Limits to the evolution of herbicide escape and tolerance in the agricultural weed Amaranthus palmeri

In response to novel anthropogenic stresses, defense strategies including
resistance, tolerance, and escape can evolve. However, if the evolution of
one or more of these strategies is limited by weak natural selection or a
lack of genetic variation, then a mixed strategy (e.g. resistance and
tolerance) is unlikely to evolve. To determine the mechanisms that limit
the evolution of defense strategies, we studied escape from and tolerance
to glyphosate herbicide in Amaranthus palmeri, an agricultural weed that
has evolved glyphosate resistance. We grew A. palmeri in fields planted
with corn, soybean, or no crop; manipulated their exposure to glyphosate;
and measured escape and tolerance. We did not detect selection or genetic
variation for glyphosate escape in any agricultural environment,
suggesting that a mixed strategy of resistance and escape is unlikely to
evolve in A. palmeri. We also did not detect selection for glyphosate
tolerance, but there was genetic variation for tolerance in a corn crop
environment, suggesting the potential for a mixed strategy of resistance
and tolerance to evolve in A. palmeri in only a subset of environments.
These results suggest that exposure to herbicides is unlikely to cause the
widespread evolution of a mixed defense strategy in agricultural weeds.