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Mentzelia L. is a New World genus centered in western North America. The genus is classified into six monophyletic sections (Hufford et al. 2003), of which section Bartonia (Torrey & A. Gray) Bentham & Hooker f. is the most species-rich, with approximately 51 species. Taxonomy of section Bartonia has been notably problematic (Thompson & Powell 1981, Holmgren & Holmgren 2002, Schenk & Hufford 2011a). Recent floristic work has clarified the species concepts within the United States as part of the Flora of North America (Hufford et al. 2016), the Californian flora in the Jepson Manual (Brokaw et al. 2012), and the North American intermountain region (Holmgren et al. 2005), but the species in Mexico have not been critically examined and substantial taxonomic confusion remains.
The most comprehensive study on Mexican Mentzelia was conducted by Thompson & Powell (1981), which focused exclusivly on species of the Chihuahuan Desert. In section Bartonia, the authors recognized three species: M. multiflora (Nutt.) A.Gray, M. saxicola H.J.Thomps. & Zavort., and M. mexicana H.J.Thomps. & Zavort. Recent fieldwork, collections, and taxonomic reevaluations, however, have revealed species in Mexico that were not previously recognized to occur there, and that M. multiflora is a misapplied name in Mexico. Schenk & Hufford (2011b), for example, recognized M. multiflora as occurring only along the eastern edge of the Rocky Mountain Front Range in the United States, which begs the question: If collections previously treated as M. multiflora are misapplied, what species are they?
Seed coat microsculpture patterns provide reliable characters that can be essential for the identification of species in section Bartonia (Schenk et al. 2016). Thompson & Powell (1981) differentiated among species based on the curvature of the testal anticlinal wall, which Schenk et al. (2016) characterized as straight, wavy, or sinuate, as well as the number of small spherical bumps on the periclinal wall of the testa called papillae. Although Schenk et al. (2016) determined that variation in seed coat microsculpturing patterns are correlated with seed size, microsculpture patterns have provided reliable characters to differentiate among species (Schenk & Hufford 2011a) and might be useful for determining what species of Mentzelia occur in Mexico.
Recent floristic work demonstrates why detailed taxonomic studies in Mentzelia are needed. Villaseñor (2016), for example, recognized M. hintoniorum B.L. Turner & A.L. Hempel, M. multiflora, M. oreophila J.Darl., M. pumila Nutt., and M. sinuata (Rydb.) R.J.Hill in Mexico, all species that are either synonyms of species that do occur in Mexico or are not recognized as occurring in Mexico by Schenk & Hufford (2020). In addition to the four previously mentioned species, Villaseñor (2016) recognized M. mexicana H.J.Thomps. & Zavort., M. reverchonii (Urb. & Gilg) H.J.Thomps. & Zavort., and M. saxicola H.J.Thomps. & Zavort. Although there is good support for the latter three species as occurring in Mexico, how species are distributed across their respective range is poorly characterized and they are often confused in herbarium collections. Issues surrounding uncertain determinations and misapplied names are not unique to Villaseñor (2016). It is because of the historic taxonomic issues in section Bartonia that similar species recognition are echoed in other treatments (e.g., Sosa et al. 2018) and especially on labels of collections in herbaria. In order to clarify the taxonomic uncertainties, a treatment of the Mexican species and their ranges is needed in section Bartonia.
In a companion article to this one, Schenk et al. (2021), surveyed the species of Mentzelia in Mexico. The authors recognized 25 species, of which six are placed in section Bartonia. In this paper, a formal taxonomic treatment of section Bartonia is presented that includes distribution maps of all taxa and an evaluation of seed coat microsculpture patterns. Through the present work, we hope to clarify persistent taxonomic issues of what species of section Bartonia occur in Mexico, what their respective ranges are, and how they are differentiated.
Materials and methods
We evaluated 313 herbarium specimens collected from Mexico for morphological variation and distribution data. Twelve collections were made from natural populations in Mexico and studied alongside herbarium specimens previously deposited at the following herbaria (herbarium abbreviations follows Thiers 2020): ARIZ, F, GH, ID, LA, MEXU, MO, N, NMC, NY, OKL, P, RSA, TEX, UCR, US, and WS. Taxonomic treatments followed Hufford et al. (2016), Schenk & Hufford (2011a), and Schenk & Hufford (2020).
Species distribution data were collated from field and herbarium collections. Latitude and longitude were taken directly from herbarium specimen labels when present, or were georeferenced with GEOLocate (Rios & Bart 2010) or GoogleEarth (Google LLC, Mountain View CA). Maps were generated in ArcMap v.10.7 (ESRI, Redland, California, USA).
We evaluated the seed coat microsculpture patterns of species, with a particular focus on the Chihuahuan Desert species that have historically been difficult to understand. Seeds were mounted onto a carbon tab and sputter coated with gold for 1 minute at 30-40 milliamps, then imaged with a JEOL JSM6610LV scanning electron microscope (JEOL Ltd., Tokyo, Japan) with 15 kV at 25X, 150X, and 200X.
Results
Six species of Mentzelia section Bartonia occur in Mexico: M. longiloba J.Darl., M. mexicana, M. procera (Wooton & Standl.) J.J.Schenk & L.Hufford, M. puberula J.Darl., M. reverchonii, and M. saxicola. Three of the four varieties of M. longiloba occur in Mexico, M. longiloba var. chihuahuaensis J.J.Schenk & Hufford, M. longiloba var. longiloba, and M. longiloba var. pinacatensis J.J.Schenk & Hufford.
