Materials and methods

We studied herbarium specimens collected from Mexico to confirm identifications and to assess their distributions. Specimens were collected from natural populations in the field, or consulted from 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)}, ^{Grissom (2014)}, ^{Brokaw & Schenk (2020)}, and ^{Schenk & Hufford (2020)}. Species distribution data were collated from field and herbarium collections and supplemented with collections on The Global Biodiversity Information Facility (^{GBIF 2012}). 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). In total, we plotted 3,343 data locality points in ArcMap v. 10.7 (ESRI, Redlands, California, USA).

Results

Five of the six Mentzelia sections were confirmed to occur in Mexico (Table 1). Mentzelia section Bartonia was determined to consist of six species in Mexico, which included three of the four varieties of M. longiloba J.Darl. Three of the six species in section Bicuspidaria were determined to occur in Mexico. The monotypic section Dendromentzelia was represented by M. arborescens Urb. & Gilg (Figure 2A) in southern Mexico. Section Mentzelia was represented by eight species, the most species among all sections in Mexico. Seven species from section Trachyphytum were confirmed to occur in Mexico. A total of 25 Mentzelia species occur in Mexico (Table 1).

Figure 2 Representative Mentzelia species from the five sections that occur in Mexico. A. M. arborescens from section Dendromentzelia, cultivated in the Washington State University greenhouse; B. M. involucrata flower with bifid stamens from section Bicuspidaria, photograph from Riverside Co., California, USA, by Joe Decruyenaere; C. M. montana fruits from section Trachyphytum, demonstrating the gradual tapering from the apex to the base, from Doña Ana County, New Mexico, USA, photograph by Patrick J. Alexander; D. M. montana leaf from Doña Ana County, New Mexico, USA, photograph by Patrick J. Alexander; E. M. montana flowers from Doña Ana County, New Mexico, USA, photograph by Patrick J. Alexander; F. M. adhaerens flower and fruit from Baja California, photograph by Patrick J. Alexander; G. M. adhaerens leaf from Baja California, photograph by Patrick J. Alexander; H. M. reverchonii fruits and pre-anthesis flower from Coahuila, Mexico; I. Leaf of M. reverchonii from Coahuila, Mexico; and J. M. saxicola leaf from Zacatecas, Mexico. 

Species of Mentzelia were distributed across all of Mexico (Figure 3). Sections Bartonia, Bicuspidaria, and Trachyphytum were distributed primarily in northern Mexico (Figure 3A). Section Dendromentzelia was distributed in southern Mexico (Figure 3B). Section Mentzelia occurred throughout the country, except for the northern limits of the Mexican Chihuahuan Desert (Figure 3B).

Figure 3 Distributions of Mentzelia sections in Mexico. A. Distribution of sections Bartonia (black circles), Bicuspidaria (gray squares), and Trachyphytum (white diamonds) in northern Mexico. B. Distribution of sections Dendromentzelia (white crosses) and Mentzelia (black triangles). 

Four taxa were determined to be Mexican endemics: M. longiloba J.Darl. var. pinacatensis J.J.Schenk & L.Hufford (section Bartonia); M. nesiotes (I.M.Johnst.) Brokaw & J.J.Schenk (section Bicuspidaria); M. adhaerens Benth (section Mentzelia; Figure 2F-G); and M. arborescens (section Dendromentzelia). All other species had ranges that either extended northward into the United States, southward into Guatemala, or were widespread across the New World (e.g., M. aspera).

Ten names are misapplied in Mexico, which include M. albescens (Gill. ex Arn.) Griseb., M. desertorum (Davidson) H.J.Thomps. & J.E.Roberts, M. dispersa S.Watson, M. isolata Gentry, M. multiflora (Nutt.) A.Gray, M. nitens Greene, M. oligosperma Nutt. ex Sims, M. oreophila J.Darl., M. pumila Torr. & A.Gray, and M. sinuata (Rydb.) R.J.Hill. Eleven taxa were inferred to be synonyms of other species that were confirmed to occur in Mexico, which included M. conzattii Greenm. (= M. arborescens), M. gracilis Urb. & Gilg (= M. lindheimeri Urb. & Gilg), M. gypsophila B.L.Turner (= M. asperula Wooton & Stanley), M. hintoniorum B.L.Turner & A.L.Hempel (= M. reverchonii [Urb. & Gilg] H.J.Thomps. & Zavort.), M. incisa Urb. & Gilg (= M. lindheimeri), M. karwinskii Urb. & Gilg (= M. hispida Willdenow), M. orizabae Urb. & Gilg (= M. hispida), M. sessilifolia Urb. & Gilg (= M. asperula), M. stenophylla Urb. & Gilg (= M. hirsutissima S.Watson), M. strigosa Kunth (= M. hispida), and M. texana Urb. & Gilg (= M. lindheimeri).

