Genetic resistance (or genetic tolerance)

Genetic resistance (or genetic tolerance) refers to the ability of certain organisms to endure environmental conditions that are extremely stressful or lethal to non-adapted individuals of the same species . Such tolerance has a genetic basis, and it evolves at the population level in response to intense selection pressures.

Genetic resistance occurs when genetically variable populations contain some individuals that are relatively tolerant of an exposure to some environmental factor, such as the presence of a high concentration of a specific chemical. If the tolerance is genetically based (i.e., due to specific information embodied in the DNA of the organism's chromosomes), some or all of the offspring of these individuals will also be tolerant. Under conditions in which the chemical occurs in concentrations high enough to cause toxicity to non-tolerant individuals, the resistant ones will be relatively successful. As time passes their offspring will become increasingly more prominent in the population. Acquiring genetic resistance is an evolutionary process, involving increased tolerance within a population, for which there is a genetic basis, and occurring in response to selection for resistance to the effects of a toxic chemical. Some of the best examples of genetic resistance involve the tolerance of certain bacteria to antibiotics and of certain pests to pesticides.

Resistance to antibiotics

Antibiotics are chemicals used to treat bacterial infections of humans and domestic animals. Examples of commonly used antibiotics include various kinds of penicillins, streptomycins, and tetracyclines, all of which are metabolic byproducts created by certain microorganisms , especially fungi . There are also many synthetic antibiotics.

Antibiotics are extremely toxic to non-resistant strains of bacteria, and this has been very beneficial in the control of bacterial infections and diseases. However, if even a tiny fraction of a bacterial population has a genetically based tolerance to a specific antibiotic, evolution will quickly result in the development of a population that is resistant to that chemical. Bacterial resistance to antibiotics was first demonstrated for penicillin, but the phenomenon is now quite widespread. This is an important medical problem because some serious pathogens are now resistant to virtually all of the available antibiotics, which means that infections by these bacteria can be extremely difficult to control. Bacterial resistance has recently become the cause of infections by some virulent strains of Staphylococcus and other potentially deadly bacteria. Some biologists believe that this problem has been made worse by the failure of many people to finish their course of prescribed antibiotic treatments, which can allow tolerant bacteria to survive and flourish. Also possibly important has been the routine use of antibiotics to prevent diseases in livestock kept under crowded conditions in industrial farming. The small residues of antibiotics in meat, eggs, and milk may be resulting in low-level selection for resistant bacteria in exposed populations of humans and domestic animals.

Resistance to Pesticides

The insecticide dichlorodiphenyl-trichloroethane (DDT) was the first pesticide to which insect pests developed resistance. This occurred because the exposure of insect populations to toxic DDT results in intense selection for resistant genotypes. Tolerant populations can evolve because genetically resistant individuals are not killed by the pesticide and therefore survive to reproduce. Almost 500 species of insects and mites have populations that are known to be resistant to at least one insecticide. There are also more than 100 examples of fungicide-resistant plant pathogens and about 50 herbicide-resistant weeds. Insecticide resistance is most frequent among species of flies and their relatives (order Diptera), including more than 50 resistant species of malaria-carrying Anopheles mosquitoes. In fact, the progressive evolution of insecticide resistance by Anopheles has been an important factor in the recent resurgence of malaria in countries with warm climates. In addition, the protozoan Plasmodium, which actually causes malaria, has become resistant to some of the drugs that used to effectively control it.

Crop geneticists have recently managed to breed varieties of some plant species that are resistant to glyphosate, a commonly used agricultural herbicide that is effective against a wide range of weeds, including both monocots and dicots. The development of glyphosate-tolerant varieties of such crops as rapeseed means that this effective herbicide can be used to control difficult weeds in planted fields without causing damage to the crop.

[Bill Freedman Ph.D. ]

RESOURCES

BOOKS

Freedman, B. Environmental Ecology. 2nd ed. San Diego: Academic Press, 1995.

Hayes, W. C., and E. R. Laws, eds. Handbook of Pesticide Toxicology. San Diego: Academic Press, 1991.

National Research Council (NRC). Pesticide Resistance. Washington, DC: National Academy Press, 1986.

Raven, P. H., and G. B. Johnson. Biology. 3rd ed. St. Louis: Mosby Year Book, 1992.