Sonké, B. & O. Lachenaud (2016). Two new of Oxyanthus DC. (Rubiaceae) from Central Africa. Candollea 71: 173–180. In English, English and French abstracts.
Two new Rubiaceae species from Central Africa, Oxyanthus doucetii Sonké & O. Lachenaud and Oxyanthus lewisii Sonké & O. Lachenaud, are described and illustrated. Oxyanthus doucetii is endemic to Cameroon and is closely related to Oxyanthus unilocularis Hiern, from which it differs in the minute calyx teeth, the stems with glabrous internodes, the dorsally glabrous stipules, the flower buds with a shorter head, and the corolla lobes not or hardly acute at apex. Oxyanthus lewisii is sparsely distributed from Cameroon to eastern Democratic Republic of Congo, and is characterised by narrow fusiform fruits, glabrous stems, and leaves lacking domatia but sparsely pubescent on the nerves beneath. Both new species are assessed as “Vulnerable” according to IUCN Red List Categories and Criteria.
Introduction
Oxyanthus DC. is a genus confined to tropical Africa, including about 35 species (Sonké, 1999; Lachenaud & Sonké, 2015). The genus is characterised by a combination of the following characters : persistent stipules, pseudo-axillary inflorescences, 5-merous flowers with long and very narrow corolla tube, contorted corolla aestivation, exserted anthers and style (except O. schumannianus De Wild. & T. Durand, with half-exserted anthers), calyx truncate or with subulate lobes, fleshy fruits with numerous seeds, and striated seed-coat.
The genus was traditionally classified in the tribe Gardenieae (Hallé, 1970; Robbrecht, 1988) of subfamily Ixoroideae. The recent molecular phylogenetic study of the Gardenieae complex by Mouly et al. (2014) led to the description of the new tribe Sherbournieae (subfamily Ixoroideae), which includes four African genera : Oxyanthus, Atractogyne Pierre, Mitriostigma Hochst., and Sherbournia G. Don. The Sherbournieae differs from the Gardenieae s.s. (sensu Mouly et al., 2014) by having a striated rather than smooth seedcoat. Within this group, Oxyanthus differs from the other three genera by its very narrow corolla tube.
The centre of diversity of Oxyanthus is in Central Africa, where 18 species are known (Sonké, 1999; Cheek & Sonké, 2000). Eight species are found in West Africa (Lachenaud &Sonké, 2015) and eleven in East Africa (Bridson & Verdcourt, 1988).
In his revision of the Central African species, Sonké (1999) reported significant variation in the widespread species O. unilocularis Hiern, particularly in the size of the calyx teeth. A reexamination of the available material shows that this character is correlated with two others (the indumentum of the twigs and stipules, and the shape of the corolla lobes), and leads to the conclusion that two different species should be recognized. The form with short calyx teeth is here described as a new species, O. doucetii Sonké & O. Lachenaud, endemic to Cameroon, while O. unilocularis remains a widespread and common Guineo-Congolian species, occuring from Sierra Leone to Uganda.
Sonké (1999) also studied a collection from Cameroon (Letouzey 4585) and another from eastern Democratic Republic of Congo (A. Léonard 3872). He recognised them as different from all species described so far in the genus, and gave them the provisional names of Oxyanthus sp. B and Oxyanthus sp. C, respectively. A reexamination of these specimens shows that they belong to the same taxon, which is also represented by three collections from Gabon, discovered since. Although mature flowers of this taxon are still unknown, the shape of the flower buds and the striated seed coat clearly place it in Oxyanthus. It is also quite distinct from other Oxyanthus species, particularly in the unusual shape of its fruits. Accordingly, we here describe this plant as a new species, O. lewisii Sonké & O. Lachenaud.
Material and methods
Herbarium material of Oxyanthus was consulted at BR, BRLU, P, WAG and YA. Measurements, colours and other details given in the descriptions are based on herbarium specimens and data derived from field notes.The distribution map was done using georeferenced specimen data derived from labels or available literature. Phytogeographical considerations follow White (1979) although we simplify his “subcentres of endemism” into Domains.The conservation status was assessed by calculating the extent of occurrence (EOO) and the area of occupancy (AOO) using the GeoCAT tool (Bachman et al., 2011) and applying the IUCN Red List Categories and Criteria (IUCN, 2012 ; IUCN SPS, 2015).
