Craterispermum gabonicum Taedoumg & De Block, 2017

Taedoumg, Herman, Sonke, Bonaventure, Hamon, Perla & Block, Petra De, 2017, Craterispermum capitatum and C. gabonicum (Rubiaceae): two new species from the Lower Guinean and Congolian Domains, PhytoKeys 83, pp. 103-118 : 105-109

Craterispermum gabonicum Taedoumg & De Block sp. nov. Figs 3 , 4

Diagnosis.

Resembling C. ledermannii K. Krause, 1912 because of the large leaves and the often robust peduncles, but differing from this species by the subcapitate inflorescences (vs branched in C. ledermannii ), the notable dimorphism between flowers and inflorescences of the different flower morphs, the secondary nerves clearly ascending and forming acute angles with the midrib (vs secondary nerves more or less perpendicular to the midrib).

Type.

GABON. Ogooué-Maritime: Rabi-Kounga , ca. 4 km N of Shell-camp, 1°55'S, 9°52'E, 19 September 1992 (fl), J.J. Wieringa & J.B. Epoma 1611 (holotype WAG [WAG0233599], isotype WAG [WAG0233600]) .

Shrub or treelet, 1.5-9 m tall; all vegetative and reproductive parts glabrous externally. Stems brownish, ca. 8 cm in diameter; young branches greenish or grayish, somewhat granular in outlook. Stipules persistent, sheath 2-7 mm long, keeled, subtruncate or with short awn <1 mm long. Leaves petiolate; petioles canaliculate, 7-19 mm long; leaf blades narrowly elliptic or narrowly obovate, more rarely elliptic or obovate, 10.5-24 × 4.4-7.2 cm, coriaceous, brownish or yellowish green and generally dull above, paler green or brown below; base cuneate; apex acuminate, acumen 8-16 mm long; margins not revolute; midrib prominent below; secondary nerves 6-9 pairs, clearly ascending and forming acute angles with midrib, obscure on both surfaces; intersecondary venation moderately prominent above, obscure on lower surface, obscure to almost invisible on both surfaces in fresh condition. Inflorescences axillary to supra-axillary, borne up to 7.5 mm above the nodes, erect, subcapitate to capitate, 4-19 × 5-20 mm; peduncle subcylindrical to flattened (up to 3.5 mm wide), robust, 1.5-4.5(-7) mm long. Inflorescences completely covered by imbricate outer bracts when young, medium green to greenish white (somewhat resembling an immature fruit); brevistylous and longistylous inflorescences dimorphous: brevistylous inflorescences very congested, 11.5-19 × 7.5-20 mm, pauciflorous to multiflorous; bracts and bracteoles broadly triangular or ovate, 6-8 × 5-8 mm and 6 × 3 mm respectively with rounded or rarely subtruncate apex and margins sometimes bearing sparse colleters; longistylous inflorescences less congested, 4-7 × 5.2-9.1 mm, pauciflorous; bracts and bracteoles broadly triangular, ca. 2 × 1.5-2 mm and 0.7-2 × 1-2 mm respectively with rounded or truncate apex and margins sometimes bearing sparse colleters. Flowers heterostylous, 5-merous, sessile; ovary and calyx pale green or whitish, somewhat violet tinged; corolla narrowly cylindrical, white; anthers and filaments white or whitish violet. Brevistylous flowers: Calyx with tube (1.5-) 3-5 mm long, sometimes bearing sparse colleters at the base inside; lobes (0.3-0.6)- 1.5 mm long, apex acute, obtuse or rounded, margins sometimes bearing sparse colleters. Corolla with tube 6-12 mm long, pubescent in the throat and upper half inside; lobes 3-6 mm long, covered with short hairs at the base inside, tips acute. Stamens inserted ca. 2 mm below the level of the throat, completely exserted from or only bases included in corolla tube at anthesis; anthers 1.1-3 mm long; filaments 1-2.5 mm long. Ovary 0.5-1.2 mm long. Style and stigma included in corolla tube at anthesis, 4-7.5 mm long; stigma bilobed, stigmatic lobes 1.5-2 mm long. Longistylous flowers: Calyx with tube 1-1.5 mm long; lobes triangular, 0.5-0.7 mm long sometimes bearing sparse colleters at the base inside. Corolla with tube 4-5 mm long, pubescent in the throat and upper third inside; lobes 3.5-4 mm long, densely pubescent at the base inside, tips acute. Stamens inserted ca. 2 mm below the level of the throat; completely included in the corolla tube or only the apices exserted at anthesis; anthers 1.8-2 mm long; filaments <0.5 mm long. Ovary ca. 1.2 mm long. Style and stigma exserted from the corolla tube at anthesis, ca. 7.5 mm long; stigma bilobed, stigmatic lobes ca. 1.5 mm long. Fruits sessile, urceolate, ca. 16 × 8 mm, colour at maturity unknown.

Taxonomic affinities.

