Null loss-of-function alleles result in embryonic
lethality. Animals succumb at the end of embryogenesis and
show homeotic transformations in the larval cuticle of the
first, second, and third thoracic segments. Specifically the
cuticle derived from parasegments 4 and 5 are transformed to a
more anterior identity such that the posterior of the first
thorax produces fragments of mouth hook material on its dorsal
surface presumably owing to a new posterior labial identity,
whereas the anterior of the second thorax resembles the first
thorax. The anterior of the third thoracic segment is weakly
transformed toward a T1-like identity. The posterior of T2 is
presumably T1 like as there are no gnathal structures seen in
this compartment. There are also partial loss-of-function
mutations which allow survival into the larval, pupal, and
adult stages. Those that allow adult survival produce animals
in which the anterior of the dorsal mesothorax shows a
transformation to prothorax. There are no other apparent
defects associated with these lesions. Those "leaky" mutants
which die in the pupal and larval stages show similar parasegmental transformations as the null alleles, except that only
the parasegment 4 to 3 homeosis is generally apparent.
Animals which survive to the pupal stage fail to evert their
anterior spiracles resulting in a blunt appearance of the
anterior pupa. This same phenotype is seen in genotypes which
survive to the adult stage. These partial mutants in many
cases are associated with chromosome rearrangements notably
deletions which approach the locus from its distal end. Moreover these mutations have been shown to complement fully other
seemingly null mutations. Subsequent molecular analyses have
shown that these results are accounted for by the presence of
two promotors, one, P1, distal to the other, P2. The partial
mutants affect the ability of the P1 promotor to initiate
transcription, while the complementing lesions inactivate P2.
Null mutants affect the transcription unit and protein encoding portion of the gene which is common to both promotors (see
below).
X-ray induced somatic clones of Antp- cells demonstrate that
the locus is required in the adult for the proper development
of the dorsal pro and mesothorax, and legs. The former is
reduced in size presumably reflecting an anteriorward
transformation while the latter are transformed to antennae.
Thus Antp+ function is required in the embryo and adult in
parasegments 4 and 5 to prevent more anterior segmental identities, specifically those normally found in the anterior
thorax and head.
The Antp locus was initially recognized by virtue of several
striking dominant gain-of-function alleles. Thirteen of these
transform the antenna of the adult into a mesothoracic leg
(Antp49, AntpB, AntpYu, AntpPw, AntpLC, AntpR, AntpWu, Antp50,
AntpRM, Antp73b, AntpCB, Antp72j, and AntpNs). Three of these
also have effects on the orbit of the eye and the vibrissal
region of the ventral head (AntpRM, Antp72j, and AntpNs).
There are also two dominant alleles (AntpCtx and AntpW) which
transform portions of the head capsule (dorsal and posterior)
and the eye to a dorsal mesothoracic identity. In some cases
this includes the production of wing tissue in the eye.
Finally, a unique dominant AntpHu produces bristles on the
normally bald propleurae just ventral to the mesothoracic
spiricle. This latter phenotype has been interpreted as the
production of sternopleural bristles on the propleurae, and
thus a T1 to T2 transformation. With the exception of AntpNs
and Antp72j all these dominant lesions are associated with
recessive lethality and gross chromosome rearrangements. All
the breakpoints fall in the interval between the distal and
proximal promotors. The dominant gain-of-function phenotype
results from the misregulation of the P2 promotor by position
affect or by the production of novel transcripts initiated in
the newly juxtaposed sequences and spliced to the downstream
Antp coding sequences. Both events result in the ectopic
accumulation of the Antp protein product in the eye-antennal
disc where the normal head repressive function of the gene
causes the observed alteration. The recessive lethality associated with these lesions falls into the partially deficient
category mentioned above. That is, these lesions show complementation with the P2 specific (Antp1 and Antp23) mutations
and in general show only strong parasegment 4 -> parasegment 3
transformations. However, there is a gradient of this affect
among the breakpoints. Those closest to P1 and furthest from
P2 are the weakest, whereas those close to P2 show the strongest phenotype and earlier lethal phase. This same result is
obtained with breakpoint mutations in the P2-to-P1 interval
which are not associated with a dominant phenotype. Therefore
this interval likely contains sequences necessary for the
proper regulation of the P2 promoter.
Three of the dominant gain-of-function lesions (AntpHu,
Antp73b, and AntpNs) have been reverted. The revertants are
either complete nulls, thus obviating the potential for ectopic expression, or are partial mutants; the latter mutants
likely remove the potential for ectopic expression by altering
the juxtaposed sequences required for abnormal P2 activity.
Both in situ hybridization and immunostaining have been used
to determine the spatio-temporal pattern of Antp expression.
Both the protein and RNA are strongly accumulated in the ventral nerve cord and more weakly in the epidermis and mesoderm
of the embryo. Protein and RNA are first detected during cellular blastoderm in a band of cells in the parasegment 4-6
anlagen. This initial spatial pattern is further elaborated
at full germ-band extension. In the ectoderm Antp products
are found starting in the region of the first thoracic segment
(parasegments 3 and 4) and extending posteriorly to the level
of the seventh abdominal segment. In the mesoderm, they are
found in parasegments 4-6. During germ band shortening the
gene products are accumulated in the CNS from parasegment 4
(posterior T1) through to the posterior end of the ventral
nerve cord. In the integument transcripts and protein are
mainly restricted to the parasegments 4-5 interval although
some weak expression can be seen in parasegments 3. As
embryogenesis proceeds, the posterior CNS expression diminishes but is still detectable at the end of embryogenesis.
The major accumulation in the CNS at this time is in the neuromeres of parasegments 4 and 5. The mesodermal expression is
found in the anterior midgut; quenching of Antp expression is
found in the posterior portion of the anterior midgut and has
been shown to be dependent on the expression of Ubx. In later
stages Antp protein can be detected in the leg, dorsal
prothoracic, and wing discs.
Males and heterozygous females show variable expression from small additional segment on the third antennal segment to a nearly complete leg including femur, tibia, and
tarsus. Arista usually present. Homozygous females lethal
but X0 males survive. Crosses involving either Apx males or
females produce many inviable embryos.
