Quantifying the Gurken morphogen gradient in Drosophila oogenesis.

Quantitative information about the distribution of morphogens is crucial for understanding their effects on cell-fate determination, yet it is difficult to obtain through direct measurements. We have developed a parameter estimation approach for quantifying the spatial distribution of Gurken, a TGFalpha-like EGFR ligand that acts as a morphogen in Drosophila oogenesis. Modeling of Gurken/EGFR system shows that the shape of the Gurken gradient is controlled by a single dimensionless parameter, the Thiele modulus, which reflects the relative importance of ligand diffusion and degradation. By combining the model with genetic alterations of EGFR levels, we have estimated the value of the Thiele modulus in the wild-type egg chamber. This provides a direct characterization of the shape of the Gurken gradient and demonstrates how parameter estimation techniques can be used to quantify morphogen gradients in development.

Quantitative analysis of the GAL4/UAS system in Drosophila oogenesis.

The GAL4/UAS system is extensively used for targeted gene expression in Drosophila, but the strength of the GAL4 drivers and their effects on target genes are rarely quantified. Quantitative information about the strength of the perturbations introduced by the GAL4/UAS system would further expand the usefulness of the GAL4/UAS system in studying gene functions and developmental processes. We have developed an assay to determine the relative level of expression for target genes tagged with green fluorescent protein (GFP). Our assay enables the relative quantitation of fluorescent proteins within specific cell types and developmental time windows in living eggs/embryos, and permits the analysis of samples from a broad expression range. We illustrate the assay using a panel of four GAL4 drivers and three UAS responder lines in Drosophila oogenesis, discuss the issues associated with the interpretation of the quantitative data, and correlate our results with the analysis of the GAL4/UAS system at the transcript level. The imaging-based strategy described here can be used to quantify other GAL4 drivers in Drosophila and other organisms.