Recognition of species with morphological attributes
Based on our results, six species are recognized as having the following combinations of characters. 1) P. alensis is characterized by shrubby habit with small areoles (2–3 mm), orange-brown spines, long hairs (7–15 cm), and large seeds (approximately 2.5 mm long). It is distributed in western Mexico, from Sonora to Jalisco. 2) P. chrysacanthus comprises treelike habit with branches measuring 7.5–12 cm in diameter, usually yellow spines, short hairs (2.5–5 cm) and flowers measuring 6 to 10 cm long. It is distributed in southern Mexico in Guerrero, Morelos, Oaxaca, and Puebla. 3) P. collinsii includes shrubby habit with elliptical areoles, a distance between areoles of 1.7–2.2 cm and dark brown spines. It is distributed in southern Mexico in Chiapas, Guerrero, and Oaxaca. 4) P. gaumeri presents treelike habit with branches measuring 4.1–5.2 cm in diameter, light green branches, and noticeably short hairs (1.7–2.3 cm). It is distributed in southeastern Mexico, in Campeche and Yucatán. 5) P. leucocephalus presents shrubby and treelike habit, usually shrubs, with branches measuring 9 to 14 cm in diameter, high ribs (1.8–3.5 cm), and hairs of 4 to 8 cm in length. It is distributed from eastern Mexico (Chiapas, Querétaro, San Luis Potosí, Tamaulipas, and Veracruz) to Central America (El Salvador, Guatemala, Honduras, and Nicaragua). 6) P. purpusii is characterized by shrubby habit with branches measuring 6 to 9 cm in diameter, low ribs (7–16 mm), areole lengths of approximately 3 mm, and yellow spines. It is distributed in western Mexico, from Sinaloa to Guerrero (see the geographical distribution of all taxa in Fig. 8). For further information, see the taxonomic key after the taxonomic treatment.
For species recognition, P. collinsii has been considered synonymous with P. purpusii because they share some characters, including a shrubby habit and similar branch diameters, radial spine lengths and rib heights (Anderson 2001; Yetman 2007; Zappi 1994). Our results confirm that they are different species. Based on the PCA, the characters with the highest weights (loadings) contributing to different groups are those associated with the areole, distance between areoles, and colors of spines (Tables 3, 4), and in the LDA, the 18 sampled P. collinsii individuals were all correctly classified in this species. Similarly, for P. purpusii, the 15 sampled individuals were all correctly classified (Table 4). P. gaumeri had previously been designated as synonym for P. royenii because they share the treelike habit, yellow spines, and flowers of approximately 7 cm long (Anderson 2001; Barthlott et al. 2015; Hunt et al. 2006; Zappi 1994). However, according to Franck et al. (2019), they are different species considering branching (divergent/ascending to strict), stem thickness (slender/thick), and fruit color (purple/red). Our morphological measurements in P. gaumeri which agree with the diagnostic characters presented by Franck et al. (2019), including the hair length of 1.7 to 2.3 cm for P. gaumeri while P. royenii has hairs up to 4 cm long. Therefore, these remarkable morphological differences between P. gaumeri and P. royenii allows us to support them as different species.
On the other hand, previous knowledge of P. cometes and P. leucocephalus shows that the main difference between them is the length of hairs, which is short (2 cm) in P. cometes and long (10 cm) in P. leucocephalus, thus considering them to be independent species (Bravo-Hollis 1978; Britton and Rose 1920; Byles and Rowley 1957; Guzmán et al. 2003). However, in this study, we found hair lengths from 4 to 8 cm for both taxa, and the LDA results show that only 2 of 6 P. cometes individuals sampled were correctly classified in this species, including an P. chrysacanthus individual and three P. leucocephalus individuals. Conversely, 22 of 24 sampled P. leucocephalus individuals were correctly classified in this species, including one P. chrysacanthus individual and one P. cometes individual (Table 4). These results support the previous proposal to consider P. cometes as a synonym of P. leucocephalus (Anderson 2001; Hunt et al. 2006; Zappi 1994). For P. chrysacanthus and P. quadricentralis, the main differences were yellow spines in P. chrysacanthus and orange-brown spines in P. quadricentralis, without recognition of central spines in P. chrysacanthus and up to four spines in P. quadricentralis, and the distances between areoles were approximately 1 cm in P. chrysacanthus and usually 1.5–1.7 cm in P. quadricentralis (Anderson 2001; Bravo-Hollis 1978; Hunt et al. 2006). Nevertheless, based on the results of mixed PCA, no clear distinction was evident between P. chrysacanthus and P. quadricentralis (Fig. 4a), and the LDA showed that 1 of 9 P. quadricentralis individuals was classified as P. chrysacanthus, while all the sampled (18) P. chrysacanthus individuals were correctly classified in this species (Table 4). Therefore, here, these species are considered a unique entity with shades of yellow to orange-brown coloration in spines, 1–4 central spines on the middle part of mature branches, and a distance between areoles of 1.2 to 2.2 cm, in addition to an areole size of 3.5 to 5.5 mm length and 3 to 5 mm width.
