COLOSI, JOSEPH C1*, JAMES J REILLY2, and WILLIAM S CURRAN3. 1Biology Department, DeSales University, 2755 Station Ave., Center Valley, PA 18034; 2Department of Anatomy, Pathology, and Cell Biology, Thomas Jefferson University, 1020 Locust Street, Philadelphia, PA 19107; 3116 ASI, Department of Agronomy, Pennsylvania State University, University Park, PA 16802. - The spread of herbicide resistance in Chenopodiaum album.
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