Plant Ecology and Evolution 153 (1): 160–166, 2020 https://doi.org/10.5091/plecevo.2020.1657 SHORT COMMUNICATION Memecylon afroschismaticum sp. nov. (Melastomataceae–Olisbeoideae) endemic to the East African Rift region of Rwanda, Burundi and western Tanzania Robert Douglas Stone School of Life Sciences, University of KwaZulu-Natal, Private Bag X01, Scottsville 3209, South Africa E-mail: r.douglas.stone@gmail.com Background – A new species of Memecylon (Melastomataceae–Olisbeoideae) from Rwanda, Burundi and western Tanzania is described in connection with preparing the family treatment for the Flore d’Afrique centrale. Methods – Standard herbarium practices were applied. Key results – Memecylon afroschismaticum R.D.Stone is described and illustrated. This new species is remarkable for being endemic to forests of the East African Rift region avoided by all but a few other species of African Memecylon. A close relationship with M. flavovirens Baker (type of M. sect. Obtusifolia Engl.) is suggested by its corolla being narrowly conical-acute in bud and anther connectives with dorsal oil-gland and acute posterior extremity. However, its elliptic-ovate and distinctly acuminate leaves resemble those of M. myrianthum Gilg (of M. sect. Polyanthema Engl.) and M. verruculosum Brenan (of M. sect. Buxifolia R.D.Stone). The known location in western Tanzania is formally protected within the Mahale Mountains National Park, but the subpopulations in Rwanda and Burundi are unprotected and presumably threatened by high human population density and subsistence agriculture. The estimated area of occupancy is also quite small (12 km2). Memecylon afroschismaticum is thus provisionally assessed as Endangered [EN B1ab(iii)+B2ab(iii)] in accordance with IUCN criteria. Keywords – Burundi; East African Rift; Melastomataceae; Memecylon afroschismaticum; new species; plant taxonomy; Rwanda; Tanzania. INTRODUCTION The genus Memecylon L. comprises > 350 species of shrubs or small to medium-sized trees (Renner et al. 2007 onwards), widely distributed in the palaeotropics and mainly occurring in the understorey of evergreen forest. In accordance with morphological and recent molecular findings (Jacques-Félix 1978; Bremer 1982; Stone 2006, 2014a; Stone & Andreasen 2010), it is now circumscribed to exclude the monospecific western and central African genus Spathandra Guill. & Perr., the palaeotropical Lijndenia Zoll. & Moritzi, and the African/Madagascan Warneckea Gilg. On the African continent, Memecylon sensu stricto currently holds 69 species including 17 recently described (Stone et al. 2006, 2008, 2017, 2019; Stone 2014b, 2015a, 2015b). This diversity is divided between the subgenus Mouririoidea (Jacq.-Fél.) R.D.Stone, a group with ovary 4-loculed and four species of western and central Africa, and subgenus Memecylon with ovary 1-loculed and 65 African species in 12 sections (Stone 2014a). © 2020 Robert Douglas Stone. This article is published and distributed in Open Access under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits use, distribution, and reproduction in any medium, provided the original work (author and source) is properly cited. Plant Ecology and Evolution is published by Meise Botanic Garden and Royal Botanical Society of Belgium ISSN: 2032-3913 (print) – 2032-3921 (online) Stone, A new Memecylon of the East African Rift region Within subgenus Memecylon, the species-groups in eastern and southern Africa are only distantly related to those of the Guineo-Congolian region (sensu White 1983), suggesting an ancient split with limited opportunities for subsequent dispersal (Stone 2014a). The geographic disjunction between the East African and Guineo-Congolian groups coincides with an “arid corridor” extending from the Horn of Africa to Namibia (including the Kenyan and Tanzanian interior) and defined by rainfall less than 10 mm per month in at least three consecutive months (Werger 1978). Until the present work, only two species of the GuineoCongolian species-group were known to occur in East Africa. First, M. myrianthum Gilg (of sect. Polyanthema Engl.) is widely distributed in forests of the Congo Basin but also extends to Rwanda (Troupin 16273, BR) and near Lake Victoria in Tanzania and Uganda (Fernandes & Fernandes 1960; Wickens 1975; Maquet 1983). Second, M. flavovirens Baker (of sect. Obtusifolia