Craterispermum capitatum and C. gabonicum (Rubiaceae): two new species from the Lower Guinean and Congolian Domains

Abstract Craterispermum capitatum and C. gabonicum, two new species of Rubiaceae, are described from the Lower Guinea and Congolian Domains. Detailed descriptions and distribution maps are provided for each species, their conservation status is assessed and their taxonomic affinities are discussed. Craterispermum gabonicum is unique within the genus because of the strong dimorphism in brevistylous and longistylous flowers and inflorescences. We hypothesize that this species shows some form of dioecy. The distribution of C. capitatum shows a wide disjunction: the species is present in the Lower Guinean and Congolian Domains but absent from Gabon and South Cameroon. An identification key for the Craterispermum species present in the Lower Guinean and Congolian Domains is given.


Introduction
The genus Craterispermum Benth. (Rubiaceae, subfamily Rubioideae) is distributed in tropical Africa, Madagascar and the Seychelles (Robbrecht 1988, Taedoumg et al. 2011, Taedoumg and Hamon 2013De Block and Randriamboavonjy 2015). Craterispermum species are shrubs or small trees with axillary or supra-axillary inflorescences, paired at the nodes and often condensed. The flowers are few to many per inflorescence, small, heterostylous and white. The ovary is bilocular with a single, apically attached, pendulous ovule in each locule. One ovule aborts and the fleshy fruit contains a single seed, shaped like an asymmetrical shallow or deep bowl. The seed has a peculiar, discontinuous seed coat, comprised of isolated cells with ring-like thickenings (Igersheim 1992). Raphides are present in all plant tissues in the genus. Craterispermum species have been shown to accumulate aluminium in leaves and stem tissue (Jansen et al. 2000); the leaves dry pale yellow or green, which is typical for aluminium accumulating species.
According to Anderson (1973), there are probably more heterostylous species in the Rubiaceae than in all other angiosperm families put together. In the genus Craterispermum heterostyly was often overlooked and plants with different floral morphs were sometimes described as separate species. For example, the type of C. congolanum De Wild. & Th. Dur. is just the brevistylous morph of C. angustifolium De Wild. & Th. Dur. De Wildeman, 1924 was one of the first to notice that some described species were just different morphs of heterostylous species. In Craterispermum and in other heterostylous Rubiaceae species (e.g. Psychotria L.), thrum flowers characteristically have included styles and exserted anthers; pin flowers have exserted styles and included anthers (complete heterostyly) (Robbrecht 1988).
Craterispermum is easily recognized at the genus level, but many of the species look similar and identification at the species level is difficult (Verdcourt 1973, Taedoumg et al. 2011. Herbarium material of Craterispermum is often poor, generally carrying only residual inflorescences. Because of the compact structure of the inflorescences, flowers and fruits fall easily during collecting, pressing, drying and mounting. Moreover, flowers are short-lived and ripe fruits do not remain on the plant for long (Taedoumg et al. 2011). The above-mentioned reasons make Craterispermum species challenging to describe.
The examination of the available herbarium material allowed us to highlight the existence of several new species. Hitherto, we have described five species from continental Africa (Taedoumg et al. 2011;Taedoumg and Hamon 2013). The present paper describes two further species from Cameroon, Gabon, Nigeria, Congo and the Democratic Republic of Congo. An identification key for the Craterispermum species present in the Lower Guinean and Congolian Domains is also given.

