Cephalocereus parvispinus S. Arias, H. J. Tapia & U. Guzmán, 2019

Arias, Salvador, Tapia, Héctor J. & Guzmán, Ulises, 2019, A new species of Cephalocereus (Cactaceae) from southern Mexico, Phytotaxa 392 (2), pp. 147-156 : 150-154

Cephalocereus parvispinus S. Arias, H. J. Tapia & U. Guzmán sp. nov. ( Figs. 1−3 )

Type: — MEXICO. Oaxaca: District Tlaxiaco, Mpio. San Bartolome Yucuañe, camino a San Juan Teita, 17º 11’ N, 97º 25’W, 1819 m, 10 Apr 2013, Tapia, Arias & Cerén 038 (Holotype MEXU!, Isotype FEZA!).

Diagnosis: — Cephalocereus parvispinus is similar to C. tetetzo in habit, but differs by the branching type (acrotonic vs. mesotonic branching in C. tetetzo ), spines size (up to 0.4 cm vs. up to 1.4 cm), shorter flowers (3.5–4.0 cm vs. 4.7–5.5 cm), inconspicuous podaria on pericarpel and receptacular tube (vs. podaria evident and decurrent), shorter and globose fruits (1.7−2.2 cm vs. 3−4 cm, ovoid). C. parvispinus is a sister species of C. euphorbioides and C. polylophus , and can be easily distinguished by the growth form (branched vs. unbranched plants in C. euphorbioides , and C. polylophus ), flower size (3.5−4 cm vs. 5−9 cm); in addition, seed size is smaller (1.4 × 0.6 mm vs. 2.4 × 1.7 and 2.6 × 1.8 mm), and micro-relief rugose (vs. none).

Description: —Tree-like, up to 8 m tall. Trunk up to 60 cm × 20−35cm, bark brown grey, without spines. Branches numerous (up to 20), 10−20 cm diameter, cylindrical, slightly curved at the base, parallels, light green; young plant unbranched up to 2 m tall, then acrotonic branching occurs; ribs 15−23, 1.4−2.1 cm high, rounded margin; areoles orbicular, 3−5 × 2−4 mm, 0.8−1.3 cm apart, radial spines 5−9, 0.1−0.4 cm long, white yellowish becoming grey when old, central spine ca. 1, up to 0.2 cm long, gray. Fertile zone undifferentiated from the infertile; ribs 18–23(–30), 1.3– 1.5 cm high; areoles 4–5 × 3−4 mm, with yellowish-brown trichomes. Flowers solitary, emerging from apical areoles, 3.5−4.0 cm long, broadly campanulate; pericarpel 0.7−1.0 × 1.0− 1.3 cm, red, with 3−4 series of areoles, podaria inconspicuous, bracts ca. 1 × 1 mm, deltoids, green reddish, without bristly spines or hairs; receptacular tube 1.4−1.8 cm, ca. 1.5 cm wide at the base of the tube, ca. 1.9 cm wide in the throat, red green, podaria almost inconspicuous, bracts 0.1−0.3 × 0.1−0.2 cm, red, without spines or hairs; perianth 1.0− 1.7 cm long, outer tepals 1.0−1.4 × 0.3−0.5 cm, linear lanceolate, reddish to white-reddish with apices acuminate, inner tepals 1.5−1.7 × 0.5−0.6 cm, oblong, white with apices acuminate; nectarial chamber in the lower part of tube, 0.5−0.7 cm; stamens of the innermost whorls 1.0– 1.1 cm long, stamens of the outermost whorls 0.7–0.9 cm long, filaments white, attached in one series, in an area of 0.6−0.8 cm long; style 2.2−2.4 × 0.1−0.2 cm, yellow-white, the stigma lobes 0.4–0.6 cm, long, light yellow. Fruits 1.7−2.2 × 1.5−1.8 cm, globose, darkred, dark brown when dried, inconspicuous podaria and bracts, naked, perianth parts persistent; dehiscing longitudinally, pulp white; seeds ca. 1.4 × 0.6 mm, ovate, dark brown, matte, interstices undifferentiated, relief low-domed on lateral region and micro-relief rugose.

Etymology: —The specific epithet refers to the extremely short size of the spines.

Phenology: —Flowering time April–May, fruiting time April–May(–early June).

