Introduction

A revision of the Coahuila’s flora checklist led us to re-evaluate some names in the family Fabaceae. Senegalia Raf. is a genus of mimosoids (^{LPWG, 2017}) closely related to Acacia Mill. It was described in 1838 by C.S. Rafinesque, as a segregation of Acacia “with legumes short stipitate, elliptic, membranaceous, compressed, with 2-3 seeds” (^{Rafinesque, 1838}). ^{Standley (1922)}, in the treatment of the Trees and shrubs of Mexico, did not include Senegalia. The genus was recognized by ^{Britton & Rose (1928)} among others; later ^{Seigler et al. (2006b)}, transferred some of the species of Acacia subgenus Aculeiferum Vassal to Senegalia, but ^{Rico-Arce (2007)} included all in Acacia.

The genus Acacia sensu lato is considered polyphyletic (^{Pedley, 1986}; ^{Maslin et al., 2003}; ^{Seigler et al., (2006a)} as supported by molecular studies (^{Miller and Bayer, 2001}; ^{Luckow et al., 2003}), including a series of subgroups. According to ^{Maslin et al. (2003)}, there is a strong evidence suggesting that Acacia should be divided in the genera Vachellia Wight & Arn., Senegalia, Acaciella Britton & Rose, Acacia and Mariosousa Seigler & Ebinger, considering that the latter is a polyphyletic group of species (^{Gómez-Acevedo et al., 2010}; ²⁰¹⁵). According to this classification, Acacia sensu stricto is restricted to the “Australian group of species” (^{Maslin, 2008}) and some others in the Pacific Islands (^{Pedley, 1978}). In America, Senegalia includes most of the species of Acacia subgenus Aculeiferum and some other new nomenclatural combinations for the genus (^{Seigler et al., 2006b}; ^{Glass and Seigler, 2006}). Some other species of the subgenus are included in the new genus Mariosousa (^{Seigler et al., 2006a}). Currently, we are accepting this segregation.

The plants named as Senegalia are distinguished by having prickles and no stipular spines, petioles with glands and absence of phyllodes, pedicellate ovaries with glands at the base, and porate pollen grains (^{Seigler et al., (2006b)}Senegalia saltilloensis was described by ^{Britton & Rose (1928)} selecting as the type E. Palmer 169, from near Saltillo, Coahuila. A Latin diagnosis is not included, neither a “holotype” was designated, and four to five duplicates are currently annotated as isotypes at A, BM, MO, NY, and US. The US herbarium sample has the annotation as holotype made by C. Glass (2003). The NY duplicate has confusing annotations, as an isotype by C. E. Glass in 2003 and as holotype by D. S. Seigler & J. E. Ebinger in 2013. ^{Glass & Seigler (2006)} designated this duplicate also as lectotype, which seems more appropriate. These annotations as different “types” is probably due to the fact that N.L. Britton at NY and J.N. Rose at US published frequently in collaboration (^{Rzedowski et al., 2009)}, making it difficult to know which sample is the original “type”.

While many of the species names in Senegalia given by Britton & Rose were reduced to synonymy (^{Seigler et al., 2006b)}, S. saltilloensis was included under S. roemeriana. Besides the type material, no other herbarium specimen was found with the name Senegalia saltilloensis. The name remained in Senegalia for almost 80 years. Then, ^{Seigler et al. (2006b)} included it as a synonym of S. roemeriana (Scheele) ^{Britton & Rose (1928)}, but ^{Rico-Arce (2007)} considered that S. saltilloensis is a different species from S. roemeriana and transferred it to Acacia, publishing it as a new nomenclatural combination. Tropicos.org from the Missouri Botanical Garden (^{TROPICOS, 2016}) and The Plant List from the Royal Botanical Gardens at Kew (^{The Plant List, 2016}) follow this nomenclature and consider it as a name in current use. To establish the identity of the species, several collections near Saltillo were made. As the type locality of Senegalia saltilloensis is not clearly defined, we followed the localities visited by another collector of those times, C.G. Pringle, who used to travel by train around Mexico. From Saltillo he moved southwards into the mountains (^{Davis, 1936}), so we assume that the lower slopes of the Sierra de Zapalinamé are the most probable places where the specimens of S. saltilloensis could be found. A detailed observation of its morphology took us to the well-known Senegalia roemeriana (Scheele) ^{Britton & Rose (1928)}, with a wide distribution and morphological variation. On this basis, we decided to compare both species.

