Re: antiquity of segmentation

[Bill Shear <bills@xxxxxxxxxxxxx>]

>Supposing (for example) that the segmentation genes were involved in doing
>something else before they started specifying segmentation: like building
>the nervous system or something.  In this case, the genes could have been
>independently co-opted in protostomes and deuterostomes.  If there is a
>shift in gene function through time, then one can have 'segmentation
>genes' without segmentation.

This is certainly a possiblity, Graham, but as you point out later in your
posting, not the most parsimonious one.  There are numerous examples of
genes for one function being 'co-opted' for another, but it might be a bit
of a stretch to suggest that the same gene which originally did something
else in animal development got switched to segmentation independently in
two lines--there are a lot of functional details that are exactly the same.
I don't know enough about what the possible constraints might be to
evaluate the probabilities.


  It also seems likely that there is 'homology' between genes
>involved in building eye structure and limb struture throughout a whole
>swathe of animals.  Are we to conclude from this that the last common
>ancestor of insects and humans therefore had legs, segmentation and eyes
>therefore?  This would create certain problems in conventional ideas of
>who these animals are related.

This is just the argument that DiRobertis presents.

>The second problem is that 'segmentation' itself is rather a vague word.
>Are onychophorans, for example, segmented?  Or kinorhynchs?  Or
>tapeworms? Lots of animals have some sort of serial repetition without
>necessarily being 'segmented' in the way that an annelid or arthropod is:
>indeed, even annelids and arthropods are probably not segmented in the
>same way as each other.

Do you have any ideas on a definition of segmentation that would
work--namely include arthropod-annelid and chordate segmentation but
exclude others?  Maybe development according to pair-rule genes would be
part of it.  What are the developmental genetics of tapeworm strobilation?
You can bet that the kinorhynchs have not be investigated on this!  I would
assume that onychophoran segmentation is in fact the same as
arthropod-annelid segmentation.


Bill Shear
Department of Biology
Hampden-Sydney College
Hampden-Sydney VA 23943
(804)223-6172
FAX (804)223-6374
email<bills@tiger.hsc.edu>