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impact of spores on evolution

Michael Coyne mcoyne at argo.net
Sat Oct 21 11:13:59 EST 1995


Ruby Estrella wrote:
> 
> Hi,
> I'm a 2nd year uni student studying microbiology 1 and I was wondering if
> anyone here is familiar with the article "Revival and Identification of
> Bacterial Spores in 25- to 40- MIllion Year-Old Dominican Amber" found in
> Science, May 1995? Well, this article describes the the discovery of
> ancient Bacillus spores in the gut of a bee in amber by scientists Cano
> and Borucki.  After analysis, they found this species to be closely
> related to a present day species, B.sphaericus. I was wondering how this
> is possible if the spores have been  dormant (and therefore not subjected
> to evolution) for millions of years and still share characteristics with
> B.sphaericus today?  Some would argue contamination, yet it was clear in
> the article that the scientists took special precautions to prevent that.
> 
> I would also appreciate if anyone could guide me to related information
> leading to references  on bacterial spores, evolution/taxonomy and
> phylogeny.
> 
> thanks very much
> Ruby Estrella
> s2156974 at cse.unsw.edu.au

Rudy -

I didn't read the Science article, but it seems to me that the answer to your 
question requires a more precise definition than "closely related".  How 
closely related?  It also depends on what genes were compared to acertain this 
degree of relatedness.  Some genes are more amendable to accumulating useful 
mutations than others; thus a very well conserved gene (perhaps a ribosomal 
RNA gene) may show little sequence divergence over millions of years, while 
other genes may accumulate mutations rapidly due to a different sort of 
evolutionary pressure (like genes directly involved in adaptation to the 
environment - surface polysaccharides, genes involved in scavaging trace 
elements, etc.).

Escherichia coli and Salmonella typhimurium (S. enterica) are considered to be 
very closely related (several clues exist: genome organization, G+C content, 
similar promoters and sigma factors, codon usage preferences, etc.) but they 
appear to have diverged several hundred million years ago.

Remember also that mutation is a two-way street.  If there is x% chance that 
base N will mutate say from C to G, there is an equal chance that this G will 
back mutate to a C, assuming there is no detrimental effect to the mutation in 
either direction.

Mike



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