Re: Question about Segmentation

From: Wirt Atmar (wirtatmar_at_aol.com)
Date: 07/26/04 

Date: Mon, 26 Jul 2004 15:10:26 +0000 (UTC)

Paul asks:

>I am not a biologist, so please use small words I can understand.
>My question is whether certain features in vertebrates, like
>ribs, vertebrae, and the abdominal "six pack" follow from segmentation
>in an ancestor to chordates.

The question you're asking deals with a phenomenon called "metamerism". A
*mere* in biology, and in science in general, is an irreducible unit. The word
"polymer", for example, means "many meric units."

The same is true of the modularization of code in the genome. Once you have the
code in place to build a finger, a vertebra, an arthropodic segment (each with
its two opposing legs), you can build as many of them as you like. All it takes
now is some form of upper level regulatory code. Thus with just a little
fiddling of the code, you can build an insect with six legs (plus antennae,
mandibular palps and caudal fins, which are just modified legs) or a millipede
(with something substantially less than a thousand legs).

Metamerism is much like polymerism, but rather than presume identically
replicated units, as polymerism suggests, each finger or vertebra is very
similar, but also slightly differently implemented as the fingers round the
hand or the vertebrae descend down the column.

That upper-level regulation isn't always perfect however. A common defect in
mammals (including humans) is extra digits. The defect is called
"polydactylism" or "polydactyly" (many fingers). You won't have any trouble
finding examples on Google of the effect, but see at least:

    http://www.guinnessworldrecords.com/index.asp?id=54815

>On a related note, what is the oldest known deuterostome?

If you're looking for a name, no one knows with any certainty, but the
protostome-deuterostome split is presumed to have occurred in the Vendian
Period. Please see:

     http://www.peripatus.gen.nz/paleontology/Vendian.html

Wirt Atmar