Central Africa; Effective population density; Gene dispersal; Iroko; Milicia excelsa; Spatial genetic structure
Résumé :
[en] In this study, we analysed spatial genetic structure (SGS) patterns and estimated dispersal distances in Milicia excelsa (Welw.) C.C. Berg (Moraceae), a threatened windpollinated dioecious African tree, with typically low density ( 10 adults ⁄km2). Eight microsatellite markers were used to type 287 individuals in four Cameroonian populations characterized by different habitats and tree densities. Differentiation among populations was very low. Two populations in more open habitat did not display any correlation between genetic relatedness and spatial distance between individuals, whereas significant SGS was detected in two populations situated under continuous forest cover. SGS was weak with a maximum Sp-statistic of 0.006, a value in the lower quartile of SGS estimates for trees in the literature. Using a stepwise approach with Bayesian clustering methods, we demonstrated that SGS resulted from isolation by distance and not colonization by different gene pools. Indirect estimates of gene dispersal distances ranged from rg = 1 to 7.1 km, one order of magnitude higher than most estimates found in the literature for tropical tree species. This result can largely be explained by life-history traits of the species. Milicia excelsa exhibits a potentially wideranging wind-mediated pollen dispersal mechanism as well as very efficient seed dispersal mediated by large frugivorous bats. Estimations of gene flow suggested no major risk of inbreeding because of reduction in population density by exploitation.
Different strategy of seed collection may be required for reforestation programmes among populations with different extent of SGS.
Amariei L (2005) Legal Compliance in Forestry Sector - Case Study: Cameroon. Report to FAO. FAO, Rome.
Atluri JB, Venkata Ramana SP, Subba Reddi C (2004) Explosive pollen release, wind-pollination and mixed mating in the tropical tree Shorea robusta Gaertn. f. (Dipterocarpaceae). Current Science, 86, 416 419.
Austerlitz F, Dick CW, Dutech C et al. (2004) Using genetic markers to estimate the pollen dispersal curve. Molecular Ecology, 13, 937 954.
Bacles CFE, Lowe AJ, Ennos RA (2006) Effective seed dispersal across a fragmented landscape. Science, 311, 628.
Bawa KS (1990) Plant-pollinator interactions in tropical rain forests. Annual Review in Ecology and Systematics, 21, 399 422.
Bittencourt JVM, Sebbenn AM (2007) Patterns of pollen and seed dispersal in a small, fragmented population of the wind-pollinated tree Araucaria angustifolia in southern Brazil. Heredity, 99, 580 591.
Bittencourt JVM, Sebbenn AM (2008) Pollen movement within a continuous forest of wind-pollinated Araucaria angustifolia, inferred from paternity and TwoGENER analysis. Conservation Genetics, 9, 855 868.
Born C, Hardy OJ, Chevallier MH et al. (2008) Small-scale spatial genetic structure in the Central African rainforest tree species Aucoumea klaineana: a stepwise approach to infer the impact of limited gene dispersal, population history and habitat fragmentation. Molecular Ecology, 17, 2041 2050.
Bosu PP, Cobbinah JR, Nichols JD, Nkrumah EE, Wagner MR (2006) Survival and growth of mixed plantations of Milicia excelsa and Terminalia superba 9 years after planting in Ghana. Forest Ecology and Management, 233, 352 357.
Bullock SH (1994) Wind pollination of neotropical dioecious trees. Biotropica, 26, 172 179.
Burczyk J, Koralewski TE (2005) Parentage versus two-generation analyses for estimating pollen-mediated gene flow in plant populations. Molecular Ecology, 14, 2525 2537.
Chen C, Durand E, Forbes F, François O (2007) Bayesian clustering algorithms ascertaining spatial population structure: a new computer program and a comparison study. Molecular Ecology Notes, 7, 747 756.
Cordeiro NJ, Howe HF (2003) Forest fragmentation severs mutualism between seed dispersers and an endemic African tree. Proceedings of the National Academy of Sciences, USA, 100, 14052 14056.
Culley TM, Wellerand SG, Sakai AK (2002) The evolution of wind pollination in angiosperms. Trends in Ecology and Evolution, 17, 361 369.
Dick CW, Etchelecu G, Austerlitz F (2003) Pollen dispersal of tropical trees (Dinizia excelsa: Fabaceae) by native insects and African honeybees in pristine and fragmented Amazonian rainforest. Molecular Ecology, 12, 753 764.
Dick CW, Hardy OJ, Jones FA, Petit RJ (2008) Spatial scales of pollen and seed-mediated gene flow in tropical rain forest trees. Tropical Plant Biology, 1, 20 33.
Doligez A, Baril C, Joly HI (1998) Fine-scale spatial genetic structure with nonuniform distribution of individuals. Genetics, 148, 905 919.
Doucet J-L (2003) L'alliance délicate de la gestion forestière et de la biodiversité dans les forêts du centre du Gabon. PhD Thesis, Gembloux Agricultural University, Gembloux.
Dutech C, Seiter J, Petronelli P, Joly HI, Jarne P (2002) Evidence of low gene flow in a neotropical clustered tree species in two rainforest stands of French Guiana. Molecular Ecology, 11, 725 738.
