virulence and selection

ctfaulkn at utkvx.utk.edu ctfaulkn at utkvx.utk.edu
Mon Dec 19 12:38:36 EST 1994


In Article <Pine.HPP.3.91.941215104410.3488A-100000 at ac852.wfunet.wfu.edu>
keasbe9 at WFU.EDU (keas brian edward) writes:
> 
>This brings up an interesting question regarding selection for virulence 
>or any other trait in parasites with complex life cycles.  Is the 
>genetic expression in one stage (e.g. adult virulence) linked to that in 
>another stage (e.g. host behavioral changes by cysticercoids)?  If this 
>is so, then selection on the genetic expression in one stage will affect 
>another. 
	(snip, snip)

>My first thoughts would be that this "linking" is not very common.  

I agree, my guess is that this would be a very expensive (risky) trait to 
maintain, and that it would limit a parasite's ability to utilize alternative
hosts for development and transmission to the DH.  Just my guess, I'm not
really well versed in eco-evolutionary parasitology. 

	(snip, snip)

>To me, this suggests that 
>selection in the adults has been completely separated from selection in 
>intermediate stages (and that different genes are activated in each 
>
	I think some of the variation in virulence of T.gondii isolates in
various intermediate hosts supports this notion, at least intuitively.  In
another post i casually mentioned that some isolates are extremely virulent in
goats, but well tolerated in mice.  We have such an isolate here, it was
associated with abortion and high mortality in the goats, but when we put it in
the mice, they said...."give us more".  


>Back to virulence....one more thought.
>
>Graham Clark writes:
>
>>all organisms, individual parasites that leave the greatest number of
>>'offspring' will be the most successful over time. The optimum level
>>of virulence becomes a balance between parasite transmission and host
>>mortality, since virulence is often directly linked to reproduction of
>>the parasite. This often leads to a low level of virulence, but not
>>always.
>
>I think that this is Ewald's point exactly, except that Ewald would 
>stress "...but not always."
>
        Recognising that high levels of virulence can only be maintained
by high rates of transmission in the different host populations.  Let's 
identify some parasite-host relationships for illustration and discussion.


**********************************
*      Charles T. Faulkner       *   When you don't know where you're
*  Univ of Tennessee, Knoxville  *   going any road will take you there.
*   (ctfaulkn at utkvx.utk.edu)     *                            Alice
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