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Refuge Begonias. Taxonomy, phylogeny and historical biogeography of Begonia sect. Loasibegonia and sect. Scutobegonia in relation to glacial rain forest refuges in Africa

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Abstract

Begonia is a genus of + 1000 species and is represented in all tropical areas. In Wageningen, under the guidance of dr J.J.F.E. de Wilde, the continental African begonias are being studied. Continental Africa has some 120 species, divided over 10 sections, and compared with the amount of species on other continents it is poor. A study on two of those sections with a total of 40 species is recorded in this work. The status of these species was in urgent need of revision, and it was questionable whether the two sections could be upheld.Some other important research questions are related to the theory on glacial rain forest refuges.The taxonomic level achieving the rank of section or genus is rather arbitrarily defined and it is being applied differently throughout the plant kingdom.In this study, species are approached according to the biological species concept. Subspecies are regarded as evolutionary entities separated in space, while varieties occur sympatrically.An extensive morphological study is the basis for distinguishing the separate species. The two sections are characterized a.o. by a creeping rhizome, a monochasial inflorescence with strongly reduced axes, indehiscent fruits, and male and female flowers with 2 perianth segments that are often bright yellow. Important diagnostic features for the separate species are found in the shape of the ovary and in the shape and the indumentum of the leaves. No clear delimitation between the two sections could be made on the basis of exclusively macromorphological characters (but see below).Former studies revealed a wide variety of leaf anatomical characters within Begonia. A leaf anatomical investigation of the species treated here was performed. Its primary objective was to acquire supplementary characters for the phylogenetic analysis, in order to obtain a more concrete and broader basis. The second objective was to investigate whether anatomical differences between the two sections could be identified.Leaf anatomy was studied by means of electron microscope research and the analysis of transverse sections. The results are considered to be satisfactory, as both goals mentioned above were reached. It was demonstrated that even within this group of closely related species a wide variety of anatomical characteristics exists, which in many cases support phylogenetic relationships already advanced upon the results of the macromorphological study.Important leaf anatomical characters are:- the presence or absence of a cuticula structure on the hairs,- the presence of short, sausage-shaped, often clustered glandular hairs or of longer, solitary, r-shaped ones,- the presence of sclerenchyma around the vasculary bundles,- the size of the epidermal cells in relation to that of the cells of the palisade parenchyma below them.Some leaf anatomical characters may be regarded as adaptations to the often deeply shaded habitat conditions in which the plants grow naturally.Anatomical characteristics of the ovary were investigated and also used for the phylogenetic analysis. Some important ones are the shape of the placentae, the shape of the septa and the arrangement of the ovules.The vascularisation within the ovary was analysed through series of cross sections. Former research suggested the activity of 2 separate meristems in shaping the ovary. This view was supported by research performed at Wageningen on the vascularisation within species of the section Tetraphila. The vascularisation of the species studied here does, however, not support this hypothesis and suggests a 'normal' development from carpels.The anatomical structure of papillae on the style also shows variation which is indicative for common descent.Micromorphological characters of the seeds were investigated earlier by colleagues in Amsterdam and they proved to be useful in delimiting sections within the genus Begonia. Some relevant characters were selected and added to the datamatrix used in the phylogenetic analysis.Species of the sections studied here have been artificially hybridized in the past, both mutually and with species from quite different sections. Some putative natural hybrids have also been observed. In only one case this possibly concerned hybridization with a species from a different section, viz. the closely related Filicibegonia.Most species of the sections Loasibegonia and Scutobegonia grow on medium to heavily shaded sites in humid tropical rain forest. They are terrestrial and occur on comparatively acid, often clayey soils with a low Mg-content.Many of the species are difficult to cultivate and demand a high relative humidity, a light, comparatively acid substrate and protection against direct sunlight.Generally, the species flower during distinct periods, usually twice a year. The flowers of several species show sleeping movements and are closed during parts of the day and night. It is still quite uncertain how pollination takes place, although insect pollination is the most likely possibility. Maturation of the fruit often takes several months. The seeds are released close to the mother plant from slowly disintegrating fruits and seed dispersal therefore does not seem to be very effective.Taxonomy strives after a stable and natural system of classification. A strict reflection of genealogical relationships in nomenclature is not compatible with its stability. The acceptance of a certain kind of paraphyletic clades as genera offers a solution.Most likely the two sections studied here form together a monophyletic group. The section Filicibegonia represents the most plausible outgroup.On account of a theoretical presupposition, a particular treatment of polytypic characters in a phylogenetic