El-Kassaby YA, Jaquish B (1996) Population density and mating pattern in western larch. The Journal of Heredity, 87, 438 443.
Epperson BK (2000) Spatial genetic structure and non-equilibrium demographics within plant populations. Plant Species Biology, 15, 269 279.
Feteke R, Nkolong E, Hubert D (2004) Plan d'aménagement des unités forestières d'aménagement n° 10 041, 10 042 et 10 044 regroupés. Pallisco, Douala, Cameroun.
FORM Ecology Consultants (2004) Plan d'aménagement durable UFA 09-021. Wijma, Douala, Cameroun.
Frankham R (1995) Effective population size adult population size ratios in wildlife - a review. Genetical Research, 66, 95 107.
Gapare WJ, Aitken SN (2005) Strong spatial genetic structure in peripheral but not core populations of Sitka spruce Picea sitchensis (Bong.) Carr. Molecular Ecology, 14, 2659 2667.
Gehring JL, Delph LF (1999) Fine-scale genetic structure and clinal variation in Silene acaulis despite high gene flow. Heredity, 82, 628 637.
Hanson TR, Brunsfeld SJ, Finegan B, Waits LP (2008) Pollen dispersal and genetic structure of the tropical tree Dipteryx panamensis in a fragmented Costa Rican landscape. Molecular Ecology, 17, 2060 2073.
Hardy OJ (2003) Estimation of pairwise relatedness between individuals and characterization of isolation-by-distance processes using dominant genetic markers. Molecular Ecology, 12, 1577 1588.
Hardy OJ, Vekemans X (1999) Isolation by distance in a continuous population: reconciliation between spatial autocorrelation analysis and population genetics models. Heredity, 83, 145 154.
Hardy OJ, Vekemans X (2002) SPAGeDI: a versatile computer program to analyse spatial genetic structure at the individual or population levels. Molecular Ecology, Notes, 2, 618 620.
Hardy OJ, Maggia L, Bandou E et al. (2006) Fine-scale genetic structure and gene dispersal inferences in 10 Neotropical tree species. Molecular Ecology, 15, 559 571.
Heuertz M, Vekemans X, Hausman JF, Palada M, Hardy OJ (2003) Estimating seed vs. pollen dispersal from spatial genetic structure in the common ash. Molecular Ecology, 12, 2483 2495.
Jøker D (2002) Milicia excelsa (Welw.) C.C. Berg. Seed Leaflet, 63. Available from http://en.sl.life.ku.dk/upload/milicia-excelsa-63-int-001. pdf.
Jump AS, Penuelas J (2006) Genetic effects of chronic habitat fragmentation in a wind-pollinated tree. Proceedings of the National Academy of Sciences, USA, 103, 8096 8100.
Latch EK, Dharmarajan G, Glaubitz JC, Rhodes OE (2006) Relative performance of Bayesian clustering software for inferring population substructure and individual assignment at low levels of population differentiation. Conservation Genetics, 7, 295 302.
Leblois R, Estoup A, Streiff R (2006) Genetics of recent habitat contraction and reduction in population size: does isolation by distance matter? Molecular Ecology, 15, 3601 3615.
Loiselle BA, Sork VL, Nason J, Graham C (1995) Spatial genetic structure of a tropical understory shrub, Psychotria Officinalis (Rubiaceae). American Journal of Botany, 82, 1420 1425.
de-Lucas AI, Robledo-Arnuncio JJ, Hidalgo E, González- Martínez SC (2008) Mating system and pollen gene flow in Mediterranean maritime pine. Heredity, 100, 390 399.
Luna R, Epperson BK, Oyama K (2005) Spatial genetic structure of two sympatric neotropical palms with contrasting life histories. Heredity, 95, 298 305.
Muller-Landau HC, Wright SJ, Calderón O, Condit R, Hubbell SP (2008) Interspecific variation in primary seed dispersal in a tropical forest. Journal of Ecology, 96, 653 667.
Nason JD, Hamrick JL (1997) Reproductive and genetic consequences of forest fragmentation: two case studies of neotropical canopy trees. Journal of Heredity, 88, 264 276.
Nathan R, Safriel UN, Noy-Meir I, Schiller G (2000) Spatiotemporal variation in seed dispersal and recruitment near and far from Pinus halepensis trees. Ecology, 81, 2156 2169.
Nichols JD, Agurgo FB, Agyeman VK, Wagner MR, Cobbinah JR (1998) Distribution and abundance of Milicia species in Ghana. Ghana Journal of Forestry, 6, 1 7.
Nichols JD, Agyeman VK, Agurgo FB, Wagner MR, Cobbinah JR (1999) Patterns of seedling survival in the Tropical African Tree Milicia excelsa. Journal of Tropical Ecology, 15, 451 461.
Nunney L (1993) The influence of mating system and overlapping generations on effective population size. Evolution, 47, 1329 1341.
Ofori DA, Cobbinah JR (2007) Integrated approach for conservation and management of genetic resources of Milicia species in West Africa. Forest Ecology and Management, 238, 1 6.
