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Embryological development and evolution


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"Evo-devo" attempts to explain the creative side of evolution - such as the origin of body plans - by looking at how embryological development (including such things as homeobox genes) affect evolution.


“The Origin of Animal Body Plans : A Study in Evolutionary Developmental Biology” Amazon.com: The Origin of Animal Body Plans: A Study in Evolutionary Developmental Biology: Wallace Arthur: Books http://www.amazon.com/exec/obidos/tg/detail/-/0521779286/qid=1119150008/sr=1-10/ref=sr_1_10/102-7361556-1866554?v=glance&s=books

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One step to different "body plans" explained embryologically is the positioning of nerve cords in animals. As a rule, invertebrates have a ventral ("belly") nerve cord whereas vertebrates have their nerve cords running along their dorsal ("back") side. The difference is due to the different expression patterns of at least 3 sets of homologous proteins between vertebrates and invertebrates.


"In fly embryos Dpp is expressed dorsally and Sog ventrally; in frog embryos BMPs are expressed ventrally and chordin dorsally. This inversion of the expression patterns of these homologous proteins along the dorsoventral axis between vertebrates and invertebrates parallels the inversion of tissue types that form along this axis during embryogenesis. For instance, in vertebrates the nervous system forms dorsally and in invertebrates it forms ventrally." (Molecular Cell Biology: Fourth Edition, Lodish, Berk, Zipursky, Matsudaira, Baltimore, & Darnell, W. H. Freeman & Co., 2000, p1011)


"FIGURE 23-9 The proteins controlling the development of the dorsoventral axis are conserved between vertebrates and invertebrates, but their expression patterns along the dorsoventral axis are reversed. Dpp and BMP4 are TGF-[beta] homologs expressed dorsally and ventrally, respectively, in Drosophila and Xenopus. Their antagonists are Sog, expressed ventrally in flies, and chordin, expressed dorsally in frogs." (Molecular Cell Biology: Fourth Edition, Lodish, Berk, Zipursky, Matsudaira, Baltimore, & Darnell, W. H. Freeman & Co., 2000, p1011)


"The combination of genetic and biochemical data in the fruit fly, frog, and zebrafish that we have considered indicate that a highly conserved pathway controls cell fates along the dorsoventral axis in vertebrate and invertebrate embryos. We can summarize the pathway as follows:


Fruit fly:

Protease: Tolloid

Inhibitor: Sog

Signal: Dpp



Protease: Xolloid

Inhibitor: Chordin

Signal: BMP2/BMP4



Protease: Minifin

Inhibitor: Chordino

Signal: Swirl


As noted earlier, the vertebrate TGF[beta] homologs (e.g., BMP2/BMP4) are expressed ventrally and chordin and its homologs are expressed dorsally, whereas the opposite expression pattern is found in invertebrates. Likewise, the expression patterns of the homologous proteases that inactive chordin and Sog are reversed, permitting maximum BMP signaling in the ventral-most regions of vertebrate embryos and maximal Dpp signaling in the dorsal-most regions of invertebrate embryos." (Molecular Cell Biology: Fourth Edition, Lodish, Berk, Zipursky, Matsudaira, Baltimore, & Darnell, W. H. Freeman & Co., 2000, p1012)

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Another step to different "body plans" explained embryologically - via homeobox genes - is the length of the body along which ribs forms.


”The study, published July 18 in the journal Science, sheds new light on “how we are put together and how we are different from other life forms,” Capecchi says. “You see, for example, a pigeon with five pairs of ribs. We have 12 pairs of ribs. The question is how do you make different body plans and how did the ability to have different body plans evolve?”


During the evolution of vertebrate animals, ancestral genes gained new uses, including suppressing development of extra ribs, which allowed the animals to move with greater flexibility to hunt for prey or escape predators.



Wellik says genes that suppressed ribs and allowed development of sacral vertebrae – those where the pelvis attaches to the spine – made it possible for mammal reproduction to evolve. Mammals deliver newborns through the pelvis.



Capecchi and Wellik studied two groups of Hox genes – Hox10 and Hox11 – and their role in skeletal development in the mouse and, by implication, in humans and other mammals.

Any animal with a spine has different kinds of vertebrae making up the spine. Mice have seven cervical or neck vertebra; 13 thoracic or chest vertebrae, to which the ribs are attached; six lumbar or lower back vertebrae; four sacral vertebrae, to which the pelvis is attached; and varying number of caudal vertebrae in the tail. (Humans have seven neck vertebrae, 12 chest vertebrae with ribs, five lower back vertebrae, five pelvic vertebrae and three to five tailbone vertebrae.


