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Rev. Mex. Biodiv. vol.79 no.1 México jun. 2008
Taxonomía y Sistemática
xPachebergia (Cactaceae), a nothogenus from western Mexico
xPachebergia (Cactaceae), un notogénero del occidente de México
Salvador Arias1* and Teresa Terrazas2
1 Jardín Botánico, Instituto de Biología, Universidad Nacional Autónoma de México, 04510 México D.F., México
2 Departamento de Botánica, Instituto de Biología, Universidad Nacional Autónoma de México, 04510 México D.F., México
Recibido: 29 junio 2006
Aceptado: 19 junio 2007
xPachebergia (Backebergia x Pachycereus), a new nothogenus from western Mexico, is proposed. Stem and seed morphology of this poorly known taxon from dry to very dry deciduous tropical forest in the Balsas River Basin, Guerrero, are described and illustrated. Its diagnostic morphological characters and relationships with Pachycereus pectenaboriginum and Backebergia militaris, its putative parental taxa, are discussed.
Key words: Backebergia, Balsas River Basin, hybrid, new nothogenus, Pachycereus.
Se propone un notogénero nuevo, xPachebergia (Backebergia x Pachycereus), del occidente de México. Se describen e ilustran los tallos y la morfología de la semilla de este taxón poco conocido de los bosques subhúmedos y deciduos de la cuenca del Balsas, Guerrero. Se discuten sus caracteres morfológicos diagnósticos y la relación con los supuestos taxones parentales, Pachycereus pectenaboriginum y Backebergia militaris.
Palabras clave: Backebergia, cuenca del Balsas, híbrido, notogénero nuevo, Pachycereus.
Reports of spontaneous intergeneric hybrids in cacti are uncommon, but now well documented (Rowley, 1994, 2004), and only 3 nothogenera have been described including columnar cacti of tribe Pachycereeae. The first of those involves a species of Pachycereus (A. Berger) Britton et Rose and Bergerocactus Britton et Rose from Baja California, with intermediate features between the massive treelike Pachycereus pringlei (S. Watson) Britton et Rose and the shrubby Bergerocactus emoryi (Engelm.) Britton et Rose, called xPacherocactus orcuttii (K. Brandegee) Rowley (Moran, 1962a; Rowley, 1994). A second intergeneric hybrid involves Bergerocactus emoryi and Myrtillocactus cochal (Orcutt) Britton et Rose, also from Baja California (Moran, 1962b), called xMyrtgerocactus lindsayi Moran. A third hybrid was reported by Glass and Foster(1964), andnamedxMyrtillenocereus, fromHidalgo state, intermediate between Myrtillocactus geometrizans (Martius) Console and Stenocereus dumortieri (Scheidw.) Buxb.
Two of the wellknown genera of treelike cacti, often becoming massive, in western Mexico are Pachycereus, the cardones, and Backebergia, the tiponches, commonly present in dry to very dry tropical deciduous forest. Pachycereus occurs from northwestern Baja California state towards the western tip of the state of Chiapas, whereas Backebergia is restricted to the central western states of Guerrero, Michoacán, and the southwestern tip of Jalisco (Fig. 1; Cattabriga, 2004). Morphological and molecular data support the recognition of both genera as members of the subtribe Pachycereinae (Arias et al., 2003; Arias and Terrazas, 2006), with Backebergia sister to Pachycereus s.s.
During a field trip to the Balsas River basin in the summer of 2003, we noticed the existence of a single individual of a treelike cactus resembling Backebergia militaris, but with only golden bristles on the tip of various branches (Mauseth et al., 2005). Vegetative characters were undoubtedly intermediate between 2 other species, Pachycereus pectenaboriginum (Engelm.) Britton et Rose and Backebergia militaris (Audot) Bravo ex SánchezMej., species that grow sympatrically on slopes in surrounding deciduous tropical forest. Comparative analysis of vegetative and seed morphology of the hybrid and the putative parents is presented.
Material and methods
We revisited the area a number of times between 2003 and 2005 to study the habitat, to search for additional individuals of the hybrid, to determine what species coexist with the hybrid as possible parents, and to locate other sites with similar associations to find new hybrid plants. Morphological features and distribution of Backebergia and Pachycereus pectenaboriginum were compiled from fieldwork and supplemented with herbarium material from CHAPA, IBUG, MEXU, NY (Arias et al., 2003; Arias and Terrazas, 2006).
