Biol Invasions DOI 10.1007/s10530-013-0486-z ORIGINAL PAPER Hybridization and introgression between the exotic Siberian elm, Ulmus pumila, and the native Field elm, U. minor, in Italy Johanne Brunet • Juan E. Zalapa • Francesco Pecori • Alberto Santini Received: 31 July 2012 / Accepted: 4 May 2013 Ó Springer Science+Business Media Dordrecht (outside the USA) 2013 Abstract In response to the first Dutch elm disease groups had higher levels of heterozygosity relative to (DED) pandemic, Siberian elm, Ulmus pumila, was U. pumila. The programs Structure and NewHybrids planted to replace the native elm, U. minor, in Italy. indicated the presence of first- (F1) and second- The potential for hybridization between these two generation (F2) hybrids and of backcrosses in the species is high and repeated hybridization could result hybrid population. The presence of healthy DED in the genetic swamping of the native species and resistant U. minor individuals combined with the self- facilitate the evolution of invasiveness in the intro- compatibility of U. minor could help explain the duced species. We used genetic markers to examine presence of F2 individuals in Italy. The presence of F2 the extent of hybridization between these two species individuals, where most of the variability present in and to determine the pattern of introgression. We the hybrids will be released, could facilitate rapid quantified and compared the level of genetic diversity evolution and the potential evolution of invasiveness between the hybrids and the two parental species. of U. pumila in Italy. Hybrids between U. pumila and U. minor were common. The pattern of introgression was not as Keywords Dutch elm disease Á Field elm Á strongly biased towards U. pumila as was previously Hybridization Á Introgression Á Microsatellites Á observed for hybrids between U. rubra and U. pumila Siberian elm in the United States. The levels of heterozygosity were similar between U. minor and the hybrids and both & J. Brunet ( ) Introduction USDA-ARS, VCRU, Department of Entomology, University of Wisconsin, 1630 Linden Drive, Madison, WI 53706, USA The repeated introduction of exotic species by humans e-mail: [email protected]; [email protected] has accelerated the range expansion of many organ- isms (Crowl et al. 2008). While various species were J. E. Zalapa USDA-ARS, VCRU, Department of Horticulture, inadvertently introduced, some introductions were University of Wisconsin, 1575 Linden Drive, Madison, intentional. For example, exotic trees have been WI 53706, USA planted as ornamental trees in urban areas or to replace native tree species decimated by a disease F. Pecori Á A. Santini Institute of Plant Protection, C.N.R., Via Madonna del epidemic (Machon et al. 1997; Cogolludo-Agustin Piano, 10, 50019 Sesto Fiorentino, Italy et al. 2000; Zalapa et al. 2010). The introduced species 123 J. Brunet et al. were often close congeners of the native species, (Ulmus pumila L.) were distributed by authorities and which implied a high potential for hybridization nurseries throughout the whole Italian territory (Go- between the native and introduced species. While the idanich 1936). In addition, seedlings of Siberian elm potential impact of hybridization on the preservation were used as rootstocks (Goidanich and Azzaroli of genetic diversity of the native species was not 1941), or scions were grafted onto Field elms (Ansa- recognized when most of these exotic species were loni 1934). Moreover, two clones of Field elm, introduced, it is now clear that hybridization can pose ‘Christina Buisman’ and ‘Villagrappa 3’, were planted an increased risk of genetic assimilation and eventual as trials because they showed enough resistance to O. loss of the native taxa (Rhymer and Simberloff 1996; ulmi. Following the first DED pandemic, Goidanich Hedge et al. 2006). This is especially true for small (1941), an Italian plant pathologist, affirmed that the populations already at risk from biotic or abiotic impact of DED was successfully controlled, partly as a stresses (Rieseberg et al. 1989; Ellstrand and Elam result of the increased use of U. pumila and the 1993; Daehler and Strong 1997; Collin 2002; Burgess selection of tolerant U. minor clones. While the et al. 2005; Prentis et al. 2007). In addition, it has been widespread distribution of Siberian elms stopped established that hybridization can increase genetic shortly after World War II, many thousands of diversity and novel gene combinations which may, in Siberian elms remained in the landscape, mainly turn, facilitate the process of adaptation and help a along roads, rivers and streams or in abandoned fields. species spread into new habitats (Ellstrand and Elm species are known to cross-hybridize (Mit- Schierenbeck 2000; Vila et al. 2000; Sakai et al. tempergher and La Porta 1991; Goodall-Copestake 2001; Rieseberg et al. 2003; Hedge et al. 2006). et al. 2005) and the ability of U. pumila to produce Many of the European and American species of fertile offspring when crossed with various DED elm, Ulmus spp. (Ulmaceae), were