Spatial Genetic Structure in the Laperrine's Olive
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Heredity (2007) 99, 649–657 & 2007 Nature Publishing Group All rights reserved 0018-067X/07 $30.00 www.nature.com/hdy ORIGINAL ARTICLE Spatial genetic structure in the Laperrine’s olive (Olea europaea subsp. laperrinei), a long-living tree from the central Saharan mountains G Besnard1, PA Christin1, D Baali-Cherif2, N Bouguedoura2 and F Anthelme3 1Department of Ecology and Evolution, Biophore, University of Lausanne, Lausanne, Switzerland; 2BP44, Laboratoire de Recherche sur les Zones Arides, USTHB/INA, Alger, Algeria and 3UMR Diversite´ et Adaptation chez les Plantes Cultive´es, IRD, Montpellier, France The Laperrine’s olive (Olea europaea subsp. laperrinei)isan four less differentiated clusters corresponded to the four emblematic species of the Sahelo-Saharan Mountains. studied mountain ranges. These results give support to the Populations of this tree are locally threatened by extinction fact that desert barriers have greatly limited long distance due to climatic vicissitudes and human activities, particularly gene flow. Within populations, pairwise kinship coefficients in Niger and Algeria. In order to study the spatial genetic were significantly correlated to geographical distance for structure and the dynamics of O. e. laperrinei populations, Niger populations but not for Algerian mountains. Historical we sampled trees in four isolated mountain ranges (Tassili factors and habitat heterogeneity may explain the differences n’Ajjer and Hoggar (Algeria), Tamgak and Bagzane (Niger)). observed. We conclude that the Hoggar acts as an important A total of 237 genets were identified using nuclear micro- genetic reservoir that has to be taken into account in future satellites. Phylogenetic reconstruction based on plastid conservation programmes. Moreover, very isolated endan- DNA data supported a maternal origin of O. e. laperrinei gered populations (for example, Bagzane) displaying evident populations in South Algeria, where a higher allelic richness genetic particularities have to be urgently considered for their was observed. Based on nuclear microsatellite data, two endemism. levels of structure were revealed: first, individuals from Niger Heredity (2007) 99, 649–657; doi:10.1038/sj.hdy.6801051; and Algeria were separated in two distinct groups; second, published online 12 September 2007 Keywords: conservation genetics; microsatellites; olive tree; plastid DNA; spatial autocorrelation; threatened species Introduction mountains, long-term persistence of some species, particularly long-living trees, is uncertain, mostly due The Saharan desert covers a large part of central Africa. to their geographic isolation, their low regeneration Since the upper Miocene, it represents a natural barrier associated with the impact of human activities and between Mediterranean and Tropical Africa which is climatic vicissitudes. For instance, Cupressus dupreziana difficult to cross for living organisms (Douady et al., A. Camus (from the Tassili n’Ajjer) is classified as 2003). The number of species present in this area is endangered taxon in the IUCN Red list (Abdoun and particularly limited (Que´zel, 1965), but the ratio of Beddiaf, 2002), while some others, such as the Laper- endemic species is relatively high in mountains that act rine’s olive, are currently distributed in a highly as refugia for plants and animals due to higher rainfall in fragmented habitat and locally threatened by extinction altitude. The Saharan flora has affinities with those from (Que´zel, 1997). Moreover, an expected increase of local adjacent regions, mainly with the Mediterranean Basin, human activities (Anthelme et al., 2006) as well as future Arabia, Sahelian and Tropical Africa (Que´zel, 1978, 1997). global changes (Williams et al., 2007) may add to the high Genetic studies have confirmed that some Saharan isolation of the Saharan flora. It could then become even species are closely related to Mediterranean taxa (for more vulnerable to a reduction of the favourable areas, example, Senecio, Coleman et al., 2003; Atriplex, Ortı´z- particularly in the Niger mountains where the altitudinal Dorba et al., 2005; Erica, McGuire and Kron, 2005). But range is smaller than in South Algeria. they have also revealed complex biogeograhic history for The populations of O. e. laperrinei have regressed with the Laperrine’s olive populations (Olea europaea L. subsp. the desert expansion during the Pleistocene (Wickens, laperrinei (Cif.) Batt. and Trab.), which may be due to 1976; Que´zel, 1978; Maley, 1980). Small populations are recurrent migration events from both Mediterranean present in mountains from central Sahara (Algeria: and Tropical Africa (Besnard et al., 2007). In the Saharan Hoggar, Tassili n’Ajjer, Mouydir; and