A comparative study of seed morphology of the hybrid and its parents was carried out. For observations with SEM, seeds were removed from fruit and subsequently washed. The dried samples were sputter coated with gold and observed in a SEM (JEOLJSM5310LV) microscope. The descriptive seed terminology follows Barthlott and Hunt (2000) and the mean values are given for seed length and width, and hilummicropylar region length. No germination experiments were carried out because most species of Pachycereus do not germinate if they are stored for more than 15 days under laboratory conditions (LozaCornejo, unpublished data), and this was the case with the hybrid since we stored seeds more than 2 months before starting our description.
Results and discussion
According to the morphological characters the plant collected is probably a first generation hybrid, with characters that are intermediate with respect to those exhibited by the parents. In particular, nonreproductive stems (habit, stem color, branch arrangement, and spines) closely resemble those of Pachycereus pectenaboriginum (Fig. 2a, d, g), and reproductive branches seem to be relatively similar to those of Backebergia militaris (Fig. 2c, f, i; Table 1).
Backebergia militaris and Pachycereus pectenaboriginum flowers are nocturnal and pollinated by bats. In the case of B. militaris, the flowers are nearly 4.25 cm long and covered by spines, bristles and trichomes of the fertile zone (cephalium) before anthesis; whereas flowers of P. pectenaboriginum are slightly larger (7.29.5 cm) and uncovered in the fertile zone. Although flowers of the hybrid are unknown, we suppose that they are also nocturnal and bat pollinated.
The fruit size of the hybrid is intermediate between B. militaris and P. pectenaboriginum, but fruit form and wall completely covered with yellow spines and trichomes are similar to those of P. pectenaboriginum (Fig. 3; Table 1).
In addition the fruit developed in areole below the apical region where the areoles were covered by large bristles a pattern different from B. militaris, in which flowers and fruits always develop within the cephalium (VázquezSánchez, 2001). The modifications observed in the apical region are intermediate with the ones occurring in the cephalium of B. militaris, lacking interareolar space and developing periderm.
The comparative seed morphology is also congruent with the hypothesis that P. pectenaboriginum and B. militaris are the putative parents (Table 1, Appendix 1), because the seed is larger than 3 mm, oval to wide oval in the 3 taxa. Seeds of most species included in the genus Pachycereus sensu Barthlott and Hunt (1993) are larger than 3 mm, smaller in few species (Arias and Terrazas, 2004). Testa relief with flat walls is a common character in all species of Pachycereinae, as is the case in the hybrid herein described (Fig. 4), whereas species classified in Stenocereinae show convex relief in different degrees. Microrelief is minutely rugose in P. pectenaboriginum, in B. militaris, and in the hybrid, as in the other species of Pachycereinae, a clear difference with members of Stenocereinae, which show different degrees of striation probably of cuticular origin (Arias and Terrazas, 2004; ArroyoCosultchi et al., 2006; Barthlott and Hunt, 2000). Moreover, mean values for seed length and hilummicropylar region (HMR) length are clearly intermediate and seed width larger in the hybrid than in the putative parents (Table 1, Appendix 1); however, the minimum and maximum seed values for the hybrid showed a narrower interval than in the putative parents (Table 1).
The only other species of cacti which grow sympatrically with the hybrid and the putative parents are Stenocereus fricii SánchezMej. and S. queretaroensis (F.A.C. Weber) Buxb. The possibility that either of these 2 species of Stenocereus could be one of the parents has been rejected, because the hybrid does not display any character that could be considered as intermediate between it and either species. These species of Stenocereus have seeds smaller than 3 mm with convex and striate relief (ArroyoCosultchi et al., 2006), and their stem architecture as well as their fruit form and spination are different (ArreolaNava andTerrazas, 2003).
Based on these evidences we propose the following:
xPachebergia S. Arias et Terrazas nothogen. nov. (Backebergia Bravo x Pachycereus (A. Berger) Britton et Rose)
Genus hybridum inter Pachycereum (A. Berger) Britton et Rose et Backebergiam Bravo intermedium.
xPachebergia apicicostata S. Arias et Terrazas nothosp. nov. (Backebergia militaris (Audot) Bravo ex SánchezMej. x Pachycereus pectenaboriginum (Watson) Britton et Rose).
Planta arborescens, ramis ad 4 m altis, sine peridermide in crescentiae constrictionibus; areolae 68 mm longae, 45 mm diametro, spinis radialibus 79, 11.6 cm longis, spina centrali 1, 12.5 cm longa. Zona fertilis distincta, 9costata, spinis setosis numerosis, 35 cm longis, flavis, apicem ramorum obtegentibus; areolae intervalo separatae.
Type: MEXICO. Guerrero: Mun. Coyuca de Catalán, ca. 30 km southwest of Ciudad Altamirano, in tropical deciduous forest, 300 m, 25May2003, T. Terrazas, S. Arias, M. Vázquez, andJ. Mauseth 673 (holotype: MEXU; isotype: CHAPA). Figures 2b, e, h, and 3b.