decimated by susceptible elm species has been exploited to develop Dutch elm disease (DED) during the 20th century, DED tolerant varieties (Smalley and Guries 1993; with a first pandemic caused by the ascomycete fungi, Santamour and Bentz 1995; Ware 1995; Santini et al. Ophiostoma ulmi (Buisman) Nannf. and a more 2002, 2007; Mittempergher and Santini 2004). The aggressive species, O. novo-ulmi, responsible for the ability for elm species to cross-hybridize suggests, second pandemic (Brasier 1988, 1991). While Euro- however, a strong potential for the formation of hybrid pean and North American species of elms were very seeds between Siberian elms and the native elm susceptible to DED, with infected trees dying within species. To date, widespread hybridization has been 1–2 years, several Eurasian species exhibited varying documented between the exotic Siberian elm, U. degrees of tolerance to both the first (Smalley and pumila, and Field elm, U. minor, in Spain (Cogolludo- Guries 1993) and the second DED pandemics (Santini Agustin et al. 2000) and between U. pumila and the et al. 2005; Solla et al. 2005). In response to the two native red elm, U. rubra Muhl in the Midwestern DED pandemics, Siberian elm, U. pumila, was planted United States (Zalapa et al. 2009, 2010). to replace the native elms in different countries, A pattern of introgression biased towards U. including Italy (Goidanich 1936) and the United States pumila, with few backcrosses between hybrid indi- (Zalapa et al. 2010). The native Field elm, U. minor viduals and the native species, was detected in both Mill., was widely used as living support for grapevine, Spain and the United States. This pattern of biased fodder for cattle, timber for construction, and firewood introgression has been attributed to the lower abun- when the first DED epidemic reached Italy in the dance of the DED susceptible native elm species over 1930s (Sibilia 1930). Field elm was also important as the landscape (Cogolludo-Agustin et al. 2000; Zalapa shadow tree in pastures and as an ornamental tree in et al. 2009, 2010). In addition, the biased introgression streets and in parks (Goidanich 1936). Given its wide has been considered a threat for the conservation of the usage, the progressive disappearance of Field elm was genetic diversity of the native elm, U. rubra in the considered disastrous, which stimulated private nurs- United States (Zalapa et al. 2009) and U. minor in erymen (Ansaloni 1934) and the scientific community Spain (Cogolludo-Agustin et al. 2000). Furthermore, (Sibilia 1932; Passavalli 1935) to look for a suitable hybridization between U. pumila and U. minor or replacement. Within a few years following the onset of between U. pumila and U. rubra has been shown to the first DED epidemics, thousands of Siberian elms increase the genetic diversity (Cogolludo-Agustin 123 Hybridization and introgression et al. 2000; Zalapa et al. 2009, 2010) and, at least in the hairless twigs; small, blunt, hairless buds; shallowly Midwestern United States, to affect the genetic furrowed, gray or brown bark; and comparatively structure of U. pumila populations (Zalapa et al. small, smooth samaras (Wyman 1951); the crown is 2010). Hybridization could, therefore, have contrib- wide from roundish to vase-shaped, secondary shoots uted to the increased range of habitats where U. pumila are generally pendulous. In contrast, Field elm leaves can establish in the United States compared to its are oblanceolate to nearly circular, coarse and pubes- native range (Zalapa et al. 2010). Moreover, hybrid- cent when young, smooth and glossy above at ization may help explain the fact that Siberian elm has maturity, glabrous beneath with axillary pubescence, become an invasive species in 41 out of 50 states in the base uneven, corky wings (Santini et al. 2008); the United States (USDA, NRCS 2002; Ding et al. 2006). crown shape varies from cylindrical to conical. Based In the current study, we use genetic markers to on these morphological descriptors, eleven trees were examine the extent of hybridization between the native classified in the field as U. pumila,41asU. minor, and Field elm, U. minor, and the exotic Siberian elm, U. 42 as putative hybrids between U. pumila and U. pumila, in Italy. We examine the pattern of introgres- minor while two trees could not be identified sion between these two species to determine whether (Table 1). In addition to these 96 samples, to facilitate introgression is biased towards U. pumila, as was the genetic identification of hybrid individuals, we previously found between U. pumila and U. minor in used as a reference population for U. pumila,49 Spain (Cogolludo-Agustin et al. 2000) and between U. accessions from China whose genetic composition had pumila and U. rubra in the United States (Zalapa et al. been previously described (Zalapa et al. 2008a). 2009). We examine the level of genetic diversity of the hybrids relative to the two parental species and Genotyping using microsatellites quantify the degree of genetic differentiation between these three groups.