Chenopodium album, a widespread dicot weed of corn and other crops was first reported to have atrazine resistance in 1975 in Canada. In most weed species including Chenopodium album, atrazine resistance is conferred by a point mutation in the chloroplast photosystem II psbA gene, resulting in a substitution of glycine for serine at position 264. This mutation is a dominant character conferred by the maternal parent. The mutation rate in Chenopodium album is much higher (10-3 to 10-4) than expected from random mutations of a chloroplast gene (10-9 to 10-12) and may be influenced by a mutator nuclear gene. French and Canadian studies of Chenopodium album with protein electrophoresis suggest that atrazine resistance has evolved frequently through mutation. Both studies found different protein patterns in resistant populations from well-separated areas. The resistant populations had low levels of within population genetic variability compared to nearby susceptible populations. We wanted to determine if the spread of atrazine resistant Chenopodium album within and between adjacent farm fields was primarily caused by frequent mutation or by rare mutations followed by migration. We collected seed from five individuals in each of 11 fields on 5 Farms in central and southeastern Pennsylvania. We measured atrazine resistance by spraying 2.7 kg/ha on three reps of seedlings at the 4 to 8 leaf stage, assessing the plants visually, and measuring dry mass. We assessed genetic relationships of individuals by RAPD's (Random Amplified Polymorphic DNA). In some fields, atrazine resistant and susceptible individuals had identical genotypes suggesting recent mutation within the field. In other fields, resistant genotypes had distinctly different genotypes from those of susceptible neighbors, suggesting immigration of the resistant biotype from elsewhere. The spread of atrazine-resistant Chenopodium album in Pennsylvania appears to be a mixture of frequent local mutation and dispersal of resistant biotypes by seed.

Key words: atatzine, Chenopodium album, herbicide resistance, mutation, weed