The most studied and the most conserved group of homeodomain protein are the Hox genes, which control segmental patterning during development; however, not all homeodomain proteins are Hox proteins.
Homeobox genes encode transcription factors which typically switch on cascades of other genes. The homeodomain binds DNA in a specific manner.
However, the specificity of a single homeodomain protein is usually not enough to recognize only its desired target genes. Most of the time, homeodomain proteins act in the promoter region of their target genes as complexes with other transcription factors, often also homeodomain proteins. Such complexes have a much higher target specificity than a single homeodomain protein.
In vertebrates the 4 paralogue clusters are partially redundant in function, but have also acquired several derived functions in particular HoxA and HoxD specify the segment identity along the limb axis.
The main interest in this set of genes stems is their unique behaviour. They are mostly found clustered together and the order they are found generally matches the order of regions they affect and their timing, a phenomenon called colinearity. Due to this balance, generally, when one gene is lost the segment becomes a more anterior one, while a gain of function mutant (ectopic) will be more posterior, famous examples of these are Antennapedia and bithorax in Drosophila.
Homeobox genes were previously only identified in bilaterians but recently, cnidarians have also been found to contain homeobox domains and the "missing link" in the evolution between the two has been identified.
The well known homeotic genes in plants (MADS-box genes) are not homologous to Hox genes in animals. Plants and animals do not share the same homeotic genes, and this suggests that homeotic genes arose independently in the early evolution of animals and plants.
|HOXA (or sometimes HOX1) -||chromosome 7||HOXA1, HOXA2, HOXA3, HOXA4, HOXA5, HOXA6, HOXA7, HOXA9, HOXA10, HOXA11, HOXA13|
|HOXB -||chromosome 17||HOXB1, HOXB2, HOXB3, HOXB4, HOXB5, HOXB6, HOXB7, HOXB8, HOXB9, HOXB13|
|HOXC -||chromosome 12||HOXC4, HOXC5, HOXC6, HOXC8, HOXC9, HOXC10, HOXC11, HOXC12, HOXC13|
|HOXD -||chromosome 2||HOXD1, HOXD3, HOXD4, HOXD8, HOXC9, HOXD10, HOXD11, HOXD12, HOXD13|
"HESX homeobox 1" is also known as HESX1.
Two examples of homeobox mutations in the above-mentioned fruit fly are legs where the antennae should be (antennapedia), and a second pair of wings.
Duplication of homeobox genes can produce new body segments, and such duplications are likely to have been important in the evolution of segmented animals.
Interestingly, there is one insect family, the xyelid sawflies, in which both the antennae and mouthparts are remarkably leg-like in structure. This is not uncommon in arthropods as all arthropod appendages are homologous.
Homeoboxes in sea anemones (Cnidaria; Anthozoa): a PCR-based survey of Nematostella vectensis and Metridium senile. (Polymerase Chain Reaction)
Aug 01, 1997; Introduction Homeobox genes encode a family of transcription factors that are characterized by the presence of a...
Suppression of embryonic lung branching morphogenesis by antisense oligonucleotides against HOM/ C homeobox factors
Sep 01, 2010; Received: 16 February 2010 / Accepted: 23 May 2010 / Published online: 10 June 2010 © The Society for In Vitro Biology 2010...
Homeobox genes go evolutionary: the genes, well known as important regulators of embryonic development, may also provide a key that helps unlock the mysteries of evolution.
Jan 24, 1992; IN THE MID-1980s, WHEN THE Homeobox genes were first identified, the discovery was hailed as a major achievement in...