Bad times for people with a purely mechanics approach of development. And there isn’t only Stuart Pivar I’m talking about, there are also more modest examples. It seems that anteroposterior polarity is determined by genes! Oh dear, how fucking normal that seems! Well, at least if you are realistic 
Two independent reports published in Science Express three days ago (20071206) discuss determinants of the anteroposterior blastema polarity [head versus tail identity] in planarian regeneration.
The two papers are complementary, with slightly different approaches and discussions. I chosen two observations:
From Petersen & Reddien, fig 2C
(C) Multiple side incisions and tail removal were made to generate a six-headed Smed-βcatenin-1(RNAi) animal (asterisks, supernumerary heads).
Each cut determines a new head when the worm is treated with Smed-βcatenin-1(RNAi) ! You can get a whole community there.
From Gurley KA et al., supporting online material, movie S4(image from movie 4)
Movie S4. Smed- βcatenin-1(RNAi) head fragments regenerate heads. The preexisting head is recognizable because of its normal brown pigmentation. The unpigmented head with noticeable photoreceptors is the newly regenerated tissue and arose where a tail would normally regenerate after amputation.
Well, that’s only a confirmation. But one could, as usually, whine that cutting the worm is a mechanical act and override the fact that Smed- βcatenin-1(RNAi) alters the outcome of the experiment, and point that those are not real animals, etc.
Well, the fun part is that you don’t have to cut the worm to get an extra-head-like structure, treatment with Smed-βcatenin-1(RNAi) is sufficient and the link points to the result/movie and the legend reads :
Smed- βcatenin-1(RNAi) intact animals transform their tails into heads. Several, apparently autonomous protrusions resembling the cephalic margin of normal heads are observed along both the left and right edges of the animal body.
I wonder what mechanical explanation the more modest example of everything mechanical will find 
And this is not an invitation for trolling in the comments.
This post dedicated to three french science bloggers, the more modest example and his favorite neocreationist.
Below the fold “links to” and “abstracts of” the two papers.
Smed-ß-catenin-1 Is Required for Anteroposterior Blastema Polarity in Planarian Regeneration
Christian P. Petersen and Peter W. Reddien
Science [DOI: 10.1126/science.1149943]
Planarian flatworms can regenerate heads at anterior-facing wounds and tails at posterior-facing wounds throughout the body. How this regeneration polarity is specified has been a classic problem for over a century. We identified a planarian gene, Smed-ßcatenin-1, that controls regeneration polarity. Posterior-facing blastemas regenerate a head instead of a tail in Smed-ßcatenin-1(RNAi) animals. Smed-ßcatenin-1 is required after wounding and at any posterior-facing wound for polarity. Additionally, intact Smed-ßcatenin-1(RNAi) animals display anteriorization during tissue turnover. Five Wnt genes and a secreted Frizzled-related Wnt antagonist-like gene are expressed in domains along the anteroposterior axis that reset to new positions during regeneration, suggesting Wnts control polarity through Smed-ßcatenin-1. Our data suggest ß-catenin specifies the posterior character of the anteroposterior axis throughout the Bilateria and specifies regeneration polarity in planarians.
ß-Catenin Defines Head Versus Tail Identity During Planarian Regeneration and Homeostasis
Kyle A. Gurley, Jochen C. Rink, Alejandro Sánchez Alvarado
Science [DOI: 10.1126/science.1150029]
Following amputation, freshwater planarians properly regenerate a head or tail from the resulting anterior or posterior wound. The mechanisms that differentiate anterior from posterior and direct the replacement of the appropriate missing body parts are unknown. Here we report that RNA interference (RNAi) of ß-catenin or dishevelled causes the inappropriate regeneration of a head instead of a tail at posterior amputations. Conversely, RNAi of the ß-catenin antagonist adenomatous polyposis coli (APC) results in the regeneration of a tail at anterior wounds. In addition, the silencing of ß-catenin is sufficient to transform the tail of uncut adult animals into a head. We suggest that ß-catenin functions as a molecular switch to specify and maintain anteroposterior (A/P) identity during regeneration and homeostasis in planarians.
OK, geek will be accepted
Why do you think that variety would be provocative? This would be the case certainly if one think of some deterministic model that exclude variety.
I do agree that planarians are weird animals, but I would like to generalize the concept, as you can find weirdness in every organism; more or less, but that’s the funny part of biology, isn’t it?
I wasn’t taking care of your editorial part, just hoping that you would add to what you have already posted. It’s laziness associated to procrastination that motivated my suggestion.
@ Oldcola :
How funny, i read “Geek” instead of “Greek” …
Do not worry, i have known about Gregor’s work for quite a long time, and i think this is great work, but this is drosophila stuff, a completely different world. Thank you for taking care of the editorial part of my blog, but i already published a little bit of special relativity in my last post; i do not want to bother people with information theory before a while
.
BTW, you are probably aware that everything which is maternally determined is highly variable when comparing different species, which i always found puzzling – for instance bicoid does not event exist in mosquitos. An interesting review about this is Peel, Chipman, Akam 2005 in Nat Rev Genet. In other words and to be a little bit provocative, things are so variable between species prior to “phylotypic stage” that all general models of development are questionable outside of the species it applies to. Besides we will all agree that planarians are really weird animals …
@ Timothée
Well, bpr3 is not yet a quality label, I hope it will become one soon. I like and support this kind of initiatives, and if you find pretentious to use it please remember that you don’t have to. I don’t think that it’s pretentious and I’ll use it when appropriate.
