In an article entitled “Pattern formation in the embryo and imaginal discs of Drosophila: What are the links?”, Adam Wilkins and David Gubb posed a question that was on the minds of many researchers at that time (1991). The embryo's segmentation hierarchy was basically understood, but it was unclear what these various genes might be doing in discs. Wilkins and Gubb argued that segment-polarity genes supply the angular values of the Polar Coordinate (PC) Model, which until then had only been an abstract formalism.

Several predictions of this “Angular Values Conjecture” were soon put to the test by molecular genetics. Chief among them was the expectation that LOF and GOF alterations in segment-polarity genes should reorganize disc anatomy. This prophecy was indeed fulfilled, and the experimental probing uncovered a trove of insights into the machinery of disc patterning.

The conjecture itself, however, turned out to be wrong. Segment-polarity genes do not paint a pinwheel on each disc. Rather, they draw a few important lines – the compartment boundaries.

The Molecular Epoch of disc research was launched in 1991

The Wilkins-Gubb paper, with its clarion call for a molecular assault on discs, provides a convenient demarcation between the Cellular and Molecular Epochs of disc research. In the 1990s, many links were patiently forged between the blastoderm and the adult. During this process, several old puzzles at the cellular level were solved by clever experiments at the molecular level.