Taxonomic treatment
Oxyanthus doucetii Sonké & O. Lachenaud, spec, nova
(Fig. 1).
Typus : Cameroon. South Region : 16 km on the road from Ebolowa to Minkok, 2°58′N 11°17′E, 28.IV.1975, fl., De Wilde 8207 (holo- : BR! ; iso- : P [P04006788]!, WAG [WAG. 1228575, WAG. 1228576, WAG. 1228577]!,YA!).
Oxyanthus doucetii Sonké & O. Lachenaud closely resembles O. unilocularis Hiern in the large and broad asymmetrical leaves with sparsely pilose lower surface, and the manyflowered inflorescences with long corollas. The species differs from O. unilocularis in the minute calyx teeth (<0.3 mm long), the stems with glabrous internodes, the stipules glabrous except sometimes on their margin, the flower buds with a shorter head 8.5–11 mm long, and the corolla lobes not or hardly acute at apex; by contrast, O. unilocularis has well-developed subulate calyx teeth 1.5–10(-14) mm long, densely puberulous stems and stipules, the head of the flower buds 12–25 mm long, and the corolla lobes sharply acute at apex.
Shrub or small tree, 2–10 m tall, with horizontal branches; stems hollow, 7–14 mm thick, glabrous or with short sparse hairs at the nodes only. Stipules 23–48 – 16–25 mm, broadly mitriform with ± acute apex, glabrous or sparsely ciliate, persistent (except at flowering nodes, where often caducous). Leaves with petiole 0.5–1.5 cm, glabrous or with short sparse hairs, and leaf blades 34–60 × 18–30 cm, very broadly elliptic, strongly asymmetrical at base with proximal side rounded to cordate and distal side acute to obtuse inserted 5–12 mm higher, acute or shortly acuminate at apex, glabrous above, sparsely pilose below with ± scabrid erect hairs 0.7–1 mm long; lateral nerves 12–13 pairs, strongly ascending (except the lower ones) and forming loops well away from the margin; tertiary veins laxly reticulate ; domatia absent. Inflorescences pseudo-axillary, one per branch and per season, corymbiform, glabrous, with several hundred flowers but usually producing only few fruits; peduncle 0.6–1.5 cm long, rhachis 5–10.5 cm long, lateral branches 1.5–4 cm long. Bracts minutely triangular, < 1(-1.5) mm, glabrous. Flowers 5-merous; pedicels 2–8 mm long, glabrous. Ovary 1.5–2 mm long, glabrous. Calyx with tube 0.7–1 mm long and minute, acutely triangular teeth 0.2–0.3 mm long, entirely glabrous. Corolla tube pale green, very narrowly cylindrical, 12.2–13.7 cm long × c. 0.2 cm wide, glabrous; corolla lobes white, lanceolate, obtuse or only faintly acute at apex, 1.2–1.7 cm long × 0.25–0.3 cm wide, glabrous outside, shortly papillose inside. Anthers exserted, ± patent, inserted at the corolla throat, linear, 3–5 × 0.5 mm including a sterile apical appendage 0.5–1 mm long, glabrous. Style exserted, exceeding corolla throat by 1.5–2 cm, glabrous, with slightly swollen elongated stigma. Fruits green, smooth, ovoid to globose, 1.3–2.7 × 1.4–2 cm when dry, glabrous, with pedicel not markedly accrescent and calyx persistent. Seeds numerous, irregularly polygonal and compressed, c. 9 × 5 mm, the surface folded into numerous closely parallel ridges.
Etymology. — The species name honours Prof. Jean-Louis Doucet, specialist in tropical forestry (Université de Liège, Gembloux Agro-Bio Tech, Belgium), to whom the first author is particularly indebted for his constant support.
Distribution and ecology. — Oxyanthus doucetii is endemic to Cameroon in the Lower Guinea Domain (Fig. 2), and is found mostly in the Central Region around Yaoundé, more rarely in the southwest (Mt. Cameroon) and South Regions (around Bipindi and Ebolowa).The species occurs in primary and secondary forests between 480 and 760 m.
