The early evolution of the - Facultad de Ciencias ... · PDF fileThe early evolution of the...
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The early evolution of the neural crest
The “transference to ectoderm” hypothesis
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The neural crest is a population of cells at the lateral borders of the neural plate - dorsal neural tube that undergo a epithelial to mesenchymal transition and develop into a variety of tissues
The neural crest that develops into skull bones is derived from the rhombomeres of the embryonic hindbrain
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The discovery of the neural crest by Horstadius challenged the traditional distinction previosly set up by Pander (1817) of the three germ layers.
It also set apart the skull from the rest of the skeleton, which questioned the “vertebral theory of the skull”
Goethe Owen
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Primitive jawless fish hardly have anything like vertebrae, but already possess a highly complex skull,
Archetypical thinking in general fell out of favor during most of the 20th century and the vertebral theory of the skull was generally dismissed.
Thomas H. Huxley had said the vertebral theory was absurd because the inverse (vertebrae as several small, simplified “skulls”) was equally plausible.
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In the 1980’s, Northcutt and Gans proposed THE NEW HEAD HYPOTHESIS on the origin of vertebrates.
The closest living relatives of the vertebrates are considered practically ‘headless”. They are sedentary filter feeders. Amphioxus lives buried in sand, and only swims to escape.
Branchiostoma (Amphioxus)
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Gans and Northcutt concluded that the neural crest was a key vertebrate innovation: important differences between the “headless” chordates and primitive fish were in crest- derived structures.
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Northcutt and Gans (1983) proposed that the neural crest evolved in the transition from filter feeding to an active, predatory lifestyle, elaborating on sensory capacities.
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The proposed absence of a neural crest in Amphioxus became conventional wisdom. However, in 1996, Holland et al. showed that a population of cells at the border of the neural plate of Amphioxus expressed Dll, a gene known to be expressed in the neural crest of vertebrates.
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Dll-expressing cells in Amphioxus extend over the neural plate, meeting at midline, roofing over the descending neural tube. They thereafter become epidermis.
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Mayor, Young and Vargas (1999) emphasized that in Amphioxus, iterative sensory “nerves” project from the dorsal neural tube. If something similar to an ectodermal, neurogenic-melanocitogenic crest was already in place in Amphioxus, this indicated that the mesoderm-like, chondrogenic crest was secondarily acquired in evolution.
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Wada (2001) emphasized the secondary acquisition of a capacity of migration into the embryo of the crest-like cells at the border of the neural plate.
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In Amphioxus, some crest genes are expressed towards the neural plate border. Others however are not expressed in the ectoderm, but are expressed in the mesoderm (Meulemans and Bronner Fraser 2004. More recently, also Tbx1/10, Mahadevan et al. 2004)
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Yu et al. (2002): FoxD3 is expressed in the neural crest of vertebrates, but it is not expressed in the ectoderm of amphioxus. Yu et al. proposed this to be a mesodermal trait acquired by the neural crest involving co-option of the expression of this gene
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Unlike vertebrates, the neural tube of Amphioxus possesses somites along its entire extension and shows no rhombomere-like morphological segmentation.
In vertebrates, morphological segmentation of the mesoderm is absent in the cephalic region, where rhombomere segmentation begins.
Several similarites exist between the cephalic ectomesenchyme and somitic sclerotomes
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Transference to ectoderm hypothesis. A mesodermal developmental system was transferred from the mesoderm to the ectoderm, bringing along chondrogenesis and hindbrain segmentation Redundancy would have allowed eventual loss of segmented mesoderm in the cephalic region.
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Gene expression patterns in Amphioxus allow to identify the putative homolog of the vertebrate hindbrain. The expression of islet revealed neural hindbrain segments in a 1 to 1 correspondence to adjacent somites (Jackman et al 2000).
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Jackman and Kimmel 2002
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Mazet and Shimeld (2002) “transfer of mechanisms regulating neural segmentation from vertical induction by underlying segmented mesoderm to horizontal induction by graded retinoic acid signalling”
“segmentation of vertebrate head mesoderm would no longer have been required, paving the way for the evolution of the unsegmented head mesoderm seen in living vertebrates”
Amphioxus
Vertebrate
Mazet and Shimeld 2002
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Bardet et al. (2005): “originally, the program for anterior segmentation in the protochordate ancestors of the vertebrates resided in the developing axial mesoderm(…) during early vertebrate evolution, this segmentation program was transferred to and controlled by the neural tube.”
