Nothodissotis (Melastomataceae), a new genus from Atlantic Central Africa, including the new species N.alenensis from Equatorial Guinea

Abstract Based on morphological and phylogenetic evidence, a new genus of Melastomataceae (Melastomateae), Nothodissotis Veranso-Libalah & G.Kadereit, gen. nov., is described from Atlantic Central Africa. Nothodissotis is distinguished from other African Melastomateae genera by its calyx-lobes that are notched at apex and asymmetrical (vs. entire and symmetrical). Nothodissotis includes two species: the type species N.barteri (Hook.f.) Veranso-Libalah & G.Kadereit, comb. nov. (syn. Dissotisbarteri Hook.f.), and the new species N.alenensis Veranso-Libalah & O. Lachenaud, sp. nov., described and illustrated here. Both species are restricted to open vegetation on rock outcrops within the forested region of Atlantic Central Africa. Nothodissotisbarteri has a scattered distribution in Cameroon, Equatorial Guinea, Gabon and Príncipe Island, while N.alenensis is endemic to the Monte Alén massif in Equatorial Guinea, an area where N.barteri does not occur. Nothodissotisalenensis differs from N.barteri by its hypanthium bearing sessile appendages with penicillate hairs (vs. stalked stellate appendages) and its staminal appendages that are much smaller in antepetalous than in antesepalous stamens (vs. subequal in all stamens). The conservation status of both N.barteri and N.alenensis is assessed as Vulnerable in accordance with IUCN criteria.


Introduction
Melastomataceae are a large pantropical family with about 4700 species in 170 genera (Clausing and Renner 2001). The majority of their species (c. 3000) occur in the Neotropics, with an important secondary centre of diversity in tropical Asia (c. 1000 species). Continental Africa is relatively poor with c. 330 species, while Madagascar has about the same number (Renner 1993). Most African representatives of the family belong to the pantropical tribe Melastomateae (excluding Marcetia DC. and allies now treated in Marcetieae), which includes about 650 species in 32 genera (Michelangeli et al. 2013;Veranso-Libalah et al. 2017a;Rocha et al. 2018). In continental Africa, around 186 species in 13 genera of Melastomateae are currently recognised (Veranso-Libalah et al. 2017a, b). Dissotis Benth. has long been regarded as the largest African genus of the tribe, with about 120 species on the continent (Renner 1993) and a single species in Madagascar (Jacques-Félix 1995). Its delimitation, however, has been problematic (Fernandes and Fernandes 1969;Jacques-Félix 1981, 1995, and phylogenetic study has shown the genus to be polyphyletic (Veranso-Libalah et al. 2017a). As a result, the genera Dissotidendron (A.Fern. & R.Fern.) Veranso-Libalah & G. Kadereit, with 11 species, and Dupineta Raf., with five species, both previously regarded as subgenera of Dissotis, have been segregated from the latter. The rest of the Dissotis species form a clade together with Antherotoma Naudin, Chaetolepis gentianoides (Naudin) Jacq.-Fél. (formerly treated in Nerophila Naudin) and African species of Osbeckia L. (sensu Jacques-Félix 1995), and are paraphyletic with respect to these three genera; the phylogenetic relationships and revised taxonomy of this group (hereafter referred to as 'Dissotis and allies') are the subject of a forthcoming paper (Veranso-Libalah et al. in prep.).
The affinities of the little-known Central African species Dissotis barteri Hook.f. were not investigated by Veranso-Libalah et al. (2017a). However, this species was included in a later phylogenetic and biogeographical study of the group (Veranso-Libalah et al. 2018) using three plastid (accD-psaI, ndhF and psbK-psbL) and two nuclear markers (nrETS and nrITS). In that study, D. barteri, together with an undescribed species from Equatorial Guinea, were recovered in a monophyletic clade separate from Dissotis and allies (see Fig. 1). Jacques-Félix (1981, 1983a had previously treated D. barteri in D. sect. Macrocarpae A.Fern. & R.Fern., but this is not supported by its morphology or by our molecular phylogenetic results (Veranso-Libalah et al. 2018). Both D. barteri and the new species from Equatorial Guinea differ from the members of D. sect. Macrocarpae (and indeed from the rest of the genus) by being deciduous (vs. evergreen) shrubs, and by their calyx lobes that are notched at apex and asymmetrical (vs. entire and symmetrical). Therefore, both molecular and morphological evidence support their exclusion from Dissotis.
The above-mentioned new species was previously misidentified as Dissotis thollonii Cogn., and was cited under this name in Parmentier and Geerinck's (2003)   and D. thollonii. While the first four species were correctly identified, D. thollonii does not occur in Equatorial Guinea, and most of the specimens cited under this name in the checklist (Parmentier & Esono 1530, 2721, 2763 and 3453) actually represent our new species. As discussed above, this species is very close to D. barteri, being a ramose shrub with stems and leaves bearing simple hairs, inflorescences few-flowered, and calyx-lobes asymmetrical, while D. thollonii is an unbranched shrub with hairs of the vegetative parts more or less branched, inflorescences many-flowered, and calyxlobes symmetrical. Parmentier and Geerinck (2003) cited two other specimens under D. thollonii, Lejoly 99/004 and 99/345, of which the former has not been traced (it is apparently not in BRLU), while the latter is sterile and cannot be identified, but differs from the other four collections in vegetative characters.
In this paper we describe a new genus of African Melastomateae, Nothodissotis Veranso-Libalah & G.Kadereit, to accommodate both Dissotis barteri and the new species from Equatorial Guinea discussed above. The former species becomes Nothodissotis barteri (Hook.f.) Veranso-Libalah & G.Kadereit, while the latter is described as N. alenensis Veranso-Libalah & O.Lachenaud. A review of relevant literature (Keay 1954;Fernandes 1969, 1978;Wickens 1975;Jacques-Félix 1983a, 1983b confirms that N. alenensis differs from all taxa of African Melastomateae so far described. Material from the following herbaria was consulted for this paper: BR, BRLU, C, EA, K, MO, P, UPS and WAG (Thiers 2018). The description of the new species is based on herbarium specimens and data derived from field notes; all measurements (except plant height) thus refer to dry or rehydrated material were made for both species, following the IUCN criteria (IUCN 2012). The extent of occurrence (EOO) and area of occupancy (AOO) were estimated using GeoCAT (Bachman et al. 2011) with a cell width of 2 km. A distribution map is provided for both species of Nothodissotis, as well as a key to the species of the genus, and a key to the currently recognized genera of African Melastomateae.
Etymology. Derived from the Greek word 'nothos' meaning false, and Dissotis, the genus which Nothodissotis most closely resembles.
Distribution and habitat. Nothodissotis includes two species in Atlantic Central Africa, both of which are restricted to rocky outcrops within the equatorial rainforest zone (Fig. 4).  Distribution and habitat. Nothodissotis barteri is sparsely distributed in Cameroon (Ebo forest), south-eastern Equatorial Guinea (near Nsork), northern Gabon, and Príncipe Island (Fig. 4). It occurs exclusively on rock outcrops at 370-1000 m elevation, mainly in low shrubby vegetation near the edge of the rocks ("manteau arbustif ") where it is locally dominant, and sometimes also as isolated plants in rocky grassland dominated by Afrotrilepis pilosa (Boeck.) J.Raynal (Cyperaceae).

