miércoles, abril 09, 2008

Deep-time dinosaur phyloepigenetics

West-Eberhardt repeatedly mentions in her book the case of the two-legged goat, born with no forelimbs, that learned to walk bipedally, and developed several hip traits, bipedal "adaptations". Certainly, to understand the development of these hip adaptations, we would be ill-advised to concentrate on the molecular mechanisms underlying cell-death in the embryonic forelimb, who have only indirect "causation", far removed from the actual mechanisms involved in the largely non-genetic influence on this trait. Comprehension can only come from observing the higher-level interactions and the direct developmental effects of emerging epigenetic interactions. It is interesting to think just how many aspects of our phenotype are, like this, only very indirectly related to the genotype (from a systems view, a mutation can be nothing but a "trigger").

I still remember when it dawned upon me in 2001; ALL vertebrate limbs, universally, grow longer with mechanical stimulation and use. I thought about early theropod dinosaur's reduced forelimbs. Even if bipedality were the result of some mutation enlarging legs or shortening arms (by the way: probably not, but behavioral), the arms would receive less mechanical stimulation when relieved from locomotion, whereas the legs now bear the entire body's weight. There is no way this is not going to decrease growth in the arm , increase it in the leg, and enhance forelimb-hindlimb size differences. Enter here the field of phyloepigenetics, which we previously named on this blog; the recopilation of cases of epigenetic explanation of evolutionary differences. This time I present a case of a deep-time epigenetic apomorphy: The fibular crest of the tibia of Theropod dinosaurs and their living representatives, the birds. Here are some photographs of this rectangular crest, that extends from the large tibia onto the thinner fibula (both elements of the "shank" or zeugopod). Since this is a trait of all theropods, the fibular crest must be at least 230 million years old. Two different birds species illustrating the fibular crest of the tibia, upwards, rectangle-shaped (from Müller and Streicher 1989)


The fibular crest is an apomorphy of theropod dinosaurs. On the left, a theropod; right, a non-theropod dinosaur (Müller and Streicher 1989)

It turns out that this crest is a sesamoid bone, that is, a bone that develops within connective tissue as a result of mechanical stimulation; first cartilage is formed in the stimulated region; this cartilage may thereafter ossify originating the sesamoid bone (The chicken patella and human knee-caps are sesamoid bones that develop from within tendons). We can say that the sesamoid bones are to connective tissue what callosities are to epidermis. All of this with plenty of experimental confirmation, such as mechanical forces in abnormal places, etc.
The fibular crest of the chicken develops as a cartilage in the narrow space between the tibia and the fibula. In theropod evolution the fibula became thinner; this may be the reason why muscles that in other reptiles pull the leg backwards and project exclusively onto the fibula, in the chicken embryo also hit the connective tissue between the fibula and the tibia, provoking the development of a new sesamoid cartilage. This cartilage therafter ossifies into the crest, tightly connecting the tibia and the fibula in the adult. This crest in birds is certainly "adaptive" since it is fundamental to have a functional leg (because the fibula in birds no longer connects distally to the foot!) Yet against the adaptationist intuitions of old-style lamarckism, or of reformed "epigenetic darwinists", nothing in all those millions of years has produced a mechanism for developing this bone without movement. It still relies on the same "good old" mechanical stimulation. If the embryo is paralized , for instance, with a postsynaptic blocker, the cartilage of the fibular crest is no longer formed. The unavoidable effects of higher-level interactions have remained the basic mechanism by which this trait is developed .


From Müller 2003

The authors of the study share some wisdom with us:

"The de novo formation of skeletal elements addresses an important but largely neglected issue in evolutionary theory: the origination of morphological novelty. This generative problem of organismal evolution is sidestepped in traditional accounts that focus on the gradual variation and adaptation of characters and calculate their population genetic underpinnings. The studied characters are usually taken as given, and their origination is tacitly assumed to be based on the same mechanisms as their variation and adaptation. There is growing awareness that this does not need to be the case and that innovation should be treated as a distinct problem of
evolution"

Müller GB and Streicher J. 1989. Ontogeny of the syndesmosis tibiofibularis and the evolution of the bird hindlimb: a caenogenetic feature triggers phenotypic novelty. Anat Embryol. 179: 327-339

Müller GB. 2003. Embryonic motility: environmental influences and
evolutionary innovation. Evol & Dev 5:1, 56–60

15 comentarios:

Cristian dijo...

Wena, son interesantes estos casos por que, si es posible que la postura sea conservada transgeneracionalmente , principalmente por el modo de vida y conducta, un desacople del modo de vida podria revertir o mover este rasgo en otra direccion.

