Euphorbia vizcainensis Maya Lastra & V.W. Steinm., 2015

Maya-Lastra, Carlos A. & Steinmann, Victor W., 2015, Two new species of Euphorbia subg. Chamaesyce (Euphorbiaceae) from Baja California Sur, Mexico and their phylogenetic relationships, Phytotaxa 221 (3), pp. 265-278 : 273-276

Euphorbia vizcainensis Maya Lastra & V.W. Steinm. , sp. nov. ( Figs. 3 and 5 )

Similar to Euphorbia polycarpa but differs in having rugose seeds with transverse ridges. Also it is most similar to Euphorbia pondii from which it is separated by features of the flattened cilia in the stipules, involucral appendages continue to enlarge throughout the maturity of the cyathium, and tubercles in the seeds.

Type:— MEXICO. Baja California Sur: Municipality of Mulegé, 40 kms al oeste de Vizcaíno, carretera Vizcaíno-Bahía Tortugas, al sureste de la laguna Ojo de Liebre, 27°29.754′ N, 113°46.446′ W, 40 m, 14 December 2013, C.A. Maya-L. 437 (holotype IEB!, isotype RSA!).

Annual herb from a well-defined, slender taproot 1–1.5 mm diam.; stems prostrate, 6.7–9.8 cm long, 0.2–0.5 mm diam., terete, glabrous, internodes 0.2–1.4 cm long. Leaves opposite; stipules interpetiolar, separate, subulate, 0.3–0.5 mm long, entire, margin ciliate, hairs elongated and appearing flattened ca. 0.1 mm long; petiole 0.5–1.6 mm long (1/4 of the lamina); lamina orbicular or ovate, rarely obovate, 1.3–3.7 mm long, 0.9–2.5 mm wide, base slightly asymmetric, margin entire, cuneate to obtuse, apex obtuse, glabrous, midrib evident beneath, faintly noticeable above, lateral veins inconspicuous. Cyathia solitary; peduncle 0.3–0.5 mm long; involucre campanulate, 0.5–0.7 mm long, 0.9–1.0 mm wide, glabrous or sparsely pilose; sinuses V-shaped, 0.1–0.3 mm deep; lobes triangular, 0.2–0.3 mm long, ciliate, sometimes glabrate, trichomes ca. 0.1 mm long, straight, margin entire; glands transversally oblong, 0.1–0.2 mm long, 0.3–0.4 mm wide, intensely reddish; appendages flabellate, lunate, divided into 2 or 3 ovate segments or reduced to a narrow rim, 0.1–0.5 mm long, 0.5–0.8 mm wide, white, margin entire or slightly undulate; bracteoles few, glabrous, terete, reddish. Staminate flowers 8–12. Ovary 3-lobed, glabrous; styles 3, reddish, bifid to middle, cylindrical, sometimes slightly dilated at the apex, glabrous, 0.3–0.4 mm long. Capsule with a pedicel 0.5–0.6 mm long; ovoid-globose, 0.9–1.1 mm long, 0.7–0.9 mm wide, green-yellowish when immature, reddish when mature, keels rounded, glabrous, gynophore exserted 0.2–0.5 mm; columella ca. 0.9 mm long. Seeds narrowly ovoid, 0.8–1.1 mm long, faintly transversally quadrangular, 0.3–0.4 mm wide, base truncate, apex acute with transversal constriction, angles pronounced, dorsal facets convex, ventral facets concave, dorsal keel low and inconspicuous, rugose with transverse ridges and low tubercles, white-grayish, ecarunculate.

Distribution and habitat:— Initially found in the Vizcaíno Desert in Baja California Sur ( Mexico), its distribution ranges much further south in the Peninsula at elevations from 20 to 250 m. The southernmost localities are near the coast, and the species has preferences for sand dunes and saline sand. However, it does not tolerate extreme salinity, and near the Ojo de Liebre Lagoon, it is found in the surrounding areas but does not occur on the salty banks.