ERR: estrogen related receptors
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Recently, the genes Id and Tbx1/10 of amphioxus have been shown to be expressed in the mesoderm-derived gill bars of Amphioxus.The gill bars of Amphioxus are made of a cartilage with low-collagen content, similar to that of primitive, jawless vertebrates.
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Does Amphioxus have anything like a sclerotome?
A Sclerocoel can be found that outpockets from the ventral somites of Amphioxus and moves around the notochord to develop into connective tissue. Ventral somites also express Tbx1/10
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Meulemans et al. (2003): “Amphioxus Id transcripts are observed in mesoderm lining the coelom of the forming pharyngeal gill bars. These cells originate as ventral outpocketings of the somites, and migrate between the pharyngeal endoderm and overlying ectoderm into the nascent gill bars (…) these cells lie in a position later occupied by cartilagenous skeletal rods
Gill bars of amphioxus probably derive from a sclerocoel-like, ventral outpocketing of the anteriormost somites
so….
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WE ARE BACK TO WHERE WE STARTED…
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Milokunmingia y Haykouichthys, del cámbrico inferior, revelan la adquisición de arcos branquiales pareados, ojos pares, cápsulas óticas, y una musculatura axial más compleja en forma de “z”, en tanto que en Amphioxus, tiene forma de “v”.
Haikouichthys
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Millokunmingia
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Millokunmingia
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Las Myxines (hagfishes) y Lampreas actuales vivientes son peces basales, sin mandíbulas ocluyentes, que divergieron hacia los comienzos de la evolución de los vertebrados. Ambos poseen una lengua especializada que permite raspar e ingresar comida, branquias en forma de bolsillo, y una piel sin escamas Los demás peces vivientes todos poseen mandíbulas ocluyentes. Descienden de formas fósiles acorazadas sin mandíbulas, conocidos como “Ostracodermos”. Los primeros peces con mandíbulas ocluyentes, los Placodermos, eran acorazados.
One of the recently published vertebrate tree topologies that entails cyclostome paraphyly (after Gess et al., 2007; possible position of myllokunmingiids modified according to Janvier, 2003); the distribution of the taxa through time is indicated by bold lines in the time scale to the left. Major synapomor- phies at nodes: 1, neural crests, epidermal placodes, fin radials; 2, dermoskeleton in mouth and pharynx; 3, extensive dermoskeleton over the entire body; 4, extensive lateral-line system enclosed in grooves and canals, vertical semicircular canals forming loops, cerebellum; 5, endoskeleton lined with calcified cartilage or perichondral bone; 6, pectoral fins in postbranchial position; 7, jaws. (Illustrations of respective taxa after Janvier, 2007b).
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Myxinoidea (anguilas babosas)
Viven enterrados en la arena y se alimentan de carroña e invertebrados No poseen línea lateral (Estructura longitudinal provista de neuronas sensoriales electroreceptoras y mecanoreceptoras). El ojo es simple y no tiene musculatura. El cerebro carece de téctum óptico, cerebelo y nervios óculo motores. Posee un oído interno simple con un solo canal semicircular. Lóbulos olfatorios del cerebro bien desarrollados.
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Ciclostomata: Petromyzontidae
Poseen dientes queratinosos en el arco branquial mas anterior (arco 1). Sin embargo este arco branquial no ocluye (cierra) la boca. Se debate si debe considerarse que poseen “mandibulas”. Las lampreas, poseen pequeñas estructuras vertebrales cartilaginosas, musculatura de las aletas, control del latido cardiaco via el nervio vago, ojos provistos de lente y de musculatura extrínseca, verdaderos linfocitos, linea lateral, adenohypófisis compleja Son anádromos
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Existe una polémica sin resolver sobre si los lampreas son más cercanos a los peces con mandíbula o a los myxines.