Habitat: — Cephalocereus parvispinus occurs in transitional areas of tropical deciduous forests and Pinus leiophylla (Schiede ex Schlechtendal & Chamisso 1831: 354) forests. It grows in cliffs, on gypsum substrate, from 1400−1900 m a.s.l.

Distribution: — Cephalocereus parvispinus occurs in the Mixteca, western cultural region in Oaxaca, Mexico.

Preliminary conservation status: —According to the IUCN Red List categories and criteria (2017), and the criterion B2a, Cephalocereus parvispinus is here assessed as endangered (EN). The species occurs in one known population, occupying an area smaller than 500 km 2.

Taxonomic notes: — Cephalocereus parvispinus shares the habit with C. tetetzo . Both species are arborescent, with a trunk up to 60 cm high and with almost parallel branches. The branching type is different in both species, because in C. parvispinus it is acrotonic ( Fig. 1B ), that is the new branches appear from or near the apex of existing one, while in C. tetetzo it is mesotonic ( Bravo-Hollis 1978: 642, Anderson 2001: 17, Arias et al. 2012: 137).

The reproductive structures also have differences. The size and shape of the flower, the absence of podaria and spines on pericarpel and receptacular tube, the size and shape of the fruit are dissimilar ( Arias et al. 2012; Fig. 2 , Table 2). The seed is also different in size, as well as in the shape and micro-relief of the testa cells ( Arroyo-Cosultchi et al. 2007; Fig 3 , Table 2).

Moreover, the phylogenetic analysis reveals no closed relationship between these two species. While C. tetetzo is sister to C. columna-trajani (Karwinski ex Pfeiffer 1837: 76) Schuman (1897: 198) and C. senilis ( Haworth 1824: 41) Pfeiffer (1838: 142) , C. parvispinus is the sister species of C. euphorbioides ( Haworth 1819: 75) Britton & Rose (1920: 33) and C. polylophus ( Candolle 1828: 115) Britton & Rose (1909: 419) , with strong support values ( Fig. 4 ).

Cephalocereus parvispinus shares with C. polylophus the undifferentiated form between the fertile and infertile branches, as well as almost the same number of radial and central spines. They are different because of their branching pattern (unbranching in C. polylophus ), the shape and size of the flower and fruit ( Fig 3 , Table 2), as well as the size and shape of the seed ( Arroyo-Cosultchi et al. 2007; Fig 3 , Table 2). The differences with C. euphorbioides are stronger, including the growth form, number and size of ribs, size and shape of the flower, fruit and seed ( Table 2).

Most Cephalocereus species grow on limestone rocks with thin and poor soils (e.g., C. mezcalaensis , and C. macrocephala ), limestone outcrops with quartz ( C. apicicephalium ), on metalimestone ( C. nizandensis ), as well as in acid soils from andesite, siltstones or in mica schist ( C. totolapensis ) ( Bárcenas-Argüello et al. 2010). This is the first report of a Cephalocereus species that grows exclusively on gypsum substrate, association that could be responsible for the restricted distributional pattern observed. This species could be an example of an edaphic endemism as occur in other desert species highly adapted to gypsum soils ( Meyer 1986).

Other specimens examined:— MÉXICO. Oaxaca: Tlaxiaco, San Bartolomé Yucuañe , 29 Oct 2011, Arias et al. 2146 ( MEXU) ; Tlaxiaco , Mpio. San Juan Teita, 04 June 2017, Sandoval et al. 1424 ( MEXU)

Figures

FIGURE 1. Main characteristics of Cephalocereus parvispinus.A. Plants in its habitat. B. Arborescent plant. C. Juvenile plants. D. Apex of infertile stem.

FIGURE 2.Reproductive structures of Cephalocereus parvispinus.A. Flower on fertile zone. B. Apex of fertile stem. C. Flower in longitudinal section, showing inner and outer sections. D. Stems and fruits. E. Fruits.

FIGURE 3. SEM micrographs of Cephalocereus parvispinus seeds. A. Lateral view. B. Hilum-micropylar region. C. Close-up of lateral view.

FIGURE 4. Phylogenetic position of Cephalocereus parvispinus based on seven chloroplast markers. Bayesian inference, posterior probabilities are indicated at the nodes.