Hox Genes: Spatial Control of Gene Expression


Many genes that alter embryos code for transcription factors.


These gene sequences are astonishingly conservative through evolution (for example have
homologous sequences in flies and in vertebrates) although they don't necessarily
have the same function. Even small changes are harmful - selective removal of mutants.


Five families of Drosophila genes

    Maternal effect - establish gradients - bicoid is example

    Segmentation: Gap (Kruppel), Pair-rule (even-skipped) (there is also an odd-skipped mutant,
    called Fushi Terazu after a kind of Japanese bamboo), Segment-polarity (engrailed)

    Homeotic (antennapedia [mutant forms legs on the head where the antennae should be],
    bithorax [mutant has an extra thoracic segment, and therefore develops an extra pair of wings])

Hox (homeotic) genes - found in "all" animals


From the Wikipedia article on Hox genes:

"A fly can function perfectly well with a chicken Hox protein in place of its own.
So, despite having a last common ancestor that lived over 670 million years
ago, the chicken and fly version of the same Hox gene can actually take
each other's places when swapped." .

"Colinearity of Hox genes In some organisms, especially vertebrates, the various Hox genes are situated
very close to one another on the chromosome in groups or clusters.

Interestingly, the order of the genes on the chromosome is the same as the
expression of the genes in the developing embryo, with the first gene being
expressed in the anterior end of the developing organism
. The reason
for this colinearity is not yet completely understood

"The homeodomain [of a Hox protein] is a 60-amino-acid-long DNA-binding domain
(encoded by its corresponding 180-base-pair DNA sequence, the homeobox)....
This consensus polypeptide chain is (typical intron position noted with dashes):


Converting this from the single-letter amino acid code, this protein consists of


Notice how many arginines and lysines there are. Having a lot of these two amino acids
makes a protein basic, and therefore able to bind to acidic molecules, i.e. DNA.
(This is all you need to remember about this sequence.)











Some important Evo-Devo research that was done here in the UNC Biology Department:

Prof. Anne Burke and her Ph.D. student, Julie Nowicki (who was a Teaching Assistant in this course for two years) collaborated with Prof. Alan Feduccia to test

whether the 4 toes of bird feet correspond to toes 1, 2, 3, & 4,

              instead of toes 2, 3, 4 & 5,

based on which D hox genes are transcribed in the toes of ostrich embryos.

This tested which reptiles birds evolved from (whether from dinosaurs, or other groups).
Their evidence contradicted the most popular "birds are dinosaurs" theories.