Yet some early vertebrates – primitive fish, amphibians and dinosaurs – had ribs growing from vertebrae extending from the neck through the chest and lower back all the way to the tail. Snakes also have ribs along the length of the body, but they evolved more recently from reptiles that had fewer ribs.


So how did mammals evolve lower back, pelvic and tailbone vertebrae without ribs?

“Hox genes have been utilized to modify the basic body plan to give rise to the different vertebrate body plans,” Capecchi says. “One way is to suppress formation of ribs beyond a certain number.”


He and Wellik bred mice with disabled versions of all three Hox10 genes, known as Hoxa10, Hoxc10 and Hoxd10. “When the whole family of Hox10 genes was knocked out, the mice had ribs from the normal ribs all the way to the tail,” says Capecchi.”


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A return to something said a while ago about the origin of eyes and their development.


Lolic: 4. you forget about the important parts like a million nerves growing from the eye and defying great odds and finding the corresponding individual nerve growing from the brain…


Here’s the general idea for vertebrate eyes.


The neural ectoderm (which is inside the developing embryo, despite it's being called ectoderm, because of the earlier process of neural tube formation) forms two evaginations (on opposite sides) called optic vesicles that project out more and more until they come into contact with the overlying outer ectoderm. The optic vesicles then induce the outer ectoderm to begin forming a lens. As the lens begins to form, it reciprocates and induces the optic vesicle to invaginate and form an optic cup. Part of the lens region also invaginates and then piches off; this portion will develop into the lens - it also induces the outer layer to develop into the cornea. The optic cup goes on to develop a retina, with some of the first cells differentiating in it being the RGCs (retinal ganglion cells).


Although there are about 7 different types of neural cells in the retina, only the RGCs send axons out of the eye, through the one optic nerve, which begins at the optic disc. Differentiation of RGCs begins in the center region of the retinal layer – that is, near the centrally located optic stalk/optic disc – and then spreads out across the retina. The RGC axons find their way to the optic disc via molecular signals, then other such signals take over to guide them the rest of the way. These visual axons running through the optic nerve take characteristic paths to 6 different regions of the brain. As of the year 2000 all the details of this pathfinding process had not been worked out, but several of the guidance molecules involved had been identified and their roles partially determined.


Here's some material on the second paragraph.



[in vertebrates,] RGCs [retinal ganglion cells] are the only retinal neurons that extend axons outside the eye. As RGCs differentiate, their axons grow laterally toward the presumptive optic nerve head in response to molecular guidance cues (Birgbauer et al., 2000; Brittis and Silver, 1994; Deiner et al., 1997) and pass outward through the optic stalk (Hinds and Hinds, 1974; Silver and Sidman, 1980). These axons travel along the optic nerve to the chiasm, where they make characteristic pathway choices and project to six specific regions in the brain, including the lateral geniculate nucleus and superior colliculus (Rodieck, 1998).



Optic nerve formation requires precise retinal ganglion cell (RGC) axon pathfinding within the retina to the optic disc, the molecular basis of which is not well understood. At CNS targets, interactions between Eph receptor tyrosine kinases on RGC axons and ephrin ligands on target cells have been implicated in formation of topographic maps. However, studies in chick and mouse have shown that both Eph receptors and ephrins are also expressed within the retina itself, raising the possibility that this receptor-ligand family mediates aspects of retinal development. Here, we more fully document the presence of specific EphB receptors and B-ephrins in embryonic mouse retina and provide evidence that EphB receptors are involved in RGC axon pathfinding to the optic disc. We find that as RGC axons begin this pathfinding process, EphB receptors are uniformly expressed along the dorsal-ventral retinal axis. This is in contrast to the previously reported high ventral-low dorsal gradient of EphB receptors later in development when RGC axons map to CNS targets. We show that mice lacking both EphB2 and EphB3 receptor tyrosine kinases, but not each alone, exhibit increased frequency of RGC axon guidance errors to the optic disc. In these animals, major aspects of retinal development and cellular organization appear normal, as do the expression of other RGC guidance cues netrin, DCC, and L1. Unexpectedly, errors occur in dorsal but not ventral retina despite early uniform or later high ventral expression of EphB2 and EphB3. Furthermore, embryos lacking EphB3 and the kinase domain of EphB2 do not show increased errors, consistent with a guidance role for the EphB2 extracellular domain. Thus, while Eph kinase function is involved in RGC axon mapping in the brain, RGC axon pathfinding within the retina is partially mediated by EphB receptors acting in a kinase-independent manner.



