The mating system of an introduced plant is critical to its establishment and proliferation. Taeniatherum caput-medusae (Poaceae) is a primarily self-pollinating annual that has invaded large areas of the western United States. A preliminary investigation of allozyme variability detected several heterozygotes; therefore suggesting limited outcrossing occurs within introduced populations. Using progeny arrays, we determined the mating system parameters for a total of ten populations in California, Idaho, and Oregon. For each population, a total of ten individuals per family and ten families were analyzed (n = 1000). Seven of the populations contained no heterozygous individuals in any family, indicating complete selfing. In three populations, a total of 23 heterozygous progeny were detected. Two populations contained 21 heterozygous individuals across four families. Genotypes of the progeny in these four families conformed to Mendelian expectations, thus indicating that the maternal plants were likely heterozygous. In one population (Emigrant Hill, OR), two heterozygous individuals occurred in two separate families, and appear to be the result of outcrossing during the progeny generation. The single-locus outcrossing rate across all populations of T. caput-medusae was very low (t ~ 0.002). Results suggest that low frequency genotypes present in several introduced populations may be the result of outcrossing, and do not represent a separate introduction into each of these populations. The creation of novel genotypes in introduced populations from outcrossing between genotypes originating from geographically disparate populations in the native range has consequences for the proliferation, success, and evolution of highly selfing invasive plants.

Key words: allozymes, invasive plant, mating system, medusahead rye, selfing, Taeniatherum caput-medusae