The Evolution and Genetics of Herbicide Resistance in Weeds

Abstract

The importance of various factors influencing the evolution of herbicide resistance in weeds is critically examined using population genetic models. The factors include gene mutation, initial frequency of resistance alleles, inheritance, weed fitness in the presence and absence of herbicide, mating system, and gene flow. Where weed infestations are heavy, the probability of selecting for resistance can be high even when the rate of mutation is low. Subsequent to the occurrence of a resistant mutant, repeated treatments with herbicides having the same mode of action can lead to the rapid evolution of a predominantly resistant population. At a given herbicide selection intensity, the initial frequency of resistance alleles determines the number of generations required to reach a specific frequency of resistant plants. The initial frequency of resistance alleles has a greater influence on the evolutionary process when herbicides impose weak selection, as opposed to very strong selection. Under selection, dominant resistance alleles increase in frequency more rapidly than recessive alleles in random mating or highly outcrossing weed populations. In highly self-fertilizing species, dominant and recessive resistance alleles increase in frequency at approximately the same rate. Gene flow through pollen or seed movement from resistant weed populations can provide a source of resistance alleles in previously susceptible populations. Because rates of gene flow are generally higher than rates of mutation, the time required to reach a high level of resistance in such situations is greatly reduced. Contrary to common misconception, gene flow from a susceptible population to a population undergoing resistance evolution is unlikely to slow the evolutionary process significantly. Accurate measurements of many factors that influence resistance evolution are difficult, if not impossible, to obtain experimentally. Thus, the use of models to predict times to resistance in specific situations is markedly limited. However, with appropriate assumptions, they can be invaluable in assessing the relative effectiveness of various management practices to avoid, or delay, the occurrence of herbicide resistance in weed populations.

Quantitative assessment of heteroplasmy levels in Senecio vulgaris chloroplast DNA

“…The co-occurrence within an individual plant of two or more chloroplast haplotypes coding for different variants of an enzyme may represent a system to allow finetuned adaptation to specific environmental conditions within the lifetime of a plant individual, as was proposed by Frey (1999). As the mutation conferring resistance to triazine imposes fitnesscosts (Gressel & Segel, 1990;Jasieniuk et al, 1996) it should be expected to go extinct unless selection to promote it occurs. If selection does occur, the frequency of resistant haplotypes is expected to increase.…”

Continuous cropping systems and weed selection

“…For instance, resistance has developed rapidly to the inhibitors of ALS, while resistance to other modes of action has been much slower to develop (e.g. resistance to paraquat) (Jasieniuk et al, 1996).…”

“…Genetic variation within and between local weed populations will determine the potential to adapt to changing environmental conditions (Jasieniuk et al, 1996;Jasieniuk & Maxwell, 2001;Maxwell et al, 1990). Weed selection ultimately depends on population fitness within a particular environment or suite of ecological conditions, relative to the success of other phenotypes that also exist in the same population.…”

ISSR variation within and among wild Brassica juncea populations: implication for herbicide resistance evolution

“…Given the genetic variation, the rate of herbicide resistance evolution will depend on the efficacy and frequency of herbicide application, the mating patterns of the populations, the relative fitness of susceptible and resistant phenotypes and the existence of gene flow processes (Jasieniuk et al 1996). Firstly, intensive use of glyphosate across wide areas has resulted in extremely high selection pressure and aided the evolution of glyphosate-resistant weeds (Culpepper 2006), and glyphosate-resistant weed populations eventually evolved (Heap and LeBaron 2001;Owen 2001).…”