Phenology. — Flower buds in October–November, mature flowers in April; fruits in March, June–July (immature) and from September to December.
Conservation status. — The EOO is estimated as c. 35,435 km2 and AOO as 44 km2. The species is known from nine locations. None of these locations are protected, and deforestation for agriculture (and also for urban extension in the periphery of Yaoundé) represents a clear threat to the species. A decline in AOO, extent and quality of habitat, number of locations and number of individuals is therefore expected, and O. doucetii is assigned a preliminary conservation status of “Vulnerable” [VU B2ab(i,ii,iii,iv)].
Notes. — As mentioned above, O. doucetii has previously been confused with the morphologically similar, and presumably closely related, O. unilocularis. It differs from the latter in having minute calyx teeth (<0.3 mm long), the stems with glabrous internodes, the stipules glabrous except sometimes on their margin, the flower buds with a shorter head 8.5– 11 mm long, and the corolla lobes not or hardly acute at apex (compare Fig. 1 and 3). Additionally, O. doucetii tends to have larger and broader stipules (23–48 × 16–25 mm) than O. unilocularis (7–35 × 4–22 mm), but this character shows significant overlap between the two species. The stipules of O. doucetii are the largest in the genus.
Two sterile collections from Mt Kolodom near Yaoundé, Sonké 120 & 121 (BR), are intermediate between O. doucetii and O. unilocularis in vegetative characters.The internodes are very sparsely pubescent, often only towards the apex, and the stipules are pubescent only at the base and margins. In the absence of fertile material, these collections cannot be referred with certainty to either species; they may represent hybrids, since both species are found in the region.
Paratypi. — CAMEROON. Central Region: Yaoundé, NE, 1939, fl. buds, Jacques-Félix 4799 (BR); Nkolakié, 3°57′N 11°21′E, VII.2010, fr. imm., Kayo 1 (BR); ibid. loc., 4.V.2003, fl. (fallen), Nguembou & Djuikouo 665 (BRLU); ibid. loc., 8.VI.2003, fl., Nguembou & Djuikouo 796 (BR, BRLU); Mont Kala, 3°53′N 11°30′E, 5.V.2004, fl. buds, Nguembou et al. 1202 (BR, BRLU); Mont Ngoa Ekele [3°51′N 11°24′E], 4.XII.1986, fr., Sonké 43 (BR, YA); Mt Akondoué [3°50′N 11°29′E], 23.IV.1987, fl. buds, Sonké 95 (BR, K, YA); ibid. loc., 29.IX.1987, fr., Sonké 116 (BR, WAG); ibid. loc., 31.X.1987, fl. buds & fr., Sonké 117 (BR); Mt Kala, 14.XI.1987, fr., Sonké 123 (BR, YA); ibid. loc., same date, Sonké 124 (BR, YA); Mt Kolodom, [3°51′N 11°22′E], 23.IV.1988, fl., Sonké 141 (BR, YA); Nkolfep, Massif du Mbaminkom, 3°58′N 11°23′E, 3.III.2002, fr., Sonké, Nguembou & Djuikouo 2758 (BR, BRLU); Ndjamtchourou (Ngoro), 4°57′N 11°20′E, 20.VII.2009, fr. imm., Sonké & Simo 5297 (BR). South Region : Bipindi, 3°5′N 10°24′E, 26.XI.2004, fl. buds & fr., Sonké & Nguembou 3598 (BR). Southwest Region: env. du village Efolofo, pentes septentrionales du Mt Cameroun, 4°21′N 9°7′E, 1.VI.1976, fr. imm., Satabié 300 (BR, P, YA).
Fig. 1.
Oxyanthus doucetii Sonké & O. Lachenaud. A. Inflorescence; B. Apex of twig with one leaf and the base of the opposite leaf (pubescence shown on a portion of the leaf); C. Apex of twig with stipules; D. Calyx; E. Detail of the apex of a flower bud; F. Detail of opened flower showing the lobes and upper part of tube; G. Fruit.