Engl.) inhabits seasonally dry, fire-prone “miombo” woodlands in south-central Africa (Angola, Zambia, Democratic Republic of the Congo [Katanga], and Malawi) but has also been collected in Burundi (Reekmans 2753, BR, EA, MO), and its distribution extends to western Tanzania and northwestern Mozambique (Wickens 1975; Stone et al. 2017; Burrows et al. 2018). In preparing the Melastomataceae treatment for the Flore d’Afrique centrale (Sosef 2016), a new species was encountered, occurring in Rwanda, Burundi and western Tanzania, which is here described as Memecylon afroschismaticum R.D.Stone. This species was previously confused with M. myrianthum and the East African M. verruculosum Brenan, but has been recognised informally in the Flora of Tropical East Africa (as Memecylon sp. B; Wickens 1975: 90) and the Flore du Rwanda (as Memecylon sp. A; Maquet 1983: 525). It is not only rare, but also unique in the sense that it is endemic to the East African Rift region (Chorowicz 2005) avoided by other Memecylon species except the widespread M. myrianthum and M. flavovirens (fig. 1). MATERIAL AND METHODS Herbarium material was studied in BM, BNRH, BR, CAS, DSM, E, EA, G, G-DC, K, L, LISC, LMA, LMU, M, MO, NU, NY, P, PRE, PRU, S, UC, UPS, WAG and YA. All cited specimens were seen, unless stated otherwise (“n.v.”). The extent of occurrence (EOO) and area of occupancy (AOO) were estimated using GeoCAT (Bachman et al. 2011), and the conservation status was assessed in accordance with the IUCN Red List Categories and Criteria (IUCN 2012, IUCN Standards and Petitions Subcommittee 2019). RESULTS AND DISCUSSION Memecylon afroschismaticum R.D.Stone, sp. nov. Fig. 2 Type – Rwanda, East Province, Kibungo Prefecture, Rusumo, downstream from the bridge, near the customs house, elev. 1350 m, gallery forest, 24 Jan 1980, Runyinya 935 (holotype: K; isotypes: BR, EA). Description – Shrub up to 4.5 m high with slender branches, the youngest branchlets bisulcate to quadrangular or narrowly Figure 1 – Geographic distribution of Memecylon afroschismaticum in comparison to that of M. flavovirens, M. myrianthum and M. verruculosum. Map created by the author using ArcGIS 10.4 software (ESRI 2016) and Natural Earth (2019) base. 161 Pl. Ecol. Evol. 153 (1), 2020 Figure 2 – Memecylon afroschismaticum. A. Flowering branch. B. Leaf. C. Section of branchlet showing axillary cyme. D. Floral bud. E. Anther dissected from floral bud, side view (left), top view (right). F. Fruit. A from Bridson 287 (BR); B–E from Runyinya 935 (K); F from Harley 9597 (K). Drawn by Sandie Burrows. 162 Stone, A new Memecylon of the East African Rift region quadrangular-alate; nodes scarcely thickened; internodes (1–) 1.5–3(–5.3) cm long; bracts of “aphyllous” nodes narrowly triangular-acute, c. 2 mm long, soon deciduous. Leaves subcoriaceous, dark green and shining on the adaxial surface, paler and dull abaxially; petioles slender, 2–3.5(–4) mm long; blades elliptic to ovate, (3.1–)3.8–5.2(–5.9) cm long, (1.2–)1.6–2.5(–2.8) cm wide, base cuneate to rounded, apex distinctly acuminate with acumen (5–)8–11(–14) mm long, acute; midnerve impressed above, subprominent beneath; transverse veins scarcely visible, 6–7 pairs oriented at an oblique angle relative to the midnerve, faintly prominent on both surfaces in dried material, confluent with the equally weak intramarginal nerves. Cymes up to c. 1 cm long, subumbellate, (1–)3(–5)-flowered, solitary or geminate in the leaf axils or sometimes at the “aphyllous” (bracteate) nodes alternating with the normal, leafy nodes; peduncles (0.5–)1– 3(–3.5) mm long; axis often extended by a short internode 0.5–1.5(–2) mm long above the peduncle; bracts narrowly triangular-acute, c. 1 mm long, rapidly deciduous; pedicels c. 1 mm long. Flowers mauve (in bud white tinted with violet); hypantho-calyx campanulate, 1.5 mm high × 1.5 mm wide, margin sinuate-dentate, teeth deltate-acute, with margins scarious; corolla twisted in bud, narrowly conical-acuminate, c. 2 mm high; anthers in bud c. 1 mm long, connective with thecae positioned at the anterior end, a conspicuous dorsal oil-gland, and posterior extremity narrowly conical-acute; open flowers not seen. Fruits ellipsoid, 7.5–9 mm long × 6–7.5 mm wide, reportedly blackish red when mature; top of ovary in fruit broadly rounded, projecting c. 0.5 mm past the appressed calyx