This species is atypical in the genus because of the flower and inflorescence dimorphism. However, it somewhat resembles C. ledermannii because of the large leaf blades and the often robust peduncles. It differs from this species by its subcapitate inflorescences (vs mostly branched in C. ledermannii ), its secondary nerves clearly ascending and forming acute angles with the midrib (vs more or less perpendicular to the midrib in C. ledermannii ) and by the fact that its inflorescences are completely covered by imbricate outer bracts when young (somewhat resembling an immature fruit) (vs never completely covered in C. ledermannii ).

Phenology.

Flowers: March-May and August-November. Fruits: February, April and August-December.

Distribution and habitat.

Craterispermum gabonicum is endemic to Gabon. It grows in primary forest but also in humid secondary forest between 150 and 400 m elevation (Fig. 4 ).

Vernacular names and uses.

Unknown.

Preliminary conservation status.

IUCN status:-Vulnerable: VU B2(iii). The extent of occurrence (EOO) of C. gabonicum is 80,847.46 km², and its area of occupancy (AOO) is 60 km² using a cell width of 2 km. The species is distributed in 7 or 8 subpopulations, 2 of which are located in protected areas: in Wonga Wongué Forest Reserve in Ogooué-Maritime Division and on the edge of the Loango National Park in West Gabon. The Wonga Wongué Forest Reserve is subject, these last years, to a very strong pressure from uncontrolled anthropomorphic activities (illegal exploitation of the resources as well as degradation of the ecosystems as a result of oil exploitation). Habitat loss outside and inside the protected areas is a serious threat for C. gabonicum .

Etymology.

This species is named after the country to which it is currently endemic.

Critical notes.

Within this species flowers and inflorescences are very variable in shape and size, more so than in other species of the genus. This variability seems to be a priori correlated with the heterostyly, which is present in all species of the genus. However, C. gabonicum does not only show the reciprocal stigma and anther position and the pollen dimorphism typical for heterostylous species and present in all continental African Craterispermum , but a further dimorphism occurs at flower and at inflorescence level. The corolla tube of brevistylous flowers is longer and wider than that of longistylous flowers (6-12 mm × ca. 3 vs 4-5 × 1.5-2 mm) (Fig. 3 H-I, K-L ). Brevistylous flowers generally also have longer and wider calyx tubes than longistylous flowers [(1.5-)3-5 mm vs 1-2 mm long) (Fig. 3J-G ). Except for the length of the filaments, possible size differences in anthers and stigmatic lobes could not conclusively be observed. In regard to the inflorescences, those with brevistylous flowers comprise more flowers than those with longistylous ones (multiflorous vs pauciflorous, respectively)(Fig. 3C-D ). Furthermore, bracts and bracteoles are larger in inflorescences with brevistylous flowers than in inflorescences with longistylous flowers (6-8 × 5-8 mm and ca. 6 × 3 mm respectively vs ca. 2 × 1.5-2 mm and 0.7-2 × 1-2 mm) (Fig. 3E-F ).

These differences in size are unknown in heterostylous species but are typical for certain dioecious ones ( Pailler et al. 1998; Lantz and Bremer 2004; Mouly and Achille 2007). In several plant groups dioecy has been shown to have evolved from heterostyly, with the functionally male flowers derived from the brevistylous and the functionally female flowers from the longistylous morphs ( Beach and Bawa 1980). This is also the case for certain Rubiaceae species, such as Chassalia corallioides (Cordem.) Verdc. ( Pailler et al. 1998) and Mussaenda parviflora Miq. ( Naiki and Kato 1999). The size difference in flowers and inflorescences observed in C. gabonicum is also reported from certain dioecious species. Fewer flowers per inflorescence are found in individuals with female flowers than in individuals with male flowers in dioecious species of the tribe Vanguerieae ( Lantz and Bremer 2004; Mouly and Achille 2007). Also, in certain dioecious species, such as Chassalia corallioides ( Pailler et al. 1998) male flowers have longer corolla tubes than female flowers. We therefore suggest that a trend towards functional dioecy could be the explanation for the dimorphism in flowers and inflorescences in C. gabonicum , with the flowers being morphologically heterostylous but functionally dioecious or evolving towards this condition.

It is very difficult to verify this hypothesis without field studies, especially since mature flowers of both morphs and mature fruits are rare on the available herbarium material and no fixed flower material was available for detailed morphological and anatomical studies. With hardly any fruit set, it is impossible to know whether only one (brevistylous) or both morphs set fruit. Furthermore, both morphs produce viable pollen (based on morphological characters) although anthers are somewhat larger and pollen more abundant in the brevistylous morph. While the calyx tube is much longer in the brevistylous morph, this is not the case for the ovaries, which rather are somewhat reduced. All ovaries of brevistylous flowers contained ovules, but these too seemed somewhat reduced in size. Because of the lack of available plant material with mature flowers and fruits, it is impossible to demonstrate with certainty the hypothesis stated here that dioecy in some form is present in C. gabonicum . The species would certainly be an ideal species for field studies focusing on breeding system and reproductive ecology.

