Evolution of virulence in a host parasite system and tragedy of the commons
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How might conflict arise between the fig wasp and the fig tree?
- by fig wasps parasitizing more seeds than optimal for the plant. male wasps don't pollinate.
- In gynodioecious figs, the female figs attract wasps to pollinate them but dont allow the wasps to breed as it only has long-styled flowers which aren't suitable for egg-laying
what is virulence
the harm done to the host by the parasite, specifically increase in mortality
Virulence is the harm measured in _________
mortality, lower fecundity etc.
What was the old idea about virulence
it is more beneficial for the parasite not to harm the host, as they will have a better chance of LT survival. when virulence occurs, it is thought to be because the parasite is new to the host
What is wrong with the old idea of virulence
it ignores the fact that individual parasites are selected to propagate their own genes, and aren't there to benefit their species or the host. As a result, there may be a high level of virulence, even in a long established system.
How does the nematode-fig wasp parasitism work according to Herre 1993?
baby nematodes crawl onto newly emerged female fig wasps and get taken to a new fig. they enter the body of this wasp and eat it then adults emerge and mate and lay eggs in the fig. These babies emerge the same time as the next gen of fig wasps in the fig.
how did herre 1993 determine the virulence of the nematodes on the wasps
compared the fecundity of wasps infected and non-infected by nematodes. Measured in single foundress broods
What were the methods Herre 1993 used
- - the wasp was parasitised if immature nematodes were found in the fig
- -the no of pollinating wasps was determined by counting wasp corpses, pre wasp emergence
- - 11 spp of fig wasp studied in panama
- - proportion of figs colonised by single female 100-24% across fig/ wasp spp
What were the results of herre 1993
relative reproductive success decline (virulence increased) as the mean proportion of single foundress broods decreased. so nematodes that parasitise wasps that always/nearly always have single foundress broods are non-virulent
what is the difference between vertical and horizontal transmission in herre's 1993 expt
vertical transmission is where the nematode is transmitted from mother to daughter wasp and horizontal is where it is transmitted from mother wasp to daughters of other foundresses in the same fig.
where only vertical transmission occurs, nematodes are
virulence increases as the opportunity for what increases
why would a virulent nematode parasitic on a wasp with only single foundress broods go extinct
its hosts would be less successful than non parasitised hosts
What explanation for this finding does Herre (1993) give?
in single foundress broods, nematodes likely to be highly related to each other. however in multifoundress broods, there is mixing of unrelated nematodes, which means there is higher selection on nematodes to increase their reproduction relative to other nematodes infecting the same host.
What response would nematodes give to this selection
they might eat more of their host sooner, increasing their own reproductive output relative to other nematodes infecting the same individual host/ other hosts in the same fig.
How does Bull (1994) put the explanation that Herre (1993) gives?
multiple foundress broods have higher virulence due to selection for increased virulence at the in-host level. some parasite genotypes are better adapted at growing in the host than others, which causes increased virulence
What is the background to Ewald's 1993 study on the evolution of virulence in human disease
humans suffer from diseases caused by micro-organisms, some of which are non virulent e.g. cold, others highly virulent e.g. cholera. Ewald gathered data on how pathogens are transmitted to other humans and found that human behaviour and mode of transmission have selected for differing levels of virulence
what are the two types of pathogens
those that rely on host mobility to be transmitted and those that don't
give an example of a pathogen that relies on host mobility and how does it work
common cold, relies on person to transmit, doesn't make person too ill to leave house so can be transmitted to others via sneezing for example.
How do pathogens that don't rely on host mobility work?
they are transmitted some other way so through an insect vector/ water. A person may be immobile but can still be bitten by an insect, or faecal contamination of bedding, clothing or toilet can still contaminate the water and spread the disease in places with poor sanitation
what did ewald show using the data from literature
pathogens that relied on human mobility were less virulent. There was a +ive correlation between virulence and proportion of outbreaks of a particular type of pathogen resulting from water borne infection
What evidence did Ewald note from India
better sanitation led to a milder form of cholera displacing a more virulent form, decreased virulence of dysentry. better sanitation reduces severity and incidence.
