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UW-Madison BOTANY 940 - Population genetic structure of mahogany (Swietenia macrophylla King, Meliaceae)

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Molecular Ecology (2003) 12 , 2875–2883 doi: 10.1046/j.1365-294X.2003.01950.x© 2003 Blackwell Publishing Ltd Blackwell Publishing Ltd. Population genetic structure of mahogany ( Swietenia macrophylla King, Meliaceae) across the Brazilian Amazon, based on variation at microsatellite loci: implications for conservation MARISTERRA R. LEMES, *† ROGÉRIO GRIBEL, * JOHN PROCTOR † and DARIO GRATTAPAGLIA ‡§ * Laboratório de Genética e Biologia Reprodutiva de Plantas, Instituto Nacional de Pesquisas da Amazônia, C.P. 478, 69011-970 Manaus-AM, Brazil, † Department of Biological Sciences, University of Stirling, Stirling FK9 4LA, UK, ‡ Laboratório de Genética de Plantas, EMBRAPA-Recursos Genéticos e Biotecnologia, C.P. 02372, 70770-900 Brasília-DF, Brazil, § Laboratório de Biotecnologia Genômica, Pós-Graduação em Ciências Genômicas, Universidade Católica de Brasília, SGAN 916 Mod. B, Asa Norte, 70790-160, Brasília-DF, Brazil AbstractMahogany ( Swietenia macrophylla , Meliaceae) is the most valuable and intensivelyexploited Neotropical tree. No information is available regarding the genetic structure ofmahogany in South America, yet the region harbours most of the unlogged populations ofthis prized hardwood. Here we report on the genetic diversity within and the differentiationamong seven natural populations separated by up to 2100 km along the southern arc of theBrazilian Amazon basin. We analysed the variation at eight microsatellite loci for 194 adultindividuals. All loci were highly variable, with the number of alleles per locus rangingfrom 13 to 27 (mean = 18.4). High levels of genetic diversity were found for all populationsat the eight loci (mean H E = 0.781, range 0.754–0.812). We found moderate but statisticallysignificant genetic differentiation among populations considering both estimators of F ST and R ST , θθθθ = 0.097 and ρρρρ = 0.147, respectively. Estimates of θθθθ and ρρρρ were significantly greaterthan zero for all pairwise population comparisons. Pairwise ρρρρ -values were positively andsignificantly correlated with geographical distance under the isolation-by-distance model.Furthermore, four of the populations exhibited a significant inbreeding coefficient. Thefinding of local differentiation among Amazonian mahogany populations underscores theneed for in situ conservation of multiple populations of S. macrophylla across its distribu-tion in the Brazilian Amazon. In addition, the occurrence of microgeographical geneticdifferentiation at a local scale indicates the importance of maintaining populations in theirdiverse habitats, especially in areas with mosaics of topography and soil. Keywords : Amazon, conservation genetics, genetic structure, mahogany, microsatellites, Swieteniamacrophylla , tropical tree Received 24 March 2003; revision received 6 June 2003; accepted 10 July 2003 Introduction The destruction of tropical forests world-wide has in-creased dramatically in recent decades (Whitmore 1997;Bawa & Seidler 1998), posing a significant threat to themaintenance of biodiversity and biological processes intropical forest ecosystems (Bawa 1994; Young et al . 1996).The genetic threat to tropical trees results from the lossof genetic diversity associated with the extinction of localpopulations, reduced population sizes, and the disruptionof mutualisms with pollinators and seed-dispersing animals(Bawa 1994; Hall et al . 1996; Nason et al . 1997; Aldrich et al .1998; Dick 2001). Moreover, selective logging may promotedysgenic selection as a result of the continuous exploita-tion of large, superior individuals and may increase levels Correspondence: Maristerra R. Lemes. Fax: + 55 (92) 6433285;E-mail: [email protected] M. R. LEMES ET AL. © 2003 Blackwell Publishing Ltd, Molecular Ecology , 12, 2875–2883 of inbreeding as a result of reduction in stand density(Bawa 1994; Murawski et al . 1994). To evaluate and mitigatethe genetic effects of deforestation and logging, it hasbecome a priority to obtain information on the naturallevels and distribution of genetic variation in populationsof tropical trees.Population genetic studies of tropical trees have shownthat most of the species investigated are outcrossed, exhibithigh levels of genetic diversity and gene flow, and carrymuch of the variation within rather than among popula-tions (Hamrick & Loveless 1989; Alvarez-Buylla et al . 1996).However, the great majority of these studies were devel-oped over relatively small spatial scales, and employedisozymes as the primary genetic markers (Hamrick &Loveless 1986; Loveless 1992, 1998; Alvarez-Buylla & Garay1994; Hall et al . 1994). In recent years, the development ofmicrosatellites for an increasing number of tropical trees(White & Powell 1997; Aldrich et al . 1998; Brondani et al .1998; Collevatti et al . 1999; Dayanandan et al . 1999; Dick &Hamilton 1999; Gaiotto et al . 2001; Lemes et al . 2002) haveallowed larger scale and more refined studies of populationgenetic structure (e.g. White et al . 1999; Collevatti et al . 2001).The central aim of this work was to characterize andunderstand the genetic structure of natural populations ofmahogany ( Swietenia macrophylla , Meliaceae) across a 2100-km transect of the Brazilian Amazon using microsatelliteloci recently developed for this species (Lemes et al . 2002).Despite the perceived importance of the Amazon basinfor tree species diversity, and as a repository for half theworld’s remaining rain forest, our investigation of maho-gany is the first population genetic analysis of a tree speciesdistributed across this vast region. Furthermore, maho-gany is of considerable interest to resource managers as itis by far the most valuable Neotropical hardwood species.One cubic metre of export-quality sawn mahogany isvalued at about US$ 700 on the international market(Verissimo et al . 1995), and Brazil alone exports about500 000 m 3 /year. A previous population genetic study ofmahogany was limited to Central America where the spe-cies is commercially extinct in most regions (Gillies et al .1999).The recent inclusion of mahogany in CITES (Conventionfor International Trade in Endangered Species, AppendixII 2002) highlights international concern regarding thefuture of South American populations. Most naturalpopulations of mahogany have been logged and there isevidence that the species does not regenerate in areas ofintense exploitation


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UW-Madison BOTANY 940 - Population genetic structure of mahogany (Swietenia macrophylla King, Meliaceae)

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