analysis is being proposed.A datamatrix for the 40 species and 132 characters was drawn up and analysed by means of cladistic methods. A first analysis yielded a very instable result with a comparatively large amount of 'weak' characters. That is why a weighting method was developed and applied. It diminishes the influence of 'weak' characters on the final cladogram structure. After application of this method 7 monophyletic subgroups could be identified. In addition, two of those were analysed separately in order to reveal their ingroup structure. The definite position of 5 species remained uncertain. The ultimately accepted phylogenetic tree is less parsimonous than the initial one.The most important conclusion that may be drawn from the genealogical relations is, that the sections Loasibegonia and Scutobegonia represent 2 monophyletic groups and therefore they should be upheld. Synapomorphic characters for the section Loasibegonia are the juicy petioles and the short, sausage- shaped glandular hairs. The presence of a thin epidermis, sclerenchyma around the tertiary nerves, a cuticula structure on the hairs, long r-shaped glandular hairs and an obtuse operculum of the seed is characteristic for species belonging to the section Scutobegonia.During the last glacial (± 70,000 - 12,000 year B.P.) in tropical Africa it was considerably cooler, + 4°C, and there was much less precipitation than at present. As a result the area of lowland rain forest presumably shrank considerably and ultimately disintegrated into a number of small refuges, situated as islands within an area occupied by more drought-resistant vegetation. Under these circumstances the area of montane rain forest probably expanded locally. Consequently, when speaking of rain forest refuges, one must make a clear distinction between lowland and montane rain forest.The Begonia species studied here are practically all confined to shaded, humid places in tropical lowland rain forest. During the last glacial, they will have survived almost exclusively in the refuges. Since, moreover, their seed dispersal seems not to be very effective, the location of the former refuges may be deduced from their present-day distribution.Begonia taxa happen to occur concentrated indeed within the main refuge localities postulated by other researchers, viz. in Liberia/Ivory Coast, in Cameroon/Gabon and in eastern Zaire. Within the Cameroon/Gabon area some 5 or 6 smaller areas with a high number of endemics can be denoted. These coincide remarkably well with the areas earlier indicated as possible refuges on the basis of other data. This strong conformity supports the view that these begonias may be regarded as dependable indicators of former refuges. As such they point to the possibility that also the Mayombe area and possibly the Doudou Mountains represent former lowland rain forest refuges.The existance of geographically isolated refuges formed a stimulant for speciation processes. Because of their short life cycle and the plasticity of their characters, the present species should be capable to evolve rapidly. Whether this has indeed happened under the influence of the development of refuges is an important question, that is studied next by means of a historical biogeographic analysis.Because it is plausible that the localities of former refuges will have retained a high degree of biodiversity into our days, knowledge about their location is of prime importance for nature conservation activities.By means of historical biogeographical research it is attempted to reconstruct the geological and climatological developments, using the phylogeny in combination with the distribution patterns of taxa. Nowadays, cladistic methods are often used, and among these Brooks Parsimony Analysis is considered theoretically the most suitable.In all, 25 areas of endemism have been demarcated. A first analysis lead to a poor result due to the comparatively large quantity of areas lacking their own endemic Begonia taxa. Eliminating these areas from the analysis yielded a better result, which was, however, still not optimal owing to the uncertain position within the areagram of the Doudou Mountains. Disregarding the latter area yielded an acceptable result. Eliminating areas seems to lead to good results in the present case, but the precise theoretical consequences of such actions need to be further investigated.A core area, composed of some 5 areas in Cameroon and Gabon, can be identified in all analyses that were performed. It coincides comfortably with the 'Lower Guinea' region often cited in floristical studies.Because the analyses show that sister species rarely occupy adjacent areas, remarkably few vicariance events seem to have occurred. If vicariance did occur during the last and previous glacials, this is perhaps concealed at present because of renewed dispersal after each occasion. The relationships between the areas seem to be more of a reflection of floristic similarity, rather than of a common history.There are indications for the presence of a demarcation line along the Sanaga River and another one across Equatorial Guinea.A revision of the sections Loasibegonia and Scutobegonia, including a key to the taxa, is presented. The taxa are accompanied by extensive descriptions, drawings, distribution maps, ecological notes and other relevant data. Recently, 14 new species have been published. In the present work the following additional new taxa are described and new combinations proposed:Begonia letouzeyi SosefBegonia prismatocarpa W.J. Hooker subsp. delobata SosefBegonia prismatocarpa W.J. Hooker subsp. petraea (A. Chev.) SosefBegonia quadrialata Warb. subsp. quadrialata var. pilosa SosefBegonia quadrialata Warb. subsp. nimbaensis SosefBegonia quadrialata Warb. subsp. dusenii (Warb.)Sosef Begonia scapigera Hook.f. subsp. australis SosefThe section Loasibegonia now comprises 19 species, 10 subspecies and 2 varieties. The section Scutobegonia has 21 species and 2 varieties.