Discussion

Twenty-five species of Mentzelia occur in Mexico (approximately 26 %), which are placed in five of the six sections, and indicates that Mentzelia, and even Loasaceae, are more diverse in Mexico than previous estimates inferred when accounting for synonymy. ^{Villanueva-Almanza (2011)}, for example, estimated that nine species of Mentzelia occurred in Mexico, just slightly over a third of what is recognized here. Mentzelia hispida was notable as not being included among the endemic taxa, as it is often regarded as a Mexican endemic (e.g., ^{Darlington 1934}, ^{Hufford et al. 2016}). Our study documented Guatemalan collections on the other side of the Mexican border (Molina 21388, F: 1661991, US: 3543407; Soto 1471, MEXU: 1435246), suggesting the species is endemic to Mexico and Guatemala. Our list of Mexican endemics is different than that of ^{Sosa et al. (2018)}, who recognized M. adhaerens, M. arborescens, M. desertorum, M. isolata, and M. sinuata. Our study overlaps in recognizing M. adhaerens and M. arborescens, but M. desertorum and M. isolata also occur in the United States. Mentzelia sinuata does not occur in Mexico, rather, its range is centered in the mid-northern Rocky Mountains in the U.S. (^{Schenk & Hufford 2020}). Our list of endemics is also different from ^{Villaseñor (2016)}, who recognized the same endemics as ^{Sosa et al. (2018)}, but also recognized M. conzattii, M. gypsophila, and M. hintoniorum, all of which are treated as synonyms of other species that occur in Mexico (see below).

The sections of Mentzelia are geographically clustered in different parts of the country (Figure 3). Section Bartonia is distributed in the two northern deserts, but only partially overlaps with section Bicuspidaria (Figure 3A), which is localized in the Baja California Peninsula and the western edge of the Sonoran Desert adjacent to the Gulf of California (Figure 3A). Section Trachyphytum occurs exclusively in the Sonoran Desert and Baja California Peninsula. Section Mentzelia is widespread across the entire country (Figure 3B), extending northward into the United States, eastward into the Caribbean, and southward into Central and South America. Section Dendromentzelia is endemic to southern Mexico (Figure 3B).

The most recent taxonomic work that focused on the Mentzelia of the Chihuahuan Desert recognized three species in section Bartonia (^{Thompson & Powell 1981}). Although we have expanded the geographical scope to include all of Mexico, we recognize the Mexican flora as including six species from section Bartonia, with M. longiloba represented by three of its four varieties. Unlike ^{Thompson & Powell (1981)}, we recognize M. multiflora as a misapplied name in Mexico. Populations of M. multiflora reported by ^{Thompson & Powell (1981)} were most likely what we now recognized as M. longiloba var. chihuahuaensis or M. procera. The only other species that occurs in the geographic area outlined by the authors is M. reverchonii (Figure 2H-I), a species that H. Thompson, who described the species, would have been unlikely to confuse.

Mentzelia section Bartonia is the most species rich section of the genus worldwide, with approximately 51 species. The section is distributed in the northern half of Mexico in the Sonoran and Chihuahuan deserts (Figure 3A). The geographic associations among Mexican species are inconsistent with phylogenetic relationships (^{Schenk & Hufford 2011}), suggesting that there have been multiple movements of species across floristic regions. Mentzelia reverchonii, for example, was sister to a clade of M. nuda and M. strictissima (^{Schenk & Hufford 2011}) that occurs in the short-grass prairies of the Great Plains in the United States. ^{Schenk (2013a)} determined a center of origin for section Bartonia to be in the Colorado Plateau, and that movement across the North American southwestern deserts to be restricted. The Mexican species of section Bartonia represent this scenario well, where all of the species are more closely related to northern species from different floristic regions than they are to each other (^{Schenk 2013a}). It can be hypothesized, although beyond the limits of this study that such distribution patterns occurred because of southward migrations driven by climate change associated with Pleistocene glacial cycles.

Mexican species of section Bartonia that occur in the Chihuahuan Desert are often associated with gypsum outcrops. The evolution of species that are limited to gypsum-rich soils has occurred at least five times in section Bartonia (^{Schenk 2013b}). It is notable that the species that occur on gypsum in the Chihuahuan Desert do not appear to be obligated to these soil types. Three species associated with gypsum soils, M. mexicana, M. reverchonii, and M. saxicola all occur on soils that show no evidence of being gypsum derived in the northern parts of their respective ranges.

