lpetrich
28 Mar 2009, 02:30 PM
A remarkable discovery of evolutionary developmental biology ("evo-devo") over the last few decades, has been deep homology, homology across animal-kingdom phyla that has been totally unexpected.
An important part of specifying identity along the anterior/posterior (head-tail) body axis is the expression of Hox genes, whose proteins control other genes. They are expressed in zones along that axis, and their proteins control the expression of other genes.
They were first identified in Drosophila fruit flies, often used in genetics research because they are easy to raise in the lab. Mutations in them and their expression control produce conditions like antennapedia, where their antennae develop like legs, and the the third thoracic segment developing like the second one, making 4-winged flies.
Since then, they have been found over much of the animal kingdom, and with similar function, as far as can be determined. Snakes lost their front limbs when some of their Hox zones got moved forward, keeping those limbs from growing.
Hox genes most likely originated by gene duplication, with different copies becoming specialized for different subdivisions of the original regions. These genes have relatives known as the ParaHox and the NK genes. By embryonic germ layer:
Ectoderm (skin, nerves) | Hox
Mesoderm (heart) | NK
Endoderm (gut) | ParaHox
Hox gene
A brief overview of Hox genes (http://scienceblogs.com/pharyngula/2006/04/a_brief_overview_of_hox_genes.php)
Hox genesis (http://scienceblogs.com/pharyngula/2006/05/hox_genesis.php)
Bilateral symmetry in a sea anemone (http://scienceblogs.com/pharyngula/2006/05/bilateral_symmetry_in_a_sea_an.php)
Loss of legs in Snakes is linked to Hox gene expression (http://www.hoxfulmonsters.com/2008/05/loss-of-legs-in-snakes-is-linked-to-hox-gene-expression/)
Turning to the next body axis, the dorsal-ventral (back-to-belly) one, evo-devo research has revived a remarkable hypothesis put forth by Geoffroy St. Hilaire in 1822: that vertebrates' body plans are inverted relative to arthropods' body plans. In effect:
Arthropod | Vertebrate
Dorsal | Ventral
Heart | Heart
Gut | Gut
CNS | CNS
Ventral | Dorsal
where CNS = central nervous system. Annelids have the same arrangement as arthropods, which suggests that the arthropod-annelid arrangement is ancestral and that vertebrates are the inverted ones.
However, there are so many details that get in the way of this nice hypothesis that it was quickly rejected. Every twenty years or so, someone would revive it, only to see it get picked apart.
But recently, some remarkable homologies have been discovered among the molecules responsible for dorsoventral patterning. Vertebrate ones were discovered in the embryos of Xenopus frogs, often used as lab animals, and were discovered to be homologous to Drosophila ones.
Fly | Frog
Dorsal | Ventral
dpp | BMP
sog | chordin
Ventral | Dorsal
dpp = decapentaplegic
sog = short gastrulation
BMP = Bone Morphogentic Protein
dpp/BMP makes cells have a dorsal/ventral fate, but is antagonized by sog/chordin, which makes cells have a ventral/dorsal fate. What is especially interesting is that the frog version of sog/chordin works in flies and vice versa.
Going further, the marine annelid worm Platynereis was discovered to have a similar patterning mechanism -- and to have a patterning mechanism for its central nervous system that closely parallels that of Drosophila and vertebrates. So despite the differences in overall shape of their respective central nervous systems, the patterning mechanisms are remarkably similar.
We have the brains of worms (http://scienceblogs.com/pharyngula/2007/05/we_have_the_brains_of_worms.php)
Geoffroy's lobster and the animal common ancestor (http://chancenecessity.blogspot.com/2009/02/geoffroys-lobster-and-animal-common.html)
Going from nervous systems to eyes, yet another bit of deep homology is in a molecule involved in early eye development. The Drosophila eyeless gene is homologous to the mouse Pax6 gene, and Pax6 can make Drosophila flies grow ectopic eyes. However, those eyes resemble fly eyes rather than mouse eyes.
And even weirder is the two types of photoreceptors in the animal kingdom, "rhabdomeric" and "ciliary". Invertebrate eyes always have rhabdomeric photoreceptors, which grow out of the skin, while vertebrate ones always have ciliary ones, which grow out of the brain.
But a remarkable "missing link" has recently been discovered. Platynereis worms have rhabdomeric-photoreceptor eyes, like other invertebrates, but they also have ciliary photoreceptors in their brains, which are possibly for setting their circadian clocks. And these ones are closest to vertebrate photoreceptors.
So the ancestor of the bilaterians likely had both kinds of photoreceptors, rhabdomeric ones on their skins and ciliary ones in their brains.
And limbs? Fruit flies have a gene called distalless, which is involved in the growth of their legs. Mutations of it can interfere with that growth, thus the name. Similar genes are involved in the growth of various appendages in other phyla, like fish fins / land-vertebrate limbs, starfish tube feet, and annelid parapodia. Though these structures are not homologous across phyla, their growth mechanisms thus have some cross-phylum homology.
And a heart? Yes, that also. Drosophila has a gene involved in heart growth called tinman, named after what mutations of it can do. Vertebrates have homologues of tinman also, the NK2 genes.
From all this, one can infer that the ancestor of most of the animal knigdom had had a front-to-rear body axis patterned with Hox genes, a belly-to-back axis, a simple central nervous system, photoreceptors, a contractile vessel for pumping internal fluids, and some sort of appendages.
