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Bromus tectorum (Poaceae) in midcontinental United States: Population genetic analysis of an ongoing invasion¹

² School of Biological Sciences, Washington State University, Pullman, Washington 99164 USA ³ Department of Biological Sciences, Boise State University, Boise, Idaho 83725 USA

ABSTRACT

Biological invasions can be substantially influenced by the genetic sampling associated with a species’ introduction. As a result, we assessed the genetic and evolutionary consequences of the entry and spread of the invasive grass Bromus tectorum (cheatgrass) across the United States midcontinent through an analysis of 54 populations, using enzyme electrophoresis. On average, these populations display 1.04 alleles per locus (A), 4.1% percent polymorphic loci per population (%P) and an expected mean heterozygosity (H_exp) value of 0.009. Heterozygotes, which have been rarely reported for B. tectorum in North America, occur in three populations in the midcontinent and are likely novel multilocus genotypes that arose postimmigration. The midcontinent distribution of multilocus genotypes suggests that plant immigrants came directly from either the native range or the eastern United States, or both. Continued dispersal of preadapted genotypes and the assembly of populations that are genetic admixtures may enhance this invasion by increasing both the genetic diversity within populations and the selection of novel genotypes arising from occasional outcrossing. The potential for postimmigration evolution in most species points to the largely unrecognized need to block the introduction of new, potentially aggressive genotypes of an alien species already in the United States.

Key Words: allozyme • Bromus tectorum • cheatgrass • invasion • multiple introductions • Poaceae • Great Plains

Received for publication 3 July 2008. Accepted for publication 24 September 2008.

FOOTNOTES

¹ The authors thank especially J. Schachner and N. Mack for their invaluable assistance in collecting these populations along with C. Brown, S. Gray, M. Haferkamp, P. Sorensen, and D. Williams. Laboratory assistance was provided by E. Murnigham, K. Burdon, A. Hodzic, and A. Ulappa. J. Nelson, M. M. Brooke, M. T. Valliant, S. Foster, C. L. Kinter, S. Louda, M. Minton, S. G. Mortenson, R. R. Pattison, A. V. Novak, S. N. Novak, M. I. Novak, C. Stern, and M. Webster provided much appreciated additional help. The authors especially thank R. Scott who prepared the final version of all figures. The manuscript was improved through the comments of several anonymous reviewers. This research was funded substantially through support from the Betty W. Higinbotham Trust at Washington State University, with additional funding provided by the Faculty Research Grant Program at Boise State University. An earlier version of this manuscript was completed while S.J.N. was on sabbatical leave at the CSIRO European Laboratory in Montferrier-sur-Lez, France, and he is grateful for the generous use of this facility.

⁴ Author for correspondence (e-mail: snovak{at}boisestate.edu)