This study reveals that the morphological variation among the species of Pilosocereus distributed in Mexico and Central America is very narrow or has overlapping ranges; for example, a diameter of branches between 7.5 and 14.5 cm, a length of longer radial spines between 1 and 2 cm, an areole length from 3.5 to 5.3 mm, and a distance between areoles from 1.3 to 2.3 cm overlap between P. chrysacanthus (= P. quadricentralis) and P. leucocephalus (= P. cometes). Nevertheless, the contributions of five qualitative characters with taxonomic importance allowed us to maximize the differences between groups and showed a clear distinction between P. collinsii and P. purpusii (Fig. 4a). In some groups of Cactaceae, several morphological characters with continuous variation have been recognized between related species, but only some characters offer information to recognize their circumscription at the species level, as in, for example, Cylindropuntia (C. multigeniculata and C. whipplei: Baker 2016), Echinocereus (E. acanthosetus and E. pulchellus: Sánchez et al. 2020), and Escobaria (E. guadalupensis and E. sneedii: Baker and Johnson 2000). Similarly, in the study group, a gradation can be observed for branch diameter and the distance between areoles, but notably, P. leucocephalus group s.s. species have areas of distribution that do not overlap, namely, they are almost completely allopatric (Fig. 8). Regarding reproductive structures, the seed is revealed as potentially informative because the length of the hilum-micropylar region is among the weightiest characters in the PCA for species group formation. Future studies on seed morphology may provide more information on its taxonomic value, as has been recognized for other groups of cacti (e.g., Melocactus: Lemus-Barrios et al. 2021; Stenocereus: Arroyo-Cosultchi et al. 2006).
Monophyly in the P. leucocephalus group s.s.
Sampled members of P. leucocephalus group s.s. were recovered in a monophyletic clade with high to low support (Figs. 5–7), with 54 terminals incorporated into the analysis and six clades at the species level distributed in Mexico and Central America (Fig. 8). According to this result, the monophyly of P. leucocephalus group s.s. reported by Calvente et al. (2016) is corroborated. For this group, two base pairs were recognized as putative synapomorphies, including one in rpl16 (an A in site 729 from the alignment) and one in petL-psbE (a G in site 277 from the alignment), and a combination of morphological characters indicated the taxonomic treatment section.
Based on our results and following the regionalization of the neotropical region (Morrone 2014), the members of P. leucocephalus group s.s. included in this study are distributed in the Mesoamerican dominion and are suggested to belong to a Mesoamerican clade that is sister to the species from the Caribbean and northern South America included in our analyses, including P. brooksianus, P. millspaughii, P. moritzianus, and P. robinii sensu Franck et al. (2019). Previous phylogenetic analyses focused on Pilosocereus mainly from South America (Calvente et al. 2016), reported that P. leucocephalus group s.s. was integrated by P. alensis, P. purpusii, P. leucocephalus, P. collinsii, P. chrysacanthus, P. quadricentralis, and P. gaumeri (as P. royenii: mistaken name; for more details, see Franck et al. (2019)). In subsequent analyses (Lavor et al. 2020) including more species, a clade integrated by P. alensis, P. chrysacanthus, P. collinsii, P. gaumeri, P. lanuginosus, P. leucocephalus, P. polygonus, P. purpusii, and P. quadricentralis was recovered, which was named a non-Brazilian species clade, and the former name—P. leucocephalus group s.s.—was omitted. Thus, although our results are in agreement with those of Calvente et al. (2016), the inclusion of a higher number of Caribbean and northern South American species can help to define whether P. leucocephalus group s.s. recovers as a natural group or is only an artifact of an artificial classification, as observed for the rest of the informal Pilosocereus groups (Calvente et al. 2016). Furthermore, wider sampling of non-Brazilian Pilosocereus species may help to clarify whether the Mesoamerican clade and the species from the Caribbean and northern South America are sister groups or whether the Mesoamerican clade is nested within the clade AII or non-Brazilian clade (Lavor et al. 2018, 2020).