Even though the morphological characteristics of the seed coat can be analyzed by means of a common stereoscopic microscope, in order to better observe its micromorphology, sixty-two seeds were visualized with scanning electron microscopy to examine their gross morphology and microsculpturing patterns (Figures 1, 2). Although seed coat traits are easiest to view at high magnification with a SEM, microsculpture traits can be observed with a 10x hand lens or stereoscopic microscope. Seed coat microsculpture patterns in section Bartonia revealed patterns consistent with previous studies. Straight to slightly wavy anticlinal cell walls were identified in the testa of M. longiloba var. chihuahuaensis, M. mexicana, M. saxicola, M. reverchonii, and M. puberula. The testal anticlinal cell walls were sinuate in M. longiloba var. longiloba, M. longiloba var. pinacatensis, and M. procera. Variation in the periclinal wall included size and shape differences of papillae, as well as differences in the number of papillae per cell. Substantial variation was observed among the varieties of M. longiloba. In Mexico, M. longiloba var. longiloba contained 34-77 papillae per cell, M. longiloba var. chihuahuaensis contained 3-7 papillae per cell, and M. longiloba var. pinacatensis contained 26-51 papillae per cell. The distantly related M. puberula contained 3-5 papillae per cell. The Chihuahuan Desert taxa consisted of M. mexicana with 3-14 papillae per cell, M. saxicola contained 3-11 papillae per cell, M. reverchonii contained 4-8 papillae per cell, and M. procera contained 32-47 papillae per cell.
Figure 1 Seeds of Mentzelia species that occur in Mexico. A. M. asperula from Mentzelia section Mentzelia (Reina 2017-566, ARIZ). B. M. hirsutissima from Mentzelia section Bicuspidaria (Hufford 2680, WS). C. M. involucrata from Mentzelia section Bicuspidaria (Hufford 4065, WS). D -K: seeds from Mentzelia section Bartonia. D. M. puberula (Schenk 1224, WS), E. M. longiloba var. chihuahuaensis (Granados Mendoza & Schenk 812, MEXU), F. M. longiloba var. longiloba (Schenk 1225, WS), G. M. longiloba var. pinacatensis (Sherbrooke s.n. April 9, 1983, ARIZ), H. M. reverchonii (Hufford 4310, WS), I. M. mexicana (Granados Mendoza & Schenk 802, MEXU), J. M. saxicola (Granados Mendoza & Schenk 813, MEXU), and K. M. procera (Schenk 1032, WS).
Figure 2 Seed coat surfaces of Mentzelia section Bartonia in Mexico. A. M. longiloba var. chihuahuaensis (Spellenberg 12305, ARIZ), B. M. longiloba var. longiloba (White s.n., ARIZ), C. M. longiloba var. pinacatensis (9 Apr 1983, Sherbrooke s.n., ARIZ), D. M. reverchonii (Hufford 4310, WS), E. M. mexicana (Granados Mendoza & Schenk 802, MEXU), F. M. saxicola (Granados Mendoza & Schenk 813, MEXU), G. M. puberula (Schenk 1224, WS), and H. M. procera (Schenk 1032, WS).
Taxa occurred in either the Sonoran or Chihuahuan deserts (Figures 3, 4). The Sonoran Desert taxa included M. longiloba var. longiloba, M. longiloba var. pinacatensis, and M. puberula (Figure 3). The Mexican endemic M. longiloba var. pinacatensis had the narrowest distribution in Mexico, likely due to specializing on the volcanic soils of the Pinacate Region. The distribution of M. longiloba var. longiloba approaches M. longiloba var. pinacatensis, but it has not been collected directly from the volcanic field. The range of M. longiloba var. longiloba is one of the most widespread when compared to other taxa of section Bartonia in Mexico (Figure 3), with a range that includes most of the Sonoran Desert and extends northward into the United States. The distribution of M. puberula is mostly associated with the Colorado River throughout its range in Mexico and the United States, with some notable extensions away from the river (Figure 3).
Figure 3 Distribution of Mentzelia section Bartonia in the Sonoran Desert. Mentzelia longiloba is abbreviated as M. l.
Section Bartonia is more diverse in the Chihuahuan Desert, where it is represented by M. longiloba var. chihuahuaensis, M. mexicana, M. procera, M. reverchonii, and M. saxicola. All five taxa are distantly related to one another based on previous phylogenetic analyses. The distributions of M. procera and M. longiloba var. chihuahuaensis are relegated to the northern portions of the Chihuahuan Desert (Figure 4). Mentzelia reverchonii is more widespread throughout the Chihuahuan Desert and northward in the United States, but its populations are sporadically distributed. Mentzelia mexicana and M. saxicola have overlapping ranges throughout northern Mexico in the Chihuahuan Desert.
Discussion
When examining herbarium collections and floristic works, numerous names of section Bartonia are misapplied in Mexico. Among the more commonly misapplied names, M. albescens (Gillies ex Arn.) Griseb., M. multiflora, M. oreophila, M. pumila, and M. sinuata are currently not known to occur in Mexico. Future collecting efforts have the potential to discover M. albescens and M. oreophila in Mexico, given that both species ranges approach the northern Mexican border from the United States. The ranges of the remaining species, all of which are endemic to the Rocky Mountains and its Front Range, are far enough north to have a low probability of also occurring in Mexico.
