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Tuesday, April 9, 2013

Rapid Evolution of Resistance to Toxic Chemicals in Atlantic Tomcod

Contributed by: Mary Morales, Rice University

The melanic form of peppered moth Biston betularia in England has been widely used as an example of natural selection in action. Before the industrial revolution in England around 1850 the melanic form of peppered moth was rarely observed in nature and most peppered moth were the light-colored wild type, but by the end of the 19th century it was the dominant phenotype as soot pollution caused the environment to darken. Even though the English peppered moth is a great example of natural selection caused by pollution, molecular basis of this evolution is still not elucidated. Furthermore, the population is not isolated due to constant migration. This makes it difficult to accurately infer the effect of environmental change on survival of natural moth populations.
As in the case of the peppered moth, the Atlantic tomcod Microgadus tomcod of the Hudson River experienced a rapid evolutionary change caused by pollution. For the Atlantic tomcod of the Hudson River, water pollution caused by the improper disposal of toxic chemicals served as the cause of natural selection. Increase in the toxicity of its environment forced the fish to adapt to an environment heavily polluted by polychlorinated biphenyls (PCBs).  Recently, the molecular basis of the evolutionary change was clarified, making the Atlantic tomcod the first well understood case of evolution driven by change of environments (1).

Figure 1. Atlantic tomcod from the Hudson River

Over a million pounds of PCBs was dumped by General Electric into the Hudson River between 1947 and 1976 before the dumping was banned (1). Since PCBs are long lasting chemicals due to their sedimentation in the soil and resistance to degradation, their influence in the Hudson River was significant, and is still significant today (2). They cause serious health issues in animals exposed to them such as cancer through their stimulation of the aryl hydrocarbon receptor (AHR), which is an important transcription factor (2). Two genes encode for the AHR (AHR1 and AHR2) (1). Although AHR1 and AHR2 are both expressed in fish, AHR2 is the more active protein (1).

Figure 2. Frequencies of AHR2-1 and AHR2-2 alleles in tomcod populations in and near the Hudson River. n indicates the number of specimens analyzed in a given location. From Wirgin et al. (1).

Wirgin et al (1) studied the Atlantic tomcod population of the Hudson River and nearby estuaries and found that there are two alleles, AHR2-1 and AHR2-2, at the AHR2 locus. These two variants are distinguished by a mutation that evokes a functional difference. The major difference between the two alleles is that the AHR2-1 exhibits a six base pair deletion causing it to be two amino acids (Phe-Leu) shorter (1).
Fish expressing the AHR2-1 protein have a significantly lower affinity for toxins, preventing them from turning on genes that should not be activated (1). Water contamination by PCBs created a strong selection force in favor of the AHR2-1 allele, as it creates a PCB resistant phenotype that enables the fish to survive and reproduce in a toxic environment (3). The frequency of the AHR2-1 allele in the Hudson River population is extremely high (99%), with the non-mutated allele (AHR2-2) only observed in heterozygotes (1). The allele frequencies of AHR2-1 were also examined in other Atlantic tomcod populations from 7 Atlantic Coast estuaries. As shown in Figure 2, the allele frequency of AHR2-1 is significantly lower in populations located more distantly from Hudson River. Specifically, the AHR2-1 allele is even absent in the four populations in distant locations. 
The different distributions of AHR2-1 and AHR2-2 alleles suggest that the wild type allele AHR2-2 is dominant in populations in non-PCBs polluted areas. The frequency of the PCB-resistant allele AHR2-1 has apparently increased rapidly from a very low level to 99% in ~ 60 years in the Hudson River.
In addition, the low frequency of AHR2-1 in other fish populations suggests that the Atlantic tomcod population of the Hudson River is relatively isolated, unlike the English peppered moth. Thus the increase in frequency of the AHR2-1 allele was not hindered by factors such as migration. The case of the Atlantic tomcod is noteworthy because for the first time the selection coefficient can be estimated if the initial frequency can be obtained in some way. Studies that aim to calculate the selection coefficient in favor of the AHR2-1 allele in the Atlantic tomcod population of the Hudson River are highly encouraged.

1.  Wirgin, I., N.K. Roy, M. Loftus, R.C. Chambers, D.G. Franks, M.E. Hahn. 2011. Mechanistic Basis of Resistance to PCBs in Atlantic Tomcod from the Hudson River. Science 331: 1322-1324
2.  “Toxic River means rapid evolution for one fish species.” 2011. Understanding Evolution. Web.  <>.
3.  Roy, N.K., S.C. Courtenay, R.C. Chambers, I.I. Wirgin. 2006. Characterization of the aryl hydrocarbon receptor repressor and a comparison of its expression in Atlantic tomcod from resistant and sensitive populations. Environmental Toxicology and Chemistry 25: 560-571.