Glad to know that you aren’t surprised by the content of those two reports. As I said: Oh dear, how fucking normal that seems! Have you considered that the more modest example guy wouldn’t accept morphogenes and their gradients as valid explanations of morphogenesis, neither animal’s manipulation, saying that those aren’t real animals? I mean, have you consider that before proposing to describe his theory as a Theory? Maybe not. I hope not anyway.
@ Tom Roud
Hi Tom, you can go on in French, English or Greek. As you like.
I think the more interesting part is the observation made by Gurley et al. that intact animals morphology change by Smed- βcatenin-1(RNAi).
I suspect that this isn’t to be as easy with more complex organisms, I mean a cocktail for RNAi allowing mice to develop two heads
But the finding is in complete opposition with some theory wishing to be purely mechanical, so I thought it was convenient to blog it. As I said, opposing proofs (if any additional is necessary) will accumulate over time.
Anyway, AP axis specification seems to be tightly related with Wnt signaling over a wide spectrum of organism. See Petersen a Reddien discussion and references:
As you mention Wolpert I suppose you read his latest essay, with Kerszberg, published this summer in Cell (doi: 10.1016/j.cell.2007.06.038 ) and you will add something on that in your blog. Two references, from Thomas Gregor et al. are probably quite interesting to include, both from Cell 2007 (dois: 10.1016/j.cell.2007.05.025 and 10.1016/j.cell.2007.05.026).
Noted that I should read them, but this will be for the next holidays period.
I think (hope) each paper will lead you one more step away from the theory.
Il faut commenter en French ou en English ?
J’invite en tous cas les lecteurs néophytes à lire sur mon blog les billets sur le French Flag Model :
http://tomroud.com/2006/05/15/genes-du-developpement-le-french-flag-model/
et sur les brisures de symétrie dans l’embryon :
http://tomroud.com/2007/01/31/brisure-de-symetrie-et-formation-de-lembryon/
pour savoir quelles sont les connaissances /théories actuelles sur le marquage antéro-postérieur…
I go on in English : i find these kind of results fascinating, but i am wondering what is their generality. It is well known that maternal markers are necessary for an embryo to develop, but in those planarians, you can build up an entire organism without them from a couple of cells. So it seems to me that evolution probably changed numerous times the way things auto-organized and developped. One of my favourite example is segmentation : in vertebrates , there is the famous segmentation clock while in drosophila there litterally is one gene per segments. It seems hard to conclude any general results on evolution, even though there may be local directions (for instance the most evolved insects such as dipteras and wasps/bees seem – at least to me- to have developped long germ segmentation while more primitive insects are short germ).
Je n’ai pas le temps de lire les papiers en ce moment. J’ai bien assez de travail avec ceux de mon domaine (plus ceux que je dois lire pour ma culture générale des domaines annexes, plus ceux de domaines qu’on voudrait explorer…)
je trouve ton comportement avec ce petit logo très intéressant. On dirait ta garantie sérieux, comme s’il te fallait absolument montrer à ton lectorat que tu sais faire autre chose que te livrer à ta petit croisade personnelle.
Et puis quand bien même je les aurai lu, mon avis n’a pas grande importance… Je ne suis pas du domaine, je n’ai pas la culture biblio qu’il faut pour en avoir une vision correcte. Je dis juste que la détermination génétique d’une polarité comme celle-là ne me surprend pas le moins du monde (tout en restant, oh sacrilège, ouvert à une approche physique qui expliquerait aussi la diffusion et les gradients).
On peut parler d’évolution après si tu veux, mais tu connais mon avis sur le sujet (et la haute-estime en laquelle je tiens les partisans de cette pseudo-théorie du dessein intelligent…).
Au fait, Timothée, as-tu lu les deux papiers ?
A force de Courteline on croirait que c’est le sujet principal ! Ton avis sur les papiers ?
Salut Timothée,
omission réparée, merci de me l’avoir signalée.
Pour le reste, comme tu as dit ne pas vouloir en discuter, je respecte ton choix, ce que tu ne sembles pas faire.
Bonne journée.
L’un des trois apprécie la dédicace…
Il se permet même d’invoquer Courteline, en passant.
Tu sais, “Passer pour un idiot aux yeux d’un imbécile,…”, et tout ça…
Celui la même te rappelle aussi qu’il est évolutionniste (au cas ou tu tenterais de le faire passer pour un suppôt de Jean Staune), mais qu’il a bouffé pas mal de génétique du développement ces dernières années (alors ce genre de résultat, il se prend à n’en être pas le moins surpris du monde).
Ah, et au fait. Tu as oublié le petit logo “Blogging on peer-reviewed science”, tellement prétentieux… Je remarque avec une certaine satisfaction que tu ne l’a utilisé qu’une fois ou presque, c’est à dire après avoir fait ton martyr à la suite de notre billet… Courteline, vous dis-je…