Nomenclatural reappraisal of the Macroptilium gracile (Leguminosae) species complex

The controversy regarding the status of the Macroptilium gracile species complex has been elucidated by the authors of this paper; nevertheless, some nomenclatural issues regarding the naming of the newly established variety have been found. In the present contribution, we discuss some nomenclatural aspects concerning the valid publication of the name Macroptilium gracile var. subcoriaceum (Benth.) Berlingeri, M.B.Crespo & Calles. After considering different nomenclatural evidence, the latter varietal name is to be considered superfluous and therefore, we establish a new combination (i.e., Macroptilium gracile var. longepedunculatum).


Introduction
Macroptilium gracile (Poepp. ex Benth.) Urb. is distributed in Central and South America, and is an extremely variable species. Ever since Bentham (1837) described its basionym Phaseolus gracilis Poepp. ex Benth., the species has been the subject of controversy. This situation has created confusion among botanists and non-botanists working with this important forage species (Cook et al. 2020).
On the basis of morphological, molecular and biogeographic data (which included revision of vouchers from the herbaria ABH, HERZU, IRBR, K, M, MY, NY, PORT, S, UOJ, VEN and W; acronyms according to Thiers 2021, continuously updated), Berlingeri et al. (2020) concluded that there is enough evidence to separate the Macroptilium gracile species complex into two distinct species: M. gracile and M. campestre (Mart. ex Benth.) Berlingeri, M.B.Crespo & Calles (= Phaseolus scolecocarpus Piper). The former is widely distributed in Central and South America, and exhibits broad morphological variation. For this reason, Berlingeri et al. (2020) described one infraspecific taxon based on the earliest legitimate name at the same rank (i.e. Phaseolus longepedunculatus var. subcoriaceus Benth.) as sanctioned by the International Code of Nomenclature for algae, fungi, and plants (hereinafter referred to as the Code) (Art. 11.4, Turland et al. 2018). This also generated an autonym (i.e., Macroptilium gracile var. gracile) as sanctioned by the Code (Art. 26.1, Turland et al. 2018). Unfortunately, this solution is in conflict with the Code because according to Article 11.6 (Turland et al. 2018) "an autonym is treated as having priority over the name or names of the same date and rank that established it".
In the present contribution, we discuss nomenclatural issues concerning the Macroptilium gracile species complex and explore ways to overcome shortcomings of the publication Berlingeri et al. (2020). As a result, a new combination is established.

Historical Background
Bentham (1837)  . The varieties latifolius and angustifolius do not have any description or diagnosis, and neither was accompanied by reference to any previous one; therefore, they were not validly published and must be considered nomina nuda. However, the varietal name subcoriaceus was validly published because it was accompanied by a description (Art. 38.1, Turland et al. 2018), and it is a legitimate name since it was established in accordance with the Code rules (Art. 6.5, Turland et al. 2018). Furthermore, Phaseolus longepedunculatus vars latifolius and angustifolius do not appear in the database of the International Plant Name Index (IPNIhttps:// www.ipni.org/), whereas var. subcoriaceus does. Addi-tionally, the detailed Latin description placed just below that name in the protologue matches the characteristics of the voucher Pohl s.n. deposited at the Royal Botanic Gardens (K-000189691!); which is original material used by Bentham (1837) and was designated as lectotype of the taxon in question by Berlingeri et al. (2020).

Nomenclatural Treatment
Berlingeri et al. (2020) have already clarified the status of the so-called Macroptilium gracile species complex; therefore, we will only address the nomenclatural issue regarding the correct name of the infraspecific taxa of M. gracile established by Berlingeri et al. (2020).
Bentham (1837) validly published the name Phaseolus longepedunculatus var. subcoriaceus, which automatically established the corresponding autonym (i.e., P. longepedunculatus var. longepedunculatus). Additionally, Bentham (1837) published two new varieties (i.e., var. latifolius and var. angustifolius) but these are considered nomina nuda because they do not have any description or diagnosis, and were not accompanied by reference to any previous one (Art. 38; Turland et al. 2018). Piper (1926) was the first to consider Phaseolus gracilis and P. longepedunculatus as extreme forms of a single species, based on the existence of abundant intermediate individuals. Consequently, he selected P. gracilis as t h e a c c e p t e d n a m e a n d s y n o n y m i s e d P. longepedunculatus. After Piper's (1926) treatment, some authors considered Macroptilium gracile (≡ Phaseolus gracilis) and M. longepedunculatum (Mart. ex Benth.) Urb. (≡ Phaseolus longepedunculatus) as distinct species (e.g., Urban 1928a;Verdcourt 1970;Maréchal et al. 1978), while other authors (e.g., Barbosa-Fevereiro 1987;Espert et al. 2007) agreed with Piper (1926).
After using multiple lines of evidence, Berlingeri et al. (2020) agreed with Piper (1926) that Macroptilium gracile and M. longepedunculatum are conspecific; however, they concluded that there was enough evidence to maintain two varieties. Unfortunately, they selected the infraspecific epithet subcoriaceum, in conflict with Article 11.6 (Turland et al. 2018). According to the Article 11.6 (Turland et al. 2018) "an autonym is treated as having priority over the name or names of the same date and rank that established it"; therefore; the correct infraspecific epithet of the taxon in question should be longepedunculatum.

Nomenclatural Note
The name Phaseolus longepedunculatus was published by Bentham (1837) and he made reference to three specimens (i.e., two collected by Carl von Martius and another collected by Johann Pohl), which should be treated as syntypes (Art. 9.6, Turland et al. 2018). We have designated Pohl's specimen deposited at K (K-000189691) as lectotype of Macroptilium gracile var. longepedunculatum based on Article 9.12 (Turland et al. 2018) and because it is the one fixing the application of Bentham's description.

Acknowledgements
The curators and staff of the herbaria cited in the text are kindly thanked. Michael H. Grayum (Missouri Botanical Garden, USA) is specially acknowledged for warning about the nomenclatural issues concerning Macroptilium longepedunculatum. Nicholas Turland (Botanischer Garten und Botanisches Museum Berlin-Dahlem, Germany) is thanked for his help with some nomenclatural precisions concerning autonyms and other issues. We would also like to thank the two anonymous reviewers who made important suggestions for improving the final manuscript.

Funding
Open Access funding provided thanks to the CRUE-CSIC agreement with Springer Nature.
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/ licenses/by/4.0/.