Chapter Study Questions

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Chapter 14

Chapter Study Questions

This activity contains 24 questions.

Match the key terms in this chapter listed below with the phrase that is the best match for it.

Using the pull-down menus, match each item in the left column to the corresponding item in the right column.


	antigenic site

	horizontal transmission

	vertical transmission

	antibiotic

	resistance

	virulence

1.1 any part of a foreign protein that an immune system can recognize and remember

1.2 a chemical that selectively kills certain bacteria by disrupting certain biochemical processes

1.3 the ability of a pathogen strain to tolerate a chemical that can kill other strains

1.4 the ability of a pathogen to damage its host

1.5 transmission of a pathogen from one host to another unrelated host

1.6 transmission of a pathogen from one host to the host's offspring

Why do pathogens evolve quickly? (There are at least four important factors)

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Some of the options below regarding hemagglutinin are correct and some are not. Decide which statements are correct.

Hemagglutinin is a protein found in the core of influenza A virus. Hemagglutinin is the major influenza protein that is recognized by the host immune system. Hemagglutinin tends to accumulate amino acid changes in antigenic sites rather than non-antigenic sites in those flu strains that survive longest. Hemagglutinin has antigenic and non-antigenic sites distributed equally over the molecule surface. Hemagglutinin is coded for by a gene that tends to accumulate more synonymous than non-synonymous substitutions at all sites, in accordance with the neutral theory. Hemagglutinin accumulates certain mutations that can be used to predict which flu strains will become most common in the next season.

Name the hypothesis described, and give an example: Evolution of virulence can occur via natural selection for debilitating or killing the current host, if this will result in efficient spread of pathogens to other hosts.

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Name the hypothesis described, and give an example: Evolution of virulence can occur via natural selection for a certain environment or host, with virulence in another host being an accidental by-product.

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Name the hypothesis described, and give an example: Evolution of virulence can occur via competition with other pathogen individuals in that one host, in the short term.

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What role did mutation and natural selection play in the case of the child who spontaneously recovered from ADA deficiency? How do we know?

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Name one component of the modern diet that appears to have caused an evolutionary response in certain human cultures. What gene(s) is involved? Why do you think this dietary component has caused an evolutionary response, while others have not?

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Which of the following statements are correct and which are incorrect?

In Strassmann's study of a society that does not use contraceptives, approximately 80% of adult women were experiencing menstrual cycles at any given time. The incidence of breast cancer is highest in European countries within the range of Mus domesticus, the mouse species in which the mouse virus is most common. Modern American women experience approximately one-third the number of menstrual cycles, over their lives, as do women in pre-modern cultures like the Dogon. Many of the risk factors for breast cancer are related to number of menstrual cycles that a woman experiences. DNA sequences similar to Mouse Mammary Tumor Virus are approximately twenty times more common in cancerous breast tissue than in normal breast tissue. Virus particles are more common in cancerous breast tissue than in normal breast tissue.

Do insect-borne diseases tend to have higher or lower virulence than diseases that are spread by direct contact? Why?

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Which of the following statements are correct?

Myopia alleles occur at high frequency today because near-sighted people can survive longer in a modern environment than they could in the past. Myopia is caused by a combination of genetic predisposition with certain environmental factors, probably close-focus activities like reading. In Barrow, Alaska, myopia is more common in recent generations that have spent more time in school. Alleles for myopia were favored historically because, although deleterious themselves, they are linked to alleles favored by selection. A higher percentage of monozygotic twin pairs than dizygotic twin pairs are discordant for myopia.

Which of the following statements regarding the findings of studies on stepchildren presented in this chapter are correct and which are incorrect?

Stepchildren, in general, are likely to be abused or ignored by stepparents. Stepchildren were, on average, ill longer than children who did not have stepparents. Biological children have higher reproductive success during early adulthood than stepchildren. Humans appear genetically wired to give care to biological children but not to stepchildren. Stepchildren are not likely to have children of their own. Stepchildren had higher levels of cortisol in their blood than biological children.

What was the likely source of the 1918 and 1968 influenza pandemic strains? Are they both thought to have arisen via recombination? How do we know?

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A recent large study of HIV patients in the United Kingdom followed 2537 patients with HIV from 1996 to 2003. None of them had received anti-retroviral drugs at the time they joined the study, and all of them were then placed on a combination of several anti-retroviral drugs. The figure below shows the prevalence over time of anti-retroviral drug resistance in HIV strains from these patients. Did any patients carry resistant strains of HIV in 1996, before they started drug therapy? Was treatment with anti-retroviral drugs associated with changes in resistance? Is this result what evolutionary theory would predict? Predict what you think will happen to the patients with resistant strains, patients whose strains are still not resistant, and the strains circulating in the general population.

Reference: UK Group on Transmitted HIV Drug Resistance. 2005. Time trends in primary resistance to HIV drugs in the United Kingdom: multicentre observational study. British Med. J. 331:1368-1374.

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Septic shock is a dangerous blood infection that can occur when bacteria from the gut get into the blood, sometimes due to an abdominal injury. It is almost always fatal if left untreated. Septic shock normally causes high fever, which is sometimes treated with fever-reducing drugs. But, could fever be beneficial? Su et al. (2005) recently studied the possible benefits of fever in septic shock in 24 sheep. The sheep were randomly divided into four groups: normal body temperature, hypothermia, mild fever, and high fever. Body temperatures of sheep in the mild fever and normal temperature groups were lowered with acetaminophen and ice packs. (The hypothermia group consisted of a few sheep that spontaneously became hypothermic while under anesthesia.) All 24 sheep died within a day or two. Survival times are shown in the accompanying figure. Bars with asterisks were statistically significantly different from the bar without asterisks.

