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First published online May 12, 2009; doi:10.3732/ajb.0800334 American Journal of Botany 96: 1116-1128 (2009) © 2009 Botanical Society of America, Inc.

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Patterns of hybridization and introgression between invasive Ulmus pumila (Ulmaceae) and native U. rubra¹

² USDA, Agricultural Research Service, Vegetable Crops Research Unit, Department of Horticulture, University of Wisconsin, 1575 Linden Drive, Madison, Wisconsin 53706 USA ³ Department of Forest and Wildlife Ecology, 1630 Linden Dr., University of Wisconsin, Madison, Wisconsin 53706 USA

ABSTRACT

Natural hybridization between introduced species and their native congeners occurs frequently and can create serious conservation concerns. Ulmus pumila (Siberian elm) is an introduced Asian elm species that has naturalized in the United States and is now considered invasive in 41 states. Red elm (U. rubra), a native to the eastern United States, often occurs in sympatry with Siberian elm, and the two species are thought to hybridize. Here, we genetically characterized reference populations of the two elm species to identify species-specific microsatellite alleles. These markers were used to classify individuals in putative hybrid zones as parental species or hybrids, assess the extent of hybridization, and track patterns of introgression. We identified nine U. rubra, 32 U. pumila, and 51 hybrid individuals in our hybrid zones. Of the 51 hybrids, 35 were classified as first-generation hybrids and 16 as backcrosses. The majority of the backcrosses (88%) were introgressed toward U. pumila. Our classification of genotypes was consistent whether we used manual classification, principal coordinate analyses or Bayesian clustering. We observed greater genetic diversity and new combination of alleles in the hybrids. Our study indicates widespread hybridization between U. pumila and U. rubra and an asymmetric pattern of introgression toward U. pumila.

Key Words: Dutch elm disease • hybrid zones • invasive species • microsatellites • red elm • Siberian elm • species-specific markers

Received for publication 2 October 2008. Accepted for publication 15 January 2009.

FOOTNOTES

¹ The authors thank D. Stevens for help identifying and collecting elms, D. Spooner and Z. Larson-Rabin for comments on the manuscript, and D. Fajardo, M. Ames, and F. Rodriguez for helpful discussion. The authors thank the National Science Foundation Minority Post-doctoral Fellowship to J.E.Z. (NSF award #0409651) and funding from the USDA-ARS to J.B.

⁴ Author for correspondence (e-mail: jezalapa{at}wisc.edu)

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