Inheritance of ALS Resistance

Callie Braley Author

07/20/2022 Added

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How to interpret "Inheritance of an ALS-Cross-Resistant Common Cocklebur (Xanthium strumarium) Biotype" by G. Anthony Ohmes and J. Andy Kendig.

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[00:00:07.970]In this video, we will look at the paper
[00:00:09.561]'Inheritance of an ALS Cross-Resistant
[00:00:12.600]Common Cocklebur Biotype'.
[00:00:14.820]We'll examine the ideas and research in it
[00:00:16.950]to better understand the genetics of resistance development.
[00:00:20.670]Before we dive into the paper,
[00:00:22.200]let's go through a few terms and definitions
[00:00:24.300]that are explained at the beginning of this paper.
[00:00:27.480]First, herbicide resistance.
[00:00:30.360]Herbicide resistance is defined
[00:00:32.100]as the ability of a biotype,
[00:00:34.260]or you can think of it as a type of plant
[00:00:36.930]that have the same genetics,
[00:00:38.790]to survive herbicide treatments
[00:00:40.530]to which the species is normally susceptible.
[00:00:44.910]Susceptible is defined as a weed population
[00:00:48.120]that can be controlled at or below the labeled use rate.
[00:00:53.130]Finally, cross-resistance.
[00:00:55.470]Cross-resistance is used to define a biotype
[00:00:58.020]that survives treatments from two
[00:00:59.730]or more different herbicide families,
[00:01:01.740]classes, or mode of action groups.
[00:01:06.240]In this study, known ALS-resistant common cocklebur plants
[00:01:09.960]were cross pollinated
[00:01:11.010]with known susceptible common cocklebur plants
[00:01:13.770]to determine the genetic control of resistance.
[00:01:18.150]Researchers identified different plants
[00:01:20.040]that were susceptible and resistant to ALS herbicides
[00:01:23.310]and these two plant types were grown.
[00:01:26.040]Once they reached a certain height,
[00:01:27.600]they were induced to flower
[00:01:28.950]and male parts of the flowers
[00:01:30.420]of the susceptible biotypes were removed
[00:01:33.210]and the plants were pollinated
[00:01:34.470]with pollen from the resistant biotypes.
[00:01:37.830]The resulting F1 seeds from these plants were planted
[00:01:40.590]and grown to produce the F2 generation.
[00:01:44.100]These F1 plants were then grown
[00:01:45.870]and allowed to self pollinate.
[00:01:48.330]The F2 seeds that were produced
[00:01:49.920]were collected and planted.
[00:01:52.170]There were two individual sets of F2 plants
[00:01:54.750]which were planted at two different times.
[00:01:57.780]One set had 782 plants, while the other had 402.
[00:02:03.150]Once these reached the two to four leaf stage,
[00:02:05.580]they were sprayed with a tank mixture of imazaquin
[00:02:08.580]at 280 grams per hectare
[00:02:10.740]and chlorimuron at 18 grams per hectare,
[00:02:13.680]both ALS herbicides.
[00:02:17.550]These two sets of F2 plants were evaluated
[00:02:20.280]based on their reaction to the ALS tank mix.
[00:02:23.700]Three distinct responses were observed,
[00:02:26.250]resistant, intermediate,
[00:02:28.800]plants with live buds, but yellowing of the leaves
[00:02:31.980]and susceptible, plants with dead buds
[00:02:34.620]as well as yellowing of the leaves.
[00:02:39.030]If this resistance was a partially dominant trait,
[00:02:42.000]a one to two to one ratio would be seen
[00:02:45.210]of resistant to intermediate, to susceptible plants.
[00:02:49.110]However, statistical tests indicated
[00:02:51.780]that this ratio did not fit the data.
[00:02:55.710]Because the intermediate plants were recovering,
[00:02:58.290]the resistant and intermediate plants were pooled
[00:03:00.810]into the same group.
[00:03:03.690]So even though there were three distinct responses
[00:03:06.000]to the herbicide,
[00:03:07.170]the plants were either considered alive or dead
[00:03:10.170]and a three to one ratio of live to dead plants
[00:03:13.110]fit the data closely.
[00:03:15.330]These tables show the results.
[00:03:17.730]In the first set, there were 587 living plants
[00:03:21.060]and 195 dead plants.
[00:03:23.880]In the second set, there were 300 living plants
[00:03:26.520]and 102 dead plants.
[00:03:29.310]Both of these numbers fit the ratio of three to one.
[00:03:32.850]This indicated that the cross resistant trait
[00:03:35.190]is a dominant to semi-dominant trait.
[00:03:39.810]Let's use a population of common cocklebur
[00:03:42.120]as an example to further clarify the genetics.
[00:03:45.540]We have the three genotypes we were looking at before.
[00:03:48.660]HR HR, HR HS, and HS HS.
[00:03:54.330]For the F1 generation,
[00:03:56.100]the genotype HR HR was crossed with the genotype HS HS.
[00:04:02.850]Let's use a Punnett square to conceptualize this.
[00:04:08.550]We have our table and our two genotypes.
[00:04:11.430]For HR HR, we'll assign each HR to its own row,
[00:04:15.210]and for the HS HS, we'll assign HS to one column
[00:04:19.260]and another HS to another column.
[00:04:22.260]What we then get is HR HS for all the offspring.
[00:04:28.410]To get our F2 generation,
[00:04:30.480]this genotype was selfed,
[00:04:32.100]as you can see in this Punnett square.
[00:04:35.250]One fourth of this offspring would be totally susceptible.
[00:04:41.430]How exactly does this tie into selection pressure?
[00:04:45.000]If producers were using the same ALS herbicide rates
[00:04:48.180]as used in this paper,
[00:04:49.980]they would be applying a lower level of selection pressure.
[00:04:53.610]The result would be survival of the cocklebur heterozygotes.
[00:04:57.990]This would keep the susceptible allele
[00:04:59.760]at a higher frequency in the population.
[00:05:03.660]Increasing the selection pressure,
[00:05:05.580]for example, using a higher rate of herbicide,
[00:05:08.460]may kill most of the heterozygotes
[00:05:10.470]as well as the homozygote susceptible plants.
[00:05:14.400]This would reduce the presence of susceptible alleles
[00:05:17.040]left in the population,
[00:05:18.540]resulting in a more resistant population.
[00:05:21.750]This is when we're in big trouble
[00:05:23.220]because ALS herbicides will provide little control.
[00:05:28.290]The connection between genetics and selection pressure
[00:05:31.020]is an important concept in understanding resistance.
[00:05:34.530]This is why it's important
[00:05:35.790]to use multiple management strategies
[00:05:37.800]when managing resistance in weeds, insects, or diseases.

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Inheritance of ALS Resistance

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