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introns and evolution

L.A. Moran lamoran at gpu.utcc.utoronto.ca
Tue Mar 8 09:38:57 EST 1994

sb9e at faraday.clas.virginia.edu wrote:

    "I have a couple of probably naive questions about the evolution of
     gene structure in animals. Many homologous genes contain more exons 
     and thus are more fragmented in higher mammals than in lower animals."

This is bionet.molbio.evolution so lets try and be precise about terminology.
It is inappropriate to use the word "higher" and "lower" when referring to
extant organisms. Just what do you mean by "higher mammals" and what are
"lower animals"? These terms suggest a false view of evolution as a ladder
with some types of living organisms on the bottom rungs and other types
(guess who!) on the top rungs. 

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.
This is certainly true if you compare the mammalian genes to those of yeast
or C. elegans.

sb9e at faraday.clas.virginia.edu asks:

     "First, is this a general tendency? Second, is this considered a 
      result of fragmentation of the gene during evolution or rather 
      a structural simplification in lower organisms?" 

Many scientists who have thought seriously about this have concluded that
some groups of organisms, such as mammals and flowering plants, can tolerate
large sloppy genomes with lots of junk. Other groups, such as unicellular
eukaryotes or smaller multicellular organisms, often have genomes that are 
smaller and might have undergone selection against excess DNA. Thus when 
introns arise by insertion they will be selected against in some species 
and not in others.

Of course there are a few people who still believe that the ancestral
eukaryote contained genes with lots of introns. They argue that the
organization of these primitive genes has been conserved in mammals and 
flowering plants but that introns have been lost in many other eukaryotes
(and prokaryotes). There is not much evidence to support the idea that
intron positions have been conserved in eukaryotic genes for over one 
billion years.

Laurence A. Moran (Larry)

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