Friday, January 21, 2011

The Evidence is in: Bedbugs have evolved!


A member of the new species of bedbug- resistant to any pesticide thrown at it.

People across the country have been bedazzled by the fact that bedbugs now appear to be almost everywhere, and nearly impossible to kill - short of burning them out. It has been conjectured by some that though the insects appear about the same size with the same features as the bugs from 50s, they are in fact a new, different species. Now the results of testing are in: THEY ARE!

These diehard bugs represent the embodiment of a feat of lightning-fast evolution, and gene restructuring is the key to why these bedbugs won’t die. (which we'll get to in a moment) This discovery was also critical in finding an extermination method that actually works.

Urban pest-management specialist Dini Miller calls the gene-restructuring process “unnatural selection.” (Though it is in reality natural selection via a key mutation, on steroids). What she's referring to is that repeated exposure to powerful pesticides merely causes certain bedbugs to produce more of the enzymes that combat poisons in their system.

According to findings published by Ohio State University entomologists in PLoS One Journal, the enzymes “modify toxic compounds into water-soluble, non-toxic compounds” that can then be flushed out of the bug via excretion.

The Ohio State University study compared bedbugs from several decades ago with bedbugs from current infestations. The older bugs - courtesy of military bug enthusiast Harold Harlan -- lived in an isolated colony and responded positively to common pesticides. (That is, they suffered and died.) Meanwhile, newer bugs survived, and analysis of the new genome revealed higher levels of the mutant, pesticide-resistant enzyme.

This new genetic evidence, coupled with the fact that bedbugs have both a better ability to protect nerve cells and thicker shells, mean that we're confronted with an entirely different species. Indeed, over the past ten years, the bedbug population has increased by nearly 500 percent in North America. Perhaps even more terrifying is the fact that the insects can now survive a pesticide dose 1,000 times greater than in decades past.

Let's examine how this natural selection to produce a new bedbug species (via micro-evolution) came to be.

In terms of the concept of fitness let B refer to a favored allele for a bedbug with total resistance (i.e. such that it produces abundant amounts of the pesticide-resistant enzyme that converts toxins to water-soluble, harmless excretions), let b refer to a deleterious allele ( one with zero capacity for such enzyme production)

Recall the measures for success of natural selection are the fitness (w) and the selective value (s). These can be measured on either absolute or relative scales, but are related algebraically on the latter by:

w = 1 – s, or s = 1 – w

Let’s say at a particular time a gene frequency ‘snapshot’ of the bedbug population under study yields: p(B) = 0.60, q(b) = 0.40, i.e. the favored allele B is reproducing bugs at the ratio 3:2 relative to the disadvantaged one, b. As before, the selective value s = 0.50. (A simple table showing the declining gene frequency of b relative to B is shown appended to this blog.)

On average over time let each bb and Bb individual produce one offspring, and each BB produce two. These average numbers can be used to indicate the genotype’s absolute fitness and to project the changes in gene frequency over succeeding generations. The relative fitness (w) is meanwhile given by:

w = 1 for BB

w = 0.5 for Bb

w = 0.5 for bb

The selection values, relative measures of the reduction of fitness for each genotype, are given respectively by:

s = 1 – 1 = 0 for BB

s = 1 – 0.5 = 0.5 for Bb

s = 1 – 0.5 = 0.5 for bb

As we expect, the pesticide-resistant bedbug genotype displays zero reduction in fitness, and hence maximum survival rate. Thus, the table provides a brief “snapshot” of how micro-evolution has been working in the bedbug population to generate billions of these pests that can’t be exterminated by ordinary pesticides – necessitating the use of varieties currently prohibited for safety and health reasons.

Let's now use the preceding template and get more specific (via defined relative populations) as to how the natural selection mechanism works in enhancing gene frequencies for survival.

We consider a diploid population of 200 individuals, and 400 genes distributed amongst them. As we see from the table given the ratio of gene frequencies between the homozygous dominant allele (B or enzyme-resistant to most pesticides) and the homozygous recessive allele b is 3:2 or 0.60: 0.40 which could manifest for the case shown via:

B = 180 (in BB) + 60 (in Bb) = 240/400 = 0.60

B = 100 (in bb) + 60 (in Bb) = 160/400 = 0.40

For the 200 individual bedbugs we may also find the genotype frequencies such that 90 are dominant (resistant to all known approved pesticides) = 90, and the heterozygous Bb = 60 and the recessive (not resistant) = bb = 50, therefore for the dominant and recessive allele distributions using genotype frequencies:

B = 0.45 BB + ½(.30Bb) = 0.45 + 0.15 = 0.60

b = 0.25 BB + ½(.30Bb) = 0.25 + 0.15 = 0.40

Now, for a STABLE population, IF the gene pool (of a selected population, whether bedbugs, roaches or fruit flies) remains constant from one generation to the next then the Hardy Weinberg theorem applies and genetic variation is retained from one generation to the next. We say the population is in “stasis” and there is NO micro-evolution occurring. (Note again, the letters p and q in the table denote the two alleles in the population, i.e. p = B, q = b, thus if B is found to enhance or increase over b then p will increase over q and we will have micro-evolution, which is exactly what we see as we go from the top down in the table).

Where can the bedbugs mutate to now, if they are to survive what we throw at them? The only way forward seems to be to become fire resistant! But that is right out of the realm of horror-scifi so let us hope it never transpires!

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