Molecular Ecology 2003 12 2875 2883 doi 10 1046 j 1365 294X 2003 01950 x Population genetic structure of mahogany Swietenia macrophylla King Meliaceae across the Brazilian Amazon based on variation at microsatellite loci implications for conservation Blackwell Publishing Ltd M A R I S T E R R A R L E M E S R O G R I O G R I B E L J O H N P R O C T O R and D A R I O G R A T T A P A G L I A Laborat rio de Gen tica e Biologia Reprodutiva de Plantas Instituto Nacional de Pesquisas da Amaz nia C P 478 69011 970 Manaus AM Brazil Department of Biological Sciences University of Stirling Stirling FK9 4LA UK Laborat rio de Gen tica de Plantas EMBRAPA Recursos Gen ticos e Biotecnologia C P 02372 70770 900 Bras lia DF Brazil Laborat rio de Biotecnologia Gen mica P s Gradua o em Ci ncias Gen micas Universidade Cat lica de Bras lia SGAN 916 Mod B Asa Norte 70790 160 Bras lia DF Brazil Abstract Mahogany Swietenia macrophylla Meliaceae is the most valuable and intensively exploited Neotropical tree No information is available regarding the genetic structure of mahogany in South America yet the region harbours most of the unlogged populations of this prized hardwood Here we report on the genetic diversity within and the differentiation among seven natural populations separated by up to 2100 km along the southern arc of the Brazilian Amazon basin We analysed the variation at eight microsatellite loci for 194 adult individuals All loci were highly variable with the number of alleles per locus ranging from 13 to 27 mean 18 4 High levels of genetic diversity were found for all populations at the eight loci mean HE 0 781 range 0 754 0 812 We found moderate but statistically significant genetic differentiation among populations considering both estimators of FST and RST 0 097 and 0 147 respectively Estimates of and were significantly greater than zero for all pairwise population comparisons Pairwise values were positively and significantly correlated with geographical distance under the isolation by distance model Furthermore four of the populations exhibited a significant inbreeding coefficient The finding of local differentiation among Amazonian mahogany populations underscores the need for in situ conservation of multiple populations of S macrophylla across its distribution in the Brazilian Amazon In addition the occurrence of microgeographical genetic differentiation at a local scale indicates the importance of maintaining populations in their diverse habitats especially in areas with mosaics of topography and soil Keywords Amazon conservation genetics genetic structure mahogany microsatellites Swietenia macrophylla tropical tree Received 24 March 2003 revision received 6 June 2003 accepted 10 July 2003 Introduction The destruction of tropical forests world wide has increased dramatically in recent decades Whitmore 1997 Bawa Seidler 1998 posing a significant threat to the maintenance of biodiversity and biological processes in Correspondence Maristerra R Lemes Fax 55 92 6433285 E mail mlemes inpa gov br 2003 Blackwell Publishing Ltd tropical forest ecosystems Bawa 1994 Young et al 1996 The genetic threat to tropical trees results from the loss of genetic diversity associated with the extinction of local populations reduced population sizes and the disruption of 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 promote dysgenic selection as a result of the continuous exploitation of large superior individuals and may increase levels 2876 M R L E M E S E T A L of inbreeding as a result of reduction in stand density Bawa 1994 Murawski et al 1994 To evaluate and mitigate the genetic effects of deforestation and logging it has become a priority to obtain information on the natural levels and distribution of genetic variation in populations of tropical trees Population genetic studies of tropical trees have shown that most of the species investigated are outcrossed exhibit high levels of genetic diversity and gene flow and carry much of the variation within rather than among populations Hamrick Loveless 1989 Alvarez Buylla et al 1996 However the great majority of these studies were developed over relatively small spatial scales and employed isozymes as the primary genetic markers Hamrick Loveless 1986 Loveless 1992 1998 Alvarez Buylla Garay 1994 Hall et al 1994 In recent years the development of microsatellites 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 have allowed larger scale and more refined studies of population genetic structure e g White et al 1999 Collevatti et al 2001 The central aim of this work was to characterize and understand the genetic structure of natural populations of mahogany Swietenia macrophylla Meliaceae across a 2100km transect of the Brazilian Amazon using microsatellite loci recently developed for this species Lemes et al 2002 Despite the perceived importance of the Amazon basin for tree species diversity and as a repository for half the world s remaining rain forest our investigation of mahogany is the first population genetic analysis of a tree species distributed across this vast region Furthermore mahogany is of considerable interest to resource managers as it is by far the most valuable Neotropical hardwood species One cubic metre of export quality sawn mahogany is valued at about US 700 on the international market Verissimo et al 1995 and Brazil alone exports about 500 000 m3 year A previous population genetic study of mahogany was limited to Central America where the species is commercially extinct in most regions Gillies et al 1999 The recent inclusion of mahogany in CITES Convention for International Trade in Endangered Species Appendix II 2002 highlights international concern regarding the future of South American populations Most natural populations of mahogany have been logged and there is evidence that the species does not regenerate in areas of intense exploitation Gullison et al 1996 Thus there is an urgent need for effective conservation and management of the remnant populations To this end estimates of population genetic parameters are essential The variability observed at microsatellite loci provides estimates of inbreeding heterozygosity gene flow and outcrossing rate all of which are important
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