Allele Frequency Changes: Mutation and Selection

Amy Hauver Author

07/20/2022 Added

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This video details the impact of mutations and selection on allele frequencies for various pesticide resistant populations.

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[00:00:09.960]In this video,
[00:00:10.950]we'll be discussing two of the four
[00:00:13.170]forces that affect the gene pool,
[00:00:15.690]mutations and selection.
[00:00:18.720]As we discussed in the first video in this module,
[00:00:22.200]a gene pool is a population of interbreeding individuals,
[00:00:25.620]and is the collection of all of the alleles
[00:00:27.780]and of all of the genes found
[00:00:29.550]within that inter breeding population.
[00:00:32.670]Each member of the population inherits alleles
[00:00:35.370]from other members of the gene pool.
[00:00:37.620]And these are the parents in that population.
[00:00:40.170]And they pass them on to other members of the gene pool.
[00:00:42.990]And these are the offspring of the population.
[00:00:46.350]There are many forces that affect and cause changes
[00:00:48.780]in the allele frequencies in a gene pool,
[00:00:51.240]but we will discuss four of them.
[00:00:53.280]In this first video,
[00:00:54.510]we'll be talking about mutation and selection.
[00:00:57.090]And in the second video,
[00:00:58.230]we'll discuss migration and drift.
[00:01:01.590]Mutations are changes in the DNA,
[00:01:03.600]and they create new alleles from the current genes.
[00:01:06.540]It occurs spontaneously, and in the population,
[00:01:09.480]and it may or may not have an effect.
[00:01:14.310]You can see here in A, there's a population of plants.
[00:01:17.790]And in B, you can see that these alleles are all the same.
[00:01:21.390]in C, a mutation occurs,
[00:01:23.130]and there is now a new allele present in the population.
[00:01:25.950]And in D, you can see the allele frequency change
[00:01:28.860]due to the presence of a new, more suitable gene.
[00:01:33.960]This is a sample population of Colorado potato beetle,
[00:01:37.110]which is a pest of most members of a Solanaceae family,
[00:01:40.590]such as potato, tomato, pepper, eggplant, and weeds,
[00:01:44.100]such as nightshades and buffalo bur.
[00:01:47.310]It has been known to defoliate entire potato fields
[00:01:49.710]in North America.
[00:01:51.900]The Colorado potato beetle has developed resistance
[00:01:55.050]to a number of insecticides, and an overexpression
[00:01:57.720]of a cytochrome P450,
[00:01:59.550]which is this detoxification enzyme,
[00:02:02.130]is associated with many of these resistants,
[00:02:06.810]imadocloprid resistance being a major one.
[00:02:10.890]The Colorado potato beetles' resistance to imadocloprid,
[00:02:14.370]caused by the overexpression of cytochrome P450
[00:02:17.610]could be compared to palmer amaranth's resistance
[00:02:19.890]to glyphosate.
[00:02:21.180]In palmer amaranth, there's an amplified number of the genes
[00:02:24.090]that produce the EPSP synthase enzyme
[00:02:26.460]that creates essential amino acids.
[00:02:28.830]We talked more about this in module one's first video.
[00:02:32.940]In the Colorado potato beetle, there's a similar phenomenon
[00:02:36.240]of an increased amount of an enzyme,
[00:02:40.620]but it all comes from the original single gene.
[00:02:45.840]Instead of having many genes creating one enzyme
[00:02:48.210]causing resistance,
[00:02:50.100]Colorado potato beetle has a single gene
[00:02:52.470]that is upregulated.
[00:02:54.060]This upregulation causes an overexpression
[00:02:56.460]of the enzyme, dramatically increasing its
[00:02:58.740]ability to detoxify insecticides, like imadocloprid.
[00:03:04.650]Selection is the preferential survival
[00:03:06.780]and reproduction of an individual in a population.
[00:03:10.380]Preferential survival and reproduction is due
[00:03:12.930]to different genotypes and their interaction
[00:03:15.030]with the environment.
[00:03:16.830]In image A, there is another population of plants.
[00:03:20.160]In B, you can see the two different genotypes,
[00:03:22.800]the pink and the green.
[00:03:24.690]If the pink represents the plant's ability
[00:03:26.880]to detoxify a herbicide application,
[00:03:29.580]then when the selection pressure
[00:03:30.900]of a herbicide application is applied,
[00:03:33.120]they will be able to survive to reproduce.
[00:03:37.560]For an example on selection acting towards
[00:03:39.990]changing the gene pool and leading to resistance,
[00:03:42.630]we'll use wheat leaf rust,
[00:03:44.430]a fungal disease that affects wheat, barley and rye.
[00:03:47.880]Let's say a producer goes out in the field
[00:03:49.770]and sees this first image.
[00:03:51.810]To protect the yield of his crop,
[00:03:53.790]he applies a fungicide,
[00:03:55.620]like a Azoxystrobin to control the rust.
[00:03:58.440]This application gives good control
[00:03:59.870]of the fungus, with only a few escapes.
[00:04:09.750]Most escapes are caused by a level of misapplication,
[00:04:12.690]but there also can be some individuals
[00:04:14.640]in the population that have standing resistance.
[00:04:17.310]This means that the gene that contributes
[00:04:19.080]to resistance is already present
[00:04:22.050]in some individuals in a population.
[00:04:24.300]And it is a matter of selection for the population
[00:04:26.820]to become resistant.
[00:04:29.490]I've highlighted one of the individuals here
[00:04:31.440]that escaped and carries the resistance gene.
[00:04:34.350]These spores are transported via wind
[00:04:36.570]to surrounding hosts,
[00:04:37.680]along with new spores from other populations.
[00:04:40.860]These spores land
[00:04:41.970]and they germinate and produce a new colony,
[00:04:46.980]this time with more resistance genes in the population,
[00:04:50.910]which I've also highlighted.
[00:04:52.740]Because the grower got such good control
[00:04:54.720]the first time applying Azoxystrobin,
[00:04:56.970]they used it again.
[00:04:58.620]This time they seemingly get good control again,
[00:05:01.410]but there are still escapes in the population,
[00:05:03.780]with many of them carrying the resistance allele.
[00:05:06.960]These spores are blown to a new host and reproduce,
[00:05:10.590]forming a population that has more resistance.
[00:05:14.580]If this grower uses azoxystrobin again,
[00:05:17.220]they're likely to have a much less control
[00:05:19.470]and eventually the product will fail.
[00:05:26.610]This population of rust has standing variation,
[00:05:29.520]the collection of all of the alleles present
[00:05:31.620]in the gene pool.
[00:05:33.090]This occurs when random mutations create new alleles.
[00:05:37.440]At the time of creation,
[00:05:38.910]these alleles have no selective advantage
[00:05:40.920]in the current environment
[00:05:42.210]where there is selection pressure applied to them.
[00:05:45.240]This means there's also no disadvantage
[00:05:47.430]to the creation of the new alleles.
[00:05:49.020]They are just simply present.
[00:05:51.120]These seemingly useless new alleles
[00:05:53.010]caused by random mutations may be necessary
[00:05:55.980]for genes to create a pesticide resistance
[00:05:58.800]under the correct selection pressure.

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