Niger: Greboun, Tamgak, Bagzane) to northeast Sudan (Jebel Marra, Gourgueil). This tree grows at relatively high altitudes Correspondence: Dr G Besnard, Department of Ecology and Evolution, (generally from 1400 to 2700 m) on volcanic or eruptive Biophore, University of Lausanne, Biophore, Lausanne 1015, Switzerland. E-mail: [email protected] rocks and its populations are often difficult to reach. It Received 17 November 2006; revised 26 July 2007; accepted 6 persists in very dry areas particularly in southern Algeria August 2007; published online 12 September 2007 where mean rainfall can be less than 50 mm per year Spatial genetic structure in the Laperrine’s olive G Besnard et al 650 (Que´zel, 1965; Me´dail et al., 2001). The persistence of this ently unconnected ramets (individualized trunks) were taxon could then be seriously affected by future sampled as already reported. Geographical coordinates environmental changes (Baali-Cherif and Besnard, of all trees were reported in each of the four mountain 2005). Today, the wood of this tree is rarely exploited ranges sampled. Additionally, for four other localities by humans but camels and goats intensively browse (Assekrem, Jabbaˆrene, In Acoulmou and Jebel Marra’ young shoots. This limits the possibility of regeneration crater), one to three ramets were also characterized of the O. e. laperrinei populations, which is considered as (Table 1). A total of 311 ramets (including four herbarium null by several authors (Maire, 1933; Que´zel, 1978). samples from the Jebel Marra and the Tefedest) were However, even if survival of young trees has never been analysed. observed, a slow regeneration of populations cannot be excluded as reported for C. dupreziana in the Tassili Genetic markers n’Ajjer (Abdoun and Beddiaf, 2002; Abdoun et al., 2005). To investigate the phylogenetic relationships between The species is also considered as an emblematic tree for populations from Niger, Sudan and Algeria, polymorph- its very long persistence and rarity (Wickens, 1976). As a ism at 20 sites (10 microsatellite or indel loci and 10 PCR- consequence, the preservation of O. e. laperrinei popula- RFLPs) of the plastid DNA (ptDNA) was investigated: tions may have an increasing importance with the ccmp5 and ccmp7 from Weising and Gardner (1999), 14 development of the eco-tourism in northern Niger and loci from Besnard et al. (2003) and four loci from Baali- southern Algeria, but to date, the means given for their Cherif and Besnard (2005). A total of 50 individuals protection are still very limited. (sampled in each mountain range) were characterized for For conservation purposes, the inventory of genetic each locus. The polymorphic loci were then used to resources of this Saharan relict tree is needed to under- characterize the whole sample. These ptDNA data stand the past and future evolution of its populations. allowed the investigation of (i) the phylogenetic relation- Studying partition of the genetic diversity between and ships between populations and (ii) the genetic differentia- within populations can help unravel the dynamic of gene tion between populations based on a maternal marker. flow (for example, Baldoni et al., 2006; Rubio de Casas Lastly, the SGS was investigated with nuclear micro- et al., 2006; Belaj et al., 2007) and assess the importance of satellite (simple sequence repeat (SSR)) markers. To some geographic and biological features, such as habitat characterize all sampled ramets from the Tassili n’Ajjer fragmentation, altitudinal distribution, clonal growth and the Assekrem, nine SSR loci were used as reported in and lifespan of individuals. For instance, one can expect Baali-Cherif and Besnard (2005) and Anthelme et al. that desert barriers may have greatly limited gene flows (accepted): ssrOeUA-DCA1, ssrOeUA-DCA3, ssrOeUA- between distant mountains and that small populations DCA8, ssrOeUA-DCA9, ssrOeUA-DCA14, ssrOeUA- are more affected by stochastic effects (for example, DCA15 (Sefc et al., 2000), GAPU45 (Carriero et al., genetic drift). Knowledge of population genetics may 2002), PA(ATT)2 (Saumitou-Laprade et al., 2000) and then provide baseline information for optimized man- EMO03 (de la Rosa et al., 2002). For these nine loci, an agement strategies (Escudero et al., 2003) through the absence or a very low frequency of null alleles was identification of areas that should preferentially bene- reported in subsp. laperrinei by Baali-Cherif and Besnard ficiate of a conservation programme, for example, (2005). A total of 307 ramets (Table 1) were characterized because of a high genetic diversity or endemism. with the nine SSR loci in the present work and in In this context, a study on the genetic diversity of previous studies (Baali-Cherif and Besnard, 2005; O. e. laperrinei populations from South Algeria and Niger Anthelme