When we talk about the fitness of weeds, it’s not about how fast they can run a marathon. We are talking about how well they grow and how many viable seeds they produce. Weeds with fitness penalties just don’t grow and reproduce as well as they should, even in the absence of herbicides.
Are there fitness penalties for Group B (e.g. Glean®; ALS inhibitor) resistant wild radish? Visiting researcher, Mei Li, along with other AHRI researchers, set out to answer this question.
Unfortunately, the answer is NO! Wild radish, with four different target-site ALS mutations, were equally as fit as herbicide susceptible wild radish.
A previous study by Dr Qin Yu found similar results in Group B resistant annual ryegrass. This is part of the reason why resistance to Group B herbicides evolves so quickly and is so widespread.
Fitness penalty research may sound simple, just grow some plants in pots and see whether some grow better than others right. Wrong! Firstly, the exact resistance mutation must be well characterised. Then the weed sample must be purified to remove other genetic differences between resistant and susceptible populations. Plants are then grown in pots, some alone and others in pots in competition with wheat plants.
Unfortunately we need to study weeds, one resistance mechanism at a time, to determine when fitness penalties occur. There are at least 22 known target-site ALS mutations that cause weeds to be resistant these herbicides. Visiting researcher, Mei Li, together with AHRI staff Dr Qin Yu, Dr Heping Han and Dr Martin Vila-Aiub studied four of these mutations.
Fitness penalties in plants can be expressed as:
Each of these factors must be measured separately to determine if a fitness penalty exists. At the end of this exhaustive research, the hypothesis that no fitness penalty exists was supported. It takes a very patient and dedicated scientist to persist with this line of research. Perhaps that explains why there is limited research in this area around the world!
There are many different known target site mutations that endow resistance to ALS (Group B) herbicides and there is a very high initial frequency of these mutations in native weed populations. It makes sense that these common mutations do not have a fitness penalty associated with them, otherwise they would not be common! If there was a fitness penalty associated with the mutation, then natural selection would have reduced the frequency of the gene in the population because the less fit plants would not have been able to compete with the fit plants.
Evolution favours plant functions and traits that maximise either growth or survival. The lack of fitness penalty in Group B resistant weeds means that once resistance has evolved it is permanent.
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