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... La diversité végétale y est inégalement distribuée et plusieurs centres d'endémisme et de diversité y ont été mis en évidence (Droissart, 2009;Lachenaud, 2019;Sosef et al., 2017). Les facteurs historiques, en particulier l'évolution de la superficie du couvert forestier au cours des différents âges glaciaires et interglaciaires du climat terrestre, ont souvent été invoqués comme une des principales causes de cette inégale distribution (Droissart, 2009;Rietkerk et al., 1996;Robbrecht, 1996;Sosef, 1994). ...
... Cependant, comme toutes les espèces ne sont pas propagées à la même vitesse et sur de grandes étendues, cela a façonné des régions où la diversité végétale forestière est restée concentrée (Sosef, 1994). Identifier ces zones pourraient ainsi permettre d'identifier les emplacements de ces refuges forestiers (Rietkerk et al., 1996;Robbrecht, 1996;Sosef, 1996). ...
... Cette corrélation a été mise en évidence pour le changement climatique du Pléistocène (Carnaval et al., 2009;Dynesius and Jansson, 2000;Feng et al., 2016;Fjeldså and Lovett, 1997;Jansson, 2003;Kier et al., 2009;Linder, 2008;Sandel et al., 2017Sandel et al., , 2011Särkinen et al., 2012;Weber et al., 2014) et semble également valable pour les périodes géologiques précédentes (Hopper and Gioia, 2004;Linder, 2008). La stabilité climatique d'une région est donc un élément fondamental qui à la fois explique le paléo-endémisme et contribue dans le même temps à créer et maintenir le néoendémisme (Harrison and Noss, 2017;Sosef, 1994). Vincens (1993) Les preuves directes du recul des forêts africaines au Quaternaire Les changements de la couverture forestière au cours du Pléistocène et de l'Holocène en Afrique tropicale ont été étudiés à travers l'étude des pollens (Dalibard et al., 2014;Dupont et al., 2000;Dupont and Weinelt, 1996;Elenga et al., 1994Elenga et al., , 2000bLézine et al., 2019;Maley et al., 1990;Maley and Brenac, 1998;Miller and Gosling, 2014;Ngomanda et al., 2007;Vincens et al., 1998), de macro-fossiles végétaux (Dechamps et al., 1988), des diatomées (Nguestop et al., 1998), des dunes (Nichol, 1999), des termitières (Lanfranchi and Schwartz, 1990), de la géochimie (Delègue et al., 2001;Giresse et al., 1994), et de la sédimentologie (Giresse et al., 2005). ...
Thesis
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The origin of the distribution of plant species endemic to western Central Africa (WCA), an area dominated by forests and among the continent’s richest in species, has long been questioned by biogeographers. It has been suggested that the ranges of forest species contracted to within a few forest refugia during periods of glacial maxima, when the climate was cooler and drier, and that some species may not have followed the re-expansion of forest when climatic conditions became more favorable. The concentration of forest species endemic to restricted areas would thus reflect the former areas of forest refugia. The identification of areas rich in endemic and/or endangered species is also crucial for implementing efficient conservation strategies. This thesis has two objectives: (i) to investigate the extent to which the current distribution of forest species endemic to WCA can be explained by their persistence within forest refugia during the Quaternary ice-ages; (ii) to analyse whether the conservation strategies adopted in Gabon, a country located at the centre of WCA, protect the country’s endemic and endangered plant species. To achieve these objectives, a compilation and verification of 19,876 occurrences of the 1,145 taxa (species and sub-species) endemic to WCA were performed. In total, about 13% of the flora of WCA is endemic to the region, 88% of which are forest taxa. The results show that the dispersal capacity of taxa and the forest refugia hypothesis seem to be explanatory factors for the differences observed in the rate of endemism between growth forms and between taxonomic groups. The analysis of the spatial distribution of endemic forest species revealed the presence of ten areas of forest endemism in WCA, six of which are partly congruent with the mountain ranges and the coastal part of the region, areas proposed in the literature as forest refugia. The results suggest, however, that other forest refugia may have existed further inland. Based on the assumption that forest refugia were maintained during glacial maxima thanks to the presence of accessible moisture present in the soil or the air, we defined four types of hydrological refugia in which the forest cover could have been maintained: (i) flat areas along major rivers, (ii) valley bottoms in hilly lowland landscapes, (iii) coastal areas, and (iv) mountainous areas exposed to oceanic air masses. Their distribution explains the presence of the different areas of forest endemism found in the region but does not fully account for the patterns of richness in endemic species. The hypothesis of hydrological refugia is thus relevant for explaining the current distribution of forest species endemic to WCA, but other factors such as the distribution of environmental gradients or the impact of humans on forests must also be considered. Finally, the preliminary assessment of the risk of extinction of species according to the Categories and Criteria of the International Union for Conservation of Nature’s Red List, using a semi-automated method developed for this work, and an analysis of species distributions, showed that the National Parks of Gabon fail to protect a significant portion of the country’s endemic and endangered species, although a lack of botanical exploration in most parks may partially explain this finding.
