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Drosophila homeotic mutants
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Homeotic (Hox) gene complexes in Drosophila
lab pb Dfd Scr Antp Ubx abdA AbdB labial Deformed Antennapedia abdominal-A proboscipedia Sex combs reduced Ultrabithorax Abdominal-B Antennapedia Complex (ANT-C) Bithorax Complex (BX-C)
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Expression patterns of homeotic genes in embryogenesis
labial Antennapedia Ultrabithorax abdominal A Abdominal B proboscipedia Deformed Sex combs reduced lab pb Dfd Scr Antp Ubx abdA AbdB
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Two types of homeotic mutants in Drosophila
dominant mutations gain-of-function Transformations “anterior to posterior“ recessive mutations loss-of-function Transformations “posterior to anterior“
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serially homologous limbs
Antennapedia - a homeotic mutant antennenal segments Arista coxa claw tarsal segments troch- anter tibia femur Antp / + antenna and leg are serially homologous limbs antenna transformed into leg
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proboscipedia - transformation of mouthparts into legs
mx mx* lb lb*
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Complex homeotic gene regulation
regulatory mutations Coding-region Bithorax Complex (BX-C)
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Ubx is expressed in T3 and A1
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Ultrabithorax binds DNA and regulates genes required for T3 and A1
Ultrabithorax expression Ubx protein OFF ON T2 specific gene OFF T3 specific gene ON A5 specific gene A1 specific gene
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All abdominal segments take on a T2 identity if the bithorax complex is deleted
ubx abdA abdB triple mutant Wild type T2 T3 A1 T2 A8 T2
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Antennapedia expression expands in flies mutant for the Bithorax complex genes
ANT-C BX-C Figures\Chapter09\DevBio7e09264.jpg
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Ultrabithorax, AbdA, and AbdB normally repress expression
of the thorax-specific “leg gene” Distalless in the abdominal segments wild-type ubx abdA abdB triple mutant abdomen T1 T2 T3
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Persistence of homeotic gene expression through development
Adult Imaginal discs larva embryo
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Maintenance of homeotic gene expression
maintenance of transcriptionally inactive state maintenance of transcriptionally active state trithorax group (Trx-G) Polycomb group (Pc-G) extradenticle realisator gene Trx-G Pc-G transcriptionally open chromatin Dfd Exd pb Scr Antp closed
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Lewis hypothesized that the duplication and diversification of homeotic master regulators underlies the evolution of an increasingly complex body plan
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Distal-less gene Drosophila Precis (butterfly)
required for limb development Drosophila Precis (butterfly)
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The human body is also built up from reiterated units (segments) with different identities along the A/P axis
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Hox gene expression in mouse embryos
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Mouse HOX gene expression patterns
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Mouse hoxa3 and hoxd3 mutants
from Nature 370: (1994)
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Wild-type mouse forelimb
hoxa-11 hoxd-11 Human hoxD13 mutation
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