Genetic Conservation in Parkia Biglobosa (Fabaceae: Mimosoideae) - What Do We Know?
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Lompo et al. ∙Silvae Genetica (2017) 66, 1-8 1 Genetic conservation in Parkia biglobosa (Fabaceae: Mimosoideae) - what do we know? D. Lompo1,2, B. Vinceti3, H. Gaisberger3, H. Konrad2, J. Duminil4,5,6, M. Ouedraogo1, S. Sina1, T. Geburek2* 1 Centre National de Semences Forestières, 01 BP 2682 Ouagadougou 01, Burkina Faso 2 Austrian Research Centre for Forests, Department of Forest Genetics, Seckendorff-Gudent-Weg 8, 1131 Vienna, Austria 3 Bioversity International, Via dei Tre Denari, 472/a, Fiumicino Rome, Italy 4 Bioversity International, Sub-Regional Office for Central Africa, P.O. Box 2008 Messa, Yaoundé, Cameroon 5 Service Evolution Biologique et Ecologie, CP160 ⁄ 12, Faculté des Sciences, Université Libre de Bruxelles, 50 Av. F. Roosevelt, 1050 Brussels, Belgium 6 Institut de Recherche pour le Développement, UMR-DIADE, BP 64501, 34394 Montpellier, France * Corresponding author: T. Geburek, E-mail: [email protected] Abstract Introduction The medicinal and food tree species Parkia biglobosa (Faba- Parkia biglobosa (Jacq.) G. Don is a highly valued multipurpose ceae: Mimosoideae) is widespread in the Sudanian savannahs tree species native to African savannahs with a large distributi- of sub-Saharan Africa, where it has a strong socio-cultural and on range extending from Senegal to Uganda (Hopkins, 1983; economic importance. Populations of this species are highly Hall et al., 1997). Commonly called “Néré” or “African locust threatened in large parts of its range due to overexploitation bean” it is one of the most important agroforestry tree species and environmental degradation. In the light of climatic chan- in West Africa (Eyog Matig et al., 2002; Nikiema, 2005). It is ges, safeguarding the genetic diversity of the species is crucial known as a multiple purpose tree widely used for medicinal to foster adaptation and to support its long-term survival. and food purposes (Ouedraogo, 1995; Hall et al., 1997). Seeds, Genetic insight is also relevant to guide sustainable harvesting. bark, roots and flowers of P. biglobosa are used to treat more This paper has the objective to review information on the spe- than 80 diseases and ailments in West Africa (Ouedraogo, 1995; cies’ geographic distribution, reproductive biology, genetic Nacoulma-Ouedraogo, 1996; Dedehou et al., 2016). Fermented characteristics and existing conservation practices, and to seeds, called “soumbala” or “dawadawa” are used as condiment, identify knowledge gaps to orientate future conservation and which is also well-known for its ability to decrease hypertensi- research focus. The literature review revealed that the species on (Ognatan et al., 2011). The sweet pulp of pods is a source of is mainly outcrossed and is pollinated by a diversity of vectors, energy and nutrients (rich in glucids, proteins, carotenoids, vit- including bats that allow long-pollen dispersal. When bats are amins A, B, C, and oligo-elements), and is important as nutriti- absent, pollination is mainly carried out by honey bees and onal and mineral supplement (Omojola et al., 2011; Dahou- stingless bees and in such case pollen-mediated gene flow is enon-Ahoussi et al., 2012; Ijarotimi and Keshinro, 2012). The relatively restricted. Data of a large-scale genetic study based species is melliferous and widely valued in traditional bee- on allozyme markers showing a moderate genetic differentiati- keeping (Nombré et al., 2009; Schweitzer et al., 2014). Due to on among populations were reanalyzed using an inverse dis- the high demand of its products, P. biglobosa is subject to tance weighted interpolation function. Three distinctive regi- unsustainable exploitation, lack of regeneration and ageing ons of diversity based on allelic richness and expected (Ræbild et al., 2012), with some tree populations disappearing heterozygosity were identified. Finally, we discuss future chal- or declining in density (Ouedraogo, 1995). Furthermore, impro- lenges for genetic conservation by emphasizing the need to ved tree management practices for P. biglobosa, such as direct use both neutral and adaptive markers in future research. sowing, protection of natural regeneration and planting, are currently not sufficiently promoted (Ræbild et al., 2012). The conservation and sustainable use of genetic resources are Key words: Parkia biglobosa, conservation, distribution, repro- critical to maintain tree resource availability in the long run, ductive biology, sub-Saharan Africa. especially in the face of significant environmental changes. A DOI:10.1515/sg-2017-0001 edited by the Thünen Institute of Forest Genetics 2 conservation strategy is based on complementary in situ and ex situ conservation, integrated in a way that relates to the sta- te of tree resources, the objectives of the strategy, and the financial resources available. In situ conservation is primarily focused on conserving the species in its natural environment by preserving its adaptation potential over the long term (FAO et al., 2001), while ex situ strategies are based on creating gene banks or field collections, transferring genetic resources off site, for the purpose of conservation or breeding (Dunster, 1996). Although P. biglobosa is officially protected by national legislation in Burkina Faso and in other West African countries (Eyog Matig et al., 2002) and cutting is prohibited, genetic con- servation efforts need to be strengthened. A sound conservati- on strategy for P. biglobosa and the promotion of its sustainab- le management should be based on scientific information about threats as well as ecological and genetic processes affec- Figure 1 ting this species. We felt the need to review and compile rele- Geographic distribution of Parkia biglobosa in relation to annual rainfall (adap- vant scientific findings on this species, published after the ted from Hall et al. 1997) monographic work by Hall et al. (1997). In this paper we (i) review existing knowledge on the spatial distribution of the species, its reproductive biology, its genetic characterization Floral biology and mating system and experiences in conservation of its genetic resources in Bur- kina Faso, with a particular focus on the activities implemented P. biglobosa is a monoecious species. The inflorescence, well by the national tree seed centre (the “Centre National de described by Hopkins (1983), Ouedraogo (1995) and Lassen et Semences Forestières”, hereafter CNSF) (Ouedraogo, 1995; Hall al. (2012), has bright red flowers, forming a capitulum in a large et al., 1997, Teklehaimanot, 1997; Sina, 2006, Ouedraogo, apical ball and a basal constricted receptacle. Different types of 2015). We also (ii) present a new analysis of allozyme data pub- flowers are found in the hermaphroditic capitulum. Hopkins lished by Sina (2006) and (iii) identify main research gaps. (1983) estimated that a capitulum has approximately 2,200 fer- tile female flowers in the ball-shaped part, 80 staminoid (male) flowers located close to the peduncle and 260 infertile nectar Natural range of P. biglobosa flowers in the ring producing nectar. Despite the coexistence of such a high number of fertile flowers in a single inflores- The geographic distribution of P. biglobosa has been described cence, only up to 25 pods are found on an infructescence (Oue- in a range map by Hall et al. (1997) showing geographic bound- draogo, 1995). Within the crown of a single tree, the capitula aries overlapped with climatic zones (Figure 1). The natural ran- tend to show different stages of flower development. Hopkins ge extends over 20 African countries north of the equator. The (1983) and Ouedraogo (1995) reported that flowers of the species is mainly found in African savannahs, particularly in the same capitulum will only be fertile during a single night. Prot- Sudanian vegetation zone, partially in the drier, more northern andry, i.e. early flowering of male and late flowering of female Sahelian zone and the wetter, more southern Guinean zone. reproductive organs, is found as a mechanism to reduce or The range covers different habitats usually on deep loamy and even totally avoid selfing (Ouedraogo, 1995). Controlled polli- sandy soils (Arbonnier, 2004) with annual rainfall varying bet- nation studies have shown that forced self-pollination results ween 700 mm in the North to 2,600 mm in the South and in healthy seeds, however, a certain proportion of seeds per exceptional levels up to 4,500 mm in Sierra-Leone and Guinea pod were aborted. Hence, self-fertilization is possible, probably (Hall et al., 1997). Tree population density of P. biglobosa varies through geitonogamy (Lassen et al., 2012). Based on controlled considerably. In the Central African Republic, densities of up to crossing experiments, Ouedraogo (1995) estimated a selfing 40 trees per hectare have been reported (Depommier and rate of 5 %. A very similar level of selfing was later confirmed by Fernandes, 1985), while in Burkina Faso, located in the centre of Sina (2006). In fact, he studied the mating system in two popu- species’ range, density varies between less than one tree per lations at Bissighin (W1.61°; N12.30°) and Nagare (W1.53°; hectare in the sub-Sahelian zone to up to 25 trees per hectare N12.30°) in Burkina Faso, examining 153 and 209 mature trees, in the southern part of the Sudanian zone (Ouedraogo, 1995). respectively. In both populations, in two consecutive years, flo- Due to the expansion of agriculture and pasture land, urbani- wering was well synchronized and the average multilocus esti- zation, mining, and also the vulnerability of the species to glo- mates resulted in high outcrossing rates of 0.941 and 0.981, bal environmental changes (Bouda et al., 2013), significant respectively. Biparental inbreeding inferred from the diffe- changes in the distribution range of P. biglobosa may have rence between single-locus and multilocus estimates of selfing taken place during the last 20 years, since occurrence data was 0 at Nagare and 0.026 and 0.086 at Bissighin in two conse- were assembled for the first time (Hall et al., 1997). cutive years showing that less than 9 % of matings involved close relatives.