The molecules used by imaginal discs to specify positional information may
be the same throughout the fly. That is, the discs may specify the respective
fates of their cell by the same mechanisms. This is called homologous specification.
Thus cells in the eye disc may respond to the same positional cues as cells
in the leg disc. Homologous specification can be seen with certain homeotic
mutants such as Antennapedia, in which antennal structures are transformed
into legs (Postlethwait and Schneiderman 1971). Occasionally, the entire antenna
becomes an entire leg, but is more common that only a portion of the antenna
becomes leg-like. In the latter cases, the replacement is absolutely position-specific.
The cells of the antenna disc that normally would have formed the distal tip
of the antenna (arista) are transformed into the most distal portion of the
leg (claw); cells specified to give rise to the second portion of the antenna
are transformed into the second portion (trochanter) of the leg. The corresponding
parts of the two structures are shown in Figure 1.
between portions of the antenna and portions of the leg. In the mutant
Antennapedia, regions of the antenna are transformed into leg structures.
The arrows show the portions of the antenna that form specific corresponding
portions of the leg. Such correspondence has also been seen in the transcription
patterns of genes such as salm. (After Postlethwait and Schneiderman
Postlethwait, J. H. and Schneiderman, H. A. 1971. Pattern formation and determination
in the antenna of the homeotic mutant Antennapedia of Drosophila melanogaster.
Dev. Biol. 25: 606-640.