Submitted to: American Journal of Botany
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/15/2009
Publication Date: 6/1/2009
Citation: Zalapa, J.E., Brunet, J., Guries, R.P. 2009. Patterns of Hybridization and Introgression Between Invasive Ulmus Pumila (Ulmaceae) and Native U. Rubra. American Journal of Botany. 96(6):1116-1128. Interpretive Summary: Elms have been extensively used in urban and suburban landscapes because of their natural beauty and their ability to withstand numerous environmental stresses. However, in the 20th century, two Dutch elm disease (DED) pandemics decimated European and North American elm species. These pandemics triggered the search for sources of DED-resistance elm species. Ulmus pumila is native to China and was introduced to the US for breeding purpose because it is tolerant to DED. After its initial introduction in the United States in the 1800’s, Siberian elms have been widely planted across the country between 1900 and 1950 after the first and second DED pandemics. In Wisconsin, U. pumila trees were mainly planted along highways and close to villages and towns to serve as wind breaks and to control erosion, and in urban landscapes to replace death or dying DED-susceptible native elms. Siberian elm has naturalized and is now considered a noxious weed or invasive species in 41 states, including Wisconsin. Red elm, Ulmus rubra, is a species native to the US that is susceptible to Dutch elm disease and is locally abundant in the Midwest. Red elm is highly susceptible to DED and surviving populations of red elms are composed of relatively few (10-50) small statured trees that seldom reach diameters greater than 20 cm before succumbing to DED. The two species, U. rubra and U. pumila, often occur in sympatry and are thought to commonly hybridize at the fringes of urban areas or wherever plantings of Siberian elm are in close proximity to wild red elm populations. The aims of this study were three fold. First we tested whether Ulmus microsatellite loci allowed the genetic confirmation of putative hybrids between U. rubra and U. pumila. Second we determined whether hybridization between these two species is a common event in sympatric populations. Third we examined the pattern of introgression in the hybrid zones because extensive hybridization and subsequent introgression between U. rubra and U. pumila could affect the genetic integrity and conservation of U. rubra populations. We found that our microsatellite loci helped us successfully identify hybrids and could even classify first and later generation hybrids with varying number of alleles from each original parent. We found common hybridization in Wisconsin between these two species. Moreover the hybrids mated more frequently back with U. pumila indicating that the U. rubra alleles were getting lost in the hybrids. The observed process of introgression could lead to the genetic assimilation of the native elm species. Thus the survival of red elm populations, U. rubra, a native elm species, is being threatened not only by DED but also by mixing (hybridization) with the invasive non native elm species, U. pumila.
Technical Abstract: Ulmus pumila (Siberian elm) is an invasive elm species, non-native to the United States, which hybridizes with Ulmus rubra (red elm), a U.S. native. While Siberian elm is highly tolerant to Dutch elm disease (DED), red elm populations in North America have been strongly affected by DED. Putative phenotypic hybrids have been recorded in Wisconsin. The principal aims of this study were three fold. First we tested whether Ulmus microsatellite loci allowed the genetic confirmation of putative hybrids between U. rubra and U. pumila. Second we determined whether hybridization between these two species is a common event in sympatric populations. Third we examined the pattern of introgression in the hybrid zones because extensive hybridization and subsequent introgression between U. rubra and U. pumila could affect the genetic integrity and conservation of U. rubra populations.To confirm hybridization and assess its evolutionary consequences, we genotyped 274 15- 30 years old trees as assessed by trunk diameter; 100 from U. rubra, 53 from U. pumila and 96 putative hybrids collected at six Wisconsin contact zones between U. rubra and U. pumila. We tested a total of 15 microsatellite loci possessing species-specific alleles for U. rubra and U. pumila. Nine loci effectively discriminated between each parental species and their hybrids. To summarize major patterns of variation in multilocus data for each species and hybrids, principal coordinate analyses (PCoA) were conducted based on individuals (of each species and their hybrids) using pair-wise squared Euclidean distances (GenAlEx Vers. 6). Our loci clearly distinguished each parental species and their hybrids and allowed us to identify F1 and backcrosses with varying number of alleles from each parent. The great majority of backcrosses occurred with U. pumila indicating that the pattern of introgression is in favor of U. pumila alleles. Because hybridization is very common in Wisconsin and introgression favors U. pumila, our data indicate that the genetic integrity of U. rubra may be in peril: the patterns of hybridization that we observed can lead to the genetic assimilation of the native elm species.