Validity of the theory of sex in relation to parasites.

Andy Peters adpeters at sunflower.bio.indiana.edu
Sat Dec 5 19:11:55 EST 1992

In article <1992Dec5.195711.22967 at u.washington.edu> xia at darwin.genetics.washington.edu (Xia) writes:
>>     Hi.  I'm looking for some information into the validity of the theory that
>>sexual reproduction has evolved as a defence against the actions of parasites.
>>Since most parasitic invaders reproduce assexually and may have many generations
>>in the course of the host organisms's life, one would expect that natural
>>selection would play a more important role in the parasite population.
>>Therefore, evolution would proceed more quickly in parasite populations. Hence,
>>the development of sexual reproduction as a means evening the odds.
>>Its a tempting theory. Does it make sense to anyone else?
>>                                               John Antonioni
>>                                               Sudbury, Canada
>No, it does not make sense, because it applies only to specialised
>host-parasite pairs. When the host is harbouring many different species
>of parasites, a recombination to avoid one species of parasite would 
>send the host to the mercy of other species of paes.

I disagree.  No matter how many parasites are coevolving with the
host, each parasite population will be tracking the most common
host genotype.  By "tracking," I mean that the parasite populations
will constantly be evolving to mimic some feature of the host (not
necessarily the same feature for each parasite).  Hence, recombining
and producing rare offspring allows the host a chance to escape from
any number of parasites.

> Also, the
>theory would require sex to evolve independently many times. 

I'm not sure why this is correct anyways, but it's not relevant
because the Red Queen applies to the _maintenance_, not the _origin_
of sex.

>the theory requires the diversified recombinants to be realized in
>progeny. For this reason it does not apply to species with low fecundity.

This is true of some other ecological-genetic hypotheses for the
maintenance of sex (i.e. the tangled bank), but I don't think it's
true for the Red Queen.  The Red Queen model states that the
production of _rare_ progeny, not _diverse_ progeny, is the benefit of
sex.  Granted, the production of large numbers of progeny will help
ensure that some of them are rare, but so will increased recombination

>I have written a paper on the issue, and 4 months has passed since
>I submitted it to Am Nat.
>Xuhua Xia
>Department of Genetics
>U of Washington

* Andy Peters                     * I borrowed Dad's jack.  I'll *
* Program in Evolution,           *   return it and his rivet    * (<-Don't
*      Ecology, and Behavior      *        gun tomorrow          *     ask)
* Indiana University, Bloomington *            -Bob              *

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