Additional specimens examined

(paratypes). GABON: Ogooé-Maritime, Toucan, 1°47'S, 9°53'E, 29 May 2002 (fl bud), H.P. Bourobou Bourobou, G. Niang-Essouma & T. Nzabi 623 (K, MO, P, WAG); 50 km SE of Lambaréné, 1°4'S, 10°30'E, 30 September 1968 (fl), F.J. Breteler 5747 (BR, K, MO, WAG); Rabi , 1°55'S, 9°50'E, 24 March 1990 (st), J. Breteler & C.C.H. Jongkind, J. Wieringa & J.M. Moussavou 9437 (BR, WAG); about 30 km E of Lastoursville, 0°40'S, 13°00'E, 20 November 1991 (fl bud), F. J. Breteler & C.C.H. Jongkind 10609 (WAG); 5-30 km NNW of Ndjolé, 0°5'S, 10°45'E, 21 April 1992 (f bud, fr), F.J. Breteler, C.C.H Jongkind. & J. Wieringa 10979 (WAG); Moyen-Ogooué, ca. 20-30 km NNW of Ndjolé, 0°3'S, 10°45'E, 1 October 1994 (fl), F.J. Breteler, B.J.M. Breteler & Klein Breteler 13110 (WAG); Ogooué-Maritime, Rabi-Kounga , route Divangui , 1°54'S, 9°46'E, 14 July 1998 (fl bud), F.J. Breteler, J.M. Moussavou, J. Nang & O. Pascal 14428 (WAG); Rabi 51, 1°55'S, 9°53'E, 1 March 2007 (st), J. Choo 1042 (BR); about 30 km NW of Doussala, in the direction of Bongo , 2°38'S, 11°38'E, 16 March 1988 (fl), J.J.F.E. De Wilde & C.C.H. Jongkind 9392 (BR, K, MO, WAG); Abanga , chantier C.E.T.A. 0°12'S, 10°11'E, 3 June 1963 (st), N. Hallé 2170 (P); Moyen-Ogooué, Camp Mboumi , 0°25'S, 10°50'E, 1 September 1999 (fl), Y. Issembe 244 (WAG); Concession Murel & Prom près du Lac Ezanga, 1°5'S, 10°13'E, 51 m, 24 November 2013 (st), O. Lachenaud, D. Ikabanga, E. Akouangou, J.Y. Serein, E. Bidault, Y. Issembe, A. Boupoya & J.D.D. Kaparadi 1609 (BRLU); 2 km SE of Forestry Camp Waka, situated ca. 32 km SE of Sindara, Waka River basin, 1°14'S, 10°53'E, 10 December 1983 (fr), A.M. Louis, F. Breteler & J. De Bruijn 1248 (WAG); Rabi (parcelle Smithsonian) code dans la parcelle: CRATGF, 1°55'S, 9°52'E, 30m, 18 March 2011 (st), D. Nguema, H. Memiague, P. Bissiemou, E. Mounoumou, G. Moussavou, L. Tchignoumba, D. Bikissa & M.W. Mbanding 1311 (BRLU); chantier CEB, ca. 50 km SW of Doussala, 2°36'S, 10°35'E, 21 August 1985 (fl, fr), J.M. Reitsma & B. Reitsma 1342 (BR, WAG); at logging site of CBG, ca. 5 km beyond checkpoint Divangui, 1°50'S, 10°0'E, 29 October 1990 (fr), I. Van Nek 152 (WAG); Nyanga, Moukalaba Doudou National Park , 2°44'S, 10°30'E, 20 February 2004 (fl, fr), J. van Valkenburg, L. Ngok Banak, Y. Issembé & T. Nzabi 2872 (BR, K, WAG); Moyen-Ogooué, Ezanga, Layon D ouest, 1°5'S, 10°14'E, sd (fl) C.M. Wilks 2466 (WAG) .

Figures

Figure 3. Craterispermum gabonicum. A Flowering branch (brevistylous morph) B Node with stipules and young inflorescences C Inflorescence (longistylous morph) D Inflorescence (corollas fallen) (brevistylous morph) E Bracteole (brevistylous morph) F Bracteole (longistylous morph) G Corolla (longistylous morph) H Longitudinal section of corolla (longistylous morph) I Corolla (brevistylous morph) J Longitudinal section of corolla (brevistylous morph) K Tip of corolla lobe showing subapical spike-like protuberance L Calyx (longistylous morph) M Calyx (brevistylous morph) N Immature fruit. A-B, D-E, G-H, M from Wieringa 1611 (WAG), C, F, I-K, L from Issembe 244 (WAG), N from Breteler 10979 (WAG). Drawn by Marijke Meersman.

Figure 4. Distribution map of Craterispermum gabonicum.