Drosophila axis extension is robust to an orthogonal pull by invaginating mesoderm

As tissues grow and change shape during animal development, they physically pull and push on each other and these mechanical interactions can be important for morphogenesis. During Drosophila gastrulation, mesoderm invagination temporally overlaps with the extension of the ectodermal germband; the latter is caused primarily by Myosin II-driven polarised cell intercalation. Here we investigate the impact of mesoderm invagination on ectoderm extension, examining possible mechanical and mechanotransductive effects on Myosin II recruitment and polarised cell intercalation. We find that the germband ectoderm is deformed by the mesoderm pulling in the orthogonal direction, showing mechanical coupling between these tissues. However, we do not find a significant change in Myosin II planar polarisation in response to mesoderm invagination, nor an effect on the rate of junction shrinkage leading to cell intercalation events. We find some impact on the orientation of neighbour exchange events, and an increased rate of growth of new cell junctions, but this makes little difference to the rate of cell intercalation. We conclude that the cellular mechanisms of axis extension are robust to the mechanical pull of mesoderm invagination.