Molecular Ecology 2003 12 1689 1702 doi 10 1046 j 1365 294X 2003 01849 x Microsatellite variation within and among North American lineages of Phragmites australis Blackwell Publishing Ltd K SALTONSTALL Department of Ecology and Evolutionary Biology Yale University New Haven USA Abstract Over the past century the spread of the common reed Phragmites australis has had a dramatic impact on wetland communities across North America Although native populations of Phragmites persist introduced invasive populations have dominated many sites and it is not clear if the two types can interbreed This study compares patterns of differentiation in 10 microsatellite loci among North American and European Phragmites individuals with results obtained from sequencing of noncoding chloroplast DNA Three population lineages native introduced and Gulf Coast were previously identified in North America from chloroplast DNA and similar structuring was found in the nuclear genome Each lineage was distinguished by unique alleles and allele combinations and the introduced lineage was closely related to its hypothesized source population in Europe Size homoplasy and diagnostic base substitutions distinguishing lineages were evident at several loci further emphasizing that native introduced and Gulf Coast North American Phragmites lineages are genetically distinct Gene flow between lineages was low and invasive introduced populations do not represent a hybrid population type Keywords hybridization invasive species microsatellite DNA markers Phragmites australis population structure Received 18 October 2002 revision received 18 February 2003 accepted 18 February 2003 Introduction Exotic invasive species pose one of the greatest threats to species and habitat diversity today second only to habitat loss Wilcove et al 1998 In addition to their impacts at the community and ecosystems levels such introductions pose a number of threats at the genetic level to closely related species and locally adapted populations of the same species Perhaps most significant are the effects of hybridization with closely related species Anderson Stebbins 1954 Lewontin Birch 1966 which can quickly change the genetic composition of native populations and cause loss of local adaptations Anttila et al 1998 Huxel 1999 It may also stimulate invasiveness in introduced populations by increasing genetic variability creating favourable gene combinations masking deleterious alleles Ellstrand Scheirenbeck 2000 Lee 2002 and causing Correspondence K Saltonstall Present address Horn Point Laboratory University of Maryland Center for Environmental Science PO Box 775 Cambridge MD 21613 USA Fax 1 410 221 8490 E mail kristin saltonstall aya yale edu 2003 Blackwell Publishing Ltd changes in ploidy levels which often increases fitness Soltis Soltis 2000 In natural plant populations microsatellites have great potential for helping to understand what determines patterns of genetic variation particularly when used in concert with chloroplast DNA cpDNA markers Their utility has been demonstrated in studies of genetic diversity Morand et al 2002 mating systems Durand et al 2000 pollination biology White et al 2002 and seedling establishment Dow Ashley 1996 Few studies have been carried out using microsatellites in analysis of population structure of polyploid species This is likely due to the problems in analysing polyploid data as well as difficulties in amplifying loci possibly because of differences in the parental genomes of polyploids R der et al 1995 An area of concern when using microsatellite frequencies to estimate population parameters is size homoplasy which can cause underestimation of the number of alleles levels of heterozygosity and genetic diversity when only allele sizes are considered reviewed in Estoup et al 2002 A number of studies have demonstrated that microsatellite alleles of the same size can arise from mutation events 1690 K S A L T O N S T A L L which either interrupt repeat units or occur in the regions flanking the repeat region This has been shown to occur both within Angers Bernatchez 1997 Viard et al 1998 and among populations Estoup et al 1995 Viard et al 1998 and closely related species Peakall et al 1998 van Oppen et al 2000 One approach to minimizing the risk of misinterpretation of genetic information is to characterize different electromorphs by sequencing particularly in cases in which other genetic data e g chloroplast or mitochondrial sequences suggest strong levels of genetic structuring that is not being detected by microsatellite analysis The common reed Phragmites australis hereafter Phragmites has a worldwide distribution and is considered native to North America However its distribution and abundance have increased dramatically over the past 150 years in North America and it is considered a nuisance species in many regions Phragmites is a perennial wetland grass that is tolerant of both freshwater and brackish conditions and typically forms a tall dense monoculture in the systems that it invades Its ability to readily colonize open habitat both by spread of seed and fragments of rhizome material has undoubtedly contributed greatly to the invasive spread of the species Phragmites shows high variation in ploidy levels throughout its distribution Throughout the genus the base chromosome number is 12 Tucker 1990 Although the diploid number 2n 24 has never been reported in a natural population a wide range of polyploid and aneuploid population types have been reported reviewed in Clevering Lissner 1999 Karyotypic studies of North American populations indicate the presence of 3x 4x 6x and 8x plants with 4x being the dominant chromosome number in modern day populations Chambers et al 1999 In this study 10 microsatellite loci were used to assess the genetic structure of Phragmites populations in North America and Europe at the level of nuclear DNA Recent evidence based on sequencing of cpDNA indicates that a single nonnative lineage of Phragmites has been introduced to North America and is likely to be responsible for the dramatic spread of the species across North America Saltonstall 2002 Native individuals persist in many locations such as in the Midwest and western parts of the country but introduced populations have overrun the Atlantic coast region and eliminated the majority of native populations Saltonstall 2003 Five diagnostic chloroplast mutations were found to distinguish native and introduced Phragmites
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