The Phylotypic stage
At the end of gastrulation all embryos appear to be similar and is thus called the phylotypic stage.
Structures that are common to the phylotypic stage of the vertebrates are
1) the notochord (an early mesoderm structure along A/P),
2) the somites (blocks of mesoderm on either side of notochord which form the muscles of the trunk & limbs),
3) the neural tube- ectoderm above notochord form a tube (brain and spinal cord).
Chordates and arthropods evolved from a common ancestor which evolved from a single-celled organism.
Comparison of ontogeny (organismal development) and phylogeny (evolutionary history) reveals important concepts.
1) Ontogeny recapitulates phylogeny.
2) Development has been modified in evolution.
The three main stages of vertebrate development
1) setting up the main body axes (A/P and D/V)
2) specification of three germ layers (endoderm, mesoderm and ectoderm)
3) germ layer patterning (mesoderm and early nervous system)
Embryonic structures have acquired new functions during evolution
Embryo's development reflects the evolutionary history of the organism.
New anatomical structures arise from existing structures.
The mammalian inner ear has 3 bones to transmit sound from the ear drum
Reptilian ancestral jaw attaches to skull via articular (jaw) bone and quadrate (skull) bone which transmit sound.
In mammals, the articular and quadrate bones have been modified into the malleus and incus bones of the inner ear
Development of vertebrate and insect wings makes use of evolutionary
Insect and bird wings are very different (two layers of epithelia versus mesenchymal core cover by ectoderm.
However, these wings (and vertebrate limbs) share signaling molecules.
Antero-posterior axis require hedgehog-like genes and TGFB (dpp & BMP-2)
Dorsal structures require apterous-like genes' Dorsal/venral boundaries require fringe family members.
Establishment of axes and patterning require a small set of conserved genes.
Mesoderm and homeobox genes
Homeobox genes are...
a large family of transcription factors.
Share a similar 60 amino acid DNA binding homeodomain which is encoded by 180 basepair homeobox sequence.
Homeobox gene family (transcription factor proteins).
Homeotic transformation is often observed in mutants of genes that have this domain.
Identified first in Drosophila (Bithorax and Antennapedia complexes) as a split cluster.
There are four separate clusters of Hox genes (subset of the homeobox genes) in vertebrates.
Hox gene clusters
Hox genes ( Hox gene clusters) are a subset of the homeobox genes of transcription factor genes.
Might have arisen by rounds of duplication of an ancestral gene, followed by a quaduplication of the cluster in mammals.
Paralogous group are composed of the most similar members of each cluster.
Partially overlapping zones of expression which vary in the anterior extent of their expression define distinct regions.
Hox genes complexes have evolved through gene duplication
Tandem gene duplication can allow retention of gene while new functions are adopted by one copy.
Hox gene (homeobox, helix-turn-helix transcription factor) cluster arose from rounds of tandem duplication.
Vertebrates have four Hoxgene complexes.
Amphioxus, a vertebrate-like chordate, has one Hox cluster which may be close to ancestral Hox complex.
Each vertebrate cluster seems to arose from chromosomal duplication of arrays generated by tandem duplication.
Some mammalian Hox genes (Clusters a-d, 10-13) not present in arthropods, seem to be products of tandem duplication before chromosomal duplication.
Changes in specification and interpretation of positional identity
have generated the elaboration of vertebrate and arthropod body plans
Hox genes control A/P axis of embryo.
The zootype is the expression of key genes along the A/P axis of the embryo.
Hox genes specify positional identity not a specific structure.
Different species of embryos interpret the values differently.
Changes in downstream targets of the Hox genes provide source of change.
Vertebrate number differences among some birds depend upon the spatial expression of Hoxc6.
In insects and crustaceans, the pattern of Hox gene expression is very different and the differences in body plan are the result.
Homeobox genes are involved in patterning the limbs and specifying
23 Hox genes are expressed in the developing chick limb.
Hoxa and Hoxd clusters are expressed in both fore and hind limb buds.
Hoxd9-13 are expressed sequently nested pattern such that Hoxd9 is expressed throughout but all Hoxd9-13 are expressed in the small posterior region.
Hoxd seem to control A/P (i.e. finger) identity.
Hoxa9-13 are expressed in a nested proximo-distal pattern.
Ie Hoxa9 only in the presumptive upper limb, Hoxa9-11 in the lower limb region and Hoxa9-13 region give rise to wrist & digits.
Limbs evolved from fins
Limbs develop after the phylotypic stage and evolved from the pelvic and pectoral fins of fish-like ancestors.
The limb pattern is highly conserved although differences exist.
Fin buds are similar to limb buds.
Proximal elements are similar but distal bony fin rays differ greatly from tetrapod limb.
Early hedgehog and Hox expression is similar but additional Hox gene expression in the distal limb bud leads to digits.
This may due to extension of cartilage forming region plus novel Hox gene patterning.
The position and number of paired appendages in insects is dependent
on Hox gene expression
The pattern of Hox gene expression along the A/P axis is conserved among insects.
Some have abdominal legs, some only one pair of wings.
Repression of Distal-less by Bithorax Complex (BXC) genes in the abdomen of Drosophila suppress limb formation.
The Hox genes can determine nature of the appendage.
Rather than new genes, new regulation relationships between BXC genes evolved to control limb and wing devekopment.
The body plan of arthropods and vertebrates is different
Despite similarities, vertebrates have an inverted dorso-ventral axis compared to arthropods.
The nerve cord is ventral in arthropods and dorsal in vertebrates.
Inversion may be directed by the position of the mouth which is specified during gastrulation.
Two related groups of proteins, chordin & sog and BMP-4 & dpp have conserved functions but result in inverted axes.
engrailed and hairy have similar roles in the Drosophila parasegments and vertebrate somites.
Changes in relative growth rates can alter the shapes of organisms.
The timing of growth leads to differences between related organisms.
Dogs have different face shapes due to relative difference in nose and jaw growth
In evolution of the horse, the central digit grew faster and longer than the others.
Once the digits were no longer touching the ground, a second event further reduced the size of the lateral digits.
Allometry is the mathematical analysis of the relative growth of various parts of an organism.
The timing of developmental events has changed during evolution
If the timing of developmental events differ from the ancestral, great effects can occur.
Heterochrony describes a difference in the timing of growth and development between organisms.
For example, acquiring sexual maturity at an immature stage or neoteny, arises from heterchronic changes.
Direct developing frogs (i.e Eleutherodactylus) do not undergo a tadpole stage and display accelerated development of adult structures.
Reduction/loss of limbs in whales and flightless birds reflect a change in timing of developmentally significant activities.
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