Tree with few branches, 5 m tall; trunk well defined, 1 m tall, ca. 30 cm diameter, without spines. Branches of first order 4 m tall, 1620 cm diameter, slightly arched at the base, dark green, the apex green glaucous when growing; constricted without bark; ribs 79, 33.5 cm high, angulate in cross section; areoles elliptic, 68 x 45 mm, distance between them 11.5 cm; radial spines 79, acicular, rigid, gray, divergent, 11.6 cm long; central spine 1, acicular, porrect, round, gray with black tip, 12.5 cm long. Fertile zone well differentiated in the taller branches; branches 1518 cm diameter, dark green; ribs 9, 22.5 cm tall; areoles orbicular, 810 x 79 mm, with abundant yellow trichomes 2 cm long; spines yellow, all bristlelike, abundant, with reddish base, 35 cm long, without color change up to the fertile zone; trichomes and bristles not covering the epidermis; interareolar spaces present. Flower unknown. Fruit globular, 2.83 cm long, 3.23.4 cm diameter; scales dry, black, 4 x 1 mm; areoles with abundant yellow trichomes and numerous yellow bristles 1.43.5 cm long; trichomes and bristles covering the epidermis; pulp red, scarce, dry at fruit maturity; seeds 4.40 x 3.25 mm, ovoid, black, glossy; testa with elongate cells, the periclinal wall flat, without microrelief.
Additional specimens examined. 1.Backebergia militaris (Audot) Bravo. MEXICO. Michoacán: Buenavista Tomatlán, Terrazas 391 (CHAPA); La Huacana, Arias 1332, 1336, 1337, 1338, 1339 (MEXU). 2. Pachycereus pectenaboriginum (Engelm.) Britton et Rose. MÉXICO. Baja California Sur: La Paz, Terrazas y Arias 478 (CHAPA). Jalisco: Autlán de Navarro, Arias y Gama 1402 (MEXU). Michoacán: Nocupétaro, Arias et al. 986 (MEXU); Apatzingán, Terrazas 466 (CHAPA). Oaxaca: Santo Domingo Tehuantepec, Arias 1353 (CHAPA); San Pedro Pochutla, Arias et al. 1351; Santa Jalapa, Arias et al. 1353 (MEXU). Sinaloa: El Fuerte, Arias et al. 1322 (MEXU); Huatabampo, Arias et al. 1321 (MEXU). Sonora: Álamos, Terrazas et al. 419a, 419b (CHAPA); Guaymas, Terrazas et al. 415 (MEXU).
To Fernando Chiang for Latin translation and useful comments, and Michael Nee for his revision and nomenclatural comments. Field work was supported by Consejo Nacional de Ciencia y Tecnología grant 33064V (T.T.). We are grateful to Gabriel ArroyoCosultchi for assistance with seed data, to Berenit Mendoza for SEM, Susana Gama for maps, and Julio César Montero for art work.
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Pachycereus pectenaboriginum (Fig. 4a, b) (Arias 986 MEXU; Terrazas 415, 419a, 419b 478 CHAPA). Seed oval, 3.354.78 x 2.243.44 mm, blackbrown, glossy, the periphery slightly keeled; border slightly expanded around hilum; cells gradually smaller toward hilum, in lateral region isodiametricpolygonal, anticlinal boundaries inconspicuous, straight; interstices uncommon on lateral region, minutely pitted in peripheral regions; relief flat; microrelief minutely rugose; HMR medium size, 1.342.27 mm, oblique, superficial.
xPachebergia apicicostata (Fig. 4c, d) (Terrazas et al. 673 MEXU). Seed oval, 4.304.78 x 2.643.46 mm, blackbrown, glossy, the periphery slightly keeled; border slightly expanded around hilum; cells gradually smaller toward hilum, in lateral region isodiametricpolygonal, anticlinal boundaries inconspicuous, straight; interstices uncommon on lateral region, minutely pitted on peripheral regions; relief flat; microrelief minutely rugose; HMR medium size, 1.421.60 mm, oblique, superficial.
Backebergia militaris (Fig. 4e, f) (Arias 1332, 1336, 1338, 1339 MEXU; Terrazas 391 CHAPA). Seed oval, 3.815.13 x 2.573.80 mm; black, glossy, the periphery keeled in the dorsal and apical region; border slightly expanded around hilum; cells gradually smaller toward hilum, in lateral region isodiametricpolygonal, anticlinal boundaries evident in all regions, straight; interstices uncommon on lateral region, minutely pitted in peripheral regions; relief flat; microrelief minutely rugose; HMR medium size, 0.791.75 mm, oblique, finely impressed.