Methods
Herbarium material of the following institutions was studied: BR, BRLU, G, K, MO, P, WAG and YA. Descriptive terminology follows Robbrecht (1988) and Anonymous (1962). Phytogeographical terminology follows White (1979). Measurements and other given details are based on the study of herbarium specimens, using a Leica MZ95 stereomicroscope, and data derived from field notes. In the descriptions and key, inflorescence size does not include the corollas, and given colours (except flower colour) are for dried material. Inflorescences are described as uniflorous (one flower only), pauciflorous (2 to 9 flowers) or multiflorous (10 to up to 50 flowers). Flowering and fruiting periods are given as cited on the collector's labels.
Specimens are cited per country, alphabetically by first collector. All cited specimens have been seen. Coordinates are given to minute-level for each specimen. In the specimen citations "sl" and "sd" indicate that collection locality and date, respectively, are missing on the herbarium label. The conservation status was assessed by applying the IUCN Red List Category criteria (IUCN 2012) using the Geospatial Conservation Assessment Tools in GeoCAT (Bachman et al. 2011). The key covers the countries Nigeria, Cameroon, Equatorial Guinea, Gabon, Congo and D.R.Congo. Taedoumg  Diagnosis. Resembling C. robbrechtianum Taedoumg & Sonké, 2011 by the coriaceous leaves, the obscure intersecondary venation especially in fresh condition and the ovoid shape of the young fruits, but differing from this species by the capitate structure of the inflorescence (vs branched and subcapitate in C. robbrechtianum), the ovoid shape of its fruits at maturity (vs asymmetrically subglobular), the granular texture of the young branches (vs smooth), and the leaf blades generally glossy above in dry condition (vs dull).
Taxonomic affinities. This species is morphologically close to C. robbrechtianum because of its coriaceous leaves, its obscure intersecondary venation especially in fresh material, the length of its peduncles and the shape of its young fruits. However, it differs from this species by the capitate structure of its inflorescence (vs branched and subcapitate in C. robbrechtianum), the ovoid shape of its fruits at maturity (vs asymetrically subglobular in C. robbrechtianum), the granular texture of the young branches (vs smooth in C. robbrechtianum), and its leaf blades generally glossy above in dry condition (vs dull in C. robbrechtianum). In addition, fruiting herbarium specimens tend to retain more fruits [(2-)4-10 fruits] (vs ca. 1-2 fruits in C. robbrechtianum).
The specimens of C. capitatum studied were almost all previously identified as C. cerinanthum Hiern. But this species clearly differs from C. capitatum by its relatively long pedunculate, branched and lax inflorescences.
Distribution and habitat. Craterispermum capitatum is known from Western Cameroon, the Democratic Republic of the Congo, South-Eastern Nigeria and the Republic of the Congo. It grows in semi-deciduous primary and secondary forest between 0 and 470 m elevation (Fig. 2).
Vernacular name and uses. Democratic Republic of the Congo -Botele bo lokonda (Turumbu); Djeli na Kupi (-). Leaves are used as fetish to avoid panthers.
Preliminary conservation status. IUCN status:-Vulnerable: VU B2b(iii). The extent of occurrence (EOO) of C. capitatum is 1,134.21 km², and its area of occupancy (AOO) is 52 km² using a cell width of 2 km. The species is distributed in 7 subpopulations, 2 of which are located in protected areas: the Omo-Oluwa-Shasha Forest Reserve located in Ondo State in Nigeria and the Korup National Park in Cameroon and Cross River National Park in Nigeria, which are in fact contiguous. Habitat loss outside the protected areas is a serious threat for C. capitatum, but loss of forest is also documented for the Omo-Oluwa-Shasha Forest Reserve and the Cross River National Park in Nigeria (Ite 1997;Adedeji and Adeofun 2014). Field study is required to fully assess the AOO of C. capitatum and, given the fact that the Democratic Republic of the Congo is not well collected, the number of locations for the species is likely to increase.
Etymology. The name of the species was chosen because of the capitate structure of its inflorescences.
Critical notes. The distribution of C. capitatum is atypical because of its absence in the South of Cameroon and in Gabon. While rare, several other Rubiaceae species show distribution patterns with a similar macro-disjunction, notably Hymenocoleus rotundifolius (A. Chev. ex Hepper) Robbr. (Robbrecht 1996) and Ixora brachypoda DC. (De Block 1998). The reason of this atypical distribution is not yet clearly determined, but in this case, it is probable that the continuous humid forest in southern Cameroon and Gabon is not an ideal habitat for C. capitatum,which occurs mostly in more semideciduous forests.
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). 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. - Stipules 5-16 mm long, with long and narrowly triangular tips, 4-13 mm long; 10-12 pairs of secondary veins; flowers 4-merous; calyx lobes unequal; tertiary and higher order venation closely and more or less regularly reticulate; leaf blades 6.  .............................................. Inflorescences subcapitate and completely covered by imbricate outer bracts (somewhat resembling an immature fruit) when young, secondary nerves clearly ascending and forming acute angles with the midrib; bracts overlapping one another at least at the base; bracts and bracteoles 6-8 × 5-8 mm and 6 × 3 mm respectively in brevistylous morph and ca. 2 × 1.5-2 mm and 0.7-2 × 1-2 mm respectively in longistylous morph; corolla tube 6-12 mm and 4-5 mm long for brevistylous and longistylous flowers respectively .