Studying the genetic variation of invasive species can provide information on their introduction and spread. Such studies benefit when they utilizing knowledge of the introduction history of a species to pinpoint localities for analysis. Taeniatherum caput-medusae (Poaceae) is a primarily self-pollinating annual that has invaded large areas of the western United States. Likely early introduction sites for this species include Roseburg, OR (1884), Steptoe Butte, WA (1901), Klamathon, CA (1903), Los Gatos, CA (1908), Mountain Home, ID (1930), Goldendale, WA (1938), Payette, ID (1944), Elgin, OR (1944), and Lapwai, OR (1946). We assessed introduction dynamics of T. caput-medusae using enzyme electrophoresis. Forty-five populations from western U.S. were analyzed for their genotypes by staining for 15 enzymes that were coded for by 29 putative loci. Across all 45 populations, a total of nine multilocus genotypes were detected. Two of these genotypes exhibited heterozygosity at one or two loci, and appear to be the product of outcrossing events. Based on the number of genotypes, we suggest a minimum of seven separate introduction events of T. caput-medusae into western U.S. When the geographic distribution of these genotypes is also considered; we believe a maximum of 14 to 15 introductions may have occurred. Five distinct multilocus genotypes were detected in eastern Washington alone, indicating that multiple introductions can occur across a relatively small geographic area. The distribution of multilocus genotypes among populations suggests that range expansion occurred primarily on a local scale. Multiple introductions into geographically distinct localities have probably facilitated the invasion of T. caput-medusae in its new range.

Key words: allozymes, invasive plant, medusahead rye, multilocus genotypes, multiple introductions, Taeniatherum caput-medusae