Interestingly, similar results to those obtained for P. leucocephalus group s.s. have been recorded in other groups of organisms with wide geographical distribution in the Americas, recognizing monophyletic groups that are distributed in the Mesoamerican dominion. For instance, among plants, Granados et al. (2017) found that in Poales, the genus Tillandsia L. subg. Tillandsia (which belongs to the clade K in the study with 82 spp.) shows a similar pattern in North and Central America. The same pattern is observed in the genus Zamia L. (Cycadales) (Calonje et al. 2019) with a group of 18 species restricted to the Mesoamerican dominion. While in animals, inside the genus Sturnira Gray (Chiroptera) the S. parvidens lineage from tropical areas of Mexico to northern Costa Rica is closely related to their South American sisters (Hernández-Canchola and León-Paniagua 2017). Similarly, in the genus Alouatta Lacépède (Primates) a Mesoamerican clade is strongly supported, comprising A. paliatta and A. pigra (Doyle et al. 2021).
Internal relationships in P. leucocephalus group s.s.
The monophyly of P. leucocephalus group s.s. is a corroborated hypothesis, but the interrelationships of its species remain quite ambiguous. For example, in the work of Calvente et al. (2016), the authors report P. gaumeri as a sister to the remaining species, whereas in Lavor et al. (2018), P. gaumeri and P. collinsii are sisters of two clades, one supporting the relationships of P. leucocephalus with P. purpusii and P. alensis and other with P. chrysacanthus (= P. quadricentralis). In both works, P. gaumeri was a basal terminal, and a close relationship was identified between P. leucocephalus, P. purpusii, and P. alensis. A more recent analysis showed basal polytomy in P. leucocephalus group s.s. (non-Brazilian species) and maintenance of the aforementioned relationships, although an additional terminal of P. leucocephalus is a sister to P. gaumeri (Lavor et al. 2020). In our study, we were not able to recover the relationships within the group using only molecular markers, but notably, the molecular data are consistent with the morphological recognition of P. alensis, P. gaumeri, and P. purpusii. The problem of the poor resolution of phylogenetic relationships between species is probably associated with the recent divergence of P. leucocephalus group s.s. (a mean divergence of 0.90 million years with an interval of 1.77–0.31; Lavor et al. 2018), incomplete lineage sorting and/or long generational times. Similar situations occur in other plant groups, such as Myosotis (Meudt et al. 2015), Astragalus (Bagheri et al. 2017), and Agave (Jiménez-Barron et al. 2020), which also show recent diversification, which complicates determination of the phylogenetic relationships between their species.
On the other hand, when jointly analyzing morphological and molecular characters, we found clear recognition of six species and their probable internal relationships within P. leucocephalus group s.s. This analysis allows us to infer the lineages P. alensis, P. chrysacanthus (including P. quadricentralis), P. collinsii, P. gaumeri, P. leucocephalus (including P. cometes), and P. purpusii at the species level. Within P. leucocephalus group s.s., the relationships lack support and differ from the results obtained in previous works (Calvente et al. 2016; Lavor et al. 2018, 2020), mainly because we identified P. leucocephalus as the sister species to the remaining species constituting the group, and P. collinsii appears to be the sister species of P. gaumeri.
