Hox Genes
Important for specification of identity along the A-P axis.
Insects
Segment identity controlled by segment identity (aka homeotic, aka selector)
genes.
Discovered through homeotic mutations. This is a mutation that causes the
transformation of one structure to another homologous structure. (Homologs
have evolutionarily related ancestry—both derived from a common ancestor
structure).
Eg. Antennapedia causes the transformation of antennae to legs;
ultrabithorax causes the transformation of halteres to wings (T3 to T2).
Drosophila homeotic genes encode transcription factors. These belong to a family
with related DNA binding domains called homeodomains. The DNA sequence
that codes for the homeodomain is called a homeobox. Genes that encode
homeodomain proteins are knicknamed Hox genes.
**It is important to note that homeotic and Hox are NOT synonomous.
Homeotic refers to a mutant phenotype and Hox refers to a sequence motif. Not
all Hox genes give homeotic mutant phenotypes and not all genes (especially in
other organisms) that give homeotic mutant phenotypes encode homeodomain
proteins.
The segment identity genes are chromosomally located in 2 gene complexes (in
Drosophila).
! The Antennapedia complex controls head and thorax segment identities
! The Bithorax complex controls abdominal segment identities
Most insects (and other segmental animals) contain all the segment identity
genes in one large complex, HOM-C. It appears that the complex was split
during Drosophila evolution. Thus, Antennapedia + Bithorax = Hom-C.
Each gene within the complex is expressed in a discrete set of segments to
control identity of that region. There is a correlation between the pattern of gene
positions within the HOM-C complex and the pattern of expression in the
embryo. Genes located toward the 5’ end of the complex tend to be expressed in
more anterior segments.
Segment identity gene expression is initially regulated by gap and pair-rule
genes. Expression patterns are refined and maintained by interactions with other
pair-rule genes. General rule is that genes expressed more posteriorly repress
genes expressed more anteriorly genes.
Eg. The normal Antennapedia gene is expressed in T2 and specifies the identity
of T2. The Antennapedia mutant is a dominant gain-of-function mutant where
the Antp gene is ectopically expressed in a head segment. This represses the
normal Hox gene in that (more anterior) head segment and converts the identity
to (more posterior) T2, causing the homeotic transformation of antennae to legs.
Eg. Ultrabithorax is normally expressed in T3, where it represses the expression
of Antp. In the ubx loss-of-function mutant, the without Ubx repression, Antp
expression expands posteriorly into T3, transforming it to T2 identity. Thus the
fly has 2 T2 segments and the halteres in T3 are transformed to wings like T2.
Segment identity genes control the expression of realizator genes—the genes
that actually direct morphogenesis. An example is the distal-less gene that
promotes leg formation.
Vertebrates
Insects: Antennapedia complex + bithorax complex = HOM-C.
Mammals: 4 copies of HOM-C = HoxA – HoxD
Order of genes in complex similar to flies
Expression pattern similar to flies
Order of genes on the chromosome is the same as the order of expression
along the A-P axis
Some are so similar to fly homeotic genes that they can be swapped
Mammalian Hox genes expressed along dorsal axis in neural tube, neural crest,
paraxial mesoderm and surface ectoderm and in derivatives of these tissues.
Experimental evidence that Hox genes specify identity along A-P axis:
1. Targeted gene disruptions result in homeotic transformations. Generally
get anterior transformations just like flies, indicating that posterior genes
inhibit anterior. Good direct evidence.
! Eg. Knockout mouse Hoxc-8 converts a lumbar vertebra to a thoracic
vertebra#get extra rib
2. Retinoic Acid teratogenesis. [RA] is normally high in Hensen’s node and
probably plays a role in axis formation. Treat gestating mice with RA and
get babies with homeotic transformations. Also see corresponding change
in homeotic gene expression pattern, and some transformations can be
mimicked by ectopic Hox gene expression. For example, ectopic
expression of Hoxa-7 generates cranio-facial defects similar to RA
treatment.
3. Comparative anatomy. Mammals and chicks have different numbers of
cervical, thoracic and lumbar vertebrae. The constellation of Hox gene
expression predicts the type of vertebra. For example, mice have 7
cervical and 13 thoracic vertebrae while chicks have 14 cervical and 7
thoracic. In both cases, the Hox-5 gene is always expressed in the last
cervical vertebra while Hox-6 is in the first thoracic.
Evolution
Hox genes appear to have played important roles in morphological evolution.
Many differences in morphology between species/phyla appear due to
evolutionary changes related to Hox genes. There are 4 major ways this could
happen:
1. Changes in the response of realizator genes to Hox genes.
Eg. Fly vs. butterfly. Fly Ubx --| genes for wing formation (get haltere in T3)
Butterfly Ubx does not block wing genes (get 2nd wing in T3)
Therefore, the Hox genes did not change but the response to them did.
2. Changes in Hox gene expression within a body segment.
Ubx --| distalless # limb formation
Drosophila, have Ubx expression throughout abdominal segments, thus no legs
Butterflies, get late downregulation of Ubx expression in small regions of
abdominal segments. This allows distalless expression and the formation of
prolegs.
3. Changes in Hox gene expression in different segments.
Eg. The different Hox gene expression patterns in mammals and birds, discussed
above, produces altered body plans with different numbers of cervical, thoracic
and lumbar vertebrae.
Eg. Snakes. Most vertebrates form forelimbs anterior to anteriormost expression
domain of Hoxc-6. Snakes express Hoxc-6 and Hoxc-8 throughout# specifies
many thoracic vertebrae and no forelimb.
4. Changes in Hox gene number. There is a correlation between the complexity
of the organism and the number of copies of the HOM-C comples. All
invertebrates have a single Hox complex
simplest = sponges (1 or 2 Hox genes)
more complex = insects Hom-C
Amphioxis – primitive invertebrate chordate (1 HOM-C)
Jawless fishes (earliest vertebrates) – 4 HOM-C ; represents a major leap in
complexity from amphioxis