Selecting Resistance Genes vs MOA

Callie Braley Author

07/20/2022 Added

10 Plays

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This video is about selection pressure, selecting for resistance genes, and mode of action in plants.

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[00:00:09.240]Welcome to the video
[00:00:10.170]Selecting Resistance Genes vs Modes of Action.
[00:00:13.650]In this video, we will discuss pesticide modes of action
[00:00:16.800]and how they relate to the genetics of resistance.
[00:00:20.460]Some things to think about before we get into this lecture are:
[00:00:24.392]How many times can you kill a pest?
[00:00:27.240]How many ways can you kill a pest?
[00:00:31.890]The answer to these questions may seem obvious,
[00:00:34.410]but keep them in mind as we go through the lecture.
[00:00:38.190]First let's review the terms
[00:00:39.480]'resistance' and 'mode of action'.
[00:00:42.030]'Resistance' in a pest refers to when that pest organism
[00:00:45.210]has a genotype that allows it to stay alive
[00:00:47.730]and complete its life cycle when it encounters
[00:00:49.950]the pesticide in its environment.
[00:00:53.370]In other words the pest is able to tolerate
[00:00:55.410]the chemical used to control it.
[00:00:58.170]It is able to survive and then reproduce,
[00:01:00.930]passing resistance genes onto its offspring.
[00:01:05.550]What about 'mode of action', which is often shortened to MOA?
[00:01:09.840]A pesticide's 'mode of action' is the way
[00:01:11.850]a pesticide works to kill or impact the biology of a pest.
[00:01:16.269]It is how the chemical operates to disrupt the pest biology.
[00:01:21.450]Different modes of action have different
[00:01:23.010]physiological targets within the pest.
[00:01:25.816]For example, some insecticides target one component
[00:01:29.280]of an insect's nervous system,
[00:01:31.230]while another targets another component
[00:01:33.240]of an insect's digestive system,
[00:01:35.490]and still another insecticide targets
[00:01:37.560]an insect's growth processes.
[00:01:40.461]Why do these terms matter?
[00:01:42.120]And how are they connected to each other?
[00:01:45.000]Farmers make choices on if and how
[00:01:47.160]they will try to control a pest population.
[00:01:50.040]These choices may involve different
[00:01:52.110]modes of action of a pesticide.
[00:01:54.870]What mode of action they choose
[00:01:56.256]and the dose encountered by the pest organism
[00:01:59.250]determines the level of selection pressure
[00:02:01.530]that is exerted on a population
[00:02:03.660]and will impact the development
[00:02:05.280]of pesticide resistant pest populations.
[00:02:08.910]In other words, the farmer can make choices
[00:02:11.130]that will determine how the pest population evolves
[00:02:14.100]pesticide resistance.
[00:02:17.060]Let's work through an example.
[00:02:18.780]Let's say we have a population
[00:02:20.250]of frogeye leaf spot in our soybeans.
[00:02:23.130]Frogeye leaf spot is a disease caused by
[00:02:25.080]the fungus Cercospora sojina,
[00:02:27.030]that can reduce yield if levels are high
[00:02:29.280]and it remains untreated.
[00:02:31.650]Since we have high levels of this fungus,
[00:02:33.690]we want to get rid of it
[00:02:34.710]so it doesn't affect crop yield.
[00:02:36.660]So we decide to spray a fungicide
[00:02:38.430]that is labeled for the fungus and for our crop.
[00:02:45.540]After spraying the fungicide,
[00:02:47.040]we see that we get about 95% control.
[00:02:50.220]No product is going to be 100% effective.
[00:02:52.950]So a farmer would be happy with that result.
[00:02:55.950]The fungicide treatment results in a high level
[00:02:58.290]of selection pressure on the population
[00:03:00.930]where any individuals that have an increased tolerance
[00:03:03.319]or resistance to that specific mode of action
[00:03:06.060]for that fungicide will have a better chance
[00:03:08.640]of surviving this treatment.
[00:03:10.830]The individuals that have the favorable genes
[00:03:13.410]are selected for.
[00:03:15.030]However, the survivors are not abundant.
