the truth about introns

aroger at ac.dal.ca aroger at ac.dal.ca
Fri Mar 11 20:04:46 EST 1994

In article <CMCooy.JsG at gpu.utcc.utoronto.ca>, lamoran at gpu.utcc.utoronto.ca (L.A. Moran) writes:
> If you are referring to differences between the genes of mammals and, say
> arthropods, then I think that the mammalian genes may contain more introns.

Not to quibble about small issues, but I'd like to point out that introns
can be extremely phylogenetically unstable.  What Larry has said about
arthropods is another example of over-generalizing....it is certainly
true that drosophila and culex have relatively few introns....but this
need not be the case for other arthropods.  For instance, the Tarantula
haemocyanin gene e contains 8 introns.....a respectable number of introns
even for a mammal!
My point is that even WITHIN phyla of animals intron density can be 
immensley variable.  This can mean one of several things.  Either 
introns are gained and lost rapidly periodically or that the ancestral
gene contained vast numbers of introns most of which were differentially
purged from extant organisms.  GEE, I WONDER WHICH IS MORE LIKELY?????

On another note, someone was mentioning that prokaryotes have relatively
few introns compared to eukaryotes--- again another misleading generalization!
Only, this time it is a much more pernicious one.
Firstly, the only introns found in prokaryotes belong to entirely different
classes of elements than spliceosomal introns commonly found in SOME 
eukaryotes.  These introns belong to the group I or group II classes
in SOME eubacteria and the 3'cyclic-phosphate-intermediate class 
in archaebacteria.  To be an intron seems to mean that a particular
element occupies a certain molecular niche.  Not all introns need 
be related by common ancestry.  So to say that prokaryotes have
relatively few introns compared to eukaryotes is like saying that
the French eat more sorbet than Canadians eat ice-cream...you can
say it but it has little meaning.

Andrew Roger
aroger at ac.dal.ca 

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