Osmaston HA (1965) Pollen and seed dispersal in Chlorophora excelsa and other Moraceae, and in Parkia filicoidea (Mimosaceae), with special reference to the role of the fruit bat, Eidolon helvum. Commonwealth Forestry Review, 44, 96 104.
Ouinsavi C, Sokpon N, Bousquet J, Newton CH, Khasa DP (2006) Novel microsatellite DNA markers for the threatened African endemic tree species, Milicia excelsa (Moraceae), and cross-species amplification in Milicia regia. Molecular Ecology Notes, 6, 480 483.
Petit RJ, Aguinagalde I, de Beaulieu JL et al. (2003) Glacial refuges: hotspots but not melting pots of genetic diversity. Science, 300, 1563 1565.
Raspé O, Saumitou-Laprade P, Cuguen J, Jacquemart AL (2000) Chloroplast DNA haplotype variation and population differentiation in Sorbus aucuparia L. (Rosaceae: Maloideae). Molecular Ecology, 9, 1113 1122.
Richter HV, Cumming GS (2008) First application of satellite telemetry to track African straw-coloured fruit bat migration. Journal of Zoology, 275, 172 176.
Robledo-Arnuncio JJ, Alia R, Gil L (2004) Increased selfing and correlated paternity in a small population of a predominantly outcrossing conifer, Pinus sylvestris. Molecular Ecology, 13, 2567 2577.
Robledo-Arnuncio JJ, Austerlitz F, Smouse PE (2006) A new method of estimating the pollen dispersal curve independently of effective density. Genetics, 173, 1033 1045.
Rousset F (1997) Genetic differentiation and estimation of gene flow from F-statistics under isolation by distance. Genetics, 145, 1219 1228.
Rousset F (2000) Genetic differentiation between individuals. Journal of Evolutionary Biology, 13, 58 62.
Rousset F (2008) genepop'007: a complete re-implementation of the genepop software for Windows and Linux. Molecular Ecology Resources, 8, 103 106.
Sato T, Isagi Y, Sakio H, Osumi K, Goto S (2006) Effect of gene flow on spatial genetic structure in the riparian canopy tree Cercidiphyllum japonicum revealed by microsatellite analysis. Heredity, 96, 79 84.
Smouse PE, Sork VL (2004) Measuring pollen flow in forest trees: an exposition of alternative approaches. Forest Ecology and Management, 197, 21 38.
Smouse PE, Dyer RJ, Westfall RD, Sork VL (2001) Two-generation analysis of pollen flow across a landscape. I. Male gamete heterogeneity among females. Evolution, 55, 260 271.
Sonke B (1998) Etudes floristiques et structurales des forêts de la réserve de faune du Dja (Cameroun). PhD Thesis, Université Libre de Bruxelles, Bruxelles.
Sork VL, Davis FW, Smouse PE et al. (2002) Pollen movement in declining populations of California Valley oak, Quercus lobata: where have all the fathers gone? Molecular Ecology, 11, 1657 1668.
Taylor DAR, Kankam BO (1999) The Role of the Straw-Colored Fruit Bat, Eidolon helvum, in Seed Dispersal, Survival, and Germination in Milicia excelsa, a Threatened West African Hardwood. Flagstaff (AZ) and Forestry Research Institute of Ghana, Northern Arizona University, Kumasi, Ghana.
Tondeur G (1939) Monographie forestière du Chlorophora excelsa Benth. et Hook. Bulletin Agricole du Congo Belge, 30, 163 198.
Troupin D, Nathan R, Vendramin GG (2006) Analysis of spatial genetic structure in an expanding Pinus halepensis population reveals development of fine-scale genetic clustering over time. Molecular Ecology, 15, 3617 3630.
Van Oosterhout C, Hutchinson WF, Wills DPM, Shipley P (2004) micro-checker: software for identifying and correcting genotyping errors in microsatellite data. Molecular Ecology Notes, 4, 535 538.
Vekemans X, Hardy OJ (2004) New insights from fine-scale spatial genetic structure analyses in plant populations. Molecular Ecology, 13, 921 935.
White MG (1966) A comparison of Chlorophora excelsa (Welw.) Benth and Cook (F.) and C. regia A. Chev., (Fam. Moraceae). The Commonwealth Forestry Review, 45, 150 153.
White F (1983) The Vegetation of Africa. Natural Resources Research, UNESCO, Suisse.
White GM, Boshier DH, Powell W (1999) Genetic variation within a fragmented population of Swietenia humilis Zucc. Molecular Ecology, 8, 1899 1909.
White GM, Boshier DH, Powell W (2002) Increased pollen flow counteracts fragmentation in a tropical dry forest: an example from Swietenia humilis Zuccarini. Proceedings of the National Academy of Sciences, USA, 99, 2038 2042.
Williams CF (1994) Genetic consequences of seed dispersal in three sympatric forest herbs. II. Microspatial genetic structure within populations. Evolution, 48, 1959 1972.
Young AG, Merriam HG (1994) Effects of forest fragmentation on the spatial genetic structure of Acer saccharum Marsh (Sugar Maple) populations. Heredity, 72, 201 208.