What is wrong with the evidence from India
it is anecdotal
What human implications did Ewald suggest this had
public health measures may reduce human suffering by reducing disease virulence
What did Bull (1994) say was more important
the direct result of public health measures- better sanitation reduces infection rate, even if virulence is unaffected.
What else does Bull (1994) stress?
although virulence can be altered by selection, some diseases that are very virulent to humans, haven't evolved with humans but are randomly transferred to humans from other animals. So diseases of animals may, by bad luck, be virulent to humans if not virulent to normal host.
What are sit and wait pathogens
Those that have long lived spores or resting stages, the pathogen may require host mobility for transfer but can be virulent because the spores can persist in the environment even after the host has died
Some pathogens may spread more easily after the host has died. What is the American Foulbrood disease?
a bacterial disease of larval honey bees caused by paenibacillus larvae that kills infected colonies. each dead larve becomes a scale which contains loads of spores which can remain viable for years. dead nests containing combs of wax cells are attractive to bees from other hives but when the colony is alive, guard bees exclude non-nestmate bees. thus, by being highly virulent and killing its host colony to remove guards, it increases its transmission to new colonies
What is the trade-off hypothesis
there's a trade off between how long the virus is able to persist in the host and the rate at which it can be transmitted. It suggest that virulence will evolve to a level where virulence and transmission is balanced, to maximise the spread of the virus
The trade-off theory says that which three epidemiological parameters are linked so that a change in one will lead to a change in the others
- - host death rate due to infection
- - transmission rate of the parasite
- - recovery rate
What is the avirulence hypothesis
selection in favour of those varieties which vegetate whence they can escape, the surviving varieties would gradually lose their highly virulent invasive qualities and adapt themselves to the conditions surrounding invasion and escape. (smith 1904)
What is wrong with the support for the avirulence theory that new host-parasite associations tend to be virulent (Read 1994)
more damaging associations are more likely to be reported. little evidence that old host-parasite associations tend to be avirulent
What did W.D. Hamilton (1964) challenge?
the idea that spp evolve to increase own persistence and that individuals should sacrifice themselves for benefit of spp.
What did Anderson and May 1982 show
if recovery and virulence are linked then intermediate virulence is favoured
Why does alizon et al (2008) believe there is a lack of evidence for the trade off hypothesis
because collecting the data to test it is complicated
what is the short sighted evolution hypothesis proposed by Levin and Bull 1994?
in diverse infections, faster growing strains are favoured because there is competition for shared limited resources. such competition can lead to increased host mortality and the subsequent extinction of the competing parasite strains (tragedy of the commons)
What support does the short sighted evolution hypothesis have?
In experiments on co-infection on rodent malaria the found that virulent strains have a competitive adv over less virulent strains in multiple infection, and this translates to higher levels of transmission to the vector (de roode et al 2005, bell et al 2006)
What stops parasites from becoming increasingly virulent if short sighted evolution is important?
not all hosts are multiply infected and in a singly infected host, being too virulent means wasting resources of the host. prevalence of multiple infection is another factor that will affect the optimal level of virulence.
How can diversity in an infection lead to decreased levels of virulence? (andre and van baalen 2007)
when strain diversity is low within a host, strains are more related, which facilitates collective action for the common good.
what is the tragedy of the commons as described in Hardin's essay in 1968
it's a dilemma that comes from a situation in which people who act independently and rationally, whilst consulting their own self-interest can deplete a shared limited resource, even when it's not in anyone's LT interest for this to happen.
what are the criticisms of the tragedy of the commons
due to the justification of privatisation using the tragedy of the commons analogy, many people have disagreed over whether individuals will always act selfishly. even self-interested individuals may find a way to cooperate as collective restraint serves the collective and individual good.
What has Elinor Ostrom done
based on many studies of user-managed fish stocks, pastures, woods, lakes and groundwater basins, she concluded that the outcomes are more often than not, better than predicted by standard theories. resource users frequently develop sophisticated mechanisms for decision making and rule enforcement to handle conflict of interest. she characterises the rules that promote successful outcomes
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