In conclusion, the group Pilosocereus leucocephalus s.s. distributed in Mexico and Central America is supported as a monophyletic group, in which we recognize six species based on morphological and molecular characters. The most important morphological characters that contribute to the formation of groups and in being able to correctly discriminate between certain species are areole length, branch diameter, distance between areoles, and spines colors, for which its potential use in other Pilosocereus species is suggested. Regarding molecular characters, only P. alensis, P. gaumeri, and P. purpusii were recovered with reciprocal monophyly using chloroplast markers, although by including a nuclear marker only P. purpusii was recovered. Given the uncertainty in the taxonomic circumscription of the closely related species in this group with the previous suggestion of a likely recent divergence, the combination of morphological and molecular characters offers good results in the delimitation of its species and reveals as one same species P. chrysacanthus and P. quadricentralis as well as P. cometes and P. leucocephalus, while P. collinsii and P. purpusii turned out to be distinct species, and P. gaumeri closely related to the Mesoamerican species, differing from the Caribbean. For future research, we suggest that other unexplored characters should be evaluated, such as chromosome numbers or anatomical information, and conduct genomic or microsatellite analyses to broaden our knowledge of this rather complex group.
The P. leucocephalus group s.s. occurring in Mexico and Central America is characterized here with amendment of the proposal made by Hunt et al. (2006) as follows: shrubby cacti, very rarely treelike, between 3.5 and 10 m high; thick branches, not very woody except at the base, often glaucous, usually 7 to 15 ribs; few to many spines, mostly thick, differentiated into central and radial; weakly differentiated floriferous areoles with long dense tufts of hairs; medium to large flowers with straight tubes, medium to large seeds (rarely small), and smooth and shiny cuticles.
Based on the results of this study, we recognize the following six species in the P. leucocephalus group s.s.:
Pilosocereus alensis (F.A.C.Weber) Byles & G.D.Rowley, Cact. Succ. J. Gr. Brit. 19: 66. 1957. ≡ Pilocereus alensis F.A.C.Weber ex Rol.-Goss., Bull. Mus. Hist. Nat. (Paris) 11(6): 508 (−509). 1905. ≡ Cephalocereus alensis (F.A.C.Weber) Britton & Rose, Contr. U.S. Natl. Herb. 12: 415. 1909. ≡ Cereus alensis Vaupel, Monatsschr. Kakteenk. 23: 23, 24, 83. 1913. TYPE: Mexico, Jalisco, Sierra del Alo, near Manzanillo, L.Diguet s.n. (holotype: P?; isotypes: US?, RB 00537920!).
Pilosocereus chrysacanthus (F.A.C.Weber ex K.Schum.) Byles & G.D.Rowley, Cact. Succ. J. Gr. Brit. 19: 66. 1957. ≡ Pilocereus chrysacanthus F.A.C.Weber ex K.Schum., Gesamtbeschr. Kakt. 178. 1897. ≡ Cereus chrysacanthus Orcutt, W. Amer. Sci. 13: 63. 1902. ≡ Cephalocereus chrysacanthus Britton & Rose, Contr. U.S. Natl. Herb. 12: 416. 1909. ≡ Cephalophorus chrysacanthus (F.A.C.Weber) Boom, Succulenta (Netherlands) 46: 107. 1967. TYPE: Mexico, near Tehuacán, Weber s.n. (not preserved). NEOTYPE (designed by Zappi, Succ. Pl. Res. 3: 144. 1994): Mexico, Puebla, Tehuacán, 30 Aug to 08 Sep 1905, J.N.Rose, J.H.Painter & J.S.Rose 9993 (neotype: US 00170926!; isoneotype: NY 02256593!).
= Pilocereus tehuacanus Weing., Z. Sukkulentenk. 3: 58. 1927. ≡ Cephalocereus tehuacanus (Weing.) Borg, Cacti (Borg), ed. 2. 150. 1951. ≡ Pilosocereus tehuacanus (Weing.) Byles & G.D.Rowley, Cact. Succ. J. Gr. Brit. 19: 69. 1957. TYPE: Mexico, Puebla, Tehuacán area, C.A.Purpus s.n., 1907 (not preserved).
= Cephalocereus quadricentralis E.Y.Dawson, Allan Hancock Found. Publ. Occas. Pap. 1: 14, tab. 3, fig. 5. 1948. ≡ Pilosocereus quadricentralis (E.Y.Dawson) Backeb., Cactaceae (Backeberg) 4: 2437. 1960. TYPE: Mexico, Oaxaca, east of Oaxaca-Chiapas, Pan-Pacific Highway, 1,000 m, 25 Jan 1947, E.Y.Dawson 3004 (holotype: AHH 8259). Note: holotype transferred to RSA 0008868!. Synon. nov.
Pilosocereus collinsii (Britton & Rose) Byles & G.D.Rowley, Cact. Succ. J. Gr. Brit. 19: 66. 1957. ≡ Cephalocereus collinsii Britton & Rose, Cactaceae (Britton & Rose) 4: 269, fig. 242. 1923. ≡ Pilocereus collinsii (Britton & Rose) F.M.Knuth, Kaktus-ABC [Backeb. & Knuth] 330. 1936. LECTOTYPE (designed by Zappi, Succ. Pl. Res. 3: 150. 1994): Mexico, Oaxaca, Tehuantepec [O.F.Cook & G.N.Collins s.n., 1902] (lectotype: US 00115537!; isolectotypes: NY 00118700!, 00120552!).
Pilosocereus gaumeri (Britton & Rose) Backeb., Cactaceae (Backeberg) 4: 2462. 1960. ≡ Cephalocereus gaumeri Britton & Rose, Cactaceae (Britton & Rose) 2: 47. 1920. ≡ Cereus gaumeri Standl., Publ. Field Mus. Nat. Hist., Bot. Ser. 3: 366. 1930. ≡ Pilocereus gaumeri (Britton & Rose) F.M.Knuth, Kaktus-ABC [Backeb. & Knuth] 330. 1936. LECTOTYPE (designed by Zappi, Succ. Pl. Res. 3: 151. 1994): Mexico, Yucatán, Progreso, 1918, G.F.Gaumer 23934 (lectotype: US 00115539!; isolectotypes: NY 00120553!, 00120554!, 00120555!).
Pilosocereus leucocephalus (Poselg.) Byles & G.D.Rowley, Cact. Succ. J. Gr. Brit. 19: 67. 1957. ≡ Pilocereus leucocephalus Poselg., Allg. Gartenzeitung (Otto & Dietrich) 21: 126. 1853. ≡ Cephalocereus leucocephalus Britton & Rose, Contr. U.S. Natl. Herb. 12: 417. 1909. TYPE: Mexico, Sonora, near Horcasitas, Poselger (not preserved). NEOTYPE (designed by Zappi, Succ. Pl. Res. 3: 147. 1994): E.Palmer 362 (neotype: US 00115543!; isoneotypes: NY 00120557!, CM 1478!, K 000062714!). Note: this neotype designated by Zappi is the type of Cephalocereus palmeri (see below).
= Cereus cometes Scheidw., Allg. Gartenzeitung (Otto & Dietrich) 8: 339. 1840. ≡ Pilocereus cometes Mittl. ex C.F.Först., Handb. Cacteenk. [Förster] 357. 1846. ≡ Pilocereus jubatus Salm-Dyck, Cact. Hort. Dyck. ed. I. 24; Lem. in Rev. Hortic. 427. 1862. ≡ Cephalocereus cometes Britton & Rose, Contr. U.S. Natl. Herb. 12: 416. 1909. ≡ Pilosocereus cometes (Scheidw.) Byles & G.D.Rowley, Cact. Succ. J. Gr. Brit. 19: 66. 1957. TYPE: Mexico, [San Luis] Potosí, [Galeotti?] (not preserved).
= Cephalocereus maxonii Rose, Contr. U.S. Natl. Herb. 12: 417. 1909. ≡ Cereus maxonii Vaupel, Monatsschr. Kakteenk. 23: 23, 26. 1913. ≡ Pilocereus maxonii A.Berger, Kakteen (Berger) 345. 1929. ≡ Pilosocereus maxonii (Rose) Byles & G.D.Rowley, Cact. Succ. J. Gr. Brit. 19: 67. 1957. TYPE: Guatemala, Jalapa, near El Rancho, 4 Apr 1905, W.R.Maxon & R.H.Hay 3769 (holotype: US 00115542!).
= Cephalocereus palmeri Rose, Contr. U.S. Natl. Herb. 12: 418. 1909. ≡ Cereus victoriensis Vaupel, Monatsschr. Kakteenk. 23: 24, 37. 1913. ≡ Pilocereus palmeri (Rose) F.M.Knuth, Kaktus-ABC [Backeb. & Knuth] 333. 1936. ≡ Pilosocereus palmeri (Rose) Byles & G.D.Rowley, Cact. Succ. J. Gr. Brit. 19: 67. 1957. ≡ Pilosocereus palmeri var. victoriensis (Vaupel) Backeb., Kakteenlexikon 367. 1966. ≡ Cephalophorus palmeri (Rose) Boom, Succulenta (Netherlands) 46: 107. 1967. ≡ Pilosocereus leucocephalus subsp. palmeri (Rose) Scheinvar, Fl. Cactológ. Est. Querétaro 192. 2004. TYPE: Mexico, Tamaulipas, near Victoria, 320 m, 01 May–13 June 1907, E.Palmer 362 (holotype: US 00115543!; isotypes: NY 00120557!, CM 1478!, K 000062714!).
= Cephalocereus sartorianus Rose, Contr. U.S. Natl. Herb. 12: 419. 1909. ≡ Cereus sartorianus (Britton & Rose) Kupper ex A.Berger, Kakteen (Berger) 157. 1929. ≡ Pilocereus sartorianus (Britton & Rose) A.Berger, Kakteen (Berger) 345. 1929. ≡ Pilosocereus sartorianus (Rose) Byles & G.D.Rowley, Cact. Succ. J. Gr. Brit. 19: 69. 1957. ≡ Pilosocereus palmeri var. sartorianus (Rose) Lodé, Fichier Encycl. Cact. Autres Succ. 19: 1776. 1997. TYPE: Mexico, Veracruz, 1908, C.A.Purpus s.n. (holotype: US 00115545!).
Pilosocereus purpusii (Britton & Rose) Byles & G.D.Rowley, Cact. Succ. J. Gr. Brit. 19: 67. 1957. ≡ Cephalocereus purpusii Britton & Rose, Cactaceae (Britton & Rose) 2: 56. 1920. ≡ Pilocereus purpusii (Britton & Rose) F.M.Knuth, Kaktus-ABC [Backeb. & Knuth] 333. 1936. LECTOTYPE (designed by Zappi, Succ. Pl. Res. 3: 150. 1994): Mexico, Sinaloa, Mazatlán, near the town overlooking the sea, 31 Mar 1910, J.N.Rose, P.C.Standley & P.G.Russell 13749 (lectotype: US 00115544!; isolectotype: NY 00120558!).
= Pilocereus guerreronis Backeb., Beitr. Sukkulentenk. Sukkulentenpflege 1: 3. 1941. ≡ Pilosocereus guerreronis (Backeb.) Byles & G.D.Rowley, Cact. Succ. J. Gr. Brit. 19: 67. 1957. ≡ Cephalocereus guerreronis (Backeb.) Buxb., Bot. Stud. 12: 101. 1961. TYPE: Mexico, Guerrero, Cañón del Zopilote, 800 m (not preserved). LECTOTYPE (designed by Zappi, Succ. Pl. Res. 3: 144. 1994): Backeberg, in ibid.: 4, photo. 1941. Synon. nov.
In previous works, P. guerreronis had been assumed to be a local form of the widespread P. alensis (Anderson 2001; Hunt et al. 2006), and in some cases had been included in P. alensis with a question mark (Korotkova et al. 2021; Zappi 1994). Moreover, its morphological traits of branches and fertile region show differences with respect to P. alensis. Herein the name P. guerreronis is recognized as a synonym for P. purpusii as by examining the protologue of P. guerreronis (Backeberg 1941) its description and distribution is more consistent with our recognition of P. purpusii by presenting branches of 7 cm diameter, ribs of 14 mm height, distance between areoles of 15 mm, discontinuous fertile region, and whitish flowers.
Species key to Pilosocereus leucocephalus group s.s.