Among the earliest events in visual system development are RGC axon pathfinding within the retina to the optic disc and subsequent axon growth through the disc into the optic stalk to form the optic nerve. Guidance mechanisms underlying these pathfinding tasks are not completely understood. Studies demonstrate that at the optic disc, the axon guidance molecule netrin-1 controls, to a large extent, proper RGC axon growth through this exit point into the optic nerve (Deiner et al., 1997). However, since RGC axons in animals lacking netrin-1 or its receptor DCC are still able to find their way to the disc, guidance cues underlying pathfinding to the disc are distinct from those which mediate RGC axon growth through the disc and into the nerve. During pathfinding to the disc, RGC axons express Ig-CAMs (Bartsch et al., 1989; Paschke et al., 1992; Silver and Rutishauser, 1984), and anti-Ig-CAM antibody injections intraocularly in vivo, or applied onto retinal eye cup preparations in vitro, cause abnormal intra-retinal axon trajectories (Brittis et al., 1995; Ott et al., 1998). However, these perturbations do not affect all RGC axons, suggesting that additional guidance cues are involved in pathfinding to the optic disc.

(bold emphasis added, http://dev.biologists.org/cgi/content/abstract/127/6/1231?ijkey=9e74c0d136326de6ca0ce20c9c8ddb3a4ac19506&keytype2=tf_ipsecsha)

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Speaking of eyes...



The old perplexing anti-evolution question: What good is half an eye?


The answer: A lot!


In most environments, an organism with “half an eye” has a great selective advantage over a similar organism with just a “quarter of an eye”.




The newer perplexing question. What good is any kind of “eye” without a brain?


The answer: A lot!


“Gehring has reviewed the possibility that the jellyfish eye may have preceded brain evolution (Gehring, 2002). He supports this view by noting that unicellular algae (i.e. Chlamydomonas) or dinoflagellates (i.e. Erythropsis) have eye organelles and no brain. Jellyfish do, however, have a number of specialized ganglia associated with the rhopalia as well as an interconnected nerve ring which may, arguably, be a type of brain for a radially symmetrical animal (Coates, 2003). An attractive feature of eye before brain is that it places sense reception before information processing. Examples of ancestral photoreception preceding a central nervous system are fascinating. One is Chlamydomonas reihhardtii (Roberts et al., 2001). The eyespot of this unicellular alga orchestrates a positive phototaxis in low intensity light and a negative phototaxis in high intensity light by directly affecting the beating pattern of the two attached flagellae. Surprisingly, the Chlamydomonas eye2-1 mutant revealed that formation of the eyespot requires a member of the thioredoxin protein family and that this developmental role does not depend on the catalytic redox capability of the thioredoxin protein. The sponge larva, Reneira sp., provides another example of coordinated phototaxis in a multicellular organism that lacks nerve cells altogether (Leys and Degnan, 2001). A posterior ring of columnar epithelial cells containing a cilia and pigmented-filled protrusions respond directly to light, leading to negative phototaxis and directed swimming behavior. Increased light intensity makes the cilia rigid and subsequently bend, shielding the pigmented vesicles; decreased light intensity reverses the process. The resulting negative phototaxis is similar to the shadow response of tunicates and the unicellular Euglena. Spectral sensitivity tests suggest that the photoreceptive pigment in the sponge larva may be a flavin or carotenoid (Leys et al., 2002). This would make the sponge larva the first metazoan not using a rhodopsin-like protein as the primary photoreceptive pigment. It is not known yet whether expression of PaxB, which is present in sponges (Hoshiyama et al., 1998), is associated with the photoreceptive cilia in the sponge larva. Clearly, detailed studies on ancestral eyes and photoresponses are a rich source of new and unexpected insights.


Planula larvae of Tripedalia also have a photoreceptive system that appears to be directly connected to cilia for steering towards particular light conditions. A series of single-cell, pigment cup ocelli, lacking neural connections, surround the posterior half of the larval ectoderm (Nordstrom et al., 2003) (see also Gehring in this issue). The positions of these ocelli vary in different species of cubozoan larvae. These light sensors apparently have photosensitive microvilli and a motor-cilium. The cilium responds directly to light and may act as a rudder to steer the larva. Thus, while ciliated and rhabdomeric photoreceptors are occasionally found in the same species, this is the first instance of the latter being reported in cnidarians.”