[A, D–F: J.J. de Wilde 8207, BR; B, G: Sonké 124, BR; C: Sonké et al. 2758, BR] [Drawing: A. Fernandez]
Fig. 2.
Distribution of Oxyanthus doucetii Sonké & O. Lachenaud (circles) and O. lewisii Sonké & O. Lachenaud (squares).
Oxyanthus lewisii Sonké & O. Lachenaud, spec. nova
(Fig. 4, 5).
— Oxyanthus sp. B, sp. C in Sonké (1999: 97, 98).
Typus: Cameroon. East Region : près Mekomo, 8 km SW confluent Dja et Lobo, [3°4′N 12°22′E], 19.III.1962, fl. buds & fruits, Letouzey 4585 (holo- : BR [BR880499] !; iso- : BR [BR880466, BR880508] !, P [P03907844] !, YA!).
Oxyanthus lewisii Sonké & O. Lachenaud has fusiform fruits, resembling those of O. robbrechtianus Sonké, O. dubius De Wild, and O. speciosus DC. subsp. stenocarpus (K.Schum.) Bridson, but more narrowly pointed at apex than in these species. It further differs from O. dubius by the shorter calyx teeth (0.4–0.6 mm, not 3–4 mm) and bracts (1.5–2 mm, not 5–8 mm), from O. speciosus subsp. stenocarpus by the shorter and few-flowered inflorescences, and from both by the nerves sparsely hairy beneath and lacking domatia in their axils. From O. robbrechtianus, it also differs in the glabrous twigs and outside of corolla, the leaves symmetrical at base, and the orange (not red) fruits without longitudinal ridges.
Shrub 1–2 m tall, with horizontal branches; stems glabrous. Stipules 4.5–12 × 2.5–7 mm, narrowly ovate with acute apex, glabrous, persistent. Leaves with petiole 0.4–1.2 cm long, glabrous to sparsely pubescent, and leaf blade 9–18.5 × 1.9– 8.5 cm, elliptic or elliptic-oblong; cuneate at base, acuminate at apex, glabrous above, sparsely and shortly pubescent on the midrib and lateral nerves beneath; lateral nerves 5–9 pairs, moderately to strongly ascending; tertiary veins rather densely reticulate; domatia absent. Inflorescences pseudo-axillary, one per branch and per season, very shortly paniculate, < 1.3 cm long, glabrous, with 5–14 flowers (usually producing 1–3 fruits); peduncle absent or very short (to 0.2 cm), rachis < 1.1 cm. Bracts subulate, 1.5–2 mm long, ciliate. Flowers 5-merous; pedicels 1–2 mm long, glabrous. Ovary 1–1.3 mm long, glabrous. Calyx with tube c. 1 mm long and short subulate teeth 0.4–0.6 mm, entirely glabrous. Corolla only known in very young bud stage, c. 0.65 cm long; tube cylindrical, glabrous, c. 0.15 cm long; lobes narrowly lanceolate, c. 0.5 cm long × 0.1 cm wide, glabrous on both sides. Anthers smooth, linear, 3.6–4 × 0.1 mm, including a sterile apical appendage ± 1 mm long. Style ± 6 mm long, with a slightly swollen elongated stigma ± 2 mm long. Fruits orange, fusiform, smooth or slightly rugose but without longitudinal ridges, 3.8– 6.5(-8) × 0.5–1.5 cm, gradually pointing into a narrow beak (0.2–0.4 cm wide at apex), glabrous, with accrescent pedicel 0.5–1 cm long and calyx usually deciduous. Seeds numerous, compressed, irregularly ellipsoid, 6–7 × 3–5 mm, the surface folded into numerous closely parallel ridges.
Etymology.— This species is named after Prof. Simon Lewis, internationally renowned British ecologist (Chair in Global Change Science, Department of Geography, University College London) who made several expeditions in Cameroon with the first author, and constantly supported his research.