... Elucidating the determinants of plant endemism in tropical areas has always been challenging due to significant gaps in our knowledge of the floras in these regions, largely because reliable data on species distributions are often scarce and/or difficult to access (Küper et al., 2006;Collen et al., 2008;Feeley & Silman, 2011;Stropp et al., 2016). The range of a species is an important indicator of historical, ecological, and evolutionary processes that have shaped the current distribution of biodiversity (Harold & Mooi, 1994;Sosef, 1994;Jansson, 2003). Endemism is a central concept in biogeography (Anderson, 1994;Brown et al., 1996) and is also highly relevant for biodiversity conservation, especially in areas where many endemic species occur (Myers et al., 2000;Brown et al., 2013;Bland et al., 2015). ...
... The rare and endemic species of the region are of significant importance for conservation . These species have also figured significantly in research that has sought to explain the current distribution of biodiversity in WCA in a historical biogeographic context in which the area occupied by African rainforest is hypothesized to have been fragmented and reduced in area and extent during the Pleistocene ice ages (Sosef, 1994;Maley, 1996;Rietkerk et al., 1996;Robbrecht, 1996). Because national boundaries in WCA do not correspond to natural or biological limits, endemic spe-Volume 107 2022 Texier et al. 3 Characteristics and Determinants of Endemic Plant Taxa in the Gabonese Area of Endemism cies tend to be concentrated toward the center of a given country, and as a consequence, considerations of endemism with regard to political or administrative areas are therefore of limited value ). ...
... The Gabonese Area of Endemism (GAE), defined to encompass the biogeographical territories connected to the ecosystems found in Gabon (see below under Materials and Methods), has a vascular plant flora comprising more than 8000 specific and infraspecific taxa, representing one of the highest levels of plant diversity in tropical Africa and making it one of the continent's richest centers for plant endemism (Linder, 2001;Kier et al., 2005;Sosef et al., 2017;Droissart et al., 2018). Although patterns of endemism within this area have been described for some taxonomic groups (Sosef, 1994;Droissart, 2009;Lachenaud, 2019), they have not been explored broadly, largely due to the lack of an up-todate and well-verified database that documents endemism for the entire flora of the area. ...
Article
Endemism is one of the most important concepts in biogeography and is often used to guide biodiversity conservation, yet our understanding of the determinants of endemism in many biodiverse tropical regions is limited. This is true for western Central Africa, a region with one of the highest levels of plant diversity in tropical Africa, where endemism is poorly documented. This study examines the Gabonese Area of Endemism (GAE) and explores the main characteristics and determinants of its vascular plant endemism with regard to taxonomy, growth form, habitat, distribution, and range size. We compiled a comprehensive, verified specimen database of vascular plant taxa restricted to the GAE, comprising 19,876 occurrences of 1145 species and infraspecific taxa, and we characterized the habitat and habit for each taxon. We then calculated the proportion of taxa in the regional flora that are endemic to the GAE. A Wilcoxon rank-sum test was used to investigate range size among endemic taxa exhibiting different growth forms, and Fisher exact tests were used to explore the association between their habit and habitat, and to test the distribution of these attributes among higher-level taxa and growth forms compared to patterns in the regional flora as a whole. We found that endemic taxa represent ca. 13% of the GAE flora, and that the rate of endemism varies considerably among taxonomic groups and growth forms. Endemism is highest among shrubs (22%) and lowest in herbs (8%), especially monocotyledons (e.g., 5% among Poales). Most endemic taxa grow preferentially in forest habitats, the dominant vegetation type of the region. Endemic trees, which structure forest ecosystems, have significantly larger ranges than endemic herbs, climbers, and shrubs. About 17% of the flora of Gabon is endemic or subendemic to the country. Our results show that the dispersal capacity of taxa and the biogeographical history of the region appear to be critical factors in explaining differences in the rate of endemism among growth forms and taxonomic groups. Our findings also highlight the benefits of carefully building a comprehensive and verified database for studying rare and range-restricted plants, and they underscore the necessity to strengthen botanical exploration throughout western Central Africa in order to develop improved and better-informed conservation strategies.
... En effet, l'endémisme permet d'obtenir des informations sur l'évolution des espèces, sur l'histoire biogéographique des aires qu'elles occupent, ainsi que sur des espèces rares susceptibles de disparaitre (Manrique et al., 2003). L'étude de l'endémisme permet ainsi de comprendre l'origine et l'âge de la flore ou de la faune d'une région ainsi que la chronologie des événements de spéciation et d'extinction (Myers et Gillers, 1988 (Sosef, 1994;Plana, 2004). La dernière période de réchauffement global s'est déroulée durant l'holocène, tandis que le dernier maximum glaciaire entrainant la fragmentation de la forêt a eu lieu durant le pléistocène (Maley, 1996) (Maley, 1987;Maley & brenac, 1987;Reynaud-Farrera et al., 1996;. ...
