Ectopic expression of transcription factors in eye-antennal discs of Drosop
hila strongly interferes with their developmental program. Early ectopic ex
pression in embryonic discs interferes with the developmental pathway prime
d by Eyeless and generates headless flies, which suggests that Eyeless is n
ecessary for initiating cell proliferation and development of both the eye
and antennal disc. Interference occurs through a block in the cell cycle th
at for some ectopic transcription factors is overcome by D-CycE or D-Myc. L
ate ectopic expression in cone cell precursors interferes with their differ
entiation. We propose that this developmental pathway interference is a gen
eral surveillance mechanism that eliminates most aberrations in the genetic
program during development and evolution, and thus seriously restricts the
pathways that evolution may take.