Etymology:— Because the first collections of Euphorbia vizcainensis were made in the Vizcaíno Desert and the species is the best represented in this area, the specific epithet refers to this floristically important region, located in the central portion of the Baja California Peninsula.

Morphology and phylogenetic relationships:— Euphorbia vizcainensis was first collected by the renowned desert ecologist Forrest Shreve in 1935. Twelve years later Howard S. Gentry made three additional collections. This species shares several morphological characters with Euphorbia polycarpa . In fact, both species are so superficially similar that two of Gentry’s collections consist of both taxa. Wheeler in 1947 recognized that these were mixed collections, determining some of the plants as E. polycarpa and others as potentially undescribed “ n sp.? ” In subsequent years, a number of additional collections of this species have been made with these identified as either E. polycarpa or E. pondii .

Careful examination of these plants shows that Wheeler was correct in distinguishing a new species. There are a number of distinct, albeit subtle characters that differentiate Euphorbia vizcainensis from E. polycarpa . The most significant feature is the seeds, which are rugose with transverse ridges and low tubercles in the former and smooth in the latter. Euphorbia vizcainensis and E. pondii are also very similar, and the two are not always easy to separate. The involucral appendages in E. pondii reach their mature size when the cyathium is very young, whereas those of E. vizcainensis continue to enlarge throughout the maturity of the cyathium. Furthermore, the seeds of E. vizcainensis are rugose with transverse ridges and low tubercles, in E. pondii instead they are rugose only, without other elements in the ornamentation of the seed coat. The stipules in both species are ciliate but in E. vizcainensis the hairs appear flattened, whereas they are usually acicular in E. pondii .

The phylogenetic analyses demonstrate that the four accessions of E. vizcainensis form a well-supported monophyletic group (PP=1). However, its relationship is not well-defined in either analysis, although in the phylogenetic hypothesis based on ITS it forms part of a clade with E. micromera , E. pondii , E. melanadenia , E. setiloba , E. arizonica , and E. polycarpa . All of these species grow on the Baja California Peninsula and are superficially similar, sharing the features of being prostrate herbs with entire leaves and reddish herbage. Its differences from E. polycarpa and E. pondii have already been presented. It can be separated from E. melanadenia , E. setiloba and E. arizonica by its glabrous stems, and from E. micromera by the presence of involucral appendages.

Other collections examined (paratypes):— MEXICO. Baja California Sur: Municipality of Comondú, near La Bocana, west of San Gregorio dune area, 26°03′23″ N, 112°17′21″ W, 30 m, 25 October 2001, J.P. Rebman 7765 (IEB!, SD). Municipality of Mulegé, Road Vizcaíno-Bahía Tortugas, 1 km to the east of cross to Bahía Asunción, south of Ojo de Liebre Lagoon, 27°25.809′ N, 114°02.878′ W, 20 m, 14 December 2013, C.A. Maya-L. 439 (IEB!, RSA!), Sand hills of the Vizcaíno Depression south and west of Scammon´s Lagoon, 16 March 1947, H.S. Gentry 7532 ( RSA!), Las Tinajas and vicinity in cerros east of Los Picachos de Santa Clara, 60–250 m, 21-23 March 1947, H.S. Gentry 7585 ( RSA!), Arroyo de Tecolote near lava flows, 60–150 m, 19–21 November 1947, H.S. Gentry 7856 ( RSA!), dunes near San Xavier, 24 February 1935, F. Shreve 6951 (ARIZ!).

Figures

FIGURE 3. Distribution map of the new species of Euphorbia in Baja California Sur (Mexico). A. Detail of Cerralvo Island.

FIGURE 5. Euphorbia vizcainensis Maya Lastra & V.W. Steinm. A. Capsule; B. seed (1. basal, 2. ventral and 3. dorsal view); C. involucral lobes detail; D. mature cyathium; E. detail of stipule trichomes; F. internode detail; G. immature cyathium; H. prostrate habit; I. detail of a flowering branch. Based on C.A.Maya-L. 437 (IEB).