Data from: Evolution and spread of glyphosate resistance in Conyza canadensis in California

Recent increases in glyphosate use in perennial crops of California, USA,
are hypothesized to have led to an increase in selection and evolution of
resistance to the herbicide in Conyza canadensis populations. To gain
insight into the evolutionary origins and spread of resistance and to
inform glyphosate resistance management strategies, we investigated the
geographical distribution of glyphosate resistance in C. canadensis across
and surrounding the Central Valley, its spatial relationship to
groundwater protection areas (GWPA), and the genetic diversity and
population structure and history using microsatellite markers. Frequencies
of resistant individuals in 42 sampled populations were positively
correlated with the size of GWPA within counties. Analyses of population
genetic structure also supported spread of resistance in these areas.
Bayesian clustering and approximate Bayesian computation (ABC) analyses
revealed multiple independent origins of resistance within the Central
Valley. Based on parameter estimation in the ABC analyses, resistant
genotypes underwent expansion after glyphosate use began in agriculture,
but many years before it was detected. Thus, diversity in weed control
practices prior to herbicide regulation in GWPA probably kept resistance
frequencies low. Regionally coordinated efforts to reduce seed dispersal
and selection pressure are needed to manage glyphosate resistance in C.
canadensis.