Quick comments on Odontochelys

Odontochelys is the new basalmost triassic turtle, with a plesiomorphic ("primitive") presence of teeth (hence the name) and a most remarkable transitional aspect: It has a perfectly formed plastron yet no carapace. Odontochelys only presents the dorsal midline of the neural plates of the carapace, that are close to the neural spines but remain separate (which is not the case in more derived turtles).
Lets briefly recall what some evil saltationists (paleos and evo-devo's) have said about this. I remember Bob Bakker's book "The dinosaur heresies" (1986). There, he mentions the fact that in turtles the pectoral girdle is actually under the ribs, and points out that this qualitative aspect is hard to imagine to have occurred in more than a single step.
What can development tell us of all this? First, the plastron is derived from the neural crest, unlike the carapace, which is derived from a mixture of dermal bone and ribs. Second, the plastron ossifies before the carapace. Odontochelys reveals that this embryological and temporal separation also reflects a phylogenetic sequence. Scott Gilbert (yes, the book guy) points out that in turtles, the ribs have shifted dorsally, to developing within the dermis, a unique trait within amniotes. If an embryonic rib is experimentally inserted in the dermis of a chicken embryo, the result is ossification of the dermis surrounding the rib. Because of this paracrine effect on the dermis, Gilbert hypothesized that a single-step shift of the ribs to the dermis could have induced the origin of a well-formed carapace.

Some regard this hypothesis as incompatible with the hypothesis that an exoskeleton of separate dermal bones preceded the origin of the carapace. This incompatibility in my opinion is not quite so; dermal bones could have existed or not previous to the shift of the ribs closer to the dermis; the shift could have led to a single carapace. Critics to Gilbert's hypothesis have pointed out that in the carapace of the early (very fragmentary) triassic turtle Chinlechelys, ribs are not "immersed" in the carapace but run immediately below the surface of the dermal plates. However it is possible that this was sufficient for the paracrine effect leading to a single carapace "shell". Unfortunately we cannot know if the pectoral girdle was already under the ribs in Chinlechelys.

Even admitting the possibility of previous ostederms, Odontochelys is certainly something unexpected from the more "gradualist" perspective, which was "hoping" early stages of dermal armor would go back to remote Pareiasaur-like ancestors (see figure below). The full plastron of Odontochelys is pretty derived, yet this species has no "coat" of osteoderms.

From Joyce et al 2007. Odontochelys indicates that "coats" of osteoderms as in a) and b) did not precede the origin of the plastron

Odontochelys is consistent with the notion that a shift in the position of the ribs to greater proximity to the dermis could have induced a"radical" extension of dermal bone, from mere dorsal midline of neural plates, to a full carapace; as a third step, superficially attached ribs and dermal bones, became fully integrated with the dermal bone conforming the "true" carapace.

References:

Li C, Wu X-C, Rieppel O, Wang L-T, Zhao L-J (2008) An ancestral turtle from the Late Triassic of southwestern China. Nature 456: 497-501

Joyce WG, SG Lucas, TM Scheyer, AB Heckert, AP Hunt (2008). A thin-shelled reptile from the Late Triassic of North America and the origin of the turtle shell Proceedings of the Royal Society B DOI: 10.1098/rspb.2008.1196

Cebra-Thomas JA, Betters E, Yin M, Plafkin C, McDow K, Gilbert SF. 2007 Evidence that a late-emerging population of trunk neural crest cells forms the plastron bones in the turtle Trachemys scripta. Evol Dev. 9(3):267-77.

Cebra-Thomas J, Tan F, Sistla S, Estes E, Bender G, Kim C, Riccio P, Gilbert SF. 2005. How the turtle forms its shell: a paracrine hypothesis of carapace formation. J Exp Zoolog B Mol Dev Evol. Nov 15;304(6):558-69.