... En outre, on cerne assez mal le rôle que les refuges auraient pu jouer dans la différenciation des espèces. Une autre question importante qui reste donc encore à résoudre, est de savoir si les refuges ont joué le rôle de moteur pour la spéciation (Sosef 1994; (1978,1981), Cremers (1986) et Robbrecht (1988). Certains éléments inhérents à la description morphologique et structurale ont été extraits de la bibliographie; la majorité des rhéophytes étant des espèces déjà décrites. ...
Thesis
Les rhéophytes sont le plus important groupe des macrophytes aquatiques d’eau douce de région tropicale où elles sont adaptées aux eaux courantes (ex., chutes, cascades, inondations à écoulement rapide). Les rhéophytes peuvent être utilisées comme indicateurs utiles du niveau de santé des eaux et ont une biodiversité menacée par les activités humaines et les changements climatiques. Les rhéophytes sont mal connues au Cameroun. La préoccupation essentielle dans le cadre du present travail est de contribuer à l’amélioration de la disponibilité des connaissances sur la flore et l’écologie des rhéophytes au Cameroun. Trente cours d’eau à courant rapide des localités du Cameroun ont été visités, durant la saison sèche et la saison pluvieuse, entre 2010 et 2014. Les données ont été collectées sur le terrain sur les conditions de l’habitat et les caractéristiques rhéophytiques de chaque taxon rencontré et complétées par les données d’Herbiers et de la littérature floristique sur le Cameroun. Les observations morphologiques et structurales, la description des échantillons rhéophytes récoltés et extraits ont été faites selon le principe classique. Les cartes ont été produites grâce au programme ArcMap 10 sur le fond de carte du Cameroun. La division phytogéographique a suivi les principes modernes de la phytogégraphie africaine. L’évaluation du statut de conservation des espèces rhéophytes s’est faite conformément aux catégories et critères de l’UICN pour la liste rouge. Une check-list de 125 rhéophytes regroupées dans 30 familles et 65 genres est disponible; parmi les 125 rhéophytes, 67 rhéophytes strictes regroupées dans 16 familles et 29 genres. La morphologie des rhéophytes recensées est caractéristique, et est marquée en général par l’aspect grégaire sur pied, un large système racinaire, tige flexible et résistante, la forme foliaire oblong - lancéolée, lancéolée, linéaire ou rubanée, l’apex foliaire acuminé ou acuté, la texture foliaire glabre et la marge entière. Au Cameroun, les rhéophytes présentent un pic de floraison et de fructification pendant les périodes de décrue. Les rhéophytes rencontrées au Cameroun sont généralement sur cailloux et sable, en eaux courantes, dans une forêt dense humide de climat équatorial. Environ 79 % des rhéophytes présentes au Cameroun sont menacées. Les menaces majeures à l’habitat rhéophytique sont: l’agriculture, la construction des barrages hydroélectriques, l’exploitation minière, le développement de grandes plantations, exploitation forestière illégale, l’urbanisme, les eaux polluées. 27 % des rhéophytes recensées sont endémiques du Cameroun et sont toutes menacées, avec 90 % présentes dans le Sud-Ouest Cameroun. Environ 67 % des rhéophytes sont confinées dans la Région Guinéo – Congolaise au Cameroun. Environ 75 % des rhéophytes affectionnent le régime à climat équatorial à 4 saisons, avec dominance dans la zone à climat équatorial à 2 saisons et 25 % le climat tropical à 2 saisons. Environ 75 % des rhéophytes présentes au Cameroun sont des herbacées et 25 % des ligneuses. La famille des Podostemaceae et des Rubiaceae possèdent le plus grand nombre d’espèces rhéophytes au Cameroun, avec respectivement le plus grand nombre d’espèces herbacées et de ligneuses. La biodiversité du Cameroun en rhéophytes est parmi les plus importantes en Afrique. Le bassin de la Sanaga abrite le plus grand nombre d’espéces rhéophytes. La biodiversité des rhéophytes diminue avec l’altitude, la longitude et la latitude.
... Long-term vicariance of these forest patches will promote allopatric speciation between isolated populations. This mechanism was applied to tropical African species based on studies of diversity/endemism patterns and palaeobotanical data (Aubréville, 1975;Diamond & Hamilton, 1980;Mayr & O'Hara, 1986;Hamilton & Taylor, 1992;Sosef, 1994;Maley, 1996;Robbrecht, 1996;Plana, 2004). However, the impact of Pleistocene climatic fluctuations on rain forest fragmentation across tropical Africa is contested (Cowling et al., 2008;Hardy et al., 2013;Levinsky et al., 2013;Lézine et al., 2019). ...