To prevent the sheep from experiencing pain or distress, and to enable ease of blood collection, all 24 sheep were kept under anesthesia during the entire experiment. Anesthesia normally lowers body temperature by 1-2 degrees, and thus all four temperature groups were defined to be 1-2 degrees lower than normal. That is, an unanesthetized sheep's body temperature is normally 38C, but in this experiment, "normal", for anesthetized sheep, was defined to be 36-37C (i.e. the second bar from the bottom on the figure).

Reference: Su, F., ND Nguyen, Z Wang, Y Cai, P Rogiers and JL Vincent. 2005. Fever control in septic shock: beneficial or harmful? Shock 23(6):516-20.

Part I: Apply the information and figure above relating to septic shock: Which group of sheep survived longest? Was mild fever beneficial? Was high fever beneficial?

Part II: Apply the information and figure above relating to septic shock: What complications did the use of anesthesia introduce to the experimental data? Would you recommend that this experiment be done without anesthesia, given animal welfare concerns? Are there other ways that this topic can be studied ethically in humans or animals?

Part III: Apply the information and figure above relating to septic shock: Suppose that your two-year-old child is ill and has a dangerously high fever. Your doctor recommends a fever-reducing drug, but you then discover that your doctor has not read Su's study, or the other studies on fever that were presented in this chapter. Do you have enough information to decide what to do? If not, what further information do you need (that is, what further studies should be done)? What would you do in the short term about your child's fever, and what would you say to your doctor? (Su's study was not published in a pediatrics journal; it was published in a specialty journal for shock.)

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Fitch et al. hypothesized that influenza strains with novel antigenic sites would enjoy a selective advantage. The graphs represent data gathered as they tested this hypothesis. Graph A shows the degree of nucleotide divergence in hemagglutinin genes from samples of influenza virus collected from the mid-1960's to 1987. Graph B is a phylogenetic tree showing the pattern of descent in the various lineages. You may refer to Figure 13.4 in your textbook for more detailed graphs. Which of the following is/are reasonable interpretations of these results?

Flu strains accumulated nucleotide substitutions in the hemagglutinin genes in clock-like fashion.The flu strains present in the mid-1980's were all the descendants of a single ancestral strain from the late 1960's.The strains that survived to the mid-1980's were selectively favored because the mutations in their hemagglutinin genes gave them novel antigenic sites.The first two choices are correct.

Which of the following support(s) Fitch et al.'s hypothesis that influenza strains with novel antigenic sites enjoy a selective advantage over strains with mutations in non-antigenic sites?

Flu lineages that had survived over many generations had more than 3/4 of their amino-acid replacements in antigenic sites, while those that went extinct had amino-acid replacements evenly divided between antigenic and non-antigenic sites.In an analysis of nucleotide substitutions in the hemagglutinin gene from 357 influenza A strains, 331 nucleotide substitutions were identified. 58% were silent and 42% were replacements.In an analysis of nucleotide substitutions in the hemagglutinin gene from 357 influenza A strains, 331 nucleotide substitutions were identified. In 18 codons in the hemagglutinin gene, significantly more replacement substitutions occurred than silent substitutions.The first and third choices are correct.

The global influenza pandemic of 1968 likely occurred because a human influenza strain acquired the hemagglutinin gene H3 from an avian influenza strain. Which of the following pieces of evidence support(s) the hypothesis that the acquisition happened after an avian and swine strain simultaneously infected a pig and swapped genes, creating a new strain that was then transmitted to humans?

Humans can acquire swine strains of influenza.Avian strains do not infect humans directly.Avian strains sometimes infect pigs.The first three choices are all correct.

Briefly describe one line of evidence in favor of the hypothesis that the use of antibiotics selects for resistance in bacteria.

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The graph below illustrates changes in resistance of Pneumococcus bacteria in children in Iceland to penicillin. The use of penicillin was strongly discouraged starting in the early 1990's with the results illustrated. According to Schrag et al.'s studies on the cost of resistance, is the trend shown on the graph guaranteed to continue? Briefly explain why or why not.

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Three hypotheses have been proposed to explain the evolution of virulence. Match each of the examples below to the hypothesis it best represents.

Using the pull-down menus, match each item in the left column to the corresponding item in the right column.


	coincidental evolution hypothesis

	trade-off hypothesis

	short-sighted evolution hypothesis

21.1 Late strains of HIV virus are more virulent than early strains within the same host.

[hint]

21.2 Histoplasmosus capsulatum is a fungus found naturally in soils and in areas contaminated with bird and bat droppings. Most people living in areas where the fungus is endemic have contracted it through airborne spores and, after a brief period of flu-like illness (called histoplasmosis), become immune to it. However, some people develop severe lung infections; in some patients, the infection can spread to other organs. These infections can be fatal if not treated. Histoplasmosis is not transmitted from person to person.

[hint]

21.3 Plasmids of E. coli selected for horizontal transmission show lower virulence and slower reproduction than do plasmids selected for vertical transmission.

[hint]

According to Ewald's analysis of the trade-off hypothesis for virulence, which of the following pathogens should be the least virulent?

Lyme disease is caused by a bacterium that is transmitted from host to host via ticks.Group A streptococcus bacteria are most commonly transmitted by direct contact with mucus secretions from the nose or throat of