(Cubozoan jellyfish: an Evo/Devo model for eyes and other sensory systems, JORAM PIATIGORSKY and ZBYNEK KOZMIK2, Int. J. Dev. Biol. 48: 719-729 (2004))

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And going back to my previous post, speaking about RGC (retinal ganglion cells) and evolutionary relationships …


”The vertebrate retina contains seven major neuronal and glial cell types in an interconnected network that collects, processes and sends visual signals through the optic nerve to the brain. Retinal neuron differentiation is thought to require both intrinsic and extrinsic factors, yet few intrinsic gene products have been identified that direct this process. Math5 (Atoh7) encodes a basic helix-loop-helix (bHLH) transcription factor that is specifically expressed by mouse retinal progenitors. Math5 is highly homologous to atonal, which is critically required for R8 neuron formation during Drosophila eye development. Like R8 cells in the fly eye, retinal ganglion cells (RGCs) are the first neurons in the vertebrate eye. Here we show that Math5 mutant mice are fully viable, yet lack RGCs and optic nerves. Thus, two evolutionarily diverse eye types require atonal gene family function for the earliest stages of retinal neuron formation. At the same time, the abundance of cone photoreceptors is significantly increased in Math5-/- retinae, suggesting a binary change in cell fate from RGCs to cones. A small number of nascent RGCs are detected during embryogenesis, but these fail to develop further, suggesting that committed RGCs may also require Math5 function.”

(Math5 is required for retinal ganglion cell and optic nerve formation, Nadean L. Brown, Sima Patel, Joseph Brzezinski, and Tom Glaser, Development 128, 2497-2508 (2001))

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More on eye development, in particular, a general principle for how axons from RGCs in the eye reach the appropriate targets in the brain. As one might expect, it's the interplay between a cell-surface receptor on the growing axons and a soluble ligand produced in the brain that binds to the receptor.


“Ephrin A Ligands Are Expressed as a Gradient along the Anteriorposterior Tectal Axis”


The soluble ephrin A ligands form a gradient in the tectum (region of the brain in lower vertebrates where the RGC axons synapse) with low levels anteriorly and high levels posteriorly. These ligands are chemical signals that bind to cell-surface receptors on axons, thereby altering their “behavior” (causing them to be attracted or repelled).



“The EphA3 Receptor Is Expressed in a Nasal-Temporal Gradient in the Retina”


This simply means that in the retina, as one moves from the nasal region (near the nose) outward to the temporal (i.e., near the temple) region, the number of EphA3 receptors in the RGC axons increases. Thus, axons originating in the nasal region will be the least sensitive to the ephrin A ligands in the brain, axons originating in the temporal region of the retina will be the most sensitive to the ephrin A ligands, and axons originating in intermediate regions of the retina will have intermediate sensitivities.



These two simple gradients – one of signal ligands and one of cell-surface receptors - allow the mapping of RGC axons to their appropriate anterior-posterior regions in the tectum.



The little quote snippets and more detail can be found at the following page:





PS: I see no reason that this general principle could not be expanded to greater levels of complexity. Consider the brain region where the axons will synapse. Imagine converting the “analog” anterior-to-posterior gradient into a “digital” one consisting of 5 bands, each with a value that represents the general level of ligand present. Here, a value of 1 indicates no ligands present, 5 indicates the maximum number of ligands present, 3 indicates an average number of ligands present ((1 + 5) / 2), and so on. So the gradient might run 1, 2, 3, 4, 5 from anterior to posterior. Thus, a single ligand could specify 5 different regions of the brain to which synapses could form. Now imagine a second 5-band ligand gradient, but this time running at a right angle to the first. Like a 5x5 tic tac toe board, there would then be 25 distinct regions, specified using only 2 ligands. Adding additional ligands and gradients (such as a ligand the originates in the center of the 5x5 matrix and diffuses outward) increases the number of distinct regions axons could be targeted to.



PPS: Well, I was right: the simple model I originally posted can be extended to incorporate another ligand gradient running at a right angle to the first.


“Thus, the nasal-temporal (NT) axis in retina is encoded by graded expression of EphA receptor tyrosine kinases by RGC axons. The recipient rostral-caudal (RC) coordinate in SC is established by graded expression of ephrin-A (Fig. 1A), which can bind and activate EphA receptors and transmit to RGC axons information about their position in SC. A similar chemical marking system, involving an EphB/ephrin-B receptor/ligand pair, exists for the mapping of dorso-ventral (DV) axis of retina to the medial-lateral (ML) direction of SC (Fig. 1B). The two approximately perpendicular expression profiles appear to be in place to determine the correct termination sites by RGC axons.” (http://arxiv.org/ftp/q-bio/papers/0503/0503001.pdf)


PPPS: Here's a more understandable presentation of this matter. It also adds information, in that it states that the original mappings based on ligands/receptors is fuzzy and gets better defined through selective pruning of synapses. http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Search&db=books&doptcmdl=GenBookHL&term=ephrin+AND+mboc4%5Bbook%5D+AND+374685%5Buid%5D&rid=mboc4.section.3963#3984

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