... Molecular dating of the African genus Begonia indicated that around half of the species sampled originated during the Pleistocene, with the other half originating earlier, during the Pliocene/Miocene . Because Begonia species are generally restricted to lowland rain forests and are poor dispersers (Sosef, 1994), this supports, at least in part, a role of Pleistocene cycles in generating plant biodiversity. In the Zingiberales lowland rain forest herbaceous genus Aframomum, most speciation events were initially proposed to have taken place during the Pleistocene (Harris et al., 2000), although a revised temporal framework for this genus indicated that only a few species originated during the last 2.5 Myr (Auvrey et al., 2010). ...
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Tropical Africa is home to an astonishing biodiversity occurring in a variety of ecosystems. Past climatic change and geological events have impacted the evolution and diversification of this biodiversity. During the last two decades, around 90 dated molecular phylogenies of different clades across animals and plants have been published leading to an increased understanding of the diversification and speciation processes generating tropical African biodiversity. In parallel, extended geological and palaeoclimatic records together with detailed numerical simulations have refined our understanding of past geological and climatic changes in Africa. To date, these important advances have not been reviewed within a common framework. Here, we critically review and synthesize African climate, tectonics and terrestrial biodiversity evolution throughout the Cenozoic to the mid-Pleistocene, drawing on recent advances in Earth and life sciences. We first review six major geo-climatic periods defining tropical African biodiversity diversification by synthesizing 89 dated molecular phylogeny studies. Two major geo-climatic factors impacting the diversification of the sub-Saharan biota are highlighted. First, Africa underwent numerous climatic fluctuations at ancient and more recent timescales, with tectonic, greenhouse gas, and orbital forcing stimulating diversification. Second, increased aridification since the Late Eocene led to important extinction events, but also provided unique diversification opportunities shaping the current tropical African biodiversity landscape. We then review diversification studies of tropical terrestrial animal and plant clades and discuss three major models of speciation: (i) geographic speciation via vicariance (allopatry); (ii) ecological speciation impacted by climate and geological changes, and (iii) genomic speciation via genome duplication. Geographic speciation has been the most widely documented to date and is a common speciation model across tropical Africa. We conclude with four important challenges faced by tropical African biodiversity research: (i) to increase knowledge by gathering basic and fundamental biodiversity information; (ii) to improve modelling of African geophysical evolution throughout the Cenozoic via better constraints and downscaling approaches; (iii) to increase the precision of phylogenetic reconstruction and molecular dating of tropical African clades by using next generation sequencing approaches together with better fossil calibrations; (iv) finally, as done here, to integrate data better from Earth and life sciences by focusing on the interdisciplinary study of the evolution of tropical African biodiversity in a wider geodiversity context.
... Dans ce cas, les refuges auraient constitué des « musées » pendant les perturbations ; 2) d'autre part, sous l'effet des fragmentations forestières en aires qui ont servi de refuges, de nouveaux taxons dérivant des plus anciens seraient apparus par spéciation. Cette hypothèse se trouve consolidée par la superposition des mailles de grande richesse spécifique aux refuges forestiers postulés par Maley (1987), Sosef (1994) et Robbrecht (1996a). ...
... Dans le domaine bas-guinéen, les mailles à richesse spécifique élevée correspondent, dans la plupart des cas, aux aires postulées comme étant des refuges forestiers (Maley 1987 ;Sosef 1994 ;Robbrecht 1996b). Les mailles ...
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Ce travail fournit une information taxonomique détaillée sur la famille des Rubiaceae, par le biais de la révision du genre Craterispermum en Afrique continentale. Il constitue parallèlement un guide méthodologique illustré pour une révision taxonomique chez les Angiospermes, décrivant étape par étape les différents aspects et composantes d’une telle révision, tant sur le terrain que dans les herbiers. Quinze espèces et 5 variétés sont reconnues au sein des Craterispermum sur le continent africain. La révision des représentants africains de Craterispermum met en lumière 7 nouvelles espèces et 3 nouvelles variétés. Au niveau générique, les spécimens sont assez aisément reconnaissables. La reconnaissance des espèces et des variétés est plus délicate. Les principaux caractères distinctifs sont de type végétatif (stipules, jeunes rameaux, feuilles) et inflorescentiel. L’analyse du complexe d’espèces difficilement distinguables constitué de Craterispermum cerinanthum, C. laurinum et C. schweinfurthii indique que C. laurinum est identifiable tant par ses caractères que par sa distribution confinée à l’ouest du domaine haut-guinéen. La confusion entre Craterispermum cerinanthum et C. schweinfurthii reste assez accentuée en Afrique centrale où les spécimens dits typiques se mêlent à de nombreux spécimens atypiques. Le domaine bas-guinéen est reconnu comme étant le principal centre de diversité et d’endémisme du genre en Afrique continentale. Les résultats de ce travail accentuent la nécessité d’établir dans ce genre une phylogénie résolue, qui constitue un élément essentiel et indispensable pour l’interprétation des autres analyses et pour comprendre les modes de spéciation et d’évolution du genre.