Paedomorphosis vs Adaptacionismo

Muchos habrán escuchado sobre uno de los rasgos más llamativos que presenta la paedomorphosis en la evolución de los seres humanos; si observamos un feto de 7 meses del chimpancé, vemos que presenta una distribución del pelaje como la de un ser humano, con el pelo concentrado sobre la cabeza. La verdad es que había escuchado este dato e incluso había visto un dibujo de la cabeza pero la verdad es que no existe reemplazo para una foto. Finalmente he encontrado una en la web, que originalmente fue publicada en el libro "The life of primates" de Adolph Schultz, (1969). Esto me ha permitido salir de algunas dudas. Puntualmente, hay pelo en la barbilla y los labios, como en el adulto humano; sin embargo, a diferencia del adulto humano, no hay pelo sobre los genitales. Aún me queda la pregunta de si hay pelo o no bajo los brazos. Otra novedad es que se observan cejas muy claramente demarcadas, que luego desaparecerán completamente (ver imagen a la izquierda). Notable!
Es curioso que, pese a que este dato se conoce desde hace ya bastante tiempo, aún se discuten diferentes hipótesis adaptacionistas para explicar la concentración del pelo sobre la cabeza del ser humano; la más popular es que esto evolucionó por selección para correr bajo el sol por largas distancias, lo cual, supuestamnete, le permitiría tener un cuerpo "refrescado" a la vez que se protegía al maratonista de la Sabana de la intensidad de los rayos del sol. Otras explicaciones citaban una supuesta menor vulnerabilidad a los parásitos, con selección sexual para explicar la permanencia del pelo sólo en la cabeza. Ninguna de estas hipóestis parece enterarse del hecho de que esta distribución se originó en la etapa fetal, donde claramente ninguno de estos factores fue relevante. Lo más gracioso es que, al confrontar a los adaptacionistas con este hecho (en discusiones en los blogs), tristemente no logran concederle ninguna relevancia; es una mera "coincidencia". No hay mejor ejemplo de cómo la ideología produce una cínica miopia intelectual incluso con los datos más evidentes. Hay quienes no se enterarían de la realidad aún si ésta les vomitase en los zapatos.

¡Ya era hora!

Ya hemos enfatizado, en este blog de afilado criterio, los graves problemas en la moda decerebrada de la detección de "selección positiva". Una de las críticas que suelo hacer ante mis colegas y en otros blogs donde dejo comenatrios es la facilidad con que se contentan con asumir cambios "ventajosos" sin hacer ninguna conección con el fenotipo, basados sólo en el test estadístico.

¿Qué pasa cuando controlamos adecuadamente si hay fenotipo asociado a las sustituciones nucleotídicas "positivamente seleccionadas"?

Un comentario recientemente publicado en PNAS menciona un interesante estudio reciente de rhodopsinas que absorben diferentes longitudes de onda en muchas diferentes especies de peces que viven a diferentes profundidades o tienen diferentes conductas relacionadas con la luz. Se caracterizan muchos cambios no sinónimos que afectan la longitud de onda que absorben las rhodopsinas y que se corresponden con ele stilo de vida de cada especie de pez (es decir son "adaptativas"; véase este post previo sobre este tema ). Resulta una instancia idónea para poner a prueba de manera biológica la famosa "prueba estadística" de la selección positiva. Nos dice Austin Hughes:

"In fact, the results showed that the codon-based methods were 100% off-target. When Bayesian methods were applied to a set of closely related rhodopsin sequences, eight sites were identified as ‘‘positively selected.’’ Yet not one of these sites was among the 12 sites known to be involved in adaptive changes in rhodopsin sensitivity. Moreover, amino acid changes at these sites were shown experimentally to have no effect and thus almost certainly to lack any adaptive significance.These results support the theoretical prediction that, because of the faulty logic in their underlying assumptions, codon-based focus mainly on statistical artifacts rather than true cases of positive selection"

Hughes concluye:

"In recent years the literature of evolutionary biology has been glutted with extravagant claims of positive selection on the basis of computational analyses alone, including both codon-based methods and other questionable methods such as the McDonald-Kreitman test. This vast outpouring of pseudo-Darwinian hype has been genuinely harmful to the credibilit y of evolutionary biology as a science. It is to be hoped that the work of Yokoyama et al. will help put an end to these distressing tendencies."

Ya era hora!!

El comentario de Hughes está en el grupo yahoo decenio (PNAS 2008Hughes). Tienen que leerselo...acá sólo puse una fracción de sus demoledoras críticas.

Refs

Austin L. Hughes. The origin of adaptive phenotypes. PNAS 2008 105:13193-13194

Yokoyama S et al. (2008) Elucidation of phenotypic adaptations: Molecular analyses of dim-light vision proteins in vertebrates. Proc Natl Acad Sci USA 105:13480–13485.