Lecture 24

Question 3

Life history theory

Answer 3

• Organisms have limited resources to allocate to these different functions

Question 4

Malaria parasites face a similar resource allocation tradeoff

Answer 4

• Growth and reproduction
• Malaria parasites allocate few resources to reproduction (invest little in transmission)
• Of all infected red blood cells, only few percent (1-2%) produce transmission stages

Question 5

Why do malaria parasites invest little in transmission

Answer 5

Hypotheses
- Greater investment means stronger transmission blocking immunity
- Greater investment means more mosquito mortality
- Within-host competition

Question 6

What investment strategies maximize R0

Answer 6

• Hypotheses depend on number of transmission stages produced, not relative investment

Question 7

Co-infections

Answer 7

• Parasites that can exploit more host red blood cells do better (low transmission investment favored)
• In experimental co-infections, parasites reduce transmission investment as predicted
• Where co-infections and within-host competition are common, expect more virulent parasites

Question 8

• Changing investment can help parasites recoup fitness lost due to drugs

Answer 8

• At high doses, increasing transmission investment provides marginal fitness benefit (“terminal investment”)
• At intermediate doses, decreasing transmission investment provides large fitness benefit ~90% (“reproductive restraint”)
• Parasites do adjust investment as predicted by theory
• More killing, less transmission

Question 9

Does altered investment help parasites evade effects of drugs

Answer 9

• Changing transmission investment can mitigate some effects of drugs
• In addition to classical resistance mechanisms, drug treatment could generate selection for altered life history traits
• By favoring parasites that invest more in proliferation vs. transmission, drugs can select for higher virulence

Question 10

Evolutionary hypotheses for coordinated development of malaria parasites

Answer 10

• Synchronicity is beneficial to parasites
• Timing is beneficial to parasites
• Synchronicity and timing both beneficial to parasites
• Neither synchronicity nor timing are beneficial to parasites (not)
• There is benefit to coordination

Question 11

Disrupting rhythms of malaria parasites

Answer 11

• In rodent malaria, ring stage parasites are most abundant during day
• Ring stage parasites used to initiate experimental infections

Question 12

Consequences for parasites of disrupted rhythms

Answer 12

• Parasites do worse
• When mismatched with host, parasites achieve ~50% lower densities (both asexual and transmission stages) compared to when matched with host
• Jet lag is bad for malaria parasites

Question 13

Consequences for hosts of disrupted parasite rhythms

Answer 13

• Mismatched hosts do better (lose fewer red blood cells)

Question 14

Potential benefit of synchronicity

Answer 14

• For malaria parasites, synchronicity may allow bursting parasites to overcome immune responses through sheer force of number

Question 15

Testing plausibility of hypothesis

Answer 15

• Under what conditions do synchronous parasites perform better than asynchronous ones
• Asynchronous parasites do best when higher # of free parasites required to trigger immune response
• Synchronous parasites do best when lower # of free parasites required to trigger immune response

Question 16

Potential benefit of timing

Answer 16

• Avoiding an unfavorable environment
• Timing may allow parasites in vulnerable stages to avoid exposure to their more damaging immune effectors
• Timing may allow parasites to match availability of resources