Segmentation and the Hox genes
Homeotic genes discovered in Drosophila melanogaster – 1 Hox gene cluster
Homeosis: transformation one organ into another (similar organ on another segment)
Homeotic genes: genes responsible for homeostasis
Homeotic mutations in Drosophila:
Antennapedia (Antp) Antp overexpression:
antenna changed into leg (homology body parts)
Ultrabithorax (Ubx) Ubx knock out: 2 wings
changed into 4 wings
Homeotic genes have a homeodomain (=homeobox): part of molecule that has a
DNA binding domain (proteins that can bind to DNA)
2 types of homeobox genes:
1. Hox genes: axial identity (tell where you are – responsible for segmenting the
body)
a. Hox1 / Hox 2
2. Selector genes: segment identity (what to do there)
Hox gene (homeotic gene with homeodomain of 60 AA)
The homeodomain encodes 3 helices
Hox proteins regulate transcription of genes with transcription factor
binding sites for Hox TFs in their promotor
Hox genes are clustered in the genome
3’ – antennapedia complex – telomeric – anterior – early
transcription
5’ – bithorax complex – centromeric – posterior – late transcription
How do these genes come in clusters?
Unequal crossing-over lead to duplication gamete and deletion
gamete
2 types of homology in duplicated genes (paralogs and orthologs)
Hox1 + Hox 2 AND Hox 1’ + Hox 2’ are paralogs (due to
duplication)
Hox2 + Hox2’ AND Hox 1 + Hox1’ are orthologs (due to common
ancestry)
Evolution of Hox gene complex
All animals have Hox genes:
Vertebrates: 4 clusters (chromosome 2 (D),6 (A),11 (B) and 15 (C)) (39 Hox
genes)
Drosophilia: 1 cluster (8 Hox genes)
Ancestral bilaterian animal: (7 Hox genes)
All Hox genes are orthologs (due to common ancestry) to each other
Vertebrates have several clusters on different chromosomes (paralogs)
Homeotic genes discovered in Drosophila melanogaster – 1 Hox gene cluster
Homeosis: transformation one organ into another (similar organ on another segment)
Homeotic genes: genes responsible for homeostasis
Homeotic mutations in Drosophila:
Antennapedia (Antp) Antp overexpression:
antenna changed into leg (homology body parts)
Ultrabithorax (Ubx) Ubx knock out: 2 wings
changed into 4 wings
Homeotic genes have a homeodomain (=homeobox): part of molecule that has a
DNA binding domain (proteins that can bind to DNA)
2 types of homeobox genes:
1. Hox genes: axial identity (tell where you are – responsible for segmenting the
body)
a. Hox1 / Hox 2
2. Selector genes: segment identity (what to do there)
Hox gene (homeotic gene with homeodomain of 60 AA)
The homeodomain encodes 3 helices
Hox proteins regulate transcription of genes with transcription factor
binding sites for Hox TFs in their promotor
Hox genes are clustered in the genome
3’ – antennapedia complex – telomeric – anterior – early
transcription
5’ – bithorax complex – centromeric – posterior – late transcription
How do these genes come in clusters?
Unequal crossing-over lead to duplication gamete and deletion
gamete
2 types of homology in duplicated genes (paralogs and orthologs)
Hox1 + Hox 2 AND Hox 1’ + Hox 2’ are paralogs (due to
duplication)
Hox2 + Hox2’ AND Hox 1 + Hox1’ are orthologs (due to common
ancestry)
Evolution of Hox gene complex
All animals have Hox genes:
Vertebrates: 4 clusters (chromosome 2 (D),6 (A),11 (B) and 15 (C)) (39 Hox
genes)
Drosophilia: 1 cluster (8 Hox genes)
Ancestral bilaterian animal: (7 Hox genes)
All Hox genes are orthologs (due to common ancestry) to each other
Vertebrates have several clusters on different chromosomes (paralogs)