... Notably, there was poor knowledge on how plant endemism patterns were affected by putative barriers to plant migration, such as the Dahomey gap, a savannah region separating Upper and Lower Guinea, or the Cameroon Volcanic line, a mountainous region in western Cameroon. Palaeoecological data revealed that Plio-Pleistocene climatic oscillations led to forest contraction at the expense of savannah vegetation during cold and dry stages (Maley 1996;Vincens et al. 1999;Bonnefille 2007), and species endemism in forest herbs and shrubs with poor dispersal abilities such as Begoniaceae (Sosef 1994) or Rubiaceae (Robbrecht 1996) provided indications for the location of Plio-Pleistocene forest refuges (Maley 1996). Nevertheless, many questions remained on the location and structure of fragmented forests during these cold stages, and their impact on the constitution of today's rainforest vegetation in Africa. ...
... Certain Detarioideae tree species have been proposed as glacial age forest refuge indicators (Rietkerk et al. 1996;Leal 2004;Tchouto et al. 2009), as well as general species richness (e.g. Aubréville 1968), the presence of certain species of Begonia (Sosef 1994) and certain species of Rubiaceae (Robbrecht 1996). Detarioideae clusters may also indicate glacial age forest refuges, especially clusters that contain both many different Detarioideae species and some Detarioideae species endemic to the cluster. ...
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Background and aims – We studied a cluster of trees in the Leguminosae subfamily Detarioideae, to: (1) determine the size, structure, and tree species composition of this cluster; (2) map the size, shape, and structure of groups of individual Detarioideae tree species in the cluster.Location – Lowland rain forest in southern Korup National Park, in the Southwest Region of Cameroon.Material and methods – Trees in permanent plots were recorded using standard plot enumeration techniques. Outside plots, single-species tree groups were recorded by a rapid technique. From this data, detailed maps of groups of trees were prepared.Key results – Detarioideae tree species occur co-dominant in a cluster of at least 32 km2 with an irregular shape. The cluster contained at least 42 Detarioideae tree species; at least 29 of these occurred in groups ranging in size from 50 to 4000 m across, depending on the species. Groups usually had circular shapes, caused by ballistic seed dispersal. In a group, trees were always mixed with trees of several other Detarioideae species. Every area within the cluster contained a specific set of Detarioideae species. The percentage of Detarioideae trees ≥ 60 cm stem diameter on 50 ha was up to 76% in Detarioideae-rich forest, to 6% in Detarioideae-poor forest. Of all trees in the centre of the cluster, 2.8% belonged to pioneer forest species, which indicates that disturbance levels were low during the past generations of trees.Discussion – The forests in the Detarioideae cluster have not been subject to substantial human and natural impacts in historic or prehistoric times. Such forests are exceptional in Africa. Detarioideae clusters may indicate glacial age forest refuges, especially clusters that contain both many different Detarioideae species and some Detarioideae species endemic to the cluster.
... The careful recognition and delimitation of areas of endemism require significant effort and they are rarely well defined in tropical countries (Colli-Silva et al., 2019;Hazzi et al. 2018). In Central Africa, several areas of endemism have been proposed but in each case they were based only on taxa belonging to a single family (Droissart, 2009;Lachenaud, 2019;Rietkerk et al., 1996;Robbrecht, 1996;Sosef, 1994). An analysis that takes into consideration the entire flora of the region is currently being undertaken by the authors of the present study in order to define the areas of endemism that occur in Gabon and thereby provide a list of species endemic to each of these areas (i.e. ...
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The High Conservation Value (HCV) concept, developed by the Forest Stewardship Council to promote sustainable forest management, is widely employed for certification of forestry and agriculture concessions, and has been adopted by many logging and palm oil companies. HCV criterion 1, which deals with “endemic, and rare, threatened or endangered species”, is rarely used in certification, mainly because lists of these species are incomplete, especially for plants, and performing threat assessments is time-consuming. The IUCN Red List Categories are often suggested as a suitable basis to define threatened taxa for the application of HCV1, but this requires the rapid and efficient assessment of large numbers of species. Using the plants endemic to Gabon as a case study, we propose a rapid, two-step procedure to identify HCV1 species. First, based on 3,298 verified and geo-referenced herbarium records, we used GIS layers and an automated computational workflow in the R environment to identify potentially threatened species using an approach aligned with IUCN Red List criteria A, B, and D. Ninety percent of the automated assessments correctly indicated the risk of extinction; errors involved incorrect assessments of species whose habitat is in reality not threatened, or occurred during the calculation of the number of locations (sensu IUCN) when a single threat impacts large areas. In a second step designed to correct these issues and comply with the Red List guidelines, we performed species-by-species verification of the automated assessments, taking into account the ecology and habitat of each species and the nature of the threats it faces. Of the 389 endemic taxa analyzed, 86% were identified as threatened (83 CR, 171 EN, and 80 VU); of these, only 35% are recorded from at least one National Park, but most are found in logging/oil palm (72%) or mining (55%) concessions, underscoring the need to improve the application of the HCV concept. To strengthen the use of HVC subcriterion 1.2 (rare, threatened or endangered species), we propose an explicit method for identifying rare species based on a quantitative threshold of the Extent of Occurrence (20,000 km²), and we examine the concept of endemicity with respect to the application of HVC subcriterion 1.3 (endemic species). The proposed methodology addresses an urgent need to develop a national interpretation of the HCV concept in Gabon, adopted as a national standard for logging concessions, and offers an efficient, reliable approach for the application of HVC1 elsewhere in Central Africa.
... African rain forest species with limited dispersal ability (e.g. ballistic fruit), such as Brachystegia, exhibit thus spatial genetic patterns generally associated with the presence of putative glacial refugia (Sosef, 1994) or microrefugia (Leal, 2001), even if west-east environmental filtering, heterogeneous topography and northsouth seasonal inversion effects could also explain part of those patterns (e.g. Dauby et al., 2014;Heuertz et al., 2014). ...
Article
Aim: Miombo woodlands form a characteristic vegetation type covering 2.7 million km² in southern and eastern Africa. Despite their wide geographical extent, their origin, floristic and spatial evolution through time remain understudied. To fill this gap, we studied the evolution of Brachystegia trees, one of the most representative genera of these woodlands (20 species), also represented in Guineo‐Congolian rain forests (8 species). Location: Tropical Africa, Guineo‐Congolian forests and Zambezian savannas. Taxon: Brachystegia genus. Methods: We used a genome skimming approach to sequence the plastomes of 45 Brachystegia samples, covering 25 of the 29 existing species, and one outgroup (Julbernardia paniculata). The phylogeny of the plastomes was reconstructed and time‐calibrated. We tested if the genetic divergence between lineages reflected taxonomic and/or geographical distances using Mantel tests. Finally, we inferred the evolutionary history of Brachystegia based on the age and spatial distribution of its lineages. Results: Surprisingly, species represented by multiple specimens appear rarely monophyletic while plastid clades display strong geographical structuring, independently of the species. Two main clades separate woodland and rain forest species, which diverged during the late Miocene–Pliocene (95% HPD = 2.78–8.59 Ma). In miombo woodlands, three subclades occur in parapatry along an East–West axis, ranging from Angola to East Africa. Their divergence started from the Plio‐Pleistocene (95% HPD = 1.17–3.69 Ma). Divergence dates (TMRCA) within miombo subclades decrease from East Africa (1.53 Ma) to Angola (0.76 Ma). Main conclusions: Brachystegia plastomes appear unreliable to identify species, probably due to species introgression leading to recurrent chloroplast captures. However, they prove very informative for tracking the past dynamics of the genus, and suggest a historical westwards expansion of miombo Brachystegia, and possibly of miombo vegetation, during the Plio‐Pleistocene. Further investigations using nuclear DNA are needed to assess the species tree as well as speciation and hybridization events between species.
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Understanding the evolutionary dynamics of genetic diversity is fundamental for species conservation in the face of climate change, particularly in hyper-diverse biomes. Species in a region may respond similarly to climate change, leading to comparable evolutionary dynamics, or individualistically, resulting in dissimilar patterns. The second-largest expanse of continuous tropical rain forest (TRF) in the world is found in Central Africa. Here, present-day patterns of genetic structure are thought to be dictated by repeated expansion and contraction of TRFs into and out of refugia during Pleistocene climatic fluctuations. This refugia model implies a common response to past climate change. However, given the unrivalled diversity of TRFs, species could respond differently because of distinct environmental requirements or ecological characteristics. To test this, we generated genome-wide sequence data for >700 individuals of seven codistributed plants from Lower Guinea in Central Africa. We inferred species’ evolutionary and demographic histories within a comparative phylogeographic framework. Levels of genetic structure varied among species and emerged primarily during the Pleistocene, but divergence events were rarely concordant. Demographic trends ranged from repeated contraction and expansion to continuous growth. Furthermore, patterns in genetic variation were linked to disparate environmental factors, including climate, soil, and habitat stability. Using a strict refugia model to explain past TRF dynamics is too simplistic. Instead, individualistic evolutionary responses to Pleistocene climatic fluctuations have shaped patterns in genetic diversity. Predicting the future dynamics of TRFs under climate change will be challenging, and more emphasis is needed on species ecology to better conserve TRFs worldwide.
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