margin, persistent calycinal crown absent; fruiting peduncles 2.5–3.5 mm long; fruiting pedicels 1.5–3 mm. Etymology – The epithet afroschismaticum is a neuter adjective meaning “of the East African Rift region.” Distribution – Southeastern Rwanda, east-central Burundi and extreme western Tanzania (fig. 1). Habitat – Riverine gallery forest and montane forest at elevations of 1300–2250 m. Associated with Drypetes gerrardii Hutch. at the riverine location in Rwanda, and with miombo (Brachystegia Benth.) at the location in Burundi. Additional collections examined (paratypes) – Rwanda: East Province, Kibungo Prefecture, Rusumo, falls of the Akagera [Kagera] river, elev. 1300 m, gallery forest, 24 Jan. 1980, Bridson 287 (BR, EA, K, WAG). Burundi: Kininya [= Kinyinya], Mosso, massif forestier fermé, 10 May 1952, Michel & Reed 1941 (BR, COI n.v., YBI n.v.). Tanzania: Kigoma Region, Kungwe-Mahali [Mahale] Peninsula, below Kungwe Mountain, c. 6°S, 30°E, elev. 2250 m, steep side of rocky ridge, dense forest, 11 Sep. 1959, Harley 9597 (BR, K, S). Preliminary IUCN Red List assessment – Memecylon afroschismaticum has an EOO of 1,296 km2 and an estimated AOO of 12 km2 (assuming a 4 km2 grid cell size). Of the three known localities, only the one in western Tanzania is formally protected within the Mahale Mountains National Park. The subpopulations in Rwanda and Burundi are evidently unprotected and also threatened by the high human population density and intensity of agricultural conversion in these two countries (Vedder 1992; Ntore et al. 2018). At the location near Kinyinya in eastern Burundi, the species was noted as “very abundant” in 1952, but recent satellite imagery (Google Earth Pro 2019) shows a patchwork of residences and smallholder farms with no sign of the closed forest described by the collectors. The Burundian subpopulation thus might be already extirpated. At the location near Rusumo in southeastern Rwanda, relatively intact vegetation can be seen along the banks of the Akagera River (Google Earth Pro 2019), and the species is presumed extant at this locality. The limited EOO and AOO of M. afroschismaticum, together with the above-described threats, would seem to indicate a status of Endangered [EN B1ab(iii)+B2ab(iii)] for this species. Diagnostic characters and similar species – A morphological comparison between M. afroschismaticum, M. flavovirens, M. myrianthum and M. verruculosum is presented in table 1. Of M. afroschismaticum, the narrowly conicalacuminate floral buds and anther connectives with conspicuous dorsal oil-gland and acute posterior extremity suggest a close relationship with M. flavovirens (type of M. sect. Obtusifolia). However, the leaves of these two species are quite different (elliptic-ovate and acuminate in M. afroschismaticum vs. ± obovate and apically rounded, truncate or emarginate in M. flavovirens). They also differ in their inflorescence position (axillary or at intervening “aphyllous” nodes vs. mainly on recently defoliated nodes of upper branchlets), pedicel length (c. 1 mm vs. 2–6 mm) and petal colour (mauve vs. white). The fruits of M. flavovirens are variable (ellipsoid to ovoid or depressed-globose, 12–14 × 8–15 mm with persistent calycinal crown present or absent), but they are consistently larger and often differently shaped in comparison to those of M. afroschismaticum (fruits ellipsoid, 7.5–9 × 6–7.5 mm and lacking a calycinal crown). The leaves of M. afroschismaticum resemble those of M. myrianthum in section Polyanthema as well as M. verruculosum in section Buxifolia R.D.Stone. The sterile collection Michel & Reed 1941, cited here as a paratype of M. afroschismaticum, was previously determined as M. myrianthum (Fernandes & Fernandes 1960: 85), and the paratype Harley 9597 was initially identified as M. verruculosum. However, the new species differs from M. myrianthum in several characters including the number of flowers per inflorescence (inflorescence subumbellate and mostly 3-flowered in M. afroschismaticum vs. inflorescence branched 2–4 times and many-flowered in M. myrianthum), peduncle length (mostly 1–3 mm vs. 5–25 mm), pedicel length (c. 1 mm vs. 2–7 mm), petal colour (mauve vs. white), and fruit shape (ellipsoid with top of ovary projecting 0.5 mm past the appressed calyx margin vs. globose with calycinal crown distinct). It differs from