Forum #11 – Exploring selective pressure and immune responses!
The genetic scripts written in DNA code constantly undergo changes, or mutations. At times, these mistakes in a gene’s message can be harmful; often, they have no significant effect. Occasionally, though, a mutation confers a survival advantage in the face of an environmental change. Most of the non-carriers of the mutation die, and those with the mutation are able to reproduce. With that powerful evolutionary selection force, the gene can become common in a population.
Scientists were astonished to find that some individuals did not become infected with HIV, even after repeated exposure to the deadly virus. For some reason, they were immune. A long and difficult scientific search, using blood samples from hundreds of HIV-resistant patients, finally teased out the genetic explanation. Resistant individuals had in their cells two copies of a mutation that disrupted the entryway through which HIV viruses entered white blood cells. People who inherited just one copy of the change could become infected, but their disease progressed more slowly. With this being such a recent epidemic, where did peoples’ immunity come from?
Another puzzle was the way this resistance is distributed throughout the world. In some Northern European populations it is relatively common. In Southern Europeans it is more rare, and it is almost entirely absent in Africans, Asians, and Native Americans. Logically, the mutation must have occurred in the past, acting as a defense against a different, previous epidemic caused — like the AIDS epidemic — by a pathogen that also targeted white blood cells.
Reading a chronological history, biologists traced the HIV-resistance gene mutation back about 700 years. That was the time at which the Black Death (bubonic plague) swept through Europe, killing one-third of the population. Then, as now, there were individuals who survived the lethal organism, perhaps because it could not enter their white blood cells. The areas that were hardest hit by the Black Plague match those where the gene for HIV resistance is the most common today. After trying to infect such resistant cells with bubonic plague bacteria to test the hypothesis that the mutation in the CCR-5 receptor gene could have thwarted the plague in the Middle Ages, as it does HIV today – results suggested a different disease might be more likely. Focus has now moved to looking at smallpox as the catalyst. This will be yet another illustration of what scientists are finding over and over in the human genome: Nature’s past successes often remain part of our genetic toolbox.
Since the reporting of the first cases of AIDS in 1981, and the discovery of its etiologic agent, human immunodeficiency virus type 1 (HIV-1), in 1983, there has been substantial scientific progress in the development of both effective antiretroviral therapy and the understanding of virus–host cellular interactions. Nonetheless, the correlates of effective immunity to HIV remain elusive, and the paucity of knowledge has hindered development of effective vaccines. One approach to understanding the immune response to HIV and why it fails in most people lies in examining those few hosts who appear to be resistant either to acquisition of the virus or to its devastation once acquired.
Scientists have considered the possibility that HIV-2, the less virulent form of HIV found in West Africa, evolved from a form of SIV (simian immunodeficiency virus) found in sooty mangabeys (SIVsm). Recently published research demonstrates that HIV-1, the form responsible for most AIDS cases, almost certainly evolved from a type of SIV (SIVcpz) that occurs in West African populations of chimpanzees (Pan troglodytes troglodytes). Researchers postulate that transmission from chimpanzees to humans occurred at least three different times, probably while human hunters butchered carcasses of chimpanzees killed for food. Evidence of the evolutionary history of the HIV virus, and the fact that chimpanzees apparently suffer few, if any, ill effects from SIVcpz infection, illustrates a successful pathogen. It is beneficial for pathogens to be sufficiently benign so as not to kill their hosts immediately, since less lethal pathogens are able to infect more hosts and increase their own reproductive success.
Need an example?
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Ebola Virus
Several strains of the Ebola virus exist. The incubation period is between 2 and 21 days. Initial symptoms are sudden malaise, headache, and muscle pain, progressing to high fever, vomiting, severe hemorrhaging (internally and out of the eyes and mouth). Patient mortality is between 50%–90% (usually within days). There is no vaccine and no cure – but we’re getting closer. |
THE TASK:
- Given the premise in this discussion, research and describe a disease for the class. Note the degree of virulence with your selected disease and whether it is increasing or decreasing in prevalence. Please try to avoid duplicating the work of other students so we get a wide distribution of examples. (15 points)
- Your post should be a minimum of 250+ words. (5 points)
- Cite resources used. (5 points)
