Class notes
BIOL2010 LT6 Homeotic Genes
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Covering the different types of Hox genes, how they function, with examples for each category. Extra reading with sources cited.
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Homeotic (HOM) genes in Drosophila and VertebratesSegments, Compartments and Parasegments
Head, T1-T3 = thorax, A1-A5 = abdomen, A7-A9 = genitalia
Mutations in segmentation genes result in Drosophila embryos lack certain segments
or parts of segments – however many mutations did not affect actual segments
- They affected the posterior compartment of one segment and the anterior
compartment of the immediately posterior segment
- Embryos delineated by parasegmental boundaries and not boundaries of the
segments
- Parasegment appears to be fundamental unit of embryonic gene expression
- Parasegmental organisation seen in the nerve cord of adult Drosophila, not seen
in the adult epidermis, nor it is found in musculature
- Adult structures are organised along segmental pattern – in Drosphila, segmental
grooves appear in the epidermis when germ band is retracted, while mesoderm
becomes segmental later in development
Transient grooves with segmental periodicity
appear during band extension and define
“parasegmental” boundaries – identified by
engrailed-lacZ
- En expressed in posterior ¼ of every
morphological segments
- En exressed in anterior ¼ of every
parasegment
- Primary pair rule genes show
parasegmental expression
- First pair rule genes expressed in parasegmental domains
- Engrailed expression defines the anterior compartment of each parasegment
and the posterior compartment of each segment
- Homeotic transformations respect parasegmental boundaries - cells of one
compartment do not mix with cells of neighbouring compartments, parasegments
and segments are out of phase by one compartment
But why should there be 2 modes of metamerism (sequential parts) in flies?
Jean Deutsch has proposed that such a 2-fold way of organising the body is needed
for coordination of movement
In every group of Arthropoda, the ganglia of the ventral nerve cord are organised
by parasegment but cuticle grooves and musculare are segmental
- View segmental border as a moveable hinge, this shift in frame by one
compartment allows the muscles on both sides of any particular epidermal
, segment to be coordinated by the same ganglion – allows rapid and coordinated
muscle contractions for locomotion
- While parts of body become secondarily organised according to segments,
parasegment is the basic unit of embryonic contstruction
- Similar situation occurs in vertebrates where posterior portion of the
anterior somite combines with anterior portion of next somite
Homeotic mutations – study done by Edward Lewis reveals how segmental identity is
established by homeotic genes
Bithorax mutations (only adult phenotype) –
transforms parts of T3 (haltere) into
equivalent parts T2 (wing) (both anterior),
bithoraxoid (poste rior)
- All bithorax mutations disrupt gene
Ultrabithorax (Ubx), deletions lethal
- Colinearity: effect matches position on
chromosome bithorax affects more than A, affects more than B – when all 3
genes deleted, all posterior segments become T2
Two complexes separated by 9.6Mb on chromosome 3 (what is between)?
Antennapedia complex Bithorax complex
First region on chromosome 3 Second region (Lewis, 1978)
Labial (lab), Antennapedia (Antp), sex 3 protein-coding genes found:
combs reduced (scr), deformed (dfd), Ultrabithorax (Ubx) – required for
proboscipedia (pb) third thoracic segment, abd A/B
Lab, pb, dfd, scr, Antp display Ubx deletions: T3 and A1 become T2
colinearity segments (distinguished by bristles
Antp: dominant gain of function and denticles)
mutation transforms antennae to Abdominal A (abd A) and B (abd B)
mesothoracic legs, normal function on changed identity of abdominal
T2 segments
Labial and deformed specify head Both Abd a/b located adjacent to Ubx
segments on Chr3 bithorax complex (specifies
Antp and scr give thoracic segments segmental identity in posterior part of
Pb appears to act only in adults, but embryo)
absence, the labial palps of mouth are
transformed into legs (Wakimoto et
al., 1984)
Mutations in labial affect most
anterior segments of head, loss of
function mutations in Antp affect T2
, Homeotic Selector Genes: after parasegmental boundaries are set, pair-rule and
gap genes interact to regulate homeotic selector genes which specify the
characteristic structures of each segment
- By the end of the cellular blastoderm stage, each segment primordium has been
given an individual identity by its combination of gap, pair-rule and homeotic
gene products
- 2 regions of Drosophila chromosome 3 contain most of these homeotic genes
Lewis’s Model + Homeotic Mutants
Demonstrate that the mutations are indeed ho
meotic – can be difficult as mutations only
affect phenotypes partially
Dominant mutations cause posterior-ward
mutations, recessive cause anterior-ward
mutations
Polycomb and extra sex comb mutants map do
not map on chromosome 3
PS identity reflects identity of poteriormost
homeotic gene active in that Ps
- Repressor: similar to a morphogen which is
strongest at the anterior end
- Repressor sensitivity becomes stronger at
the more posterior segments
- Bx has the lowest repressor sensitivy
Bx-C deletion elimantes all homeotic gene
activity – all PS4
Pc or Esc mutations inactivate the repressor –
all PS14
In pbx mutants, the locus is inactivity – PS5 =
PS6 both have same identity
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