M. verruculosum in petiole length (mostly 2–3.5 mm vs. leaves subsessile or petioles up to 1.5 mm), number of flowers per inflorescence (mostly 3 vs. 1), shape of corolla in bud (narrowly conical-acuminate vs. rounded-apiculate), pedicel length (c. 1 mm vs. 0.1–0.5 mm), petal colour (mauve vs. white), and fruit shape (ellipsoid with top of ovary projecting 0.5 mm past the appressed calyx margin vs. globose with calycinal crown distinct). In addition, M. afroschismaticum is found only in the region of the East African Rift and is wholly allopatric from M. verruculosum (of the 163 Pl. Ecol. Evol. 153 (1), 2020 Table 1 – Morphological comparison between Memecylon afroschismaticum and other African Memecylon species (including its presumed closest relative M. flavovirens as well as two others, M. myrianthum and M. verruculosum, with which it has been previously confused). Note: the geographic distribution of each species is plotted in fig. 1. M. afroschismaticum M. flavovirens M. myrianthum M. verruculosum Habit shrub to 4.5 m shrub or much-branched tree to 7.5 m shrub or tree 2–12(–22) m shrub to 3 m Bark unknown thick, deeply longitudinally fissured thin, finely longitudinally fissured thin bisulcate to quadrangular or narrowly quadrangular-alate quadrangular terete to subquadrangular quadrangular elliptic to ovate obovate (rarely obovateoblong to elliptic or narrowly elliptic) elliptic ovate mostly 3.8–5.2 × 1.6–2.5 cm mostly 2.5–8.5 × 1–5.5 cm 3.5–12 × 2.5–6 cm 2–3.8 × 0.8–2 cm (to 5 × 3 cm in largeleaved form in Eastern Arc Mts of Tanzania) Leaf base cuneate to rounded cuneate cuneate rounded to subcuneate Leaf apex acuminate rounded, truncate or emarginate acuminate acuminate 2–3.5(–4) mm ± absent (leaves subsessile) or to 2 (–4) mm 2–5(–7) mm ± absent (leaves subsessile) or to 1.5 mm Inflorescence position axillary or sometimes at the “aphyllous” (bracteate) nodes alternating with those bearing normal leaves mainly on the recently defoliated nodes of upper branchlets at both leafy and defoliated nodes axillary or at the “aphyllous” (bracteate) nodes alternating with those bearing normal leaves No. of flowers per infl. (1–)3(–5) 3(–5) many (infls. branched branched 2–4×) 1 Peduncle length (mm) mostly 1–3 mm ± absent (infls. subsessile) or to 3(–8) mm 5–25 mm 1–4 mm Shape of corolla in bud narrowly conicalacuminate narrowly conicalacuminate conical-acute rounded-apiculate Pedicel length c. 1 mm 2–6 mm 2–7 mm 0.1–0.5 mm Petal colour mauve white white white colour unknown, dorsal gland conspicuous, posterior extremity narrowly conical-acute deep purple in colour, dorsal gland conspicuous, posterior extremity narrowly conical-acute bright blue in colour, dorsal gland present, posterior conical-acute white in colour, dorsal gland present, posterior extremity obtuse ellipsoid ellipsoid to ovoid or depressed-globose globose globose Fruit size 7.5–9 × 6–7.5 mm 12–14 × 8–15 mm diam. 6–8 mm diam. 5–6 mm Persistent calycinal crown absent, top of ovary projecting 0.5 mm past the appressed calyx margin variably present or absent present present Character Young branchlets Leaf shape Leaf dimensions Petiole length Anthers Fruit shape 164 Stone, A new Memecylon of the East African Rift region Shimba Hills and vicinity in southeastern Kenya with relatively large-leaved forms occurring in the Eastern Arc Mts of Tanzania; see fig. 1). The sharply acuminate leaves of M. afroschismaticum also resemble those of an as-yet undetermined Memecylon from the Ituri forest in northeastern D.R. Congo (represented by the sterile collection Bytebier et al. 3281, BR, EA). This collection has been misidentified as M. nodosum (Engl.) Gilg ex Engl. or M. accedens R.D.Stone, Ghogue & Cheek (both of M. sect. Afzeliana Jacq.-Fél.), but in my nrETS and ITS analyses it was resolved as sister to M. flavovirens (Stone 2014a). In view of these morphological and molecular findings, M. afroschismaticum is provisionally placed near to M. flavovirens in section Obtusifolia. Possibly also belonging to this section is M. poggei Gilg, a poorly known Congolian species with type material (Pogge 1066) kept in B and evidently destroyed during WWII. Further evaluation of the affinities of M. afroschismaticum must await additional collections, especially of DNA samples and material with open flowers. 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