wide) parallel to the axis, and spaced 180º apart, peduncle 4.6–9.7 cm long. Staminate flower pedicellate, pendulous, 1 bract subtending each flower, 4.8–7.4 × 2.9–5.7 mm, membranaceous, broadly cymbiform, margin toothed to lacerated, base cordate to auriculate, apex acuminate, longitudinally multi-nerved, light-green when fresh, yellowish-green when dry; pedicel 1.7–3.2 cm long, filiform, flattened when dry, torus ca. 2 mm diam., discoid, flattened; perianth rotate to slightly crateriform, 1.5–3 cm wide, tepals 6, in two whorls of 3 in flower buds, 7–8 × 6–8 mm, free, inserted in the margin of the torus, subequal or undifferentiated, broadly obovate to very broadly obtrullate, base cuneate, margin erose, apex broadly obtuse, glabrous, multi-nerved (ca. 7 main veins and intricate network of secondary veins), glossy green when fresh, yellowish when dry; fertile stamens 6, filaments inserted on a conical column when fresh, subcylindrical when dry, ca. 2 mm long when fresh, ca. 1.5 mm long when dry, broader at base, occupying the entire torus, color similar to tepals, white-rugose bands transversely oriented, anthers inserted at the top of the column, ca. 1 mm long, oblong, dorsifixed, slightly curved, following curvature of the column apex, bright orange when fresh, yellowish when dry, staminodes absent; pistillodium absent. Pistillate inflorescence restricted to the apical portion of the stem, solitary in each node, 9.7–13.3 cm long, in simple spikes, few-flowered (less than 10 flowers), patent, axis flattened in cross section when dry, with membranous wings (ca. 1 mm wide) parallel to the axis, and spaced 180º apart, peduncle 8.2–11.5 cm long. Pistillate flowers sessile, densely grouped at the apex of the inflorescence, 1 bract subtending each flower, 1–2 × 0.5–1 mm, oval to triangular, margin irregular to lacinate, base truncate to cordate, apex apiculate, onenerved, yellowish when dry; ovary 3.5–4.0 × 2.6–3.8 mm, glabrous, ellipsoid to oblong, markedly tri-locular, with membranous wing projected from the dorsal edge of each carpel, parallel with the longitudinal axis of the ovary, wings dark brown brightening towards the margins when dry, brownish-green when fresh, torus ca. 2 mm diam., discoid, flattened; perianth rotate, tepals 6, in two whorls of 3 in flower buds, 1.5–2.5 × 1–2 mm, free, inserted in the margin of the torus, recurved during anthesis, undifferentiated, ovate to oblong, base truncate, apex obtuse, glabrous, one-nerved, outer face smooth and inner face papillose, mate green when fresh, yellowish when dry; styles 3, ca. 1.5 mm long, fused at base for ca. 2/3 of their length, above free and spreading, stigmatic surfaces entire, erect; staminodes absent. Mature capsules not seen, immature capsules 2–2.5 × 1.5–2 cm in outline, light green, oblong to slightly obovoid, base rounded and apex concave, the dorsal carpel wings becoming cartilaginous in fruit and expanding to ca. 4 mm long and smaller wings expanding between the valves to ca. 2 mm long, giving the obovoid shape; mature seeds not seen, immature seeds slightly elongated towards the base.

Distribution and Habitat: — Dioscorea magnibracteata is only known from Cerro Azul and adjacent Cerro Blanco ( Fig. 2 ), in conserved seasonally deciduous dry forests, between 280 to 400 m. Those low elevation forests are located at the southernmost tip of the cordillera Chongon-Colonche that reaches the western outskirts of the city of Guayaquil, in the Province of Guayas, Ecuador. This site is part of the Western Ecuador Province (comprising tropical humid forest, seasonally dry tropical forest, xerophytic scrublands and mangroves), having low diversity when compared to the wet forest, but with a great number of local and regional endemics ( Morrone 2014). Dioscorea magnibracteata occurs in the understory of the forest and is endemic to Guayaquil flora. Even though the area is part of the Bosque Protector Cerro Blanco and Prosperina in the adjacent Cerro Azul, both are private reserves that encompass 6.078 and 332,3 hectares, respectively. This is one of the last remnants of this type of vegetation in the Ecuadorian coast, being a fragmented environment still severely threatened mainly by mining and urban development pressure of the city of Guayaquil.

Phenology:—Flowering material has been collected four times, only once at the end of January and the remaining three collections were made in February, all exclusively during the rainy season, which seems to be a flowering pattern for several regional endemics ( Cornejo 2009; Delannay et al. 2019). These records and field observations of the paratype locality, visited several times throughout years by the second author, suggest that Dioscorea magnibracteata sprouts or germinates in early January at the beginning of the rainy season (January to May), with a markedly seasonal phenology, apparently with aerial shoots absent during the dry season (June to December).

