Mechanism of morphogen gradient scaling in growing tissue
In plain words
The signaling molecule Dpp forms a concentration gradient across the fly wing, and its decay length grows in proportion to the wing as it grows. Which chemistry keeps this proportion is disputed.
Precise statement
In the Drosophila wing disc, the Dpp gradient decay length $\lambda$ scales with disc length $L$ during growth, $\lambda/L \sim \mathrm{constant}$ (Wartlick and co-workers, 2011). Identify which parameter in $\lambda = \sqrt{D/k}$ ($D$ = effective diffusion constant, $k$ = degradation rate) is regulated and by which feedback (expander-repressor via Pentagone, intracellular recycling of Dpp, dilution by growth, receptor regulation), with a model that predicts $\lambda(L)$ in mutants.
What would settle it
Measurement of $D$ and $k$ versus disc size in wild type and feedback mutants, showing which one changes to keep $\lambda / L$ fixed.