Development Reveals the Unity of Life

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

Development Reveals the Unity of Life

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