Memecylon afroschismaticum sp. nov. (Melastomataceae–Olisbeoideae) endemic to the East African Rift region of Rwanda, Burundi and western Tanzania

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.


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 2006Stone , 2014aStone & 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.

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 Guineo-Congolian 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 (holo type: K; isotypes: BR, EA). Description -Shrub up to 4.5 m high with slender branches, the youngest branchlets bisulcate to quadrangular or narrowly  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. Preliminary IUCN Red List assessment -Memecylon afroschismaticum has an EOO of 1,296 km 2 and an estimated AOO of 12 km 2 (assuming a 4 km 2 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.  (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   (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.

Stone, A new
Further evaluation of the affinities of M. afroschismaticum must await additional collections, especially of DNA samples and material with open flowers. As of this writing, the Burundian collection is sterile, the Tanzanian material has only fruits, and the two collections from Rwanda have only floral buds in a fairly early stage of development.