Genetic variation in bromus tectorum (Poaceae): Comparison between native and introduced populations

The amount and distribution of genetic variation in 51 native (Eurasian and northern African) populations of Bromus tectorum were assessed at 25 loci using starch gel electrophoresis and were compared with our previous results for introduced (North American) populations of this predominantly cleistogamous grass. More alleles and variable loci were detected across populations in the native range than in North American populations. Within populations, however, the level of polymorphism is higher in the introduced range than in the native range. Deviation from Hardy-Weinberg expectation among native populations is almost as severe as for introduced populations. Eurasian populations exhibit greater genetic differentiation than those in North America; populations from Southwest Asia are the most genetically differentiated. Comparison of single-locus genotypes suggests possible source populations in both Europe and Southwest Asia for the populations now so prominent in western North America. Genetic differences between introduced and native populations of B. tectorum stem from two opposing factors: The reduction in genetic variability across populations produced by founder effects combined with an increase in the within-population component of genetic variation from multiple introductions.