Branchial arches or biomechanical flexion folds?

I heartily agree. There is more than enough molecular evidence to strongly support claims for homology in the pharyngeal arches, their similarity amongst species is clearly not a mere side effect of biomechanical movements.Another relevant paper was brought up a while ago discussing the Gcm2 gene which shows homologies between both the pharyngeal pouches and the organs of calcium homeostasis in teleosts, chondricthyans and tetrapods (Okabe and Graham, 2004).TTFN,WK

Eh? How has any superficial appearance argument been shot down? You aren't saying that they don't appear similar, you simply have an alternative explanation, and one which fails to explain a substantial amount of data. Can you tell us which vertebrates don't? And do these vertebrates still have external pharyngeal structures? They look at four species, which does not include humans, how many do you want? Did you actually re-read the paper or are you just going by some vague memory of the discussion on the previous thread?Bernd has already cited references looking at a number of different moleclar markers. A recent review looking at the development of the pharyngeal arches and noting a number of molecular markers in chick which I would be very surprised if there expression was not conserved Bmp7, Shh and FGF8 (Okabe, et al., 2005). They also discuss conservation of the Tbx1 gene's role in the development of pharyngeal structures in humans and zebrafish. People have shown conserved functions, Okabe and Graham for one,TTFN,WK

No thanks, I look at developing embryos every day. There is no 'unfolding effect', in fact the very opposite is the case for many vertebrates. Can you explain to me how going from a stage 10 to a stage 17 chick embryo could conceivably be described as unfolding? Presumably this means that you can't name a vertebrate without pharyngeal pouches. Well that is a claim that you will have to substantiate, in fact that is the claim this entire thread gives you a very specific opportunity to substantiate. This is true, which rather begs the question of why you seem to think endlessly repeating your 'biomechanical fold' mantra had any relevance to the discussion of homologies in the developing pharyngeal structures. Ahhh, fair enough. Thats a totally ludicrous number you just pulled out of your hat but at least it wasn't evasive. Do you have any idea of the level of work needed to clone and perform in-situs on 100 different vertebrate species? Of course you don't, you don't have the first clue about embryology or developmental biology. Let me assure you that it is a lot of work. cloning and characterising the expression of a homologous gene in a new species, especially one whose genome has not been sequenced, is a considerable task, it could make up a substantial proportion of a Ph.D. thesis, that is at least 3 years of work.As long as we all know your expectations are totally divorced from reality. Yes, there are, but these others domains of expression are also largely conserved. I can't think off-hand of an example where a gene is expressed exclusively in 2 unrelated organs in differing species, if you can then please let me know. Despite the fact that there is a clearly conserved function in calcium homeostasis? Have you re-read the paper Randman, because you are making some fairly fundamental mistakes in your understanding if you have. Yes, that is almost exactly 'the rule'; homologous structures will have homologous genes. The second part is not neccessary, there may be genes which are not shared, it is the ones that are, and their function, that are the basis of claims of homology. Genes that don't appear homologous throughout should fit into expected patterns of descent however, we wouldn't expect to find many gene expression patterns shared by humans and chick but not by humans and mouse or humans and cows.TTFN,WK

Perhaps you should do what Bernd suggests and actually explain in detail what this concept means to you, since your previously explanation was obivously framed with 'poor terminology'.Care to actually support any of this with data? I was looking at the pharyngeal pouches on some mice just today, and they don't look consistent with the degree of tissue deformation one would expect merely from the angle of curvature between the head and trunk.I will link a really seminal paper for you, one of the 500 most cited papers of all time. This is the paper describing the 'Normal Stages' of chick development by Viktor Hamburger and Howard Hamilton, it is the seminal work for chick developmental biology. A reprinted version is available as a PDF here .This goes into exacting detail over the angle of the head relative to the neck and trunk and the development of the pharyngeal arches, called visceral arches in this paper, over the entire time course of chick development. I would specifically direct your attention to plate 8, on page 35 of the PDF, which deals specifically with the development of the visceral arches.Can you use this essentially raw data to show how it is 'biomechanical flexion' that results in the visceral arches rather than any other tissue specific developmental program which may influence something such as cell growth rates, for instance, leading to the outgrowth of these structures. We will overlook for the moment that these structures share very specific anatomical features which do not appear in other nearby regions not subject to this 'biomechanical flexion' we can perhaps deal with when we understand the basis of your ideas a bit more precisely. You can try and make a case for it if you like, but you may be hard pressed. Do they give rise to structures similar to those which develop from the pharyngeal arches? Do they express genes in patterns characteristic of the 'gill pouches' such as Gcm2, Tbx1, rostrally and caudally positioned Fgf8 or BMP7 (or their respective homologues such as Dpp and Branchless)? If not then you may be mistaking a mere surface resemblance of morphology for actual homology of the features in question.TTFN,WK

AS no doubt it is typical of 'creos' to dismiss whole bodies of substantial evidence with an airy wave of the hand and a swift shifting of the goalposts. If you were merely avoiding making conclusions no one would object, instead you are concluding that evolutionary biologists are effectively lying about similarities in the developmental programs of various organisms and their relationships as determined through purely morphological studies or molecular/genetic studies.You always seem to want more details, no matter how many there are, and not even be prepared to tentatively accept the hypothesis which virtually all of the current evidence supports in the meantime. Thats strange, because laymen accept things as being true with barely any data at all most of the time. On an entirely unrelated matter I would really appreciate you finally addressing the actual account from the Sabom book in relation to van Lommel's claims on the NDE thread.TTFN,WK

I would once again recommend to Randman that he sign up with Google to access the Google Book Search.A translated edition of 'The Ontogenetic Basis of Human Anatomy' is available through Google books. For copyright reasons you can't read through the whole book, but searching for specific key words or phrase should allow you to read a significant amount of the relevant passages.TTFN,WK

No, but it certainly removes any argument that the folds are not therefore homologous structures. If the same folds occur in essentially the same place due to the same processes controlled by the same genes then what do you have apart from an excellent example of a homologous structure?TTFN,WK

That isn't much of an argument though. What molecular data we have supports the hypothesis which was developed before molecular data became available, which is pretty surprising if the structres are unrelated as you claim. But as you are now putting them forward this is a meaningless distinction. Its like saying that the structures are composed of cells and thats all they are, cells.Both Bernd and I have put forward som pretty specific posts addressing your understanding of 'biomechanical folds' and their origins, perhaps you could devote some time to addressing those posts.TTFN,WK

Bleating that we don't know enough to say anything, in the face of all the evidence that we do have, is also pretty poor.Your attitude might be more convincing if you actually addressed any of the data rather than simply demanding more once some data is presented to you.TTFN,WKThis message has been edited by Wounded King, 16-Dec-2005 07:53 PM

No they aren't. You dont need 100 other in situs in order to accept my claim that there are specific genes with conserved patterns of expression in the pharyngeal arches consistent with a hypothesis of common descent, all you need is to look at the data in respect of those particular genes.TTFN,WK

Given that the rate of growth is governed by gene exression this appears to be conceding Bernd's position rather than arguing against it.In support of your contention, that the embryo is not neccessarily curved when the first pharyngeal/visceral arches appear, the same section says... So the embryo has begun to curve by day 28.There is a higher detail image of day 29 on the side of the web page where it says 'picture' on the second page covering the fifth week. This highlights the developing arches and the begining of the head/neck flexion is clearer. The day 30 embryo picture gives a very clear view of the developing arches and I really think it would be hard to make a case for such protuberant structures being the result of the flexion of the head and neck. These folds are almost as wide again as the neck itself. Because other than Blechshmidt's word for it there is no data to base a conclusion that tension is the source.This all seems rather academic if you are prepared to concede, as you stated earlier, that it is simply a question of nomenclature for you and that similar patterns of gene expression and gene function in no way affect your preference for 'biomechanical flexion folds' as the correct term and that the terminology does not have any relevance to the question of homology.If you are prepared to accept that these genes can control the development leading to the formation of the 'biomechanical flexion fold' then you fundamentally diagree with Blechshmidt's approach to embryology, even if you prefer his terminology in this instance.TTFN,WKThis message has been edited by Wounded King, 19-Dec-2005 11:15 PMThis message has been edited by Wounded King, 20-Dec-2005 12:05 PM

Please see my above post.This is not the case, the embryogenesis web page states that flexures begin to impart the 'c' shape on day 28.You can even see on the day 29 embryo that the head has begun to slightly curve around towards the heart loop.By day 30 the flexions are simply very pronounced.TTFN,WK