Ecological and Environmental Genetics
The field of environmental genetics can be viewed from three perspectives: the ways that the environment affects gene expression, how the environment changes the individual genome, and how specific environment factors bring about gene frequency changes.
There are numerous ways that gene expression is affected by environmental conditions. Certain factors in the external or internal environment of an organism will, in effect, turn on or induce the reading of certain genes. Possibly the best known of these is the lac operon model of Jacob and Monod.
A well-known general type of environmental induction can occur through chemicals that cause cancers and other abnormal growths. While some carcinogens cause cancer through actual change in genetic structure others cause tumors. Diethylstilbestrol (or DES) acts as an estrogen and is a potent cause of tumors. DES was used to help women conceive in the mid-twentieth century, until it was found to cause vaginal tumors in the daughters of those mothers. Estrogen-like substances, foreign to the body are called xeno-estrogens and there are a number of examples of these.
Another famous case where maternal environmental altered gene expression is thalidomide. This drug was prescribed in certain European countries in the 1950's and 1960's to pregnant women as a means of controlling morning sickness. However, it was found to cause the fetuses to develop with serious limb abnormalities; for example: normal hands or feet but no legs or arms. Alternatively, it has recently been found that pregnant women who take the vitamin folic acid will reduce the chance of their babies being born with neural tube defects (e.g., spina bifida). It is important for these women to take the vitamin early in their pregnancies when the embryonic central nervous system is being formed.
Not all changes in gene expression occur early in an organism's life. Rather, this is a life-long phenomenon as the living being adjusts to the ebb and flow of the natural world around it. Environmental conditions can change the actual genetic make-up (i.e., mutations) or the arrangement of genes (i.e., translocations) on the chromosome. Many chemicals can change the DNA or the number of chromosomes. For example, taxanes cause microtubule bundles to fail to function in cell division. Ionizing (such as gamma) radiation from earthly or cosmic emissions can cause point mutations in the DNA.
Certain viruses, called bacteriophages, attack bacteria and as they do, may incorporate DNA from a previous host into the new host's genome. This action, called transduction, can greatly alter the characteristics of that new bacterium. The public health implications of transduction are enormous because some genes spread in this manner impart antibiotic resistance. Transduction can cross species lines so that relatively harmless, antibiotic-resistant Escherichia. coli can have its genes passed to seriously pathogenic bacteria. When antibiotics are given ineffectually to people who have viral intestinal infections, their E. coli populations are more likely to evolve to a greater degree of antibiotic resistance. If at some point the person is exposed to a dangerous bacterium, the E. coli may have their resistance factors transduced to that harmful organism. Antibiotics will then be of little benefit to the person where this has taken place.
Finally, human viruses, such as the Epstein-Barr virus (causing cold sores, mononucleosis, and several other maladies) can cause translocations in the host genome. At least one of these changes in gene arrangement has been shown to cause Burkett lymphoma. Viruses are part of the environment and are a way that genomes can change.
Environmental factors also cause changes in gene frequencies. Selective forces favor certain characteristics while acting against others. The Hardy-Weinberg Equilibrium examines this principle in some detail.
An example of the effect of environmental factors on gene frequencies involves the Peppered moth, or Biston betularia,. The moth's population before the onset of the Industrial Revolution in the British Isles was characterized as a medium-sized (ca. 3 cm. in wingspan) white moth predominately with black speckles over its dorsal surfaces. Visually, the moth appeared to be a patch of light lichen (fungus) and thus, escaped predation from visually hunting birds. Rarely, a darker, autosomal recessive, form appeared but these were quickly spotted by their predators and eaten.
As the Industrial Revolution progressed, coal burning increased and, with it, greater quantities of sulfur dioxide. This gas is toxic to lichens and they disappeared in the polluted air. Because the lichens disappeared, the white form of the Peppered Moth became more noticeable as an unusual feature in the environment. Concomitantly, dark or melanic forms became less obvious against the now-darker woods. Eventually, in the most industrialized areas of the United Kingdom, the dark forms of the moth were the only ones to be found. Scientists found, however, that in unpolluted, upwind sites, the white form was still protected and hunting insectivorous birds missed them while fairly quickly spotting melanic moths. Subsequent investigators have many similar examples in other industrialized countries in other animals of what is now called industrial melanism.
What future problems might there be in the interactions between genes and their environment? Probably the most controversial aspects are the newly developed, genetically engineered agricultural varieties of crops. Artificial varieties, per se, are nothing new. With few exceptions, every agricultural item is somewhat different from what would be found in the natural, non-human environment. There is no place where you can find "wild" broccoli or corn, to name just a few things. Corn, in fact, is thousands of years old and was first bred from a wild grass by Mexican Indians. Corn varieties were selected and continue to be selected over many generations for favorable qualities. However, new forms of rape seed (the source for canola oil) and other field crops have been brought into production after a brief development period. This rapid introduction rate has been possible because of advances in genetic engineering. Some could argue that this more rapid pace does not give scientists a very long time to make adequate environmental impact assessments. This concern, while valid, must be balanced against the need to produce varieties that are resistant to rapidly evolving pests. Other points to ponder are these: Better producing varieties mean that more food can come from the same amount of land so that more people can be fed. However, the genes that give greater efficiency to crops might be transferred to weed species, thus helping to form "super-weed" varieties. It is possible to breed plants so that they will include extra vitamins or human disease-resistance factors in fruit. Can it be said that new genetically engineered crops are bad when balanced against children dying of diarrhea or going blind from vitamin A deficiency? To paraphrase Chief Seattle: We do not know what the future will hold but we do know it will hold change.
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