CMI and Whale Leg Genes

Yeah I think it’s rare to find a gene expressed uniquely in a particular limb. Maybe more specialized tissues found in specific organs like the brain, but there’s always going to be some low levels of activity in other tissues even if the result of spurious transcription.
Then there’s the fact that evolution often occurs by already existing genes being coopted to partake in other functions, so whatever the gene in question “is for”, it’s unlikely to be only for that and nothing else.

No one said it did. But it is present in extant cetaceans and is responsible for the large part of hind limb development, or would be if it were expressed.

Genes that aren’t expressed tend not to last very long, since they accumulate deactivating mutations. Are you sure that Tbx4 isn’t expressed in whales? It would be surprising if it had been maintained in working yet dormant form since the Eocene. More likely that it’s expressed somewhere in the whale at some time.

It’s not expressed in forming hind limbs in whales. It may be expressed in them at other places but that’s not relevant to CMI’s claim.

I’d say it’s highly relevant. Also relevant is the silly idea that the term “leg genes” is meaningful.

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Are you really going to back CMI’s silly idea science says there must be only one “leg gene” which produces only legs? I though you were smarter than that. It’s a fact extant cetaceans retain unexpressed genes for hind limb development. Do you disagree?

This is only a “gotcha” against creationists if tbx4 functions exclusively in forming hind limbs, but since this is unlikely to be the case, the creationists can say “whales never had hind limbs, they only have the tbx4 gene for the other functions it has”.

No. I doubt they’re saying anything quite that silly, and I have no idea where you got the notion that I would back it up.

I don’t disagree so much as that I’m skeptical of any such claim. Genes unexpressed for 50 million years tend not to work any more. Are you sure that Tbx4 is both unexpressed and still functional?

This seems like a fruitless semantical argument. What exactly is meant by the concept of a “leg gene”? Since nobody who has used the word has defined it properly from the beginning, anyone can just disagree with the use of the term and insist that’s not a good definition, that whatever is being said is not what they meant all along, and take turns insisting that the oppositions use or understanding of the term is wrong or counterintuitive. Yawn.

I think you may be misinterpreting that snippet. The argument is that there is purifying selection operating in Tbx4 in cetaceans, which means it has some actual function now, though that function doesn’t include influencing hindlimb development.

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Cetaceans do have hindlimb buds early in development. So what does that mean?

Don’t know. When is Tbx4 expressed in ordinary hindlimb development? Is it expressed in cetacean hindlimb buds? Other places?

It might still might have a function in hindlimb development, since whale embryos show some hints of hindlimb initiation early on, before the hind limb buds are reabsorbed.

What is your explanation for cetaceans born with atavistic hind limbs?

Apparently it’s normally expressed prior to and during the hindlimb bud initiation stage, so since whale embryos get their hindlimb buds all the way to producing a transient apical ectodermal ridge (AER), which normally requires the expression of fgf10, which is normally induced by tbx4, it seems to me fairly likely that tbx4 might be functioning in the whale hindlimb development, as brief as it is. I’m not aware of any expression data showing tbx4 in whale hindlimb buds though.


That it’s not unexpressed.

Mutant mice (mammals) lacking tbx4 have lethal defects in the allantois in addition to the lack of hindlimbs, suggesting that even if it played zero part in hindlimb development in extant whales, it might be maintained in the genome solely for this other essential function.

It also suggests that limb buds might have some brief developmental function in whales, influencing other developing structures that don’t disappear.

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That what isn’t unexpressed?

If so, it would likely be the pelvic structures that remain in whales, as hindlimb bud ablation can lead to defects in the pelvic tissues in chickens:

and pelvic defects have also been observed in tbx4 mutants: