Genetic variation in Bromus tectorum (Poaceae): introduction dynamics in North America

The Eurasian grass Bromus tectorum was collected first in its 19th century invasion of western America in six widely separated locales. We used allozyme variability to evaluate whether populations at these locales and elsewhere in North America stem from different founders. By identifying the same geographically restricted alleles in both introduced populations and potential source populations and tracing the distribution of these alleles in western North America, we identified a minimum of five or six independent founder events. These alleles were often at their highest frequencies in populations at or near the earliest collection sites. Founder events likely occurred near Cache Creek, B.C., Ritzville, Wash., Juniper Flat, Nev., Emigrant Pass, Nev., and either Dubois, Idaho, or Provo, Utah, or both. Multiple introductions and the spread of allelic variants produced a mosaic of genotypes throughout western North America and partially offset the reduction in genetic variation this alien grass would have probably incurred during intercontinental migration. Key words: Bromus tectorum, biological invasions, multiple introductions, shared alleles, enzyme electrophoresis.