An important part of specifying identity along the anterior/posterior (head-tail) body axis is the expression of Hox genes, whose proteins control other genes. They are expressed in zones along that axis, and their proteins control the expression of other genes.
They were first identified in Drosophila fruit flies, often used in genetics research because they are easy to raise in the lab. Mutations in them and their expression control produce conditions like antennapedia, where their antennae develop like legs, and the the third thoracic segment developing like the second one, making 4-winged flies.
Since then, they have been found over much of the animal kingdom, and with similar function, as far as can be determined. Snakes lost their front limbs when some of their Hox zones got moved forward, keeping those limbs from growing.
Hox genes most likely originated by gene duplication, with different copies becoming specialized for different subdivisions of the original regions. These genes have relatives known as the ParaHox and the NK genes. By embryonic germ layer:
Ectoderm (skin, nerves) | Hox
Mesoderm (heart) | NK
Endoderm (gut) | ParaHox
Hox gene
A brief overview of Hox genes (http://scienceblogs.com/pharyngula/2006/04/a_brief_overview_of_hox_genes.php)
Hox genesis (http://scienceblogs.com/pharyngula/2006/05/hox_genesis.php)
Bilateral symmetry in a sea anemone (http://scienceblogs.com/pharyngula/2006/05/bilateral_symmetry_in_a_sea_an.php)
Loss of legs in Snakes is linked to Hox gene expression (http://www.hoxfulmonsters.com/2008/05/loss-of-legs-in-snakes-is-linked-to-hox-gene-expression/)
Turning to the next body axis, the dorsal-ventral (back-to-belly) one, evo-devo research has revived a remarkable hypothesis put forth by Geoffroy St. Hilaire in 1822: that vertebrates' body plans are inverted relative to arthropods' body plans. In effect:
Arthropod | Vertebrate
Dorsal | Ventral
Heart | Heart
Gut | Gut
CNS | CNS
Ventral | Dorsal
where CNS = central nervous system. Annelids have the same arrangement as arthropods, which suggests that the arthropod-annelid arrangement is ancestral and that vertebrates are the inverted ones.
However, there are so many details that get in the way of this nice hypothesis that it was quickly rejected. Every twenty years or so, someone would revive it, only to see it get picked apart.
But recently, some remarkable homologies have been discovered among the molecules responsible for dorsoventral patterning. Vertebrate ones were discovered in the embryos of Xenopus frogs, often used as lab animals, and were discovered to be homologous to Drosophila ones.
Fly | Frog
Dorsal | Ventral
dpp | BMP
sog | chordin
Ventral | Dorsal
dpp = decapentaplegic
sog = short gastrulation
BMP = Bone Morphogentic Protein
dpp/BMP makes cells have a dorsal/ventral fate, but is antagonized by sog/chordin, which makes cells have a ventral/dorsal fate. What is especially interesting is that the frog version of sog/chordin works in flies and vice versa.
Going further, the marine annelid worm Platynereis was discovered to have a similar patterning mechanism -- and to have a patterning mechanism for its central nervous system that closely parallels that of Drosophila and vertebrates. So despite the differences in overall shape of their respective central nervous systems, the patterning mechanisms are remarkably similar.
We have the brains of worms (http://scienceblogs.com/pharyngula/2007/05/we_have_the_brains_of_worms.php)
Geoffroy's lobster and the animal common ancestor (http://chancenecessity.blogspot.com/2009/02/geoffroys-lobster-and-animal-common.html)
Going from nervous systems to eyes, yet another bit of deep homology is in a molecule involved in early eye development. The Drosophila eyeless gene is homologous to the mouse Pax6 gene, and Pax6 can make Drosophila flies grow ectopic eyes. However, those eyes resemble fly eyes rather than mouse eyes.
And even weirder is the two types of photoreceptors in the animal kingdom, "rhabdomeric" and "ciliary". Invertebrate eyes always have rhabdomeric photoreceptors, which grow out of the skin, while vertebrate ones always have ciliary ones, which grow out of the brain.
But a remarkable "missing link" has recently been discovered. Platynereis worms have rhabdomeric-photoreceptor eyes, like other invertebrates, but they also have ciliary photoreceptors in their brains, which are possibly for setting their circadian clocks. And these ones are closest to vertebrate photoreceptors.
So the ancestor of the bilaterians likely had both kinds of photoreceptors, rhabdomeric ones on their skins and ciliary ones in their brains.
And limbs? Fruit flies have a gene called distalless, which is involved in the growth of their legs. Mutations of it can interfere with that growth, thus the name. Similar genes are involved in the growth of various appendages in other phyla, like fish fins / land-vertebrate limbs, starfish tube feet, and annelid parapodia. Though these structures are not homologous across phyla, their growth mechanisms thus have some cross-phylum homology.
And a heart? Yes, that also. Drosophila has a gene involved in heart growth called tinman, named after what mutations of it can do. Vertebrates have homologues of tinman also, the NK2 genes.
From all this, one can infer that the ancestor of most of the animal knigdom had had a front-to-rear body axis patterned with Hox genes, a belly-to-back axis, a simple central nervous system, photoreceptors, a contractile vessel for pumping internal fluids, and some sort of appendages.