Synoptic revision of Kolobopetalum and Rhigiocarya (Menispermaceae) with the description of a new Kolobopetalum species from Gabon, and a new specific combination in Rhigiocarya from West Africa

Background and aims – In the framework of the Menispermaceae treatment for the Flore du Gabon, the closely related genera Kolobopetalum and Rhigiocarya are synoptically revised. Material and methods – Standard practices of herbarium taxonomy have been applied to study the relevant herbarium material from B, BM, BR, BRLU, HBG, K, L, P, U, and WAG. The relevant collecting data are stored in the Botany Section of the Naturalis Biodiversity Center, Leiden, The Netherlands. Key results – The distinction between the genera Kolobopetalum and Rhigiocarya is further investigated and improved. As a result, Kolobopetalum leonense is transferred to Rhigiocarya. The Gabonese collections of Kolobopetalum contained a new species named K. synsepalum, which is described and illustrated. Rhigiocarya nervosa is resurrected as a distinct species. The distribution of all species is mapped. Illustrations are provided to facilitate the use of the key for the two genera and their species.


INTRODUCTION
The genera Kolobopetalum Engl. and Rhigiocarya Miers of the Menispermaceae are closely related, which is confirmed by molecular research by Ortiz et al. (2016). It is also illustrated by the transfer of Rhigiocarya chevalieri Hutch. & Dalz. to Kolobopetalum (Troupin 1949) and, in this paper, by the transfer of Kolobopetalum leonense Hutch. & Dalz. to Rhigiocarya.
Rhigiocarya was described by Miers in August 1864 with one species R. racemifera, based on a fruiting specimen from Nigeria (Miers 1864a). Earlier in the same year, in June 1864,  published the nomen nudum Chasmanthera nervosa based on a male flowering specimen from Sierra Leone and announced that the details of the species would be given in the third volume of the Contributions to Botany. They indeed appeared in the said Contributions in 1871 (Miers 1871). However, three years earlier, in 1868, Oliver in the Flora of Tropical Africa validated Miers' name Chasmanthera nervosa by providing it with a description, which validation has not been taken into account by all authors dealing with this species (Oliver 1868). Engler (1899) discovered that Chasmanthera nervosa did not belong to Chasmanthera and created for it the new genus Miersiophyton. Diels (1910) found that the fruiting type specimen of Rhigiocarya racemifera from Nigeria belonged to the same species as the male flowering specimen from Sierra Leone known as Miersiophyton nervosum and placed the latter in synonymy of the former name. This was followed by Chevalier in 1920 (Chevalier 1920). But in 1938, the same author combined Chasmanthera nervosa in Rhigiocarya and treated R. racemifera as its synonym, apparently based on the earlier date of publication of the nomen nudum Chasmanthera nervosa (Chevalier 1938). Rhigiocarya remained monotypic until Hutchinson & Dalziel (1927a) described R. chevalieri from Côte d'Ivoire, followed by  who described the third species R. peltata, also from Côte d'Ivoire. In the same paper, Miège stated that Rhigiocarya chevalieri should be transferred to Kolobopetalum, which was already done by Troupin in 1949. In the present paper, Rhigiocarya nervosa is resurrected as a species distinct from R. racemifera, and Kolobopetalum leonense is transferred to Rhigiocarya bringing its number of accepted species to four.
Kolobopetalum was described by Engler in 1899 with K. auriculatum as its only species. Engler (1899) based it on material from Togo and Cameroon, a mixture of specimens with three or six stamens. Stapf (1905) published the second species K. ovatum from Liberia with six stamens. Winkler (1908) described Kolobopetalum exauriculatum based on his own collection with six stamens from West Cameroon. Winkler's name was not recorded by Diels (1910) who published the new species K. veitchianum, also with six stamens and also from West Cameroon (see note under Kolobopetalum auriculatum). Diels (1910) lectotypified Engler's Kolobopetalum auriculatum by a specimen with three stamens. He also described the new species Kolobopetalum suberosum, which has been transferred to a new monotypic genus Sarcolophium by Troupin in 1960. Exell (1926 described the new Kolobopetalum mayumbense from Angola which was later transferred to the new monotypic genus Leptoterantha (Troupin 1949). In 1932, Exell described Kolobopetalum salmonicolor, another new species with six stamens, from Angola. It was synonymized under Kolobopetalum auriculatum by Troupin in 1962. Together with the new Kolobopetalum synsepalum described in this paper, the genus Kolobopetalum counts three accepted species.

MATERIAL AND METHODS
Classical methods of herbarium taxonomy have been followed. This study is based on the relevant herbarium specimens from the herbaria B, BM, BR, BRLU, HBG, K, L, P, U, and WAG. The Index Herbariorum (Thiers continuously updated) is followed as regards the herbarium acronyms. Specimens cited but not seen are marked with an asterisk. The relevant data of all specimens are stored at the Botany section of Naturalis Biodiversity Center, The Netherlands. Morphological illustrations were prepared from the specimens mentioned in the captions. The software ArcMap (ArcGIS Desktop v.10.7.1; Esri 2019) is used to produce the maps.

Morphology
The species of both genera are subwoody, usually winding lianas, glabrous in all their parts, except for Rhigiocarya nervosa where the inflorescence has an obscure papillate-like indumentum. A milky exudate may be present. The flowers are unisexual and dioecious. They are arranged in axillary or cauliflorous, simple or compound racemes (panicles), solitary in female inflorescences and mostly in fascicles of (2-)3-4(-5) flowers in male inflorescences. The pedicel is articulated at the top and the flowers of both sexes have two whorls of three sepals and also two whorls of three petals. The androecium mostly consists of two whorls of three stamens. Some Kolobopetalum auriculatum have three stamens only. The male flowers have no pistillode. The female flowers have, as far as known, three carpels and six staminodes. The fruits are drupaceous with a slimy, sticky mesocarp. The endocarp is spiny. The three species of Kolobopetalum and the four of Rhigiocarya are confined to the Guineo-Congolian forest region (White 1979) (figs 1, 2). Kolobopetalum ovatum is the only species present in White's three subdivisions, in Upper Guinea, Lower Guinea, and Congolia. In the latter subdivision, it is the only species present. Upper Guinea has three endemics: Rhigiocarya leonensis, R. nervosa, and R. peltata. Lower Guinea has two endemics: Kolobopetalum synsepalum and Rhigiocarya racemifera. All the species have more of less the same ecology, i.e. have been collected mostly from secondary vegetation in the forest area.

Provisional
IUCN conservation assessment -Kolobopetalum synsepalum is assessed as Endangered: EN B2ab(i,ii,iii,iv,v). The species is known so far from 6 collections, from 3 locations (2 river systems in Gabon and a single location in Congo-Brazzaville). None of these locations is currently protected, and the most eastern location in Gabon is currently under severe threat to be converted into oil palm plantations. Based on 2 km cells, the species has an AOO of 24 km 2 , and an EOO of just over 9200 km 2 . Given that 3 of the 6 collections (and hence 12 km 2 of the AOO) is from the threatened area, a 50% decline in AOO, even more in EOO (becomes 700 km 2 without this eastern location), and a strong reduction in individuals and suitable habitat, as well as a reduction in the number of locations can be foreseen.  (Exell 1926: 13) = Leptoterantha mayumbensis (Exell) Troupin (Troupin 1949: 428). Kolobopetalum suberosum Diels (Diels 1910: 163) = Sarcolophium suberosum (Diels) Troupin (Troupin 1960: 30).