Disintegration of the genus Prosopis L. (Leguminosae, Caesalpinioideae, mimosoid clade) Author Hughes, Colin E. Department of Systematic & Evolutionary Botany, University of Zurich, Zollikerstrasse 107, 8008 Zurich, Switzerland colin.hughes@systbot.uzh.ch Author Ringelberg, Jens J. https://orcid.org/0000-0003-0567-5210 Accelerated Taxonomy Department, Royal Botanic Gardens, Kew, Richmond, Surrey, TW 9 3 AE, UK Author Lewis, Gwilym P. https://orcid.org/0000-0003-2599-4577 Unidad Ejecutora Lillo, Consejo Nacional de Investigaciones Cientificas y Tecnicas - Fundacion Miguel Lillo, Miguel Lillo 251, 4000 S. M. de Tucuman, Argentina Author Catalano, Santiago A. https://orcid.org/0000-0001-9153-1365 Facultad de Ciencias Naturales e Instituto Miguel Lillo, Universidad Nacional de Tucuman, Miguel Lillo 205, 4000 S. M. de Tucuman, Argentina text PhytoKeys 2022 2022-08-22 205 147 189 http://dx.doi.org/10.3897/phytokeys.205.75379 journal article http://dx.doi.org/10.3897/phytokeys.205.75379 1314-2003-205-147 1396FDE670D4506385C78B2620B2BD5B Neltuma Raf., Sylva Tellur.: 119. 1838. Prosopis sect. Algarobia DC. Prodr. 2: 446. 1825. Mitostax Raf., Sylva Tellur.: 120. 1838. Algarobia (DC.) Benth., Pl. Hartw.: 13. 1839. Prosopis sect. Monilicarpa Ruiz Leal ex Burkart, J. Arnold Arbor. 57(3): 230. 1976. Type . Neltuma juliflora (Sw.) Raf. [= Mimosa juliflora Sw.]. Description. Spiny, erect to prostrate subshrubs, shrubs and small trees, (0.1-) 4-10 (-20) m high, usually with a short trunk to 40-60 (->100) cm diameter, branching lax with a spreading rounded or flat-topped crown, twigs cylindrical, flexuous, often arched downwards, glabrous, green or reddish, often with rather long internodes, armed with uninodal axillary, solitary or paired, straight, strong, cylindrical, subulate spines (Figs 2 and 3E ), these not necessarily at all nodes, 0.2-15 (-33) cm long x 0.2-1.4 cm in diameter and sometimes thicker than the subtending twig, or with spinescent rigid straight cylindrical branchlets 8-50 cm, brachyblasts congested, blackish. Stipules small, triangular and dry. Leaves with 1-3 (-8) pairs of pinnae, the petiole (0.2-) 2-7.5 cm long, the pinnular rachises (0.2-) 4-19 (-24.5) cm long, with (1-) 2-30 (-50) pairs of opposite leaflets, these linear, ovate-oblong, oblong-linear or lance-ovate, more or less acute, palmately pinnativeined or almost without veins, (0.15-) 2.5-10 x 0.05-3.5 cm, puberulous to scarcely ciliolate or glabrous, or sometimes aphyllous or subaphyllous ( N. sericantha , N. kuntzei ), the leaves small and soon falling off the young developing shoots which become spinescent. Inflorescences axillary, solitary or fascicled, spicate, (1.5-) 3-15 cm long with 20-250 flowers on short 1.6 mm pedicels. Flowers white, yellow, greenish-yellow or occasionally red, often perfumed, sometimes some functionally male flowers; calyx 1-2 mm long; corolla 3-5 mm long, the petals almost free, pubescent, usually villous within; stamens and style exserted, anthers with a minute caducous incurved claviform gland arising from the connective. Fruits linear moniliform or compressed turgid (Figs 6 and 7H-I ), straw yellow, sometimes tinged reddish-maroon or black, 1-several per infructescence, indehiscent, glabrous, mostly straight to subfalcate, S- or C-shaped or annular with 1-3 very lax open spirals, acuminate, (2-) 5-29 cm in length x 0.5-2.6 cm diameter, margins often thickened and undulate, valves striate corrugate or smooth, exocarp crustaceous, mesocarp thin or more usually thick and pulpy, mealy or spongy, dry, usually sweet, endocarp hard and bony or coriaceous, with convex faces and acute extremities, segmented in longitudinal or transverse subquadrate closed seed chambers. Seeds brown, compressed ovate, 5-10 x 3-6 mm. See also Johnston (1962) . Geographic distribution. Potentially up to 43 species, but probably somewhat fewer (see below). Widespread across seasonally dry tropical and arid regions of the Americas with a pseudo-amphitropical bicentric pattern of greatest species diversity in the Mexican-Texan and Argentinian-Chilean-Paraguayan regions, especially diverse and abundant in the Chaco, with an outlying disjunct occurrence of Neltuma ruscifolia of questionable nativity in the Caatinga in north-east Brazil ( Burkart 1976 ; Oliveira & Queiroz 2020) and extending into warm and some colder temperate areas in Texas and Nevada in the north and Patagonia in the south, where N. denudans Benth. reaches 48 °S (Fig. 8 ). Habitat and uses. Dominant across large tracts of the Gran Chaco in mixed sub-xerophyllous woodland, also in Monte vegetation, open desert forests in quebradas along seasonal rivers, in Stipa -dominated pampas and semi-desert shrub steppe with hot summers and cold winters in Patagonia as far as 48 °S, some species capable of surviving extreme drought; spanning a wide range of substrates and edaphic conditions including stony and sandy mesas, coastal and inland sand dunes and deep black seasonally inundated, sometimes saline, clay vertisols. Some species weedy and invasive, both within their native ranges and where introduced (see Introduction). The wood generally hard, dense, durable and flexible and widely used for fence posts, parquet flooring, barrels, firewood and charcoal and the fruits are eagerly consumed by all forms of livestock (see Introduction). Etymology. Possibly derived from the common name Mulla Thumma in the Dravidian language Teluga in the Indian states of Andhra Pradesh and Telangana, where Neltuma juliflora is introduced. Affinities. Neltuma is sister to, but deeply divergent from, the combined Strombocarpa + Xerocladia clade (Fig. 1 ). Thirteen species of Prosopis have been described since the publication of Burkart's (1976) monograph. One of these, Prosopis bonplanda P.R. Earl & Lux, was already placed in synonymy under P. glandulosa by Palacios (2006) . All of the rest can be confidently placed in Neltuma (= Prosopis sect. Algarobia + Prosopis sect. Monilicarpa ), based on morphological descriptions and illustrations from their respective protologues. We provide new combinations in Neltuma for all these names, listing potentially up to 43 species for the genus, but we suspect that some of these new species may be no more than regional variants of the widespread and taxonomically difficult N. pallida / N. juliflora species complex. Given the difficulties of species delimitation across parts of Neltuma , we suggest that a detailed molecular study with complete sampling of species and dense sampling of multiple accessions, representing intraspecific diversity, is needed to properly re-assess species boundaries and possible hybridisation. The Mimobaits gene set of Koenen et al. (2020) would be an ideal tool for such a study.