Friday, March 28, 2014

Voracious Worm Evolves to Eat Biotech Corn Engineered to Kill It

Voracious Worm Evolves to Eat Biotech Corn Engineered to Kill It - Wired Science

Any scientist that pays attention to evolution should know this would happen. But I suppose that this isn't well accepted by industry and/or farmers.

"After years of predicting it would happen — and after years of having their suggestions largely ignored by companies, farmers and regulators — scientists have documented the rapid evolution of corn rootworms that are resistant to Bt corn."

As one evolutionary ecologist commented, "this is yet further confirmation of Malcolm’s Law." (great video clip ;) )

A friend asked me in response to this, “Evolve or adapt? Is this a new species? Has its DNA changed?”

My response was as follows:

Very good question! The short answer is: Yes to both evolve and adapt. No, these resistant insects are not a new species.  To explain in detail would require a long discussion because it is a complex subject. But I'll see if I can adequately summarize.

Evolution and adaptation are not mutually exclusive. Not all adaptations result in one species evolving into a new species. On the other hand, the first and foremost driver of evolution, and therefore part of the process of evolution, is adaptation. The important factor is the degree and expanse of adaptation.

Resistance may be defined as “a heritable change in the sensitivity of a pest population that is reflected in the repeated failure of a product [including GMO plants] to achieve the expected level of control when used according to the label recommendation for that pest species.” Cross-resistance occurs when resistance to one insecticide confers resistance to another insecticide, even where the insect has not been exposed to a different insecticide.

Because insect populations are usually large in size and they breed quickly, there is always a risk that insecticide resistance may evolve, especially when insecticides are misused or overused. In reference to the scenario of the corn rootworm, these animals have evolved the ability to resist mortality from the toxins that are genetically incorporated into the corn plants and which confers the plants’ abilities to repel insects that feed on them, or have potential to feed on them.

Resistance can be acquired in several ways, depending on the interacting organisms. You may have heard about the increased and widespread resistance of bacteria to antibiotics, especially Staphylococcus and the mycobacteria that cause tuberculosis.  Bacteria acquire resistance very quickly because they are single-celled organisms, increase rapidly in population, and they readily evolve, including changes in their DNA via horizontal gene transmission (whereby genes from one organism are laterally transferred to another without traditional reproduction). This is a very basic tool in most biologicaland molecular labs. However, with higher organisms, this all becomes more complicated.

Insects are the second most rapidly evolving organisms, especially in their resistance to pesticides and other toxins. And they have evolved several mechanisms of acquiring that resistance.

  1. Metabolic resistance: This is when resistant insects alter or destroy the toxin faster than susceptible insects, or quickly rid their bodies of the toxic molecules before it can reach its site of action (humans possess many of these same enzymes for the same purpose, such as cytochrome p450). Resistant insects may have higher levels or more efficient forms of such enzymes. Additionally, these enzymes may have broader spectrum of activity conferring detoxification of many different insecticides. Metabolic resistance is the most common mechanism and often presents the greatest challenge.

  2. Target-site resistance: In this form of resistance the insect accommodates the chemical by altering one or more physiological functions. This may involve reduced neuronal sensitivity to insecticides, altered sensitivity by other enzymes and systems, even decreased penetration of the insecticide through the body wall, and increased excretion or sequestration of insecticide preventing it from reaching the site of action.

  3. Behavioral resistance: This involves changes in behavior by which insects avoid insecticides. Resistant insects may detect or recognize a danger and avoid the toxin. Changes in behavior and subsequent resistance have been reported for several classes of insecticides. Insects may simply stop feeding if they come across certain insecticides, or leave the area where the toxin is present.
Two events greatly influence pesticide resistance: dosage and time. Continued applications over time can induce resistance to the insecticide and the resistant pests become increasingly difficult to control at the labeled rate and frequency. This in turn often leads to more frequent applications of the insecticide. The intensity of the resistance and the frequency of insecticide-resistant individuals in the population both increase further and problems of control continue to worsen as yet more product is applied. This can also equate with monoculture of a genetically modified crop containing a gene for toxins targeted at a pest, especially when the same GMO crop is plantedrepeatedly in successive years.

What the researchers in this study with the corn rootworm discovered is that the dosage of the toxin in the GMO plants (aka the expression of the gene that enables production of the toxin in the corn) was below the level where a large percentage of the population is killed. This is called a ‘low-dose event’. Consequently, the surviving population was able to exponentially increase their progeny that carry resistance. A resistant population was at high levels within three generations of these insects.

Another discovery was that in the resistant population, the genes that confer their resistance were not recessive. What that means is that even if many of the resistant insects mated with others outside of the range of the GMO crop, aka non-resistant insects, their progeny would still inherit the resistance from their resistant parents. (I won’t go into pre-adaptation resistance)

When resistance begins to show in crops, managers often switch to another pesticide if one is available.  To reduce acquisition of resistance by insects, another gene from Bt bacteria (which is the original source bacteria for all these Bt GMO transgenics) was inserted into another strain of corn. This strain of corn carries a gene for higher expression of the toxin (‘high-dose event’), to which the insects are more susceptible with higher mortality rate. However, those insects that already had resistance to the low-dose event strain of GMO corn were also more resistant to the high-dose strain of corn, which then compounded their total resistance.

The genetics of the heritable resistance traits and the intensive and repeated presence of the GMO crop containing the Bt toxin together are responsible for
the rapid build-up of resistance in the insects. I cannot answer your last question asking if the DNA of the insects has changed. The authors of this study on the corn rootworm did not (from my initial perusal of the published paper) examine changes in their DNA. They did, however, discover that the transcription of several genes has changed. What this means is that there are changes in how the DNA is expressed, resulting in increased levels of enzymes or other proteins that are coded by DNA. This could suggest changes in some of their DNA, including epigenetics, for genes that control how other genes are expressed. Also, horizontal gene transfer from plant chemicals and insecticides has been documented (published in 2013) in predatory mites (feeding on plants), but apparently the authors did not investigate this.

Speciation and taxonomical classification of species is considerably dependent on morphological differentiation and changes between closely related organisms. These resistant rootworms have not changed their appearances and bodily structures; only their metabolism (and possibly their target sites) and, less so, their behavior. This is why the answer to your question regarding a new species is negative.

I hope this answers your questions.

1 comment:

  1. Very cool blog! Glad I found it as part of the A to Z Challenge. Looking forward to reading it!