Lo mejor es que estos casos es que se pueden encontrar en caleta de organismos distintos y relacionados con monton de rasgos.

Sanders dijo...

q weno q le gustó, friendly
Lo del bipedismo en el origen de los dinosaurios da para su propio post; obviamente, un origen faculativo, como lo es la conducta del correr bípedo en algunas lagartijas.

Cristian dijo...

hoy estuve en un seminario sobre evolucion de orquideas con relacion a la interaccion con los polinizadores
y resulta que las orquideas que simulan los olores y las formas de hembras de abejas de distintas especies han diversificado super rapido con super poco cambio genetico, en comparacion con las otras orquideas, es increible, me voy a conseguir unos cuantos paipers para hacer un post de eso
a ver q tal...

Chico dijo...

O caso é ótimo porque não foi "internalizado", o que mostra que não precisa internalizar! Ademais, se tivesse sido internalizado, o que isto significaria? Que perdeu plasticidade, simplesmente. Não significaria que aqueles indivíduos que tinham genes que os livravam da dependencia de um determinado estimulo ambiental foram selecionados e blá, blá, blá...

Chico dijo...
Este comentario ha sido eliminado por el autor.
Chico dijo...

West-Eberhard fala da relação entre bipedalismo e cambios na caixa toráxica. Por falta de pressão exercida pelas patas dianteiras elas teriam sido modificadas, como em hominidea. Aconteceu algo similar em dinossauros?

Chico dijo...

O conceito de "acomodação fenotípica" somente faz sentido desde uma perspectiva "filogenotípica". É preformacionista. Um câmbio inicial só necessita ser acomadado se suponho que este câmbio ocorreu em um gene para o fenótipo e que outros elementos precisam se ajustar. Assim, a acomodação fenotípica é apresentada como uma propriedade extra que dá evolvabilidade ao sistema.
O fenômeno se torna mais parcimonioso desde uma perspectiva filoepigenética, pois uma mutação é um cambio na estrutura que tem consequencias para o processo desenvolvimental. Assim, assimilação fenotíca é um conceito redundante e desnecessário. Como diria o velho Ockham: "entia non sunt multiplicanda praeter necessitatem".

Sanders dijo...

Es la principal diferencia entre la visión de la deriva natural y los "darwinistas epigenéticos". Ellos buscan a como de lugar llevar la discusión a los genes y su selección. La visión de sistemas sabe muy bien que la cosa va bastante más allá del genotipo y de toda la estructura inicial; Las interacciones y propiedades emergentes de un nivel más macroscópico, y la lógica propia de ellas, es un importante determinate del resultado fenotípico y de la herencia. Mu gusta mucho su caráter de "inescapable" porque en este marco es que se insertan las mutaciones, y la lógica de estas interacciones de alto nivel es la que determina si un nuevo fenotipo es viable o no. El genotipo sigue al fenotipo...

Sanders dijo...

La pérdida del extremo distal de la fíbula es un ejemplo de un cambio que sólo puede ocurrir en conservación de la adaptación, gracias al desarrollo por movimiento de la cresta fibular.

Cristian dijo...

todo ese proceso que podemos llamar conservacion de la adaptacion tiene un componente epigenetico y contexto dependiente super clarito.
Por ejemplo, tomemos el caso de los polinizadores terriblemente "fit" para determinadas plantas como podria ser la abeja de la miel.
Bueno resulta que en algunas plantas, donde ademas de abejitas, llegan abejorros a las flores, estos no pueden entrar en las flores y frequentemente hacen perforaciones a un costado para robar el nectas. Bueno, pues adivinen que, cuando pasa esto las "super mutualistas" abejas de miel dejan de "prestar servicios"* e polinizacion a las flores y se dedican a chupar nectar desde las perforaciones que hacen los abejoros. Plop!


Cristian



(* Termino merms usado en ecol exportado del lenguaje economico)

Sanders dijo...
Este comentario ha sido eliminado por el autor.
Sanders dijo...

lo que dices del mimetismo de la orquídea me recuerda que el mimetismo entre especies de mariposas con buen y mal sabor, tambien, según estudios de cruzas, sólo está sujeto a la influencia de unos pocos genes.

Chico dijo...

Simplesmente desnecessário:

"This position does not deny than mutation and selection are involved in innovation but agues that their effects are indirect and that the major role of these processes is ensuring inheritance and variation of the phenotypic characters that originate primarily from epigenetic properties of developmental systems undergoing evolutionary modifications."

Müller GB. 2003. Embryonic motility: environmental influences and
evolutionary innovation. Evol & Dev 5:1, 56–60

Roberto E. Yury Yáñez dijo...

estupendo ejemplo

Sanders dijo...
Este comentario ha sido eliminado por el autor.