Three names have been misapplied in Mexico for section Bartonia. Mentzelia albescens, which is amphitropically disjunct between Texas in the United States and Argentina and Chile in South America (^{Schenk & Saunders 2017}), approaches the Mexican border, but no collections have been made, as far as we know, in Mexico. Mentzelia oreophila occurs in the Sonoran Desert in the United States, but collections are not known from Mexico. The misapplication is likely the result of ^{Prigge's (1993)} treatment of M. puberula, a species that does occur in Mexico, as a synonym of M. oreophila. ^{Brokaw et al. (2012)} later recognized M. puberula as a distinct species, a treatment followed here. Mentzelia procera had historically been treated as a variety of the Rocky Mountain endemic M. pumila (^{Schenk & Hufford 2010}), and previous works and herbarium collections have recognized M. procera as M. pumila, which is misapplied in Mexico.

Mentzelia section Bicuspidaria consists of six annual species worldwide, three of which occur in Mexico: M. hirsutissima, M. involucrata S.Watson (Figure 2B), and M. nesiotes. The three species are each other's closest relatives, with M. hirsutissima being more closely related to M. involucrata than the morphologically similar M. nesiotes (^{Brokaw et al. 2020}). Despite the cryptic differences between M. hirsutissima and M. nesiotes, the two species are geographically isolated and occur in ecologically distinct climates (^{Brokaw & Schenk 2020}). All three species occur in the California Peninsula and/or the western edge of Sonora. Although the ranges of M. involucrata and M. hirsutissima overlap in southern California and Baja California, M. nesiotes is allopatric in the Vizcaíno fog desert west of the Peninsular Ranges (^{Brokaw & Schenk 2020}). We were unable to locate and verify the collection of M. hirsutissima reported from near Coxcatlán, which was considered introduced into the area (^{Villanueva-Almanza 2011}). We agree with ^{Brokaw et al. (2020)}, ^{Brokaw & Schenk (2020)} in treating M. stenophylla as a synonym of M. hirsutissima on the basis of overlapping characters that prohibit clear species boundaries as well as phylogenetic results that recovered representatives mixed within a clade.

The monotypic section Dendromentzelia is located in southern Mexico (Figure 3B), and it is notably different compared to sections Bartonia, Bicuspidaria, and Trachyphytum in being a small tree with opposite leaves, versus herbaceous to subshrubby with alternate leaves. Mentzelia arborescens has long been associated with section Mentzelia (^{Darlington 1934}), but more recent phylogenetic studies have identified support for its placement as sister to section Mentzelia (^{Hufford et al. 2003}, ^{Grissom 2014}, Figure 1). We continue to observe the importance of the tree habit in recognizing M. arborescens within Dendromentzelia. Mexican collections have historically been treated as M. conzattii, but we agree with ^{Darlington (1934)} and ^{Weigend (2007a)} in recognizing the taxon as a synonym of M. arborescens.

Mentzelia section Mentzelia is the most diverse section in Mexico with eight species. ^{Grissom (2014)} identified a Sonoran Desert origin for the Mexican species of section Mentzelia. From the Sonoran Desert, lineages dispersed into the Mexican Highlands, West Indian region, Central America, and South America. The Sonoran Desert origin of section Mentzelia contrasts with section Bartonia, which has an estimated origin further north in the Colorado Plateau of the United States (^{Schenk 2013a}).

Mentzelia hispida, M. aspera L., and M. lindheimeri from section Mentzelia are especially widespread throughout Mexico and represent considerable taxonomic challenges (^{Grissom 2014}). The concept of M. hispida is one of the most confused among section Mentzelia because of its wide range that encompasses much phenotypic variation (^{Villanueva-Almanza 2011}). ^{Darlington (1934)} recognized M. strigosa, which she identified as a Mexican endemic, but did not cite a single specimen in her monograph. We treat M. strigosa as a synonym of M. hispida, as well as M. karwinskii Urb. & Gilg and M. orizabae Urb. & Gilg, both of which have been synonymized in other works (^{Darlington 1934}, ^{Calderón de Rzedowski 1992}, ^{Villanueva-Almanza 2011}).

Mentzelia aspera is considered a tropical weed (^{Thompson & Powell 1981}) and has the widest distribution of all species in Mentzelia (^{Reyes 1999}, ^{Weigend 2007b}, ^{Hufford et al. 2016}), occurring from Arizona in the United States southward to Argentina, and extending eastward into Brazil. Mentzelia aspera is commonly confused with other annual species from section Mentzelia, a trait that appears to have evolved multiple times from perennial ancestors throughout the genus. In Mexico, M. aspera is common in all places except the Chihuahuan Desert, where it rarely occurs along its periphery. Differentiating M. aspera from M. isolata can be difficult, but M. aspera tends to have longer pedicled fruits and more stamens (see key below).

