Keetia nodulosa sp. nov. (Rubiaceae — Vanguerieae) of West-Central Africa: bacterial leaf nodulation discovered in a fourth genus and tribe of Rubiaceae

From Firenze University Press Journal: Journal of Plant Taxonomy and Geography (Webbia)

Martin Cheek, Herbarium, Royal Botanic Gardens, Kew, Richmond, Surrey

Jean Michel Onana, University of Yaoundé I, Faculty of Science, Department of Plant Biology

Keetia E.Phillips was segregated from Canthium Lam. by Bridson (1985, 1986). Restricted to sub-Saharan Africa, and extending from Guinea in West Africa (Gosline et al. 2023a; 2023b) also Senegal to Sudan in the North and East (Darbyshire et al. 2015) also Ethiopia, and S. Africa in the South (Brid-son 1986), this genus of about 40 accepted species (POWO, continuously updated) are mainly forest climbers, distinguished from similar Canthioid genera in west Africa by their pyrenes with a fully or partly-defined lid-like area around a central crest and endosperm streaked with granular patches (Bridson 1986). In a phylogenetic analysis of the tribe based on morphology, nuclear ribosomal ITS and chloroplasttrnT- F sequences, Lantz & Bremer (2004), found that based on a sample of four species, Keetia was mono-phyletic and sister to Afrocanthium (Bridson) Lantz & B. Bremer with strong support. Highest species diversity of Keetia is found in Cameroon and Tanzania, both of which have about 15 taxa (Onana 2011; POWO, con-tinuously updated). In contrast, neighbouring Gabon has only 10 species, although most specimens recorded remain unidentified to species, Sosef et al. 2006). Sev-eral Keetia species are point endemics, or rare national endemics, and have been prioritized for conservation (e.g. Onana & Cheek 2011; Couch et al. 2019; Murphy et al. 2023; Darbyshire et al. 2023) and one threatened species, Keetia susu Cheek has a dedicated conservation action plan (Couch et al. 2022)Bridson’s (1986) account of Keetia was preparatory to treatments of the Vanguerieae for the Flora of Tropical East Africa (Bridson & Verdcourt 1991) and Flora Zambesiaca (Bridson 1998). Pressed to deliver these, she stated that she could not dedicate sufficient time to a comprehensive revision of the species of Keetia outside these areas: “full revision of Keetia for the whole of Africa was not possible because the large number of taxa involved in West Africa, the Congo basin and Angola and the complex nature of some species would have caused an unacceptable delay in completion of some of the above Floras” (Bridson 1986). Further “A large number of new species remain to be described.” Several of these new species were indicated by Bridson (1986), and other new species by her arrangement of specimens in folders that she annotated in the Kew Herbarium. One of these spe-cies was later taken up and published by Jongkind (2002) as Keetia bridsoniae Jongkind. In the same paper, Jong-kind discovered and published Keetia obovata Jongkind based on material not seen by Bridson. Based mainly on new material, additional new species of Keetia have been published by Bridson and Robbrecht (1993), Bridson (1994), Cheek (2006), Lachenaud et al. (2017), Cheek et al.(2018a) and Cheek and Bridson (2019).In the course of formally publishing new species to science from Cameroon so that they could be Red Listed and considered for inclusion in the Cameroon Important Plant Areas programme (e.g. Murphy et al.2023), numerous new species to science have been published (see below), mainly based on species informally identified as new in the course of a series of surveys for improved conservation management of plant species and habitats conducted mainly in western Cameroon in the 1990s (Cheek et al. 2006). This paper continues the endeavour.

In this paper, a remarkable new species of Keetia, K. nodulosa Cheek is described.Keetia nodulosa is unique in its genus and tribe for having conspicuous bacterial nodules on its abaxial leaf blade surfaces, resembling those seen in species of the genus Pavetta L., which also have conspicuous black nodules often at nerve junctions. The presence of bacte-rial nodules was first reported in the conservation check-list “The Plants of Mount Kupe, Muanenguba and the Bakossi Mts” (Cheek et al. 2004: 375). Rod like bacteria were then confirmed as present in the nodules by micro-scopic examination (B. Spooner pers. comm. to Cheek). The specimens Etuge 2798 and Etuge 2829 (both Mt Kupe) were matched with specimens from Cameroon, that had been included in the protologue of Keetia purse-glovei Bridson (Bridson 1986), Zenker 2986 (Bipinde) and Zenker & Staudt 415 (Yaoundé). However, the two Etuge specimens concerned had been annotated as “vel sp. aff.”, indicating that they might represent another but relat-ed species. Further research showed that all the Ugan-dan material of Keetia purseglovei, including the type, lacked bacterial nodules, and while very similar to the Cameroonian material, differed in several morphologi-cal characters. In searching all other material of Keetia at K, and other herbaria, for bacterial nodules, an additional specimen, Emwiogbon FHI 65823 from Nigeria, close to the Cameroon border, was found. This matched the Cameroonian material of K. nodulosa. It had been identified as a second specimen of Keetia inaequilatera (Hutch. & Dalz.) Bridson. While similar to the type and only other known specimen of that species, characters were found that separated this specimen from the type of that species including the presence (vs absence) of bacterial nodules. Finally, just before the paper was completed, a further specimen, with flower buds, Gereau et al. 5639from the Rumpi Hills, that had been identified as K. cf. hispida, was encountered and also placed in K. nodulosa in view of having bacterial nodules and other concordant characteristics. Further searches on gbif.org revealed that addition-al specimens had been identified as Keetiapurseglovei, mainly from Gabon, Central African Republic, R.D. Congo and Congo-Brazzaville. However, these differed from K.nodulosa, and only one of these, Texier 216 4, possessed visible bacterial nodules (see notes below) so were discounted.In this paper it is shown that two specimens from Cameroon previously ascribed to Keetia purseglovei in Bridson (1986) together with additional specimens, are specifically distinct from the Ugandan material of that species, including the type. The Cameroonian taxon,which extends to Nigeria, is formally characterized and named as Keetia nodulosa sp. nov. Within Africa, Cameroon remains a major source of discovery for new species to science of vascular plants, with more species new to science published per annum than any other country in tropical Africa (Cheek et al.2020a). Recent novelties range from forest trees (Quinta-nar et al. 2023; Cheek et al. 2022a; 2023a), shrubs and small trees (Couvreur et al. 2022; Gosline et al. 2022; Stone et al. 2023; Cheek et al. 2023b), lianas (Jongkind and Lachenaud 2022), rheophytes (Cheek et al. 2022b), terrestrial herbs (Cheek et al. 2021), to epilithic herbs (Janssens et al. 2022; Cheek et al. 2023c) and ferns (Shang and Zhang 2023; Dubuisson et al. 2022).

DOI: https://doi.org/10.36253/jopt-15946

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