Below, a taxonomic treatment for the species of section Bartonia modified from Schenk & Hufford (2020) is presented. Selected Mexican specimens that were evaluated as part of this project are indicated in supplementary data. A key to identify species in section Bartonia, as well as other Mentzelia species in Mexico, is provided in Schenk et al. (2021).
Taxonomic treatment of Mentzelia section Bartonia in Mexico.
Mentzelia longiloba J.Darl. var. chihuahuaensis J.J. Schenk & L.Hufford. Madroño 57: 256. 2011. Type: United States, Texas, Brewster Co., Rte 118, S of 898, on W side of rd with E exposure, 30.098 °N, 103.596366 °W, 1385 m elevation, 7 Aug 2004, J. Schenk 901 (holotype: WS!; isotypes: ARIZ!, NY!, TEX!, UNM!, US!).
Biennial herbs, 3.2-5.5 dm tall. Stems primary aerial axis arising at or above the soil surface, erect, straight; axillary branches on distal half of primary axis or along the entire primary axis. Leaves 35-110 × 7.4-27.1 mm, intersinus distance at widest point 2.4-15.3 mm; leaves on proximal third of primary axis oblanceolate to elliptic, dentate with 12-20 teeth 8.4-12.5 mm apart, teeth opposite, perpendicular to leaf axis, regular, 2.3-9 × 4-5 mm with rounded to acute apices; leaves on distal third of primary axis lanceolate, serrate to pinnatilobate with 8-14 teeth or lobes 6.1-9.4(14.1) mm apart, lobes opposite, slightly angled towards leaf apex, regular, 2.9-7.5 × 3.7-4.8 mm with rounded to acute apices. Petals golden yellow, narrowly spatulate, 11.3-16.3 × 3.1-5.1 mm. Androecium golden yellow; anthers remaining straight after dehiscence; outer whorls of stamens staminodial and fertile, five outermost stamens in median antesepalous positions petaloid, narrowly spatulate, 11.2-15.4 × 2.4-4 mm, without anthers, second whorl of stamens all fertile. Capsule cup-shaped to cylindrical, 9.1-15(20.5) × 6.1-9.1 mm, base round to tapering; hypanthium 0.9-2 mm. Seeds 2.9-3.2 mm long; seed coat anticlinal cell walls straight (occasionally slightly wavy), central papillae generally 3-7 per cell.
Distribution. Throughout the entire Chihuahuan Desert, occurring at 548-2,378 m in Mexico (Figure 4).
Habitat. Sandy and/or rocky soils, along roadsides, flats, hills, dunes, or in arroyos, but also occasionally associated with clay or volcanic soils.
Common names. Chihuahuan blazingstar, pega ropa.
Mentzelia longiloba var. chihuahuaensis has been recognized as M. multiflora, a name misapplied to populations across North America and now more narrowly recognized as being localized to the eastern edge of the Rocky Mountain Front Range (Schenk & Hufford 2011a). Phylogenetic analyses recovered M. longiloba var. chihuahuaensis in a polytomy with other M. longiloba varieties, plus M. nuda (Pursh) Torr. & A.Gray, M. sinuata, M. speciosa Osterh., M. strictissima (Wooton & Standl.) J.Darl., and M. reverchonii (Schenk & Hufford 2011b), the latter of which also occurs in the Chihuahuan Desert. We follow Schenk & Hufford (2011a) in continuing to recognize the taxon as a variety of M. longiloba.
Mentzelia longiloba var. chihuahuaensis is unique among the varieties of M. longiloba in seed testal cells. Unlike the other varieties that contain sinuate anticlinal cell walls with over 26 papillae per cell, M. longiloba var. chihuahuaensis has straight walls with 3-7 papillae per cell (Figure 2). Schenk & Hufford (2020) reported 4-6 papillae per cell, but our estimates widen the range slightly more.
Mentzelia longiloba J.Darl. var. longiloba. Ann. Missouri Bot. Gard. 21: 176. 1934; Mentzelia multiflora (Nutt.) A.Gray subsp. longiloba (J.Darl.) Felger, Desert Plants 2: 108. 1980. Mentzelia multiflora (Nutt.) A.Gray var. longiloba (J.Darl.) Kartesz, Synth. N. Amer. Fl. version 1, 1999. Type: United States, California, hayfields, 15 Sep 1931, M.E. Jones s.n. (holotype: MO!; isotype: UC!).
Biennial herbs, 2.6-6.3 dm tall. Stems primary aerial axis arising at or above the soil surface, erect, straight; axillary branches on distal half of primary axis or along the entire primary axis. Leaves 38-112 × 10-24.6 mm, intersinus distance at widest point 3.6-13.7 mm; leaves on proximal third of primary axis oblanceolate or elliptic, pinnatilobate with 12-30 lobes 4.2-15.8 mm apart, lobes opposite, perpendicular to leaf axis, irregular or regular, 1.4-6.6 × 1.9-6.1 mm with rounded to acute apices; leaves on distal third of primary axis lanceolate, pinnatilobate with 10-28 lobes 5-15.8(20.8) mm apart, lobes opposite, perpendicular to leaf axis, irregular or regular, 2.3-8.6 × 1.8-5.8 mm with rounded to acute apices. Petals golden yellow, narrowly spatulate, 12.2-20.4 × 3.7-7.2 mm. Androecium golden yellow; anthers remaining straight after dehiscence; outer whorls of stamens staminodial and fertile, five outermost stamens in median antesepalous positions petaloid, narrowly spatulate, (6.7)10.6-15.5 × 1.9-5.2 mm, without (occasionally with) anthers, second whorl of stamens all fertile. Capsule cup-shaped to cylindrical, 9.6-16.4 × 6-9.2 mm, base rounded; hypanthium 0.7-1.7 mm. Seeds 3.3-4 mm long; seed coat anticlinal cell walls sinuate, central papillae generally 67-106 per cell.
