Anonychium (Benth.) Schweinf., Reliq. Kotschy.: 7. 1868.

Hughes, Colin E., Ringelberg, Jens J., Lewis, Gwilym P. & Catalano, Santiago A., 2022, Disintegration of the genus Prosopis L. (Leguminosae, Caesalpinioideae, mimosoid clade), PhytoKeys 205, pp. 147-189 : 147

Anonychium (Benth.) Schweinf., Reliq. Kotschy.: 7. 1868.

Prosopis section Anonychium , Benth. Hook. J. Bot. 4: 347. 1842.

Type.

Prosopis oblonga Benth. Benth., J. Bot. (Hooker) 4: 348. 1842, a synonym of Anonychium africanum .

Description.

Unarmed trees 4-20 m high, branches lacking axillary brachyblasts. Stipules inconspicuous, long-lanceolate, pubescent, caducous as young leaves develop, absent from most herbarium sheets. Leaves somewhat pendulous, 1-4 pairs of pinnae, the petiole 3-5 cm long, the rachis 5-9 cm long, the pinnular rachises 6-15 cm long, with 4-13 pairs of opposite leaflets, these 1.3-3.5 × 0.4-1.5 cm, glabrous or finely pubescent, mid-vein subcentric. Inflorescences spicate, 5-9 cm long, axillary, solitary or in pairs, densely flowered; pedicels 0.5 mm. Flowers small, yellowish or greenish-white, sweetly scented; calyx ca. 1 mm long; corolla ca. 3.5 mm long, the petals linear, free, glabrous on both sides; anthers apically broadened with an unusual anther gland borne ventrally between the thecae and forming a triangular hood-shaped protrusion made up of papillate cells; pollen with costae on the pores and a smooth (perforated) tectum; ovary and style pilose or villous. Fruits indehiscent, straight or sub-falcate, dark reddish-brown to blackish, shiny, subterete, 10-20 × 1.5-3.3 cm, exocarp hard, 1-2 mm thick, mesocarp spongy, thick, dry, endocarp segments thin, longitudinal, in one row (Figs 5B and 7A ). Seeds many, dark, shiny, ovate compressed, 8-10 × 4-9 mm, rattling within the pod when ripe.

Geographic distribution.

Monospecific. Widespread across Sahelian Africa, from Senegal in the west to Sudan and Ethiopia in the east (Fig. 8 ).

Habitat and uses.

Anonychium africanum is native across the whole Sahelian savannah belt. Trees are maintained and managed by farming and pastoralist communities in traditional silvo-pastoral systems throughout the African Sahel, providing essential products, including wood, fuel, food, livestock fodder and medicines and enhancing soil fertility ( Weber et al. 2008). Seeds are widely dispersed by browsing animals, such as camels, cattle and goats at the end of the dry season ( Tybirk 1991) and perhaps also by humans who collect the pods to feed to their animals, and cow dung (containing viable seeds) to fertilise their fields.

Etymology.

Anonychium literally meaning the absence of nails or claws from the Latin or Greek ‘onych’ = ‘ónyx’ meaning nail or claw, refers to the lack of armature of this genus.

Affinities.

Prosopis africana has long been considered anomalous within the genus and was placed in its own section Anonychium by Bentham (1842) and later this was upranked to its own genus, Anonychium by Schweinfurth (1868; under the name A. lanceolatum Schweinf.). Unlike almost all other species of Prosopis s.l., P. africana lacks armature, has internally glabrous petals, pollen with costae ( Guinet 1969), V-shaped anthers with small stomia forming short pockets on the ventral surface of the anthers and anther glands that are apparently morphologically unique within mimosoids ( Luckow and Grimes 1997). The anther glands of Anonychium africanum (as P. africana , Luckow and Grimes 1997: Figs 25-27) stand out as quite different from the typical mimosoid claviform anther glands of the remaining species of Prosopis s.l., being sessile, borne ventrally between the thecae, rather than stipitate borne apically or dorsally from the connective between the thecae as in most other mimosoids and forming triangular hood-shaped protrusions made up of papillate cells which are also unique amongst mimosoid anther glands ( Luckow and Grimes 1997). Alongside the robust molecular evidence for placement of P. africana distantly related to the rest of Prosopis (Fig. 1 ), this suite of morphological differences amply justifies segregation of P. africana as a distinct monospecific genus.

Anonychium is a phylogenetically isolated lineage that subtends the grade of other unarmed, mainly species-poor genera, Plathymenia , Fillaeopsis and Newtonia which is paraphyletic with respect to the core mimosoid clade of Koenen et al. (2020) (Fig. 1 ; Ringelberg et al. 2022). This is in line with pollen of Anonychium which shows similarities to Newtonia ( Guinet 1969).