Evidencia molecular
Evidencia morfológica
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Mientras que las myxines se desarrollan de maneras directa, las lampreas tiene una forma larval, la larva amoceta, que vive enterrada y filtra, muy similar a Amphioxus
Lamprea de río
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Los conodontos, conocidos desde el cámbrico por sus dientes fósiles, son más cercanos a los gnathostomos que las myxines y lampreas. Presentan dientes con esmalte (enamel) y grandes ojos con musculatura asociada. Importantemente, los dientes presentan evidencia de desgaste.
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Los dientes en conodontos se encuentran en la linea media (lado isquierdo contra derecho en vez de “arriba” contra “abajo”). La relación de estos dientes con los arcos branquiales cartilaginosos sólo es inferida; se cree que el primer arco puede haber sostenido los elementos más anteriores. Si bien presenta dientes, la arquitectura general no puede homologarse facilmente a la de una verdadera mandíbula. No se les considera como verdaderos mandibulados (a diferencia de los Gnathostomados actuales )
anterior
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Antiguamnete, Myxines, Lampreas y Ostracodermos se designaban como “agnata” debido a la ausencia de mandibulas. Actualmente se habla de los ostracodermos como “stem gnathostomes” (mandibulados troncales) para recalcar que estos son mas cercanos a los peces mandibulados modernos (gnatostomos) que a las formas vivientes sin mandibula (lampreas y myxines) Los Ostracodermos son más cercanos a los gnatostomos que a los ciclostomos, entre oras cosas debido a que comparten con gnatostomos la capacidad de producir distintos tipos de hueso
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Grupos de Ostracodermos
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En Ciclostomos y Ostracodermos, Se destaca la presencia de un ducto nasohypofisial que se comunica con las narinas y a partir del cual se desarrolla la hipófisis.
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Anáspidos Ostracodermos de agua dulce. Cola dirigida hacia abajo. Algunos anáspidos poseen escamas, otros son “desnudos” Sus fósiles se han recuperado en Europa y América del Norte. Jamoytius comparate con las lampreas un anillo circular alrededor de la boca, ojos grandes y arcos branquiales similares. Poseen aletas laterales pareadas en forma de cinta, detrás de la línea de aperturas branquiales
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Arandáspidos Placa dorsal y ventral en la cabeza y están separadas por varias placas branquiales. Placas dermales compuestas de diminutas unidades poligonales que se añadían a los bordes de las placas (incapaces de crecer por si mismas). Esta podría representar una forma primitiva de crecimiento ya que está hecho de hueso sin células (aspidina). Se han hallado en América del Sur, Australia.
Astraspis
Sacabambaspis
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Heterostraci Son de aguas costeras semi saladas y algunos depósitos posiblemente de agua dulce. Se han hallado en: América del Norte, Europa, Siberia. Aparentemente carecen de endoesqueleto Esqueleto dérmico bien desarrollado, cubre cabeza, cuerpo y cola. Placa dorsal y ventral en la cabeza. Cada placa está hecha de un tejido de hueso sin células llamado Aspidina (también presente en escamas de otros ostracodermos, incluyendo anáspidos y galeáspidos). Poseen una sola apertura branquial
Doryaspis
Drepanaspis
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Telodontos No poseen placas y tienen escamas pequeñas de diversa morfología. Existen dos grupos de telodontos: aplanados con cola hacia abajo y profundos con la cola bífida. Se han hallado escamas en todo el mundo.
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Galeáspidos Poseen la apertura nasohipofisial justo frente de los ojos, como en mixinoideos, y al igual que ellos, comunica con la faringe. Aperturas branquiales por la superficie ventral Sus fósiles se han encontrado en China y Vietnam.
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Aletas pareadas Cola doblada hacia arriba, flexible Campos sensoriales en los márgenes de la coraza cefálica y justo detrás de los ojos, asociados a la línea lateral Poseen una sola gran coraza dorsal. La boca y las aperturas braquiales se disponen ventralmente. Se han hallado en: América del Norte, Europa, Siberia y Asia Central
Osteostraci y Pituriaspidos: el origen de las aletas pares
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Pituriáspidos No tienen ducto nasohipofisial. Se desconoce la ubicación de los narinas. Se han hallado en Australia.