Distribution. Occurs throughout the Sonoran Desert in Mexico and the Colorado Plateau in the United States. In Mexico, populations occur in Baja California and Sonora at 15-1,470 m in elevation (Figure 3).
Habitat. Sandy compacted soils or loose sand dunes.
Common names. Dune blazingstar, hierba del piquete, hierba pegajosa.
Mentzelia longiloba var. longiloba has been recognized as M. multiflora ssp. longiloba (Felger 2000). Schenk & Hufford (2011b) inferred M. longiloba to be distantly related to M. multiflora, supporting its recognition at the specific level and they further recognized four varieties that exhibit notable geographic, phylogenetic, and morphologic variation.
Seed coat microsculpture patterns were consistent with Schenk & Hufford (2020) in having sinuate anticlinal cell walls. Along the periclinal wall, however, we identified additional variation. Schenk & Hufford (2020) reported 34-106 papillae per cell, while we identified 34-77 papillae per cell (Figure 2). The discrepancy in papillae number suggests variation across populations and the range of M. longiloba var. longiloba.
Mentzelia longiloba J.Darl. var. pinacatensis J.J.Schenk & L.Hufford. Madroño 57: 257. 2011. Type: Mexico, Sonora, Pinacate Region, 1.1 km N of Pinacate Peak, 31.7847222 °N; 113.4902778 °W, 950 m elevation, 13 Oct 1986, R.S. Felger & G.E. Joseph 86-432 (holotype: ARIZ!; isotypes: ARIZ!, RSA!).
Biennial herbs, 1.5-5 dm tall. Stems primary aerial axis arising at or above the soil surface, erect, straight; axillary branches along the entire primary axis. Leaves 24-78 × 5.5-13.8 mm, intersinus distance at widest point 1.9-7.3 mm; leaves on proximal third of primary axis elliptic, dentate to pinnatilobate with 12-50 teeth or lobes, 2.9-18.2 mm apart, teeth or lobes opposite, perpendicular to leaf axis, irregular or regular, 2.8-6.9 × 1.5-3.1 mm with acute apices; leaves on distal third of primary axis elliptic to lanceolate, dentate to pinnatilobate with (6)8-28 teeth or lobes 2.1-5.5 mm apart, teeth or lobes opposite, perpendicular to leaf axis, irregular or regular, 1.7-8.9 × 1.3-3.1 mm with acute apices. Petals golden yellow, narrowly spatulate, 11.9-14.1 × 4.7-6.7 mm. Androecium golden yellow; anthers remaining straight after dehiscence; outer whorls of stamens staminodial and fertile, five outermost stamens in median antesepalous positions petaloid, narrowly spatulate, 9.7-13.6 × 3.3-5.9 mm, without anthers, second whorl of stamens all fertile. Capsule cup-shaped, 7.6-13.2 × 5.8-8.5 mm, base rounded; hypanthium 0.5-1.1 mm. Seeds 2.9-3.4 mm long; seed coat anticlinal cell walls sinuate, central papillae generally 26-51 per cell.
Distribution. Endemic to the volcanic Pinacate Region of northwestern Sonora at 130-950 m in elevation (Figure 3).
Habitat. Only on volcanic cinder cone soils.
Common name. Pinacate blazingstar.
Among the varieties of M. longiloba, M. longiloba var. pinacatensis has the most limited range. Populations are localized in the Sierra Pinacate, a volcanic field in the northern Sonoran Desert, which is also home to other endemic plants and animals (Felger 2000).
Phylogenetic analyses recovered M. longiloba var. pinacatensis as sister to M. longiloba var. yavapaiensis (Schenk & Hufford 2011b), a taxon that is currently considered endemic to Arizona (Schenk & Hufford 2020). In Mexico, M. longiloba var. pinacatensis is most likely confused with M. longiloba var. longiloba, but the two taxa can be differentiated by the larger leaves and fruits of M. longiloba var. longiloba (Figure 5).
Figure 5 Representative leaf outlines of Mexican taxa of Mentzelia section Bartonia. Mentzelia longiloba var. chihuahuaensis is based on the isotype and Granados Mendoza & Schenk 812 (MEXU), M. longiloba var. longiloba is based on Gentry & Engard 23284 (MEXU), M. longiloba var. pinacatensis is based on Felger 86-432 (ARIZ, RSA), M. mexicana is based on Henrickson 24086 (MEXU), M. procera is based on Wooton s.n. (P), M. puberula is based on Schenk 1224 (WS), M. reverchonii is based on Granados Mendoza & Schenk 806 (MEXU), and M. saxicola is based on Granados Mendoza & Schenk 813 (MEXU).
