Photoactivatable GFP resolves Drosophila mesoderm migration behaviour.

Mesoderm migration is a pivotal event in the early embryonic development of animals. One of the best-studied examples occurs during Drosophila gastrulation. Here, mesodermal cells invaginate, undergo an epithelial-to-mesenchymal transition (EMT), and spread out dorsally over the inner surface of the ectoderm. Although several genes required for spreading have been identified, our inability to visualise mesodermal cells in living embryos has left us to speculate about the cell rearrangements involved. Several mechanisms, such as chemotaxis towards a dorsally expressed attractant, differential affinity between mesodermal cells and the ectoderm, and convergent extension, have been proposed. Here we resolve the behaviour of Drosophila mesodermal cells in live embryos using photoactivatable-GFP fused to alpha-Tubulin (PAGFP-Tub). By photoactivating presumptive mesodermal cells before gastrulation, we could observe their migration over non-fluorescent ectodermal cells. We show that the outermost (outer) cells, which are in contact with the ectoderm, migrate dorsolaterally as a group but can be overtaken by more internal (inner) cells. Using laser-photoactivation of individual cells, we then show that inner cells adjacent to the centre of the furrow migrate dorsolaterally away from the midline to reach dorsal positions, while cells at the centre of the furrow disperse randomly across the mesoderm, before intercalating with outer cells. These movements are dependent on the FGF receptor Heartless. The results indicate that chemotactic movement and differential affinity are the primary drivers of mesodermal cell spreading. These characterisations pave the way for a more detailed analysis of gene function during early mesoderm development.