Distribution and ecology. — Oxyanthus lewisii is sparsely distributed in southern Cameroon, northern Gabon, and eastern D.R. Congo in the Lower Guinea and Congolian Domains (Fig. 2). It appears to have a widely disjunct range, with a gap of over 1,000 km between the D.R. Congo locality and the rest of the range; similar disjunctions are known in other species, e.g. in Chassalia pleuroneura (K.Schum.) O. Lachenaud (Lachenaud &Jongkind, 2010).The species is apparently rare; it occurs in lowland forest between 380 and 750 m, at least sometimes in riverine or periodically inundated formations.
Phenology. — Flower buds in March; fruits from December to April.
Conservation status. — The EOO is estimated at c. 371,433 km2 and the AOO as 20 km2. The species is known from five locations, none of which are protected. Logging activities and deforestation for agriculture (and also for mining in the eastern part of its range) represent potential threats to the species. A decline in AOO, extent and quality of habitat, number of locations and number of individuals may therefore be expected, and Oxyanthus lewisii is assigned a preliminary conservation status of “Vulnerable” [VU B2ab(ii,iii,iv,v)].
Notes. — Oxyanthus lewisii is remarkable by the long fusiform fruits, which are somewhat variable in shape, being particularly narrow in Letouzey 4585 and Le Testu 9033, but distinctly broadened in the middle in Dauby et al. 2293. The only other taxa with fruits approaching in shape those of O. lewisii, although not so pointed at apex, are O. robbrechtianus Sonké, O. dubius De Wild, and O. speciosus subsp. stenocarpus (K.Schum.) Bridson. Oxyanthus lewisii differs from O. dubius by the shorter calyx teeth and bracts, from O. speciosus subsp. stenocarpus by the shorter and few-flowered inflorescences, and from O. robbrechtianus, it also differs in the glabrous twigs and outside of corolla, the symmetrical leaf base, and the fruits lacking longitudinal ridges. Oxyanthus speciosus subsp. stenocarpus also has a different habitat (submontane forests, 1000–2000 m).
In the absence of fruits, O. lewisii might be confused with O. gracilis K. Schum., O. pallidus Hiern and O. subpunctatus (Hiern) Keay, all of which have the leaves fairly similar in size and shape, but either entirely glabrous beneath (O. pallidus) or pubescent only on the nerve axils (O. gracilis and O. subpunctatus).
Paratypi. — Gabon. Estuaire: Parc des Mts de Cristal, région d'Akoga, 0°51′50″N 10°29′53″E, 13.XII.2015, fr., Boupoya & Issembé 1211 (BRLU). Ogooué-Ivindo: Concession CEB, N de la Zone de Milolé, 2°14′55″S 12°44′19″E, 12.II.2010, fr., Dauby et al. 2293 (BRLU). Woleu-Ntem: Aloum [2°09′N 11°42′E], 14.III.1933, fr., Le Testu 9033 (BR). D.R. Congo. South Kivu: Kingulube [2°39′S 28°02′E], fr., 15.IV.1959,A. Léonard 3872 (BR).
Fig. 3.
Oxyanthus unilocularis Hiern. A. Stipules and fragment of twig; B. Calyx; C. Apex of flower bud. [Wieringa et al. 2479, BR] [Drawing: A. Fernandez]
Acknowledgements
BS's visit to Europe in 2015 was supported by the project AUF “Les changements climatiques et leurs impacts sur l'évolution de la biodiversité des forêts tropicales humides d'Afrique Centrale: implications pour le futur”. He particularly wishes to thank Prof. Pierre François Docgoue (AUF) and Dr.Thomas Couvreur (IRD), for assistance, facilities and proofreading. His visit to Belgium in February 2016, during which this paper was completed, was funded by Nature+ asbl (Gembloux, Belgium). We are grateful to Antonio Fernandez (Botanic Garden Meise) for the illustrations of the species. The herbarium curators of BR, BRLU, P, WAG and YA are thanked for their assistance while working in their institutes. We also thank Gilles Dauby for his photograph of O. lewisii, Lise Zemagho for her help with the IUCN assessments, and Ehoarn Bidault (Paris) and Roxali Bijmoer (Naturalis, Leiden) for their help in finding the isotypes of O. doucetii.The comments of an anonymous reviewer were helpful for a final revision of the paper.