Mentzelia mexicana H.J.Thomps. & Zavort. Wrightia 4: 21. 1968. Type: Mexico, Coahuila, La Rosa, 7 Sep 1940, F. Shreve & E. Tinkham 9908 (holotype: GH!; isotypes: ARIZ!, MICH).
Biennial herbs, 1-4 dm tall. Stems primary aerial axis arising at or above the soil surface, erect, straight; axillary branches on distal half of primary axis or along the entire primary axis. Leaves 24-82 × 11.7-29.1 mm, intersinus distance at widest point 3.4-18.8 mm; leaves on proximal third of primary axis oblanceolate or elliptic, dentate, serrate, or pinnatilobate with 6-16 teeth or lobes 5.7-13.3 mm apart, teeth or lobes opposite or nearly opposite, slightly angled towards leaf apex or perpendicular to leaf axis, regular, 1.4-5.1 × 2.4-6.4 mm with rounded to acute apices; leaves on distal third of primary axis elliptic to lanceolate with usually non-clasping bases, margins dentate, serrate, or pinnatilobate with 6-12 teeth or lobes 5.8-13.1 mm apart, teeth or lobes opposite or nearly opposite, slightly angled towards leaf apex or perpendicular to leaf axis, regular, 2.5-6.7 × 1.8-4.9 mm with rounded to acute apices. Petals golden yellow, narrowly spatulate, 10.2-15.3(17) × 2.5-6.2 mm. Androecium golden yellow; anthers remaining straight after dehiscence; outer whorls of stamens staminodial and fertile, five outermost stamens in median antesepalous positions petaloid, narrowly spatulate, 9.9-14.4(16.7) × 2-4.8 mm, without anthers, second whorl of stamens all fertile. Capsule cup-shaped, 6.5-12.8 × 5.6-8.4 mm, base rounded; hypanthium 0.4-1.2 mm. Seeds 2.2-2.9 mm long; seed coat anticlinal cell walls straight, central papillae generally 3-14 per cell. See Figure 6.
Figure 6 Inflorescences of Mentzelia section Bartonia from Mexico. A. Mentzelia mexicana from Coahuila (Henrickson 24086, MEXU), B. M. saxicola from Zacatecas (Granados Mendoza & Schenk 813, MEXU), and C. M. reverchonii from Coahuila (Granados Mendoza & Schenk 807, MEXU).
Distribution. Eastern Chihuahua, Coahuila, and Nuevo León at 740-1,525 m in Mexico (Figure 4).
Habitat. Clay soils largely composed of gypsum, often growing on steep hillside slopes, roadcuts, or arroyos.
Common name. Mexican blazingstar.
Mentzelia mexicana is a Chihuahuan Desert endemic that occurs along the northeastern Mexican states and extends northward into southwestern Texas in the United States. The distribution of M. mexicana overlaps substantially with M. saxicola (Figure 4). Thompson & Powell (1981) emphasized elevational differences among M. mexicana (their report: 600-1,300 m), M. saxicola (their report: 800-1,900 m), and M. longiloba var. chihuahuaensis (treated by the authors as M. multiflora, their report: above 1,400 m). Although the authors' general sentiment of M. mexicana and M. saxicola occurring at lower elevations is supported here, additional collections since their work have revealed much greater overlap in elevations. In Mexico, M. mexicana occurs at 740-1,525 m, M. saxicola occurs at 850-1,843, and M. longiloba var. chihuahuaensis occurs at 548 -2,378 m. Relative to M. mexicana and M. saxicola, the elevation of M. reverchonii does not overlap, occurring at 1,860-2,050 m in Mexico (but it occurs at 50-2,134 in the United States). Given the paucity of M. procera collections, we do not have a good sense of the elevational variation that populations occur at, but we know that it occurs at 970-2,470 m throughout its range in the United States and our two collections indicate an elevational range of 1,250-2,406 m in Mexico, suggesting populations are situated at higher elevations than M. mexicana and M. saxicola.
Thompson & Powell (1981) described chromosome polymorphism in M. mexicana. Populations in the northern edge of the species range from Big Bend National Park in Texas, United States, were reported as n = 9. Populations from Coahuila, however, were reported as n = 10. The authors identified no morphological distinctions between the n = 9 and n = 10 populations, a result corroborated here. Schenk & Hufford (2011b) identified evidence for recent hybridization between M. mexicana and an unknown member from their clade-1e. If the true chromosome number of M. mexicana is n = 10, individuals from clade-1e that could be the parental cross include M. longiloba var. chihuahuaensis and M. reverchonii because of their geographic proximity and their chromosome number of n = 9. If the true chromosome number of M. mexicana is n = 9, however, the parental cross would likely be with M. saxicola, which overlaps in its range and has a chromosome number of n = 10.
In this study, we sought to determine how populations of M. mexicana and M. saxicola were distributed (Figure 4). We determined the two species to have overlapping ranges throughout their distribution. For example, Granados Mendoza & Schenk 802 and 803 (MEXU) were identified as M. mexicana and M. saxicola, respectively, both of which were collected at the same locality in Nuevo León. In the aforementioned population, M. saxicola was less common and occurred along the flat area at the base of a roadcut, while M. mexicana occurred along the base of the roadcut slope, thus, there remained some spatial separation syntopically. Whether introgression occurs is yet to be determined, but remains a possibility given their proximity, and introgression might be an explanation for why the two distantly related species are so difficult to differentiate morphologically.
Mentzelia procera (Wooton & Standl.) J.J.Schenk & L.Hufford. Madroño 57: 247. 2011; Nuttallia procera Wooton & Standl., Contr. U.S. Natl. Herb. 16: 150. 1913; Mentzelia pumila Torr. & A.Gray var. procera (Wooton & Standl.) J.Darl., Ann. Missouri Bot. Gard. 21: 169. 1934. Type: United States, New Mexico, White Sands, 18 Aug 1907, E.O. Wooton & P.C. Standley s.n. (holotype: US!; isotype: NMC!).
Biennial herbs, 2.7-9.4 dm tall. Stems primary aerial axis arising at or above the soil surface, erect, straight; axillary branches on distal half of primary axis or occasionally along the entire primary axis. Leaves 38.3-108 × 5.3-14.9 mm, intersinus distance at widest point 1.7-3.9 mm; leaves on proximal third of primary axis oblanceolate to elliptic, pinnatilobate with 14-22(26) lobes 5.6-12.1 mm apart, lobes opposite, perpendicular to leaf axis, regular, 1.8-4.5 × 1.3-3.8 mm with rounded to acute apices; leaves on distal third of primary axis elliptic to lanceolate with non-clasping bases, pinnatilobate with 6-18 lobes 3.4-12.8 mm apart, lobes opposite to alternate, perpendicular to leaf axis, regular, 1-4.5 × 1.2-3.8 mm with rounded to acute apices. Petals light to occasionally golden yellow, narrowly spatulate, 9.5-16.8 × 3.4-5.6 mm. Androecium light yellow; anthers remaining straight after dehiscence; outer whorls of stamens staminodial and fertile, five outermost stamens in median antesepalous positions petaloid, narrowly spatulate, 8.2-14.6 × 2.3-3.4 mm, without anthers, second whorl of stamens all fertile. Capsule cup-shaped to cylindrical, 9.8-18.8 × 5.2-7.3 mm, base tapering; hypanthium 0.6-1.3 mm. Seeds 2.5-3.1 mm long; seed coat anticlinal cell walls sinuate, central papillae generally 28-68 per cell.
Distribution. The range of M. procera is more extensive in the United States, but spans the Mexican border into the northern parts of Chihuahua and Coahuila, at 1,250-2,400 m in elevation (Figure 4).
Habitat. Sandy soils, roadsides, arroyos, and hillsides.
Common name. Upright blazingstar, pega-pega.
The concept of M. procera has changed through time, being first described as a variety of M. pumila, it was later recognized at the specific level by Schenk & Hufford (2011a), who emphasized that the two taxa are distantly related. The range of M. procera is almost entirely in the United States and only extends slightly into Mexico. Mexican populations have been difficult to locate in herbarium collections and more directed field sampling is needed to better understand the range of M. procera in Mexico.
In northern Mexico (and throughout the overlapping ranges in the United States), M. procera is likely to be confused with M. longiloba var. chihuahuaensis and M. reverchonii. Seed characters best differentiate M. procera from M. longiloba var. chihuahuaensis, where the anticlinal walls of the former are sinuate and the periclinal walls have 28-68 papillae per cell, compared to the straight anticlinal walls and 4-6 papillae per cell on the periclinal walls of the latter (Figure 2). The petals of M. procera tend to be a lighter yellow, which differentiates it from both aforementioned species. Capsules of M. procera exhibit a tremendous amount of variation in length, but tend to be smaller on average than those observed in M. reverchonii.
Mentzelia puberula J.Darl. Ann. Missouri Bot. Gard. 21: 177. 1934. Type: United States, California, San Bernardino Co., Kane Springs, Ord Mtns., 1 May 1906, H.M. Hall & H.P. Chandler 6824 (holotype: UC!).
Short-lived perennial herbs, 1.4-5.4 dm tall. Stems primary/renewal aerial axes arising from a subterranean branching caudex, erect, straight; axillary branches along the entire primary axis. Leaves 18.8-62.7 × 11-36 mm, intersinus distance at widest point 8.9-28.8 mm; leaves on proximal third of primary axis broadly elliptic to obovate, dentate to serrate with 8-12 teeth 5-11.5 mm apart, teeth opposite, slightly angled towards leaf apex or perpendicular to leaf axis, regular, 1-4 × 1.7-6 mm with rounded to acute apices; leaves on distal third of primary axis oval, or obovate with non-clasping bases, dentate to serrate with 6-12 teeth 3.1-12.7 mm apart, teeth opposite, slightly angled towards leaf apex or perpendicular to leaf axis, regular, 1-3.5 × 2.6-7.3 mm with acute apices. Petals golden yellow, narrowly spatulate, 6.7-9.5(12.6) × 2.3-5.8 mm. Androecium golden yellow; anthers twisted after dehiscence; outer whorls of stamens all fertile, five outermost stamens in median antesepalous positions petaloid, slightly clawed, narrowly spatulate, 5.4-9.2(11) × 1.3-2.7 mm, with anthers borne on stalks, second whorl of stamens all fertile. Capsule cup-shaped, 5-9.8 × 5-8 mm, base rounded; hypanthium 0.9-1.8 mm. Seeds 2.4-2.8 mm long; seed coat anticlinal cell walls straight, central papillae generally 3-5 per cell.
