The Extent of Hybridization and Its Impact on the Genetic Diversity and Population Structure of an Invasive Tree, Ulmus Pumila (Ulmaceae)

Authors: ZALAPA, JUAN - UW MADISON; Brunet, Johanne

Submitted to: Evolutionary Applications
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/10/2009
Publication Date: 3/1/2010
Citation: Zalapa, J.E., Brunet, J. 2010. The Extent of Hybridization and Its Impact on the Genetic Diversity and Population Structure of an Invasive Tree, Ulmus Pumila (Ulmaceae). Evolutionary Applications. 3(2):157-168.

Interpretive Summary: Along with the destruction of natural habitats and overexploitation, introduced species are a major cause of ecosystem degradation. In new environments and without their natural checks and balances, introduced species may spread rapidly and become invasive, usually disrupting their adopted community. Uncontrolled invasive species can cause significant harm to agriculture and human health and are responsible for a high rate of species extinction and degradation of natural environments. If enough genetic variability exists, invasive species may evolve in response to the natural selection pressures generated by their new environment and become even more invasive. An additional problem arises when natural hybridization occurs between introduced species and their native congeners. In fact, the transfer of genes important for local adaptation (from a native to an introduced species) may play a role in the evolution of increased invasiveness resulting in the evolution of “super invasive” species. Siberian elm, native to East Asia, was introduced in the U.S. in the 1900’s because of its high tolerance to Dutch elm disease (DED). However, Siberian elm has spread rapidly following its introduction and has now become one of the most invasive woody species in the U.S. (declared invasive in 41 states), alongside Russian olive and tamarisk. In addition, Siberian elm is cross-compatible with red elm (U. rubra) and natural hybridization has been confirmed genetically in our laboratory. A first goal of the current study was to compare the genetic diversity of Siberian elm between accessions from its native range in East Asia and accessions collected throughout the U.S. We wanted to know whether there was any evidence of founder effects/genetic bottlenecks (i.e., reduced diversity) or an increase of genetic diversity during the invasion process, especially in the context of hybridization. A second goal of this study was to examine the genetic structure of invasive Siberian elm (Ulmus. pumila L.) populations in the United States. After genotyping 325 individuals at seven microsatellite loci, we observed comparably high level of heterozygosity, high genetic diversity and low coefficients of inbreeding in both U.S. and East Asian accessions. Although sampling was made from phenotypically typical U. pumila populations, we found that six out of eight naturalized populations contained hybrids (U. pumila x U. rubra) at frequencies between 15 and 53 % which indicated widespread hybridization with U. rubra in Wisconsin, Illinois, and South Dakota. Thus, the high genetic diversity and frequent hybridization with U. rubra raise concern on the negative impact of this invasive species on native species. This research provides critical information for ecologists and government officials to deem Siberian elm as a serious ecological concern and to consider control/removal and revegetation with native tree species.

Technical Abstract: Siberian elm (Ulmus pumila L.), native to East Asia, was introduced in the U.S. in the 1900’s because of its high tolerance to Dutch elm disease (DED). Siberian elm has spread following its introduction and has now become one of the most invasive woody species in the U.S., alongside Russian olive and tamarisk. In addition, U. pumila is cross-compatible with other diploid native elms and hybridization with red elm (U. rubra) has been confirmed genetically. In this study, we used seven microsatellite markers to compare the level of genetic variability of Siberian elm accessions collected over the U.S. versus original collections from East Asia. We then examined how genetic diversity was distributed within and among eight naturalized populations of U. pumila in Wisconsin. We observed comparably high level of heterozygosity, high genetic diversity and low coefficients of inbreeding in both U.S. and East Asian accessions. Principal coordinate analysis differentiated all of our populations. Although sampling was made from phenotypically typical U. pumila populations, we first tested each tree with species-specific markers to classify it as genotypically “pure” U. pumila or hybrid (U. pumila x U. rubra). Six of the naturalized populations contained hybrids at frequencies between 15 and 53 % which indicated widespread hybridization with U. rubra in Wisconsin, Illinois, and South Dakota. An analysis of molecular variance (AMOVA) indicated that most of the genetic variation in naturalized U. pumila populations was within (93%) as opposed to between (7 %) populations. Moreover, analyses of the microsatellite data using the computer program STRUCTURE to perform Bayesian cluster analysis and the graph theoretic approach implemented in the Population Graphs computer program indicated, respectively, that the presence of hybrids increased the degree of clustering and modified the level of connectivity among U. pumila populations. The high genetic variability in naturalized Siberian elm populations in the U.S. and the connectivity among populations coupled with frequent hybridization with other elm species should increase the ecological concern for this exotic, invasive tree species.