When exotic poplars and native Populus balsamifera L. meet on the Canadian Prairies: Spontaneous hybridization and establishment of interspecific hybrids Patricia Talbot a,b , William R. Schroeder c , Jean Bousquet b , Nathalie Isabel a,b,⇑ a Natural Resources Canada, Canadian Forest Service, Laurentian Forestry Centre, 1055 du P.E.P.S., P.O. Box 10380, Station Ste-Foy, Québec, QC, Canada G1V 4C7 b Canada Research Chair in Forest and Environmental Genomics, Forest Research Centre and Institute for Systems and Integrative Biology, Université Laval, Québec, QC, Canada G1V 0A6 c Agriculture and Agri-Food Canada, Agroforestry Development Centre, P.O. Box 940, No. 2 Government Road, Indian Head, SK, Canada S0G 2K0 article info Article history: Received 22 May 2012 Received in revised form 24 July 2012 Accepted 24 July 2012 Keywords: Hybridization Introgression Balsam poplar Russian poplar Single nucleotide polymorphisms (SNPs) Windbreak abstract As the interest in plantations of fast-growing tree species increases, concerns regarding the possibility of gene flow between plantations of exotic trees and their wild relatives are being raised. In Canada, the Prairie ecozone provides a unique opportunity to examine spontaneous hybridization and the spread of exotic genetic material because of the historical introduction of large numbers of exotic poplar species and their hybrids. In this region, poplar shelterbelts bearing exotic components such as Populus laurifolia Ledeb. and Populus nigra L. have been in contact with the native populations of Populus balsamifera L. since the 19th century. Given the ability for poplar species to hybridize, the objective of this study was to estimate the rates of spontaneous hybridization between one common poplar cultivar planted in shelterbelts, #‘Assiniboine’ ($‘Walker’ [$Populus deltoides Marsh.  #(P. laurifolia  P. nigra)]  #P. del- toides), and its neighbouring native congener, $P. balsamifera, on two study sites. To distinguish the genetic contributions of the species under study, a set of 26 diagnostic single nucleotide polymorphisms (SNPs) representative of 23 genes was used. We genotyped 2116 seeds sampled from P. balsamifera and identified paternal alleles specific to P. laurifolia, P. nigra and P. deltoides, which are typical genetic com- ponents of poplar shelterbelts. Surprisingly, the highest hybridization rate (2.3%) was observed at the site where 100-year-old Russian poplar shelterbelts (P. laurifolia  P. nigra) were found. A preliminary study of the P. balsamifera stands established in an 8 km radius around the study site confirmed introgression of P. laurifolia and P. nigra alleles at a rate of 4.6%. These results indicate that spontaneous hybridization between shelterbelts of exotic cultivars and native P. balsamifera does occur in the Canadian Prairies and that interspecific hybrids can establish and survive in this landscape. This study suggests that biolog- ical factors such as the genetic composition of the native population and the gender of the exotic cultivars should be taken into consideration in the management strategies of this ecozone. Crown Copyright Ó 2012 Published by Elsevier B.V. All rights reserved. 1. Introduction Understanding and detecting the spread of exotic species or ge- netic material is imperative to ensure effective management and mitigation of their impacts on native populations. Spontaneous hybridization between exotic and native species can result in the formation of invasive hybrids/genotypes that may spread through the native gene pool via subsequent introgression (Abbott, 1992; Ellstrand and Schierenbeck, 2000; Laikre et al., 2010). Numerous examples have been documented in plants (Mercure and Bruneau, 2008; Schierenbeck and Ellstrand, 2009; Zalapa et al., 2009), fish (Normandeau et al., 2009), insects (Pinto et al., 2005), and fungi (Brasier et al., 1999). Escaped hybrids can have a negative impact on native populations by threatening the genetic integrity of the native species (i.e., by limiting local adaptation), by showing ‘wee- dy’ tendencies (i.e., by spreading rapidly, out-competing parent populations and colonizing new habitats) (Rhymer and Simberloff, 1996; Lenormand, 2002; Schierenbeck and Ellstrand, 2009) and by affecting biodiversity at the level of communities and ecosystem processes (Whitham et al., 1999). Alternatively, the benefits of exo- tic hybrids have long been exploited by forestry, agriculture and biotechnology industries to develop cultivars with improved sur- vival, resistance and performance, and to reduce reliance on pesti- cides. For example, improved genotypes of poplars and willows are used for phytoremediation of contaminated soils (Kopp et al., 2001; Zalesny et al., 2007) and exotic species are used in tradi- tional breeding programs to improve resistance against diseases (Smalley and Guries, 1993). In the context of bioenergy production and carbon sequestration, there is an increasing interest in 0378-1127/$ - see front matter Crown Copyright Ó 2012 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.foreco.2012.07.036 ⇑ Corresponding author at: Natural Resources Canada, Canadian Forest Service, Laurentian Forestry Centre, 1055 du P.E.P.S., P.O. Box 10380, Station Ste-Foy, Québec, QC, Canada G1V 4C7. Tel.: +1 418 648 7137; fax: +1 418 648 5849. E-mail address: nisabel@rncan.gc.ca (N. Isabel). Forest Ecology and Management 285 (2012) 142–152 Contents lists available at SciVerse ScienceDirect Forest Ecology and Management journal homepage: www.elsevier.com/locate/foreco