Materials and Methods

A total of 62 samples of Senegalia roemeriana from the herbaria ANSM, ASU, CFNL, DES, ENCB, HIN, MEXU, and USU were examined (^{Thiers, continuously updated}) (Table 1). These collections are a good representation of this species. In addition, eight samples from the Saltillo area, including the annotated “type” material of Senegalia saltilloensis were included (ANSM, NY, US). The revision of the herbarium specimens had the purpose of comparing and analyzing the morphological variation of the two related taxa. A series of selected characters that can be evaluated from herbarium samples were taken from each specimen.

A total of 70 samples were evaluated as representative of the morphological diversity of both species and Senegalia saltilloensis and S. roemeriana were considered as OTUs (Operational Taxonomic Units). A number of 21 characters were selected as elements of informative value (Table 2). A stereomicroscope Labomed model CSM2 (U.S.A.) and a vernier Mitutoyo series 1505 (Japan) were used to observe details and take measurements. The data were analyzed by the Detrended Correspondence Analysis (DCA), using the CANOCO version 4.5 software (^{ter-Braak and Šmilauer, 2002}).

Results and Discussion

We graphically presented axis I (eigenvalue of 0.085 and a cumulative percentage of variance of 29.2%) and axis II (eigenvalue of 0.051 and a cumulative percentage of variance of 46.6%) of the DCA. The ordination diagram shows that the specimens analyzed are positively correlated with axis I (Figure 1), most samples of Senegalia saltilloensis and S. roemeriana are mixed and placed close to the center of the ordination diagram. The group located at the extreme of both axes indicates morphological variation proper of the species. Some of the numbers at the upper right (19, 49, 59, 47, 48, 39, 46) are specimens without fruits and the ones on the right (68, 57, 38, 25, 43, 41, 16, 33) have no flowers.

Figure 1: DCA showing the grouping results of Senegalia roemeriana (Scheele) Britton & Rose (black circle) and Senegalia saltilloensis Britton & Rose (blue diamond), following a similarity analysis. 

Additionally, Figure 1 shows all the elements grouping in one aggregate, including the samples from the Saltillo area. The detailed examination of the lectotype (sample 32) and isolectotype (33), as well as the original description of Senegalia saltilloensis led us to consider the presence of prickles on the stem and the fruit slightly curved, flat and with the cuneate base as the key characters to separate this taxon from S. roemeriana. The revision of the herbarium specimens of S. roemeriana showed that the abovementioned characters are part of the morphoplasticity of the species. The plants range from having branches armed with sparse very small fine prickles to abundant stronger prickles. Additionally, the leaf rachis can also be prickly, the leaflets glabrous to pubescent, the legume 2-4 cm wide, glabrous to puberulent, with the base cuneate to rounded.

Conclusions

The results support the synonymy published by ^{Seigler et al. (2006b)}, considering the names Senegalia saltilloensis and the combination Acacia saltilloensis published by ^{Rico-Arce (2007)} as synonyms of the previous name Senegalia roemeriana.

As a consequence, the nomenclature of the species should be:

Senegalia roemeriana (Scheele) ^{Britton & Rose, N. Amer. Fl. 23(2): 115. 1928} ≡ Acacia roemeriana Scheele, Linnaea 21(4): 456. ¹⁸⁴⁸. TYPE: United States of America. Texas, western Texas, Austin, Römer s.n. (holotype: B, probably destroyed).

= Senegalia saltilloensis^{Britton & Rose, N. Amer. Fl. 23(2): 115. 1928}. Type: Mexico. Coahuila, near Saltillo, E. Palmer 169 (lectotype: NY, designated by ^{Glass & Seigler (2006)}; isolectotypes: BM, MO, US) ≡ Acacia saltilloensis (Britton & Rose) L. Rico, Amer. Sp. Acacia 166. 2007.

Authors contributions

JAVQ y EEC designed the study. JAV y JAED made the analysis. All the authors contributed to the revision and discussion of the manuscript.

Financial support

The study was supported by the Universidad Autónoma Agraria Antonio Narro (project 40012100/Plantas endémicas y raras de Coahuila).