We conservatively treat M. gracilis, M. incisa, and M. texana as synonyms under M. lindheimeri. ^{Hufford et al. (2016)} recognized the latter two taxa as synonyms of M. lindheimeri, but ^{Grissom (2014)} advocated for their continued recognition, in addition to M. gracilis. Representatives of M. gracilis, M. incisa, and M. lindheimeri formed a clade in phylogenetic analyses of ^{Grissom (2014)}, which also included M. floridana Nutt. ex Torr. & A.Gray. The aforementioned clade has a unique biogeographic pattern, forming a band around the northern portion of the Gulf of Mexico (^{Grissom 2014}). Additional systematic work is needed to assess the species status of the taxa, as well as their relationships.

We treat M. sessilifolia and M. gypsophila under M. asperula. ^{Grissom (2014)} recovered all three taxa in a clade, but the relationships within the clade were weakly supported. Mentzelia sessilifolia had been treated as a synonym of M. hispida by ^{Darlington (1934)}, but representatives analyzed by ^{Grissom (2014)} were recovered as being too distantly related to be consistent with Darlington's treatment. Additional work is needed in this clade to determine whether M. sessilfolia and M. gypsophila should be recognized as distinct species in regard to M. asperula.

Many of the reported occurrences of M. oligosperma in the northeastern portion of Mexico (e.g., ^{Turner 2004}) were likely M. pattersonii B.L.Turner. Mentzelia oligosperma approaches the Mexico border in southwestern Texas, but current herbarium collections provide no evidence that it ever crosses the border (although future collecting efforts might reveal that it does indeed occur in Mexico). The two taxa are sister (^{Grissom 2014}) and can be differentiated by the greater number of stamens in M. oligosperma (15-45 versus approximately 10). Turner (2004) cited the larger petals (15-20 × 8-10 mm) of M. pattersonii as a distinguishing character, but we note that although M. pattersonii petals can be slightly larger than M. oligosperma, those measurements overlap greatly with those reported by ^{Hufford (2016)} as (6-)8-18.5 × (3-)4-10.5 mm. Mentzelia oligosperma and M. pattersonii form a clade that is closely related to the Chihuahuan endemic M. pachyrhiza I.M.Johnston (^{Grissom 2014}).

Mentzelia section Trachyphytum is the second most species-rich section in Mentzelia world-wide with approximately 22 species. Individuals are all annual herbs, about half of which are polyploids, including tetraploids, hexaploids, and octoploids (^{Brokaw & Hufford 2010}). Mentzelia veatchiana Kellogg, for example, is hypothesized to be an allohexaploid (6x) between the tetraploid M. montana (Davidson) Davidson & Moxley (or possibly M. ravenii H.J.Thomps. & J.E.Roberts) and the diploid M. pectinata Kellogg (^{Brokaw & Hufford 2010}). Such complex evolutionary histories render distinguishing species on the basis of morphological characters challenging (e.g., see M. veatchiana and M. montana in key below). In Mexico, section Trachyphytum is represented by seven species: M. affinis Greene, M. albicaulis (Dougl.) Dougl. ex Torr. & A.Gray, M. desertorum, M. micrantha (Hooker & Arnott) Torr. & A.Gray, M. montana (Figure 2C-E), M. obscura H.J.Thomps. & J.E.Roberts, and M. veatchiana. All seven species occur in either Baja California Peninsula and/or the Sonoran Desert (Figure 3A). Given that the center of diversity in section Trachyphytum is in California, it is not surprising to observe representatives of the section in the peninsula and western Sonora Desert of Mexico.

Distinguishing among M. albicaulis, M. desertorum, and M. obscura is challenging because of the great degree of morphological variation in the species and because M. obscura is morphologically intermediate between M. albicaulis and M. desertorum (^{Brokaw 2016}). The species occur sympatrically, creating further challenges to their identifications, but do not appear to be currently hybridizing (^{Brokaw 2016}). The most reliable characters to distinguish among the three taxa are seed characteristics. When observing the seeds at 10X magnification, the periclinal walls of the cells can be seen as being either flat to slightly convex or domed. The wall shapes are best observed when looking at cells that occur along the ridge of the polygonal seed.

Specimens collected from Sonora have been identified as M. nitens and M. dispersa. We determined those names to be misapplied. Collections attributed to M. nitens belonged to section Mentzelia, which can be distinguished on the basis of the broad lobing patterns on the proximal leaves. The collections labeled M. dispersa were also better placed in section Mentzelia for the same reason as above, but others were reidentified as M. montana. Mentzelia montana can be differentiated from M. dispersa on the basis of seeds situated in two rows within capsules (vs. one row in M. dispersa), the usually white bases of bracts subtending flowers (vs. green bases), and capsule lengths 6-17(-20) mm (vs. 7-30 mm; ^{Brokaw 2016}). We continue to recognize both M. nitens and M. dispersa as species that occur only in the United States.

Key to Mentzelia species in Mexico.