Evolvability, not in terms of just
changing, but in terms of adaptation is a prerequisite to
life (although not the only one), or at least one of the
interesting aspects of life.
I do not think that Scrapie has evolvability.
I've been trying to formulate the argument is a simple, convincing way,
and have not really succeeded ;-)
If, as the main hypothesis goes, diseases as Creutzfeld-Jacob, Kuru
and Scrapie, are mediated by a protein that transforms proteins
to take on the shape of itself, I do not see how those proteins
have evolvability. The prion proteins in the sheep or humans
do have evolvability, and mutations might arise in those that
would increase the host range of one of those proteins in their
scrapie shape. (given
that it happens to be in an organism that gets infected with an
earlier version of the Scrapie version of the protein).
Such changes, however, can hardly be called adaptations, unless
one takes the standpoint that Sheep/Cows/Mice etc. are just another
means of the protein (Scrapie shape or not) of replicating itself.
I think that that standpoint is farfetched, the prions do serve a
function in the brain tissue, and are not just happy parasites
along for the ride.
Another way of making the same argument:
version of the protein is a different "something" than the non-scrapie
version. It uses prions (non-scrapie) versions of the protein to
make copies of itself. Or, in other words, the non-scrapie versions
are just "food" for the scrapie versions, that happen to have the same
or a similar sequence. A scrapie population can never adapt to a
mutation in the prions of the mammals that renders it immune to the
shape-transformation. The only thing that can happen is that a
mutation occurs in the prions themselves. This might give rise to a
new version of the Scrapie protein. This does NOT constitute an
adaptation in the Scrapie population, but a new "birth" of a Scrapie
"species". (unless on takes the above mentioned point of view that
Sheep/Cows/Mice are just a means of the prion protein of replicating
Wayne Lanier mailed me the following:
>Well, them Prions must've come from somewhere! Maybe they did "evolve"
>once and might again.
My point basically is that the repeatedly coming into existence of the
Scrapie version of the protein in such a scenario is not an adaptation.
One does expect a selection against the possibility
of being transformed into a Scrapie shape (if this is at all possible,
given the other constraints on the prion protein).
Cows never used to eat sheep-meat, hence there was never a selection
pressure for being resistant to this shape-transformation
by the sheep-scrapie version of the protein.
Are there any records of Scrapie like diseases in meat-eaters ??
Best, Martijn Huynen