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Re: Limb Evolution - "Old" genes

Jeff and list,

Nice story. I noticed this (Jeff's words):
The paddlefish /Polyodon spathula/ is often referred to as a "living fossil", an organism that is similar to no known species apart from fossils.

I hadn't come across this way of thinking about living fossils before, but it's an interesting one. There have been lots of attempts to define this term. This one would certainly make /Latimeria /a living fossil, but not /Crocodylus/. All good so far. So where does it leave /Carcharias, Hexanchus, Sphenodon, Lingula/? What about the Wollemi pine? I guess it's all in how you use the word 'similar'.

Has anyone got a definition of Living Fossil that they think works?

Also, check out the enlarged version cartillainous stain of the paddlefish - does that thing look like a shark (especially in the tail) or what?


Jeff Hecht wrote:
I wrote a news story for New Scientist

It looks like very solid work -

At 11:32 AM +0100 5/24/07, Luis Azevedo Rodrigues wrote:
Any comments?

Nature 447, 473-476 (24 May 2007)

An autopodial-like pattern of Hox expression in the fins of a basal actinopterygian fish

Marcus C. Davis, Randall D. Dahn & Neil H. Shubin


Comparative analyses of Hox gene expression and regulation in teleost fish and tetrapods support the long-entrenched notion that the distal region of tetrapod limbs, containing the wrist, ankle and digits, is an evolutionary novelty. Data from fossils support the notion that the unique features of tetrapod limbs were assembled over evolutionary time in the paired fins of fish. The challenge in linking developmental and palaeontological approaches has been that developmental data for fins and limbs compare only highly derived teleosts and tetrapods; what is lacking are data from extant taxa that retain greater portions of the fin skeletal morphology considered primitive to all bony fish. Here, we report on the expression and function of genes implicated in the origin of the autopod in a basal actinopterygian, Polyodon spathula. Polyodon exhibits a late-phase, inverted collinear expression of 5' HoxD genes, a pattern of expression long considered a developmental hallmark of the
autopod and shown in tetrapods to be controlled by a 'digit enhancer' region. These data show that aspects of the development of the autopod are primitive to tetrapods and that the origin of digits entailed the redeployment of ancient patterns of gene activity.

Colin McHenry
School of Environmental and Life Sciences (Geology)
University of Newcastle
Callaghan NSW 2308
Tel: +61 2 4921 5404
Fax: + 61 2 4921 6925