Weeds growing in agricultural ecosystems are mostly annual species. In addition to crop competition, weed populations are subjected to powerful anthropogenic disturbances aiming at eradicating them. These disturbances can be divided into agricultural practices that are applied before or after crop sowing. Pre-sowing practices such as soil cultivation or non-crop-selective herbicide applications eliminate early-germinated weed seedlings. In conventional agriculture, post-sowing practices essentially consist of crop-selective herbicide applications that eliminate weed seedlings germinated after the crop. Selective pressures exerted by agricultural practices foster the evolution of adaptive traits in weed populations. Germination has a prominent role in adaptation, because adequate germination timing allows seedlings to elude pre-sowing practices and to emerge under the best possible conditions for growth. The ability to withstand herbicide effects, i.e. herbicide resistance, is another major adaptive trait. Both traits are crucial for weed success in agricultural ecosystems.
A recent paper in Annals of Botany examines the effect of herbicide resistance alleles on seed germination dynamics and seedling emergence in the grass weed Alopecurus myosuroides Huds. (black-grass). The results show that pleiotropic effects directly associated with different alleles vary substantially with the allele, and can also vary with the environment. Interestingly, such results are not unique to plants. The deleterious effects on winter survival of four mutant alleles conferring the same adaptation (insecticide resistance) in the mosquito Culex pipiens has been shown to vary with the allele, and in the yeast Saccharomyces cerevisiae, the pleiotropic effects of several mutant alleles endowing fungicide resistance were studied individually for each of several genes. Variation of associated pleiotropic effects with the resistance allele and with the environment may be thus be a general rule.
Délye, C., Menchari, Y., Michel, S., Cadet, É., & Le Corre, V. (2013) A new insight into arable weed adaptive evolution: mutations endowing herbicide resistance also affect germination dynamics and seedling emergence. Annals of Botany 111(4): 681-691. doi: 10.1093/aob/mct018
Selective pressures exerted by agriculture on populations of arable weeds foster the evolution of adaptive traits. Germination and emergence dynamics and herbicide resistance are key adaptive traits. Herbicide resistance alleles can have pleiotropic effects on a weed’s life cycle. This study investigated the pleiotropic effects of three acetyl-coenzyme A carboxylase (ACCase) alleles endowing herbicide resistance on the seed-to-plant part of the life cycle of the grass weed Alopecurus myosuroides. In each of two series of experiments, A. myosuroides populations with homogenized genetic backgrounds and segregating for Leu1781, Asn2041 or Gly2078 ACCase mutations which arose independently were used to compare germination dynamics, survival in the soil and seedling pre-emergence growth among seeds containing wild-type, heterozygous and homozygous mutant ACCase embryos. Asn2041 ACCase caused no significant effects. Gly2078 ACCase major effects were a co-dominant acceleration in seed germination (1·25- and 1·10-fold decrease in the time to reach 50 % germination (T50) for homozygous and heterozygous mutant embryos, respectively). Segregation distortion against homozygous mutant embryos or a co-dominant increase in fatal germination was observed in one series of experiments. Leu1781 ACCase major effects were a co-dominant delay in seed germination (1·41- and 1·22-fold increase in T50 for homozygous and heterozygous mutant embryos, respectively) associated with a substantial co-dominant decrease in fatal germination. Under current agricultural systems, plants carrying Leu1781 or Gly2078 ACCase have a fitness advantage conferred by herbicide resistance that is enhanced or counterbalanced, respectively, by direct pleiotropic effects on the plant phenology. Pleiotropic effects associated with mutations endowing herbicide resistance undoubtedly play a significant role in the evolutionary dynamics of herbicide resistance in weed populations. Mutant ACCase alleles should also prove useful to investigate the role played by seed storage lipids in the control of seed dormancy and germination.