Tetramerium pauciflorum T.F. Daniel & V.W. Steinmann, 2016

Daniel, Thomas F. & Steinmann, Victor W., 2016, Tetramerium pauciflorum (Acanthaceae: Justicieae): a New Species from the Basin of the Río Balsas in Michoacán, Mexico, Proceedings of the California Academy of Sciences 63 (14), pp. 567-573 : 568-572

Tetramerium pauciflorum T.F. Daniel & V.W. Steinmann , sp. nov.

Tetramerium pauciflorum is distinguished from its congeners by the combination of its shrubby and somewhat thorny habit; linear to lance-linear leaves; pubescent (eglandular and glandular trichomes), lanceolate to lance-subulate, and entire bracts that are 6–10 mm long; 5-lobed calyx; pseudopapilionaceous and whitish (with purple markings) corollas; and pubescent capsules that are 4.5– 5.5 mm long.

TYPE.— MEXICO. Michoacán: Mpio. La Huacana , ca. 4 km SE of San Pedro Barajas, along MEX 37, hills W of El Limoncito , 18°46ʹ59ʺN, 102°01ʹ17ʺW, 315 m, scrublike tropical deciduous forest, 21-II-2002, V. Steinmann, E. Carranza, & E. Pérez 2310 (holotype: MEXU!; isotypes: CAS!, RSA!) . Figures 1 , 2 .

Spindly and intricately branched shrub to 1 m tall, distal branches spreading at angles of 45° or more, sometimes sharp-pointed at apex. Young stems pubescent with retrorse eglandular trichomes 0.05–0.1 (–0.3) mm long, trichomes ± evenly to 2-fariously disposed. Leaves often absent during anthesis, subsessile to petiolate, petioles to 3 mm long, blades linear to lance-linear, 12–30 mm long, 0.6– 3 mm wide, 10–25 × longer than wide, acute at apex, acute at base, surfaces (especially along midvein abaxially) pubescent with erect to antrorse eglandular trichomes 0.05– 0.2 mm long, margin usually ciliate with similar trichomes. Inflorescence of densely bracteate terminal (mostly at apex of short axillary branches) 1–2-flowered spikes to 13 mm long (excluding flowers), 4–6 mm in diameter near midspike, rachis (if present) not visible. Bracts erect, lanceolate to lance-subulate, 6–10 mm long, 0.7– 1.2 mm wide, 6–8.6 × longer than wide, attenuate and mucronate at apex, abaxial surface with midvein (only) evident and prominent, puberulent with erect eglandular, subglandular, and glandular trichomes to 0.05 mm long and sparsely pubescent with an overstory of flexuose eglandular trichomes 0.1– 0.4 mm long, margin and adaxial surface densely shaggy pubescent with flexuose eglandular trichomes to 1.3 mm long. Bracteoles lance-subulate, 4–5 mm long, 0.4– 0.6 mm wide, abaxial surface and margin pubescent like bracts. Calyx 5-lobed, 3 mm long, lobes 2– 2.5 mm long, abaxially and marginally pubescent like bracts (although glandular trichomes sometimes more conspicuous). Corolla ± pseudopapilionaceous, white with maroon veins outlining light purple regions on upper lip, 12.5–16. 5 mm long, externally glabrous (margins of lobes ciliolate), tube 3–5 mm long, 0.8– 1 mm in diameter at base, 0.8– 1.4 mm in diameter at apex, upper lip spatulate to obovate, 7–11 mm long, 3–4 mm wide, lower lip 7. 5–12 mm long, lateral lobes 7–11. 5 mm long, 2.5– 4 mm wide, lower-central lobe 6.5–10. 5 mm long, 3.5– 4.4 mm wide. Stamens 6–8 mm long, filaments pubescent near base, thecae 1.2– 1.5 mm long; pollen 3-colporate, 6-pseudocolpate, polar diameter ( P) 50–52 µm, equatorial diameter ( E) 26–27 µm, P: E = 1.84–1.98. Style 10–11 mm long. Capsule 4.5– 5.5 mm long, pubescent with erect to flexuose eglandular trichomes 0.05– 0.1 mm long, head 3–4 mm long. Seeds plano-convex, 1.6– 1.7 mm long, 1.3– 1.4 mm wide, surface and margin covered with ± conic papillae.

PHENOLOGY.— Flowering: November, February, May; fruiting: February, May. Plants collected in September bear abundant leaves, but lack flowers and fruits; plants collected in November, February, and May are leafless or nearly so, but fertile. Flowering likely occurs during the entire dry season from November through May.

DISTRIBUTION AND HABITAT.— Mexico (south-central Michoacán). Plants occur on nearly vertical rocky cliffs in tropical deciduous forest (with Hechtia and Agave ) at elevations of 250– 400 m.

CONSERVATION ASSESSMENT.— Tetramerium pauciflorum is endemic to the western portion of the basin of the Río Balsas, where it has been collected from two regions ca. 30 km apart – in the valleys of the Río Marquez in the north and the Río Tepalcatepec in the south. Two collections (Steinmann & Ramírez-Amezcua 5414 and 8089) were made from nearby sites at the northern locality, and the type and remaining paratypes were made at the southern locality. The extent of occurrence (EOO) is ca. three km 2 and the area of occupancy (AOO; with grid cell area of four km 2) is eight km 2. Field observations reveal that the species is rare within the EOO, but it can be locally abundant where it occurs ( Fig. 2E ). Although there is considerable agricultural development in the vicinity of the northern collections, plants occur on (or are possibly restricted to) steep (nearly vertical) and rocky slopes where habitat destruction is less than in surrounding areas. Furthermore, the southern locality is in the Zicuirán-Infiernillo Biosphere Reserve, where active efforts to conserve the vegetation are underway. Because no known threats, declines in population size/habitat quality, or extreme fluctuations can be identified at present, in spite of its limited distribution based on the calculated EOO and AOO, this species is provisionally assessed as Least Concern (LC) based on the IUCN criteria and guidelines ( IUCN 2016).