Etymology:— The epithet of this new species refers to the large size of the bracts of the male inflorescence, which can reach up to ca. 8 mm long, a colossal size when compared to the rest of the neotropical species of the genus (average 1–3 mm long). The name was written on annotation labels signed by Temple Clayton (1914–1978) at the Naturhistoriska riksmuseet (Swedish Museum of Natural History - S).

Additional specimens (paratypes):— ECUADOR. Guayaquil : Bosque Protector Cerro Blanco , Bosque seco Tropical, 79°58’W 02°10’S, 20 February 1994, X. Cornejo & C . Bonifaz 1771 (♂, GUAY!); Cerro Azul, Bosque Protector Prosperina, 79°58’W 02°09’S, 6 February 2021, X. Cornejo & J . Josse 9363 (♂ ♀, GUAY!) .

Notes:— Temple Clayton came across the specimen Asplund 15394 during the analysis of collections of the S herbarium and suggested it as a new species on his annotation labels. He also indicated on the labels the most important diagnostic characters to identify this species, informing that the male part has bracts of exceptional size and large anthers arranged in a column, that it is clearly a monoecious plant and the female flowers are very congested at the apex of the inflorescence. Temple Clayton’s notes on D. magnibracteata herbarium specimens also state that it is related to Dioscorea chiquiacensis R.Knuth (1917: 196) [= Dioscorea hieronymi Uline ex R.Knuth (1917: 197) ] and Dioscorea fuliginosa R. Knuth (1925: 78) , both species belonging to Dioscorea sect. Cycladenium Uline (1897: 83) . According to Knuth’s 1924 classification, this section comprises a fair amount of species with pedicellate staminate flowers in a raceme, with 3 stamens disposed in a fleshy disc and without staminodes. This broad delimitation clusters dioecious and monoecious species over a very large morphological spectrum, especially in relation to floral characters, and is not supported by phylogenetic studies, as shown by Viruel et al. (2018) and Couto et al. (2018).

In relation to the morphological proximity of D. magnibracteata , we understand that the unique characteristics of this species easily differentiate it from any other species of the genus, although it is probably a member of the NW II Clade (Viruel et al. 2016, 2018; Couto et al. 2018). The closest species to D. magnibracteata regarding morphology would be those belonging to D. section Monadelpha and D. margarethia G.M.Barroso, E.F.Guim. & Sucre (1970: 42) , a group of monoecious species with showy flowers and three stamens in column (six in D. margarethia ). Is also possible to highlight species from Knuth’s Cycladenium and Centrostemon sections (e.g.: D. coriacea Humb. & Bonpl. ex Wild. (1806: 795) , D. piperifolia Humb. & Bonpl. ex Wild. (1806: 794) and allies), which may exhibit some form of monoecy (with plasticity in sex expression) and similar floral morphology (six central stamens and relatively larger size when compared to other species of the genus), respectively in the sections. However, none of the species in these sections have all the main characters observed in D. magnibracteata , and its simple separation from these others, especially for its monoecy, flowers larger than most species of the genus, six stamens in column and for its large bract.

Regarding the placement of D. magnibracteata in any section or infrageneric classification, recent phylogenetic studies suggest that major revisionary work is required in the NW II Clade and it is therefore premature to assign the new species to any existing section. Notwithstanding that most of the monoecious species of Dioscorea sequenced so far have appeared in the clade Monadelpha ( Viruel et al. 2018, Couto et al. 2018), we understand that the positioning of this new species is not phylogenetically verified, particularly because almost all of the monoecious species, except D. margarethia G.M.Barroso et al. , have only three stamens and not six as in D. magnibracteata . Thus, we leave D. magnibracteata without formal infrageneric taxonomic placement, until it can be tested in a phylogeny.

Figures

FIGURE 1. Dioscorea magnibracteata: A. Vine with staminate flowers and leaf blade, abaxial view. B. Tuber, bearing one meristematic point.C. Staminate flower.D. Terminal part of inflorescence bearing two flower buds and one open mature flower (note the relatively large floral bracts and valvate aestivation of corolla, both from abaxial view); the androecium composed of six stamens with orange anthers at upper right. E. Terminal part of pendulous pistillate inflorescence. F. Immature fruits on the axis of the infructescence. A–F, photos by Xavier Cornejo, from the paratype Cornejo & Josse 9363 (GUAY).

FIGURE 2. Geographic distribution of Dioscorea magnibracteata, showing the protected areas in the occurrence region.