Evolution of the immune system

Eric Cabot ecec at quads.uchicago.edu
Fri Dec 11 16:00:26 EST 1992

In article <1992Dec11.153054.22186 at midway.uchicago.edu> ecec at midway.uchicago.edu writes:
>In article <92345.091058FORSDYKE at QUCDN.QueensU.CA> <FORSDYKE at QUCDN.QueensU.CA> writes:
>>Larry says (ref.1)
>>    It does not follow that because many genes which are functionally related
>>are not linked, that functional relationships never require close linkage.
>>One obvious example are meiotic drive loci, often within inversions, where the
>>drive gene and its target are closely linked. If two proteins must be finely
>Please allow me to stick my oar in here a moment.  I don't really agree
>that the participants in meiotic drive loci are really as tightly linked
>as you imply.  Take the Drosophila melanogaster SD system, for example,
>the Sd and responder loci are on different sides of the centromere
>and linkage disequilibrium between the two has not been demonstrated.
>Sd and rsp can be separated by recombination with reasonable ease
>(but with dire consequences for the Sd bearing chromosome).
>Furthermore, these loci are not bound up by an inversion. Think
>of the consequences if they were!  There are often inversions associated
>with some drive loci, but this might be a case of hitch-hiking.
As a followup to my earlier posting (shown above) I'd like to clarify that
the major players in the segregation distorter system are linked
but not necessarily because of fine-tuning of the gene products.
More likely the linkage is necessary to ensure that chromosomes
with Sd will not easily loose the insensitive responder allele (and 
gain a sensitive allele) by recombination.  The looser the linkage,
presumably, the harder it would be for SD chromosomes to persist.
A pericentric inversion  occurring on an SD chromosome
would tighten this linkage and would probably have some fitness advantage
over other SD chromosomes which could become sensitive to drive 
(that is "suicidal") as a result of recombination. 

I'd also like to note that there are in fact several known examples
of SD chromosomes that have the Sd and responder loci bound
up within inversions (SD-Roma, for example).  In otherwords,
I was incorrect in my initial posting.  That's what I get for
posting to the net before having my first cup of coffee!

    Eric Cabot                         |       "Non Nobis Nati Solum" 
ecec at midway.uchicago.edu               | 

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