PARATYPES.— MEXICO. Michoacán: Mpio. La Huacana , ca. 4 km SE of San Pedro Barajas, along MEX 37, hills W of El Limoncito , 18°46ʹ59ʺN, 102°01ʹ17ʺW, V . Steinmann 3907 ( CAS, MEXU), V . Steinmann et al. 2688 ( CAS, MEXU); Mpio. Múgica , 5.5 km NE de la salida a Nueva Italia por la autopista Lázaro Cárdenas – Morelia, cañada que baja al Río El Marqués debajo del puente de la autopista, 19º02ʹ00ʺN, 102º03ʹ20ʺW, V . Steinmann & Y . Ramírez-Amezcua 5414 ( CAS, MEXU); Mpio. Múgica , 5.5 km NE de la salida a Nueva Italia sobre la autopista Morelia-Lázaro Cárdenas, 19º01ʹ57ʺN, 102º03ʹ12ʺW, V . Steinmann & Y . Ramírez-Amezcua 8089 ( CAS, MEXU, RSA).

Flowers of Tetramerium pauciflorum are typical for species in sect. Tetramerium , in which the somewhat keel-like lower lip of the pseudopapilionacous corolla acts both as a landing platform for floral visitors (including pollinators) and as a structural device to aid or ensure self-pollination as the corolla falls from the plant. This pattern was described and illustrated in some detail by Daniel (1986), especially for T. nervosum Nees. Herkogamy in T. pauciflorum can be somewhat more extreme than commonly seen in other species of sect. Tetramerium with the stigma extended beyond the anthers by up to 4 mm during anthesis ( Fig. 2b ). Pollen of T. pauciflorum ( Fig. 3 ) resembles that of other species of the genus ( Daniel 1986, 1998) in both size and sculpturing.

A characteristic of this species that appears to be unique in the genus is the terminal inflorescences that bear only one or two flowers in what appear to be mature spikes. Other species have from 4 (rare) up to 40 or more flowers per spike. Morphologically, T. pauciflorum appears similar to two other congeners that occur in the Balsas basin, T. vargasiae T.F. Daniel & Cruz Durán and T. butterwickianum T.F. Daniel. Whereas the former species is known only from Guerrero (ca. 300

Table 1. Morphological characteristics of three species of Tetramerium . km southeast of the nearest locality of T. pauciflorum ), the distribution of T. butterwickianum overlaps that of T. pauciflorum in the Municipio of La Huacana. Although these two species occur within about 3.5 km of each other near Nueva Italia, they have not been found growing together. These three species, which pertain to section Tetramerium ( Daniel 1986) , all are somewhat thorny shrubs with ± divaricate branches, linear to lanceolate leaves, and white corollas. Table 1 summarizes distinctions among them, and the following key also serves to identify them:

1a. Calyx 4-lobed; leaf blades 4–8 × longer than wide; bracts 5–6. 5 mm long, 2.3–3.3 × longer than wide, with an apical mucro 0.2– 0.5 mm long, prominently 3-veined.... T. butterwickianum

1b. Calyx 5-lobed; leaf blades 10–25 × times longer than wide; bracts 6–19 mm long, 6–10 × longer than wide, with an apical mucro 0.5– 1.2 mm long, only midvein evident or obscurely to prominently 3-veined...................................................... 2

2a. Young stems pubescent with retrorse trichomes; capsule pubescent; inflorescence spikes 1–2- flowered, to 12 mm long; corolla 12.5–16. 5 mm long; bracts 6–10 mm long, 0.7– 1.2 mm wide, with an apical mucro 0.8– 1.2 mm long, and with only 1 vein (midvein) evident and prominent on abaxial surface....................................... T. pauciflorum

2b. Young stems pubescent with antrorse to antrorsely appressed trichomes; capsule glabrous; inflorescence spikes 4- or more-flowered, to 45 mm long; corolla 20 mm long; bracts 16–19 mm long, 1.8– 3.2 mm wide, with an apical mucro 0.5– 0.6 mm long, and obscurely to prominently palmately 3-veined on abaxial surface............................ T. vargasiae

Figures

FIGURE 1. Tetramerium pauciflorum. A. Habit. B. Leaves, proximal leaf on right, distal leaf on left. C. Inflorescence with flower. D. Bract. E. Bracteole. F. Calyx. G. Detail of pubescence of calyx lobe. H. Anther. I. Capsule. J. Detail of pubescence of capsule. A, D–J from Steinmann 2310; B from Steinmann et al. 2688; C from Steinmann 3907. Drawn by Nicole Bollinger.

FIGURE 2. Tetramerium pauciflorum. A. Inflorescence with flower from front showing colored markings on upper lip and spreading lower lip of corolla. B. Inflorescence with flower from side showing separation of anthers and stigma and cottony pubescence of bracts, bracteoles, and calyx. C. Sterile plant with leaves in September. D. Close up of leafy shoots in September. E. Habitat on steep, rocky slopes and numerous leafless plants of T. pauciflorum in May (e.g., see red arrows). A, B, E from Steinmann & Ramírez-Amezcua 5414; C, D from Steinman & Ramírez-Amezcua 8089.

FIGURE 3. Pollen of Tetramerium pauciflorum (Steinmann 3907). A. Apertural view. B. Interapertural view.