Distribution. Baja California and northwestern edge of Sonora near the United States border, occurring at elevations of 90-460 m in Mexico (Figure 3).
Habitat. Near the bases of granite rock outcrops or occasionally in sandy arroyos.
Common name. Pubescent blazingstar.
Mentzelia puberula is the most distinct species of section Bartonia in Mexico. Phylogenetic analyses conducted by Schenk & Hufford (2011b) inferred that all species of section Bartonia belong to one of two sister clades that they referred to as clade-1 and clade-2. Mentzelia puberula was recovered in clade-2, which primarily consisted of short-lived perennials that have a woody caudex that branches underground. All other species of section Bartonia in Mexico were recovered in clade-1. Mentzelia puberula, consequently, is more distantly related to all other species of Mentzelia section Bartonia in Mexico and is the southernmost representative of clade-2. Only a few specimens of M. puberula from Mexico exist, and it is likely that additional field surveys in the Sonoran region will discover novel populations and expand its known range.
Mentzelia puberula was treated as a synonym of M. oreophila by Prigge (1993) and Felger (2000). The accounts of M. oreophila in Mexico (e.g., Sosa et al. 2018) likely reflect the treatment of M. puberula as a synonym of the former taxon. Although the two taxa are closely related (Schenk & Hufford 2011b), they are easily distinguished by the clasping leaf-bases of the upper leaves in M. oreophila versus the non-clasping bases in M. puberula. We follow Brokaw et al. (2012) in advocating for the recognition of the two taxa as distinct species. Mentzelia oreophila is a Mojave Desert endemic, distributed in California and Nevada, and it is not known to occur in Mexico.
Mentzelia reverchonii (Urb. & Gilg) H.J.Thomps. & Zavort. Wrightia 4: 24. 1968; Mentzelia pumila Torr. & A.Gray var. reverchonii Urb. & Gilg, Nova Acta Acad. Caes. Leop.-Carol. German. Nat. Cur. 76: 94. 1900; Nuttallia reverchonii (Urb. & Gilg) W.A.Weber, Phytologia 55: 9. 1984. Type: United States, Texas, Callahan Co., May 1882, J. Reverchon 322 (lectotype: US!; isolectotypes: MO!, P!).
Mentzelia hintoniorum B.L.Turner & A.L.Hempel, Phytologia 79: 298. 1995. Type:
Mexico, Nuevo León, Mpio. Galeana, San Roberto to "Y," 24.6986111 °N, 100.1761111 °W, 2,015 m, 5 Sep 1995, G.B. Hinton et al. 25495 (holotype: TEX!; isotype: TEX!).
Biennial to short lived perennial herbs, 1.6-8.5 dm tall. Stems primary/renewal aerial axis arising at or above the soil surface, erect, straight; axillary branches on distal half of primary axis. Leaves 24.2-54 × 6.6-29 mm, intersinus distance at widest point 5.4-13.5(22.4) mm; leaves on proximal third of primary axis oblanceolate to elliptic, dentate to pinnatilobate with 10-20 teeth or lobes 4.6-9.3 mm apart, teeth or lobes opposite, perpendicular to leaf axis, regular, 1.1-5 × 2.4-5.3 mm with rounded to acute apices; leaves on distal third of primary axis lanceolate with non-clasping bases, margins dentate with 8-14 teeth 2.8-8.4(11) mm apart, teeth opposite, perpendicular to leaf axis, regular, 0.6-3.8 × 1.6-5.8 mm with rounded to acute apices. Petals golden yellow, narrowly spatulate, (9.6)11.1-14.1(22) × 2.5-4.2(5.5) mm. Androecium golden yellow; anthers remaining straight after dehiscence; outer whorls of stamens staminodial and fertile, five outermost stamens in median antesepalous positions petaloid, narrowly spatulate, 9.4-13.4(20) × 2.2-3.7(5) mm, without anthers, second whorl of stamens all fertile. Capsule cup-shaped to cylindrical, 9.4-25 × 6-10 mm, base tapering; hypanthium 1-1.8 mm. Seeds 3-3.1 mm long; seed coat anticlinal cell walls straight, central papillae generally 5-8 per cell. See Figure 6.
Distribution. Coahuila and Nuevo León at 1,860-2,050 m in Mexico, but 50-2,134 m in the United States (Figure 4). Populations have not been collected in the northwestern Tamaulipas panhandle, despite being collected on the other side of the border in Texas, and we recommend targeted collections in this area to identify potential populations.
Habitat. Occurring in a myriad of soil types, include loam, gypsum, clay, sandy, and rocky soils, along roadsides, in grasslands, alluvial river banks, and hills.
Common name. Reverchon's blazingstar.
We recognize M. hintoniorum as a synonym of M. reverchonii following Schenk & Hufford (2020). Mentzelia hintoniorum was not compared to M. reverchonii in the original description (Turner & Hempel 1995), despite documented collections of M. reverchonii from Nuevo León that existed in MEXU at the time it was described. When evaluating the morphological variation among the two taxa, the only distinguishing character between them is the enlarged woody base of M. hintoniorum, which is a wound response similar to other species of section Bartonia, such as M. nuda, M. rusbyi Wooton, and M. strictissima (Schenk & Hufford 2020).
