The modular Gal4 system has proven to be an extremely useful tool for conditional gene expression in Drosophila. One limitation has been the inability of the system to work in the female germline. A modified Gal4 system that works throughout oogenesis is presented here. To achieve germline expression, it was critical to change the basal promoter and 3'-UTR in the Gal4-responsive expression vector (generating UASp). Basal promoters and heterologous 3'-UTRs are often considered neutral, but as shown here, can endow qualitative tissue-specificity to a chimeric transcript. The modified Gal4 system was used to investigate the role of the Drosophila FGF homologue branchless, ligand for the FGF receptor breathless, in border cell migration. FGF signaling guides tracheal cell migration in the embryo. However, misexpression of branchless in the ovary had no effect on border cell migration. Thus border cells and tracheal cells appear to be guided differently.

For all animals, cell migration is an essential and highly regulated process. Cells migrate to shape tissues, to vascularize tissues, in wound healing, and as part of the immune response. Unfortunately, tumor cells can also become migratory and invade surrounding tissues. Some cells migrate as individuals, but many cell types will, under physiological conditions, migrate collectively in tightly or loosely associated groups. This includes invasive tumor cells. This review discusses different types of collective cell migration, including sheet movement, sprouting and branching, streams, and free groups, and highlights recent findings that provide insight into cells' organization and behavior. Cells performing collective migration share many cell biological characteristics with independently migrating cells but, by affecting one another mechanically and via signaling, these cell groups are subject to additional regulation and constraints. New properties that emerge from this connectivity can contribute to shaping, guiding, and ultimately ensuring tissue function.

Genetic screens in Drosophila have lead to the discovery of many genes important for patterning and signal transduction in diverse organisms. Traditionally, the phenotypic effects of loss-of-function mutations are analyzed. As an alternative way to link genes and function,