[00:03:17.850]In a future growing season, conditions promote
[00:03:20.034]the Cercospora sojina population to reproduce
[00:03:23.160]and build back up.
[00:03:24.930]But wait, those individuals in our population,
[00:03:27.479]selected for resistance to the fungicide, are reproducing.
[00:03:31.710]So resistant individuals now make up a greater percentage
[00:03:34.380]of the population, depicted by the red individuals.
[00:03:38.820]This resistance could have naturally been
[00:03:40.800]in the population perhaps due to a past mutation
[00:03:44.220]and we just simply selected for it
[00:03:45.990]when we applied the fungicide.
[00:03:48.270]The fungicide did not create the resistance.
[00:03:50.790]It selected for it.
[00:03:54.210]Let's take a closer look inside the fungus
[00:03:56.400]to see what is happening genetically.
[00:03:59.430]As we know, inside each cell is a nucleus.
[00:04:03.000]Inside this are chromosomes.
[00:04:05.700]Each chromosome is made up of DNA,
[00:04:08.460]which is composed of nucleotides.
[00:04:11.430]Specific sequences of these DNA nucleotides are genes.
[00:04:19.140]Genes encode for specific proteins that will determine
[00:04:22.110]the chemical characteristics at the target site.
[00:04:25.620]Therefore, a change or mutation of the DNA
[00:04:28.680]can alter the chemical characteristics
[00:04:30.600]at the target site for a specific mode of action
[00:04:33.360]and potentially eliminate the effectiveness
[00:04:35.520]of that mode of action,
[00:04:37.200]making the pesticide no longer effective on the fungus.
[00:04:41.580]This could be due to enzymes being produced
[00:04:44.160]that will increase the breakdown of the pesticide
[00:04:53.280]or protein differences alter the target site
[00:04:56.160]and reduce or eliminate the effect
[00:04:57.960]of the pesticide at that target site.
[00:05:01.440]Let's look at a depiction of this target site change.
[00:05:10.770]When we apply the pesticide to a fungus
[00:05:12.600]without the resistance genes,
[00:05:14.850]the pesticide works like it was intended
[00:05:17.190]and kills the fungus.
[00:05:19.530]However, if the target site is changed due to a mutation
[00:05:23.880]and we apply our pesticide,
[00:05:26.010]it no longer binds to the target site
[00:05:28.260]and therefore does not work like it was intended.
[00:05:31.740]Because the resistant fungus has an advantage,
[00:05:34.470]it is more likely to survive
[00:05:36.000]and pass on the resistance gene to the next generation.
[00:05:39.180]As long as environmental conditions are favorable.
[00:05:43.020]If we continue to use the same mode of action
[00:05:45.180]on this fungal population, we will keep applying
[00:05:47.970]a high level of selection pressure.
[00:05:50.670]This means we will continue to select for
[00:05:52.620]the individuals that have an increased tolerance
[00:05:55.050]or resistance to this mode of action.
[00:05:58.140]The frequency of resistant individuals
[00:06:00.090]will continue to increase in the population.
[00:06:04.860]Let's revisit these questions
[00:06:06.270]from the beginning of the lecture.
[00:06:08.550]Remember, you can kill a pest once,
[00:06:11.280]but you can kill a pest organism in many different ways.
[00:06:16.410]If the farmer changes modes of action,
[00:06:18.420]they're changing the way they kill the pest.
[00:06:21.085]They're no longer applying selection pressure
[00:06:23.520]on the population for resistance
[00:06:25.140]to the same mode of action.
[00:06:27.660]Changing to a different mode of action
[00:06:29.280]provides the pest control benefit
[00:06:30.990]the farmer is looking for.
[00:06:34.110]To summarize this video, we've discussed the terms
[00:06:36.660]'resistance' and 'mode of action',
[00:06:38.970]how these terms are connected,
[00:06:41.130]the genetics of the resistance in our population
[00:06:44.370]and how choosing modes of action can increase
[00:06:46.860]or decrease the level of selection pressure
[00:06:49.080]applied on a population.

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Selecting Resistance Genes vs MOA

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