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8/3/2019 Development Reveals the Unity of Life
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Development Reveals the Unity of Life
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y All of chapter 20Homeotic genes directthe morphogenesis oftissues that are appropriate to a specific
region ofthe body
The homeotic genes ofDrosophila have a shared feature, the homeobox
y Homeobox: a sequence of 180 base pairs in the protein-codingregion of homeotic genes that specifies a DNA-binding region of the
homeotic protein called the homeodomain
The homeobox encodes a homeodomain, which is a DNA binding domain
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The position ofa homeotic gene within the homeotic gene cluster correlates withthe
anterior boundary of its expression domain and unique combinations ofhomeoticgenes specify unique cell types, tissues, and structures ofthe adultfly
The homeobox sequence was used to searchfor similar genes in other organisms
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The homeobox sequence was used to searchfor similar genes in other organisms
Homeotic genes are conserved throughoutthe animal kingdom and carry outthe
same role ofspecifying axial position
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A master gene for eye development also reveals deep homology
Drosophila Mouse Human
Eyeless Pax6 PAX6
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y PAX6 can substitute for eyeless in Drosophila and direct thedevelopment of compound eyes
y The early signals (or general instructions) for making eyes areancient and have been used repeatedly during animal evolution
The eyeless and Pax6 genes/proteins have diverged a lot yet retained some strong
sequence identity in the encoded protein
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One gene in invertebrates is often represented by a family ofhomologous genes in
vertebrates (particularly true ofregulatory genes)
Invertebrates Vertebrates
Eyeless Pax3, Pax6, Pax7
Hedgehog Sonic hedgehog, Indian hedgehog,
Desert hedgehog, Tiggywinkle
Fringe Lunatic fringe, radical fringe
Distal-less Dlx1, 2, 3, 4, 5, 6
8 homeotic genes 39Hox genes
Sonic Hedgehog Effect
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Sonic Lacking motor neurons
Indian Shortened middle digits
Desert Testicular disgenesis
Tiggywinkle Retinol cell development
Lunatic Fringe Fertility, control of borders of somites, inner ear development
DLZ Development of the middle ear
The extraordinary conservation ofclustered homeotic genes provides one ofthe best
pieces ofevidence for deep homologies
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Gene duplication and subsequent divergence accounts for the increase in the number
ofregulatory genes in vertebrates
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Gene duplication and divergence is an important driving force behind the evolution of
new kinds ofanimals
In other lineages the ancestral gene is not duplicated but continues to evolve
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A current phylogeny depicting the relationships among animals
Similar trees can depictthe evolutionary relationships among genes
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y Gene Orthologues genes in different organisms that haveevolved from a common ancestral gene
y Gene Paralogues related genes in an organism that have arisenby gene duplication
The DNA sequences oforthologous genes (between species) are more similar than the
sequences ofparalogous genes (within the same species)
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The instructions that regulate embryogenesis are provided by orthologous genes in
vertebrates and invertebrates, even thoughthe structures formed fromthose
instructions are very different