WEEDY ADAPTATION IN SETARIA SPP .2. GENETIC DIVERSITY AND POPULATION GENETIC-STRUCTURE IN S-GLAUCA, S-GENICULATA, AND S-FABERII (POACEAE)

Setaria glauca (yellow foxtail), S. geniculata (knotroot foxtail), and S. faberii (giant foxtail) are important cosmopolitan weeds of temper ate and tropical regions. Isozyme markers were used to investigate gen etic diversity and population genetic structure in 94 accessions of ye llow foxtail, 24 accessions of knotroot foxtail, and 51 accessions of giant foxtail, collected mainly from North America and Eurasia. Giant foxtail populations were nearly identical genetically, with only one p opulation exhibiting isozyme polymorphism. Yellow and knotroot foxtail populations had low genetic diversity but marked population different iation. Although the latter species are similar morphologically, they are readily distinguished electrophoretically, with Nei's genetic iden tity being 0.83. In both species, genetic divergence between accession s from Eurasia and North America was minimal. Populations from the nat ive ranges had slightly greater genetic diversity than those from the respective introduced ranges. Yellow foxtail populations genetically c lustered into Asian, European, and North American groups. Within North America, yellow foxtail populations from Iowa were genetically divers e whereas populations collected from other North American locations we re nearly monomorphic for the same multilocus genotype. Knotroot foxta il populations in North America were genetically differentiated into n orthern and southern groups on either side of a line at approximate to 37 degrees N latitude. No genetic patterning was evident in knotroot foxtail populations from Eurasia. In both yellow and knotroot foxtail, patterns of population genetic structure have been influenced by seve ral factors, including genetic bottlenecks associated with founder eve nts, genetic drift, and natural selection.