Key to the species of Nothodissotis
Phenology. Flowering recorded mainly from November-February, once in August; fruits in March, May-June, August, October and December.
Conservation status. Vulnerable [VU B2ab(iii)]. The EOO of Nothodissotis barteri is estimated to be 82,625 km 2 (above the upper limit for Vulnerable status under sub-criterion B1) and its AOO to be 48 km 2 (within the limit for Endangered under sub-criterion B2). The species is sparsely distributed in Cameroon, Equatorial Guinea, northern Gabon and Príncipe island, and is restricted to rocky outcrops where it occurs in low shrubby vegetation or grassland. It is known from 21 collections representing eleven subpopulations, most of which (except three on Príncipe island) lie outside protected areas. In most of its range, bushfires and agriculture (mostly pineapple plantations) represent the main threats to its habitat; planned tourism development in Príncipe is another threat. A decline in habitat extent and quality is therefore expected. The eleven subpopulations represent a total of ten locations (sensu IUCN 2012), fall- ing within the limit for Vulnerable status, and the species is therefore assessed as Vulnerable under these conditions B2ab(iii).
Notes. This species, originally described from Príncipe Island (Hooker 1871), has since been reported from Gabon (Jacques-Félix 1983b) and Equatorial Guinea (Parmentier and Geerinck 2003). The collections cited above from Cameroon are the first for the country and represent an important range extension northwards.
The seeds of this species have not been described previously (e.g. Jacques-Félix 1983b). They are cochleate, c. 0.5 × 0.35 mm, and bear rounded tubercles arranged in parallel rows (Fig. 2 E-H).
Two specimens, probably from the same field collection by Barter in 1859, are housed in K, with neither of them designated as the holo-or isotype. For this reason, we designate the specimen K000313101 as the lectotype and K000313102 as the isolectotype. Diagnosis. This new species differs from N. barteri by its hypanthial appendages that are sessile with penicillate hairs (not stipitate with a crown of stellate hairs) and its more strongly dimorphic stamens, the staminal appendages being much longer in antesepalous stamens than in antepetalous ones (vs. staminal appendages ± equal in length in all stamens).
Phenology. Flowering in May.

Conservation status. Vulnerable [VU D2
]. Nothodissotis alenensis is endemic to Monte Alén National Park in Equatorial Guinea, where it has been collected four times and is known from two rock outcrops, representing two subpopulations. Its EOO can- Figure 5. Nothodissotis alenensis, A habit B, B´ leaf adaxial surface C, C´ leaf abaxial surface D floral buds in different developmental stages; cl = calyx-lobes, ia = intersepalar appendages, p = petals E stamens of the outer (left) and inner (right) stamen whorls (drawn from Parmentier & Esono 1560, 2721, 2763. Illustration by Doris Franke. not be calculated (since only two sites are known) while its AOO is estimated to be 8 km 2 , within the limit for Critically Endangered status under criterion B2. The species occurs in a remote area within a national park, and there is no evidence of an immediate threat or of a population decline. However, its extremely limited range makes it vulnerable to any threat that might arise in the future, e.g. climatic change or introduction of invasive species; it is therefore assessed as Vulnerable according to criterion D2.
Key to African Melastomateae genera 1 Calyx either truncate or with lobes not contorted, leaving the corolla exposed in bud; trees or shrubs with 4-merous flowers; seeds often provided with dorsal hyaline papillae .