Figures

Figure 1. A Phylogeny of the Caesalpinioideae showing the placement of the Prosopis grade (boxed in red) within the subfamily, based on analyses of DNA sequences of 997 nuclear genes (Ringelberg et al. 2022) B the part of the phylogeny that includes all elements of Prosopis s. l. Genera recognised in the new generic system presented here are in bold. Pie charts show the fraction of gene trees supporting that bipartition in blue, the fraction of gene trees supporting the most likely alternative configuration in green, the fraction of gene trees supporting additional conflicting configurations in red and the fraction of uninformative gene trees in grey. Numbers above pie charts are Extended Quadripartition Internode Certainty (Zhou et al. 2020) scores. Branch lengths are expressed in coalescent units and terminal branches were assigned an arbitrary uniform length for visual clarity, see Ringelberg et al. (2022); the root is not drawn to scale C, D the two most likely alternative tree topologies which would allow for a monophyletic Prosopis s. l., either without (C) or with (D) Xerocladia and Indopiptadenia. In C and D numbers above pie charts = number of gene trees supporting the species tree, numbers below pie charts = number of gene trees conflicting with the species tree C lack of gene tree support (just 69 gene trees) for the alternative species tree topology where sections Algarobia + Monilicarpa (≡ Neltuma) are sister to section Strombocarpa (≡ Strombocarpa) vs. 573 genes supporting a sister group relationship between Strombocarpa and Xerocladia (as shown in D) D lack of gene trees (zero gene trees) supporting a monophyletic Prosopis s. l.

Figure 5. Fruits of Prosopis, Strombocarpa, Xerocladia and Indopiptadenia A Prosopis cineraria B Anonychium africanum C Strombocarpa palmeri D Prosopis farcta E Strombocarpa ferox F S. strombulifera G S. pubescens H S. abbreviata (2 examples) I S. tamarugo J S. torquata K S. burkartii L Xerocladia viridiramis M Indopiptadenia oudhensis A-G, M (5 cm scale bar) H-L (1 cm scale bar with asterisk). All specimens at K A drawn from Gazanfar SG 4332 B Dembele & Sanogo ML- 146 and longitudinal section of fruit from Barter 1193 C Hughes et al. 1552 D van der Maesen 1627 E Atahuachi et al. MA 1147 F Hunziker 2036 G Acocks 1788 H Tweedie s. n. (from 2 type specimens) I Aronson 7742 J Vuilleumier 1019 K Acosta & Rosas 748 L Kolberg & Tholkes HK 2493 M Bajpai & Babu 264498. Drawn by Andrew Brown, July 2021.

Figure 7. Variation in fruits across Prosopis s. l. and allies A indehiscent pods of Anonychium africanum with thick pulpy mesocarp collected as fodder for livestock B plano-compressed pods of Indopiptadenia oudhensis lacking a thickened mesocarp and dehiscent along both sutures C indehiscent fruits of Prosopis farcta with a thick pulpy mesocarp D tightly coiled indehiscent screwbean fruits of Strombocarpa strombulifera E indehiscent pods of Strombocarpa ferox with a thick pulpy mesocarp F indehiscent fruits of Strombocarpa palmeri G small reniform to flabellate, flattened, indehiscent, 1 (- 2) - seeded, winged fruits of Xerocladia viridiramis which are unique within mimosoid legumes H indehiscent fruits of Neltuma articulata with a thick mesocarp and a hard bony segmented endocarp which remains closed I. Unripe indehiscent pods of Neltuma kuntzei with a thick pulpy mesocarp, these turning dark blackish-brown when ripe, reminiscent in colour to fruits of Anonychium. Photos courtesy of Marco Schmidt (A) (see Dressler et al. 2014), Dr. Omesh Bajpai and Dr. Lal Babu Chaudhary (B), Zeynel Cebeci (C) (https // en. wikipedia. org / wiki / Prosopis _ farcta), Dick Culbert (D) (see https // eol. org / pages / 640506, Colin Hughes (E, F, H, I), and Herta Kolberg (G) (see Plants of Namibia https // herbaria. plants. ox. ac. uk / bol / namibia).

Figure 8. The distributions of Indopiptadenia, Prosopis s. s., Anonychium, Xerocladia, Neltuma and Strombocarpa, based on 6,469 quality-controlled species occurrences from GBIF (www. gbif. org), DryFlor (www. dryflor. info), SEINet (www. swbiodiversity. org / seinet) and several other data sources (Ringelberg et al., in prep.). Map created using R packages ggplot 2 (Wickham 2016), sf (Pebesma 2018) and rnaturalearth (South 2017). The eight occurrence records, mapped in Bahia Brazil, are of Neltuma ruscifolia which is considered potentially native to that region (Burkart 1976 Oliveira & Queiroz 2020), while records of N. juliflora from Bahia, which is introduced and naturalised in that region, have been eliminated.