Mentzelia reverchonii is morphologically similar to M. longiloba var. chihuahuaensis in the Mexican Chihuahuan Desert. The easiest way to differentiate between the two species is the larger leaf lobes that are pointed to the distal end of the upper leaves in M. longiloba var. chihuahuaensis compared to the perpendicular lobes in M. reverchonii (note that lower leaves are both perpendicularly situated). Other features that help to differentiate the two species, despite overlapping characters, include M. reverchonii being a larger plant, 1.6 -8.5 dm tall (vs. to 3.2-5.5 dm tall in M. longiloba var. chihuahuaensis), lower primary leaves up to 54 mm long (vs. up to 110 mm long), and fruits that are up to 25 mm long (vs. up to 15[20.5] mm long).
Mentzelia saxicola H.J.Thomps. & Zavort. Wrightia 4: 22. 1968. Type: United States, Texas, Hudspeth Co., 3 mi W of Van Horn on US 10, 100 yards W of the Hudspeth-Culberson Co. line, 17 Aug 1964, P.R. Atsatt 500 (holotype: US!; isotypes: LA! [×2]).
Biennial herbs, 1.8-3 dm tall. Stems primary aerial axis or axes arising at or above the soil surface, erect, straight; axillary branches on distal half of primary axis or along the entire primary axis. Leaves 21-44.4 × 5.8-11.2 mm, intersinus distance at widest point 1.6-3.4 mm; leaves on proximal third of primary axis oblanceolate to elliptic, pinnatilobate with 6-14 lobes 4.1-11.6 mm apart, lobes opposite or alternate, slightly angled towards leaf apex, regular, 2.4-3.9 × 1.8-3 mm with rounded or acute apices; leaves on distal third of primary axis oblanceolate, elliptic, or lanceolate with non-clasping bases, pinnatilobate with 4-12 lobes 5.3-10.5 mm apart, lobes opposite or alternate, slightly angled towards leaf apex, regular, 1.8-3.9 × 1.3-3.1 mm with acute apices. Petals light to golden yellow, narrowly spatulate, 11.7-16.6 × 3-4.8 mm. Androecium light to golden yellow; anthers twisted or remaining straight after dehiscence; outer whorls of stamens staminodial and fertile, five outermost stamens in median antesepalous positions petaloid, narrowly spatulate, 10.4-14.6 × 2-3.8 mm, generally without anthers, second whorl of stamens all fertile. Capsule cup-shaped, 7-11.9 × 5.7-8.2 mm, base rounded; hypanthium 0.8-1.4 mm. Seeds 2.7-3.3 mm long; seed coat anticlinal cell walls straight, central papillae generally 3-13 per cell. See Figure 6.
Distribution. Endemic to the Chihuahuan Desert and occurring in eastern Chihuahua, Coahuila, and Zacatecas, at 850-1,843 m elevation in Mexico and 510-2,100 m in the United States (Figure 4).
Habitat. Sandy or gypsum-clay soils, occurring in arroyos, cliffs, roadcuts, and steep hillsides.
Common name. El Paso blazingstar.
Despite the two taxa being distantly related (Schenk & Hufford 2011b), differentiating between M. saxicola and M. mexicana can be difficult in some collections (Thompson & Powell 1981) because flower, stamen, and capsule sizes overlap greatly (Figure 6). Characters of the primary leaves differentiate between them best if the leaves are on the plant when it is collected. The primary leaves of M. saxicola are generally smaller (and particularly narrower) than those of M. mexicana, 21-44.4 × 5.8-11.2 mm (vs. 24-82 × 11.7-29.1 mm), and the lobes tend to be smaller on proximal leaves, 2.4-3.9 × 1.8-3 mm (vs. 1.4-5.1 × 2.4-6.4 mm; Figure 5). Leaves along the main shoot of M. mexicana are dentate to pinnatilobate, but almost always are pinnatilobate in M. saxicola. The intersinus distance of primary leaves along the main stem are narrower in M. saxicola (1.6-3.4 mm) than they are in M. mexicana (3.4-18.8 mm). Secondary leaves (those positioned along secondary branches that tend to persist longer than primary leaves throughout the growing season) overlap too greatly to be of utility in differentiating the species and are often the only leaves remaining on the plant if collected late in the season.
Schenk & Hufford (2020) estimated 38-48 papillae per cell for M. saxicola periclinal surfaces of the seed coat, a range that is markedly different than 3-11 papillae per cell reported here (Figure 2). Their estimates were based on seeds collected from the type locality, which as Thompson & Zavortink (1968) noted, is a hybrid population. Although Thompson & Zavortink (1968) suggested that M. saxicola was hybridizing with M. multiflora, Schenk & Hufford (2020) determined that M. multiflora was a misapplied name in Texas and that M. saxicola was hybridizing with M. procera at the type locality. Mentzelia procera has 26-68 (32-47 in our estimates from Mexico populations) papillae per cell (Figure 2), and it is likely that the Schenk & Hufford (2020) number reflects an introgression of papillae number from M. procera, and that the true number is much closer to those estimated by Thompson & Zavortink (1968; reported as 3-5) and here. It is likely that Thompson & Zavortink happened upon seeds that did not possess introgressed traits, as Schenk & Hufford (2020) did in their work.
Supplementary